Near infrared organic semiconducting materials for bulk heterojunction and dye-sensitized solar cells.

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Near Infrared Organic Semiconducting Materials for Bulk Heterojunction and Dye-Sensitized Solar Cells Surya Prakash Singh*[a] and G. D. Sharma*[b] Inorganic & Physical Chemistry Division, CSIR-Indian Institute of Chemical Technolog, Hyderabad 500607 (India) E-mail: [email protected] [b] R & D center for Engineering and Science, JEC group of Colleges, Jaipur Engineering College Campus, Kukas, Jaipur (India) E-mail: [email protected]

[a]

Received: November 13, 2013 Published online: ■■

ABSTRACT: Dye sensitized solar cells (DSSCs) and bulk heterojunction (BHJ) solar cells have been the subject of intensive academic interest over the past two decades, and significant commercial effort has been directed towards this area with the vison of developing the next generation of low cost solar cells. Materials development has played a vital role in the dramatic improvement of both DSSC and BHJ solar cell performance in the recent years. Organic conjugated polymers and small molecules that absorb solar light in the visible and near infrared (NIR) regions represent a class of emering materials and show a great potential for the use of different optoelectronic devices such as DSSCs and BHJ solar cells. This account describes the emering class of near infrared (NIR) organic polymers and small molecules having donor and acceptors units, and explores their potential applications in the DSSCs and BHJ solar cells. DOI 10.1002/tcr.201300041 Keywords: donor-acceptor systems, materials science, near-infrared, organic polymers, solar cells

1 Introduction Infrared (IR) radiation is the electromagnetic radiation whose wavelength is longer than the longest wavelength of the red color in visible light but shorter than microwave. The span of the IR region can be divided into three parts: near IR, mid IR and far IR. The near infrared (NIR) falls in the wavelength region that covers 750 nm to 2500 nm. NIR materials are defined as the substances that interact with NIR light via absorption and reflection, and emit NIR light under external stimulation such as photo-excitation, electric field and chemical reaction. NIR materials can generally be classified into two groups: inorganic materials including metal oxides and semiconductor nanocrystals, and organic materials including metal complexes, ionic dyes and extended π-conjugated chromophores and donor–acceptor charge transfer chromophores.

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Research on NIR materials and technology is motivated by curiosity in the fundamental study and practical applications in a number of important sectors such as energy, communications, bio-imaging, sensing and advanced optoelectronics devices. As can be seen from Figure 1, nearly 50% of solar energy falls in the NIR region, therefore, it is neceassry to develop materials which absoprb light in NIR region of the solar spectrum. NIR absorbing organic materials and their application are diverse and extensive in many technological sectors, such as NIR absorbing pigments for laser printers and digital copying machines as charge generation materials[1] and in optical disks (e.g. CDR) as information storage materials.[2] Due to the environmental constraints and predicatable exhaustion of fossil fuel resources, it is one of the major

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challenges in the 21st centurary to find clean and renewable energy. Solar energy which has the potential to act as renewable and clean sources is believed to be one of the promising and long term alternatives to replace fossil energy in the future. Solar cell is a device that converts solar energy into electrical energy. Currently, solar cells based on silicon have been practically used; however, the high cost hinders their wide application. Organic solar cells are promising candidates for photovoltaic applications, because of their light weight and low cost solution processing through roll to roll printing.[3] Organic solar cells have been developed by the bulk heterojunction (BHJ) concept[4] which consists of a blend of the small

molecules or conjugated polymer as donor and fullerene derivatives as acceptor component for the photoactive layer thin film. The resulting BHJ structure not only provides a large area donor–acceptor interface for charge separation, but also leads to a bicontinuous interpenetrating network for charge transport. At present, one of the key limiting factors for the power conversion efficiency of organic solar cells is the mismatch of absorption spectrum of photoactive layer with the solar spectrum. The photon flux of sunlight displays a maximum at around 690 nm (1.8 eV).[5] Thus to achieve ideal spectral overlap of the absorption of active material used in solar cells with solar irradiation, materials with broad absorption band ranging from visible to near IR region are required.[6] Moreover, photoactive layers with NIR absorption render transparent solar cells for sun shading and solar power windows applications.[7] The electrical and optical properties of organic material are primarily governed by the electron donor (D) units and electron acceptor (A) units linked via π-conjugated bridge. Photo-excitation leads to intramolecular charge-transfer from the D unit to the A unit.[8] Such a D-A combination allows for band gap tuning through hybridization of the highest occupied molecular orbital (HOMO) of the donor moiety with the lowest unoccupied molecular orbital (LUMO) of the acceptor moiety. Such a redistribution of electron density leads to absorption bands that can extend from the ultraviolet to well into the near infrared regions of the spectrum, depending on the electronic offset between the donor and acceptor components and the overall delocalization length.[9] This review article summarizes the various types of small molecules and polymers for BHJ organic solar cells and DSSCs.

Dr. Surya Prakash Singh is a Scientist at CSIR-Indian Institute of Chemical Technology, Hyderabad, since Sept. 2011. He studied chemistry at the University of Allahabad, India, and obtained his D. Phil. degree in 2005. Later, he joined CSIR-Indian Institute of Chemical Technology, Hyderabad as Research associate. After working at Nagoya Institute of Technology, Japan, in the group of Prof. Takeshi Toru, as a postdoctoral fellow (2006–2008), he joined Osaka University, in 2008 as an Assistant Professor. He worked as a researcher at Photovoltaic Materials Unit, National Institute for Materials Science (NIMS), Tsukuba, Japan (2010–2011). He has been involved in the design and synthesis of materials for dyesensitized solar cells, organic solar cells and quantum dot solar cells and has published over 80 papers and reviews in peerreviewed journals and patents, edited two books and authored one book chapter.

G. D. Sharma was born in 1959. He received his Ph.D. degree in Physics from Indian Institute of Technology, New Delhi, India. After that he joined as Assistant Professor in November 1985 at Department of Physics, JNV University, Jodhpur and was promoted to the post of Professor in same department in 2004. During 1990–1991, he worked as postdoctoral fellow in the Department of Electrical Engineering, The State University of New Jersey, USA, and worked on organic photovoltaic devices. At present, he is working as Director (Academic and Research) of JEC group of Colleges, Jaipur Engineering College campus, Kukas, Jaipur. His research area includes organic solar cells based on conjugated polymers, small molecules, and dye sensitized solar cells. He’s had national and international research collaboration, published more than 170 research papers in international journals. More than 14 students have completed their Ph.D. degree under his supervision.

Fig. 1. Spectrum of solar light.


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Near-IR Organic Semiconducting Materials for Solar Cells

The band gap of conjugated organic materials can be decreased by extension of conjugation length, to a certain extent. It has been shown by theoretical calculations that energy levels of oligothiophenes vary with different number of the repeated units. By adding every extra thiophene unit, the energy level hybridization continues, leading to a decrease in the energy gap.[10] For series of rylene diimides extension of π-conjugation also leads to gradual bathochromic shift of the absorption maxima.[11] By having a different number of fused naphthalene units in the rylene diimide dyes, the energy gap can be tuned over a quite large range, e.g., from roughly 1.25 eV to 2.15 eV. However, the energy gap of a conjugated π-system can only be lowered to a certain value by increasing the chain length, as the overall π-system is governed by the effective conjugated length. Therefore, change of the conjugation length is effective in tuning the energy gap level within a certain range but is still unknown or challenging to realize its limitation in terms of the lowest energy gap, especially for small molecules and oligomers.[12]

single bond, or a so-called Peierls gap.[13] Peierls predicted that smaller bond length alteration results in a lower value of energy gap of a conjugated compound. For example, polyacetylene would be a metallic conductor if the distance between all carbons atoms were identical.[14] Therefore, minimizing the bond length alternation is an important step towards the reduction of the energy gap of conjugated molecules. The π-conjugated systems mainly include linear alternant polyene (CH)x and polyarylene compounds. The major difference between the former and the latter is the resonance energy of the monomer unit. In the former, the electron can delocalize along the chain if there is no deformation or defect, while in the latter system there exists a competition for π-electrons between confinement within the rings and delocalization along the chain due to the aromaticity.[15] The aromaticity resonance energy, which is defined as the energy difference between the aromatic structure and a hypothetical reference consisting of isolated double bonds,[16] is thus a measure for the stability and delocalization of a conjugated monomeric unit, which in turn decides the final energy band gap of conjugated compounds and oligomers. As a result, the energy gap of aromatic molecules is larger than that of polyene molecules. Accordingly, the resonance energy of a conjugated building block should be taken into account for the molecular design. Another difference between polyene and polyarylene π-conjugated systems is the non-degenerate ground state. The latter has two limiting mesomeric forms, aromatic and quinoid, which are not energetically equivalent.[17] In most cases, the quinoid form has a smaller energy band gap.[18] Therefore, an increase in the quinoid character in the design of organic materials can be expected to effectively reduce the energy gap. At the same time, the quinoid structure represents an increased double bond character of the bonds between the repeat units, leading to the reduction of bond length alternation. This approach has been routinely taken for the synthesis of NIR organic semiconducting materials.[19]

2.2 Effect of Bond Length Alternation

2.3 Effect of Donor–Acceptor Units

In a typical π-conjugated system, the energy gap originates from the bond length alternation between the double and

Introduction of a strong electron donor (D) and a strong electron acceptor (A) in conjugated small molecules and polymers has been employed as a common strategy for the lowering of the energy band gap.[20] The D-A types of chromophores exhibit two resonance forms (D − A↔+D = A−), which gives rise to an increased double bond character between D and A units, thus reducing the bond length alternation, resulting in a decrease in the Peierls gap. The hybridization of energy levels of donor and acceptor can raise the energetic level of the HOMO higher than that of the donor and lower the energetic level of the LUMO below that of the acceptor, leading to small HOMO-LUMO separation,[21] as shown in Figure 3. For example, the unsubstituted bis-azomethine compound exhibits

2 Molecular Design and Band Gap Tuning of Organic Materials The optical and electrical properties of organic materials are determined by their energy gap, which is the energy separation between the HOMO and LUMO energy levels, and also called as HOMO-LUMO energy band gap. The red shift in the absorption and emission spectra results from the narrowing of the band gap. Therefore the band gap narrowing is the key factor in the design of low band gap polymers and small molecules. For the tuning of the band gap of organic materials, several factors such as conjugation length, bond length alternation, and donor–acceptor charge transfer should be considered as shown in Figure 2. 2.1 Effect of Conjugation Length

Fig. 2. Factors affecting the optical band gap of the organic material.

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3 Applications of NIR Absorbing Organic Materials

Fig. 3. Interaction of energy levels of a donor (D) and acceptor (A) leading to a narrower band gap.

maximum absorption and emission at 440 nm and 534 nm, respectively. However, by substituting the nitro and diethylamino groups at both ends, the push-push and pull-pull compounds (DD and AA) give a red shift of 110 nm and 41 nm in absorption, respectively, relative to the unsubstituted compound. Dufresne et al. have reported that in the presence of one donor and one acceptor at each end, the push-pull compound has the most pronounced bathochromic shift in absorption (148 nm) and emission (126 nm).[22] The spacer unit between the donor and acceptor also play an important role in controlling the energy band gap of organic semiconducting materials, because it can change the interaction of the electronic orbital of donor and acceptor moieties.[23] Moreover, the D-A charge transfer interaction can be further enhanced by attaching multiple electron donating groups at the appropriate positions of the acceptors or with different donor –acceptor topology.[24]

2.4 Other Effects of Band Gap Tuning Intermolecular interactions such has hydrogen bonding, molecular stacking and charge transfer also cause a change in the band gap of organic small molecules and conjugated polymers in solid state thin film form.[25] The polar effect of heavy atoms on the energy gap should also be considered while designing NIR absorbing organic materials. It is reported that the lowering of energy gap involves the replacement of sulfur and oxygen atoms with heavier ones such as selenium and tellurium in a conjugated system.[26] A red shift of 67 nm in absorption maximum was observed in a simple di-phenylbenzobisthiadiazole compound when sulfur was replaced with selenium.[27]


In photochemistry, a NIR absorbing material is a compound that readily undergoes photo-excitation under NIR light and then transfers its energy to other molecules, thus making the mixture in a reaction or a chemical process more sensitive to the light at specific wavelengths. Using NIR photosensitizing organic materials in both dye sensitized solar cells and small molecule /conjugated polymer bulk heterojunction (BHJ) solar cells has recently received tremendous interest, aiming to increase the power conversion efficiency (PCE) by capturing and converting more than 50% of solar energy that falls in the NIR spectral region (above 700 nm). Since the first report of an organic photovoltaic cell by Tang[28], the research and development of organic solar cells has been pushed by potential commercial uses as justified mainly by the low cost of both materials and fabrication processes at low temperature[29] and encouraged by the promising and steadily improving performance of dye sensitized solar cells and bulk heterojunction solar cells.[30] 3.1 Dye Sensitized Solar Cells Based on NIR Metal Free Dyes Dye sensitized solar cells (DSSCs) have attracted considerable attention in recent years.[31] At present, state-of-the-art DSSCs based on ruthenium(II)-polypyridyl complexes as the active material have an overall power conversion efficiency (PCE) approaching 11% under standard (Global Air Mass 1.5) illumination.[32] The conventional DSSCs typically contain four components, namely a photoanode consisting of mesoporous nanocrystalline semiconductor metal oxide film, sensitizer (dye), an electrolyte or hole transporter material (HTM) and a counterelectrode. In DSSCs, incoming light is absorbed by the sensitizer containing an achoring group, which is anchored to the surface of a nanocrystalline metal oxide semiconductor film (generally TiO2 or ZnO and other metal oxide). Charge separation takes place at the interface through the photo-induced electron injection from the excited state (LUMO) of the dye into the conduction band of TiO2 or ZnO. Holes are created into the ground state (HOMO) of the dye, which is further regenerated through the reduction of electrolyte or HTM, which itself is regenerated at the counter electrode by electrons through an external circuit. In general, for efficient DSSCs, the regeneration of sensitizer by an electrolyte or HTM should be faster than the recombination of the conduction band (CB) electrons with oxidized sensitizers. Additionally, the HOMO of the should lies below the energy level of HTM or electrolyte, so that the oxidized dye formed after the electron injection into the conduction band of TiO2 can be effectively regenerated by accepting the electrons from

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Fig. 4. General operating principle of DSSCs, where TCO stands for transparent conducting oxide.

the HTM or electrolyte. The general operating principle of DSSCs is shown in Figure 4. On the basis of the accumulated knowledge of the organic dyes used as sensitizer for DSSCs, following are the requirements for the molecular design of organic dyes: (1) The organic dye must have at least one anchoring group (e.g., –COOH, –SO3H, –PO3H2, –OH) for adsorption onto the TiO2 surface. In particular, a carboxy group can form an ester linkage with the TiO2 surface to provide a strongly bound dye and good electron communication between them. (2) To achieve efficient electron injection from the excited dye to the CB of TiO2, the energy level of the LUMO of the dye must be higher (more negative) than the conduction band (CB) of the TiO2 electrode. On the other hand, to achieve efficient regeneration of the oxidized state by electron transfer from the I 3− I − redox couple in the electrolyte, the energy level of the HOMO of the dye must be lower (more positive) than the I 3− I − redox potential. (3) To give high light-harvesting efficiency, the organic dye must have high molar absorption coefficients over the wide region of sunlight, to provide a large photocurrent. (4) To achieve a durable DSSC, the organic dye must have chemical stability in its photo-excited state and in the redox reactions throughout the reaction cycle. (5) Dye aggregation on the TiO2 surface, leading to low conversion efficiency of the DSSC, should be avoided. In particular, donor–acceptor π-conjugated dyes are liable to undergo π-stacked aggregation on the TiO2 surface, which leads to reduction in electron-injection yield from the dyes to the CB of the TiO2 owing to intermolecular energy transfer. (6) Recombination of injected electrons with dye that result in dark current should be suppressed.

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In last 15–20 years, a lot of efforts has been devoted to the synthesis and investigation of materials for DSSCs. Efforts in synthesis of sensitizers for DSSCs can be grouped into two broad areas: (a) Functional ruthenium(II)-polypyridyl complexes such as N3, N719 (TBA+ = tetra-n-butylammonium), Z907 and black dye (the chemical structure along with the PCE of DSSCs using these dyes as sensitizers are shown in Figure 5). These materials contain expensive ruthenium metal that requires careful synthesis and tricky purification steps. (b) Metal free dyes, which can be prepared rather inexpensively by following established design strategies. The major advantages of these metal-free dyes are their tunable absorption and electrochemical properties through suitable molecular design.[33] One of the drawbacks of the ruthenium based dyes is the limited absorption in the near IR region of the solar spectrum. Porphyrins and phthalocyanine systems exhibits intense spectral response bands in the near IR region and posses good chemical, photo- and thermal stability, providing good potential candidates as sensitizers for DSSCs. The introduction of porphyrins and chlorophylls as photosensitizers on DSSCs is particularly interesting given their primary role in photosynthesis. Owing to appropriate LUMO and HOMO energy levels and very strong absorption of the Soret band in the 400–550 nm region, as well as the Q-band in the 500–800 nm region,[34] porphyrin derivatives can be suited as panchromatic photosensitizers for DSSCs. Several studies have demonstrated that porphyrin dyes can show efficient photo-induced electron injection into the conduction band of TiO2.[35] 3.1.1 Porphyrin Dye Based DSSCs Kay and Grätzel, in pioneering work on meso-porphyrin dye (Por1) (Figure 6), reported high IPCE values of over 80% and an efficiency value of 2.6%.[36] They pointed to the importance



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stabilized and concentrated onto the substituent through π-conjugation, causing enhancement and red shifts of absorp-

Fig. 5. Molecular structure of the ruthenium based dyes i.e. N3, N719, Z907 and black dye used as sensitizers in DSSCs.

of using co-adsorbers to prevent unfavorable aggregation of this dye. Wamser and Cherian obtained IPCE values of 55% at the Soret peak and 25–45% at the Q-band for similar porphyrin dyes, achieving an overall energy conversion efficiency of about 3%.[37] Nazeeruddin and co-workers developed a new family of porphyrin dyes with different central metal ions (Cu(II) or Zn(II)) and different anchoring groups (-COOH or -PO3H2).[38] They concluded that for porphyrins with carboxylic anchoring groups, the diamagnetic metalloporphyrins containing zinc showed very high IPCE values in comparison to that of the paramagnetic metalloporphyrins containing Cu. Moreover, porphyrins with a phosphonate anchoring group showed lower efficiencies than those with a carboxylate anchoring group. The DSSCs based on dye with carboxylate anchoring group, fabricated in a THF dye bath, achieved a higher IPCE value of 75% and overall efficiency of 4.8%. Through modifying the meso-tetraphenylporphyrins by substitution at the β-position with functional groups, one could extend the π-systems to enhance the red-absorbing Q-bands due to the splitting of the four frontier molecular orbitals. Officer and co-workers synthesized a series of porphyrin photosensitzers[39] and used them for DSSCs. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations reveal that the electrons in the LUMO orbitals of porphyrin dyes are


tion spectra and increasing the possibility of electron transfer from the substituent. The cells based on porphyrin dye of this structure yielded close to 85% IPCE with a corresponding overall efficiency of 5.6% under AM 1.5 G irradiation.[40] As a consequence, they further extended the π-system in the β-position and obtained more-efficient porphyrin sensitizers.[41] Porphyrin dye (Por2) (Figure 6) exhibited an IPCE value of up to 75% and an impressive efficiency of 7.1% in DSSCs with a liquid electrolyte and achieved a conversion efficiency of 2.4% in solid-state DSSCs. It proved that modification of the β-position of porphyrin is an effective strategy to optimize the energy levels and improve the efficiency of DSSCs. Mozer and co-workers studied the reason why the porphyrin dyes showed lower photovoltage compared with Ru sensitizers.[42] Their results revealed that a significantly reduced electron lifetime in porphyrin-based DSSCs is the main reason for their generally lower open circuit voltage. The role of para-alkyl substituents on mesophenylporphyrin sensitized DSSCs was studied by Ballester and co-workers, who discovered that the presence of hydrophobic alkyl chains on the molecular structures of porphyrins could decrease the recombination between the injected electrons and the electrolyte without influencing the electron recombination between the injected electrons and the oxidized dye and dye regeneration.[43] Kim and co-workers investigated electronic and photovoltaic properties of functionalized porphyrins at meso and β-positions with different carboxylic acid groups in DSSCs and claimed that the effective electronic coupling through the bridge played an important role in the charge injection process.[44] The longer distance between the dyes and the TiO2 surface exhibited better performance due to a slow charge recombination rate. Furthermore, the doubly diene substituted porphyrin at two β-positions achieved better photovoltaic performance (PCE about 3.0%) than that of the mono-β-dienyl-porphyrin. Yeh and co-workers also designed and synthesized a series meso- or β-porphyrins with a carboxyl group and used as sensitizers for DSSCs.[45] Through adjustment of photophysical and electrochemical properties with different substituents, these compounds showed the best photovoltaic properties with 6.0% efficiency under AM 1.5 G illumination. Tan and co-workers reported three new porphyrin dyes (Por16, Por17 and Por18 as shown in Figure 8) with a D-π-A structure: the porphyrin acted as a donor and cyanoacrylic acid acted as an acceptor as well as anchoring group; different thiophene derivatives acted as a π-bridge to broaden the absorption of the dyes.[46] Three methylphenyl groups are introduced to reduce aggregation between neighboring porphyrins on the TiO2 surface by sterically hindering the porphyrin core. Among the three porphyrin dyes, Por17 exhibits the maximum PCE of 5.14%, which is 64% of the value for N719

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Fig. 6. Molecular structure different Por1-Por7 porphyrin dyes.

under similar conditions. These results show that a thiophene π-conjugation unit improves the light harvesting capabilities of porphyrin dyes, and the length as well as the alkyl chain of this unit influence the charge transport efficiency, both of which are crucial for the photovoltaic performance of the DSSCs. Recently, Grätzel et al. have reported[47] a DSSC made from “push-pull” porphyrin dye Por3 (R=C6H13) with a PCE of 11%, which is very close to the values characteristic of ruthenium-based DSSCs, making this porphyrin the most efficient green dye for DSSC applications. The elongation of the π-conjugated system of porphyrin monoacids produces a decrease in the HOMO–LUMO gap,

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with a concomitant red shift in both the Soret and Q-band absorptions. It was therefore predicted that improvement of the visible light harvesting efficiency in porphyrin dyes would lead to better photovoltaic responses of DSSCs. Indeed, several authors have demonstrated this effect by comparing the photovoltaic responses of π-elongated porphyrin monoacids with respect to derivatives lacking this property. For example, Imahori and co-workers have compared porphyrins Por4 and Por5,[48] in which a naphthyl bridge introduced at the meso position is additionally fused to the porphyrin core in the case of compound Por5, producing an efficient elongation of the π-system. The PCE values of DSSCs made of Por5 were



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Fig. 7. Chemical structures of push-pull type porphyrin dyes (Por8-Por12).

significantly improved (4.1% versus 2.8% in the case of porphyrin Por4) as a consequence of enhanced photocurrent generation in the region between 650 and 800 nm. Lin et al. have demonstrated that porphyrin absorption properties can be significantly affected by including different acene units into the π-conjugated system of porphyrin Por6.[50] In particular, the incorporation of benzene, naphthalene, anthracene, tetracene and pentacene units in porphyrins Por6 (Figure 6) produces a systematic increase in the porphyrin Q-band broadening and red-shifting. The best balance between improving light harvesting properties by increasing the number of acene units and performance of the DSSCs was found for derivative Por6c (PCE of 5.4%). The remarkably poor performance of compound Por6e, which exhibits very broad absorption bands, has been explained to be caused by an inefficient electron injection into TiO2 due to nonradiative relaxation of the molecule in the singlet excited state. β,β-fused mono-carboxylic porphyrins exhibiting a π-elongated system such as Por7 have also been employed as sensitizers in DSSCs.[50] A high PCE value of about 6.3% has been obtained, further confirming the influence of the light harvesting properties in the improvement of the photovoltaic performance of porphyrin sensitized solar cells. The incorporation of “push-pull” porphyrin derivatives as sensitizers in DSSCs has led to remarkable progress in this


family of sensitizers, reaching PCE values comparable to that of the ruthenium bipyridyl complexes. Among them, derivative Por3 (R=tert-butyl) (Figure 6), in which a diarylamino group is attached to the meso position of the porphyrin, opposite to the anchoring group, has allowed for the preparation of solar cells exhibiting 6.0% overall efficiencies.[45] In fact, this success has been attributed to a good compromise between suppression of dye aggregation by introducing bulky tert-butyl groups onto the aryl substituents, extension of the π-conjugated system and enhancement of the charge transfer directionality in the excited state by introduction of an electron donating group opposite to the anchoring group, thus optimizing the device performance. Indeed, the benefit of introducing amino groups on the aryl substituent opposite to the binding group of porphyrins Por8 (Figure 7) has been demonstrated by comparing the photovoltaic responses of derivatives bearing different electron-donating or withdrawing groups.[51] The superior performance of derivative Por8a (6.1% efficiency), in contrast with the low efficiency values obtained with compound Por8d (1.1%), has been justified by the strong electron-donating effect of the Me2N substituent, which additionally produces a broadening and red-shifting of the absorption spectrum. Recently Yeh et al synthesized as series of porphyrin dyes similar to Por3. In one of them, the two R groups in the diarylamino substituent were replaced by hexyl chains, improv-

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Fig. 8. Chemical structures of Por13-Por18 porphyrin dyes having different anchoring groups.

ing the device performance to up to 6.6% of power conversion.[52] In other examples, for porphyrins Por9 and Por10, the influence of the nature and the position of the electron donating group on the photovoltaic performance of these porphyrin sensitized solar cells has been tested. In general, increasing the number of diarylamino and/or triarylamino groups enhances the light harvesting efficiency of the porphyrin through the extension of π-conjugation, which is an essential requirement for DSSCs. However, dye aggregation on the TiO2 surface may be the reason for the modest device efficiencies (4.2 to 5.5%) obtained with these derivatives.[53] The phenyl linker group in porphyrin Por3 has been replaced either by naphthalene (Por11)[49] or anthracene (Por12) in order to produce a redshift in the Soret and Q-bands as the π-system was expanded.[54] Remarkable PCE values of 6.7%, beyond that of N719 (6.1%) with no added scattering layer, were obtained with the green sensitizer Por11. With the addition of a scattering layer, the cell performance of porphyrin Por11 was comparable to that of N719 (PCE = 6.9 vs. 7.3%). In

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continuation of their work in these porphyrin dyes as sensitizers, Yeh el. al have reported PCE values of 6.8% and 7.0%.[52b] More recently, new porphyrin sensitizers Por13 and Por14 have been prepared in which the carboxylic acid based anchoring group has been modified.[39] In addition to stronger acidity, the introduction of an electron-withdrawing α-CN group in the anchoring unit of porphyrin Por14 provokes an enhancement on the light-harvesting properties and concomitant photovoltaic performance, in comparison with the acrylic acid derivative Por13 (5.2% vs. 4.0%). On the contrary, the introduction of a phthalic unit as an anchoring group in porphyrin Por15 did not produce any beneficial effect in the photovoltaic properties. As mentioned above, the introduction of a malonic acid binding group in porphyrin derivatives such as Por2 has led to excellent overall conversion efficiencies between 5% and 7%,[55] depending on the aryl group attached to the porphyrin meso position, and are still considered one of the best porphyrin based sensitizers.



THE CHEMICAL RECORD

Liu et al, have investigated a series of unsymmetrical transD-B-A porphyrin based dye molecules (D-donor, B-bridge, A-acceptor), with a different donating group substituent on the opposite side of the acceptor and used as sensitizer for DSSCs.[56] These molecules contain a porphyrin unit as a π-bridge, a substituted phenyl group as an electron donor and cyanoacrylic acid or carboxylic acid as an electron acceptor as well as an anchoring unit. Among all porphyrin dyes, the dye having conjugated diphenyl aniline at the opposite side of anchoring group (cyanoacrylic acid) gives a PCE of 2.38%. Recently, Tan et al.[57] designed a series of meso-position modified porphyrin sensitizers with porphyrin moieties as donor, thiophene derivatives, namely 4-methylthiophene (MT), 4-hexylthiophene (HT) or 3,4-ethylenedioxythiophene (EDOT) groups, as π-conjugated linkers and cyanoacrylic acids both as acceptors and anchoring groups and used these as sensitizers for DSSCs. These sensitizers exhibit broader absorption up to NIR as compared to porphyrins without thiophene, resulting in a higher photocurrent. Different thiophene derivatives employed as π-linkers for connecting the porphyrin donor (D) and anchor (A) groups, not only extended the light absorption region but also affected the electron injection character of the dyes into the TiO2 surface. By introducing the electron-rich EDOT moiety, the light absorption of porphyrin with EDOT has been broadened and slightly red-shifted compared to porphyrin with MT and HT. Therefore, the porphyrin with EDOT exhibits the maximum power conversion efficiency of 6.47% with Jsc = 15.59 mA/cm2, Voc = 0.64 V and FF = 0.65. Since electron injection efficiencies of the three porphyrin dyes are more or less similar, the light-harvesting ability and electron collection efficiencies of porphyrin sensitizers have strong influence on the photovoltaic performances. Based on the structure similar to shorter links, they have incorporated further acenes in the bridge between the porphyrin core and –COOH anchoring group to improve the light harvesting capability, enabling the overall PCE of DSSC with an anthracene modified porphyrin to attain ∼80% of the performance of a DSSC based on N719 dye. Lin et al. have prepared a series of Zn porphyrins bearing a phenylethynl, naphthalenylethynyl, antracenylethynyl, phenanthrenylethynyl or pyrenylethynyl substituent and used these as photosensitizers for DSSCs. The DSSC based on the Zn porphyrin with pyrenylethnyl substituent exhibits an overall PCE of 10.06%. This value is superior that of the N719 based DSSC, which is attributed to the broader and more red-shifted absorption band toward the NIR region that makes the IPCE spectrum cover broadly across the entire visible region 400– 800 nm.[58] To increase the electron donating ability of the D-π-A system based on zinc porphyrin derivatives, a porphyrin with a carbazole containing triphenylamine (TPA) moiety was developed by Kim et al.[59] and used as sensitizer for DSSC, achieving a PCE of 5.0%. Recently Diau et al. designed highly


efficient porphyrin sensitizers with two phenyl groups at the meso positions of the macrocycle bearing two ortho-substituted long alkoxyl chains for dye-sensitized solar cells; the orthosubstituted devices exhibit significantly enhanced photovoltaic performances with the best porphyrin, LD14, showing Jsc = 19.167 mA/cm2, Voc = 0.736 V, FF = 0.711, and an overall power conversion efficiency of 10.17%.[60] 3.1.2 DSSCs Based on Squaraine Dyes Squaraine dyes are well known for their intense absorption in the far-red/NIR region, and as a result have been widely investigated as red/NIR sensitizers.[61] Chemical structures of some of the squaraine dyes (SQ1-SQ6) are shown in Figure 9. However, when squaraine dyes were used in DSSCs, low IPCEs and PCE values were obtained. Squaraine dyes are well known to easily form H-aggregates (H-aggregates cause the blue shifts of the absorption bands relative to monomeric forms), which significantly affect their efficiency in DSSCs. Zhang and co-workers reported that a DSSC based on the symmetrical squaraine dye SQ1 gave a PCE value of 3.9%.[61a] Furthermore, they reported four symmetrical squaraine dyes (SQ1–SQ4, λmax abs = 643–656 nm in DMSO) in DSSCs; those based on SQ3 and SQ4 gave PCE values of 3.2 and 3.4%, respectively.[60b] In two reports they showed that the symmetrical squaraine dyes SC1, SQ3 and SQ4 formed H-aggregates on TiO2 electrodes, which resulted in increases in their sensitization efficiencies in DSSCs. Grätzel and co-workers reported the highly efficient symmetrical squaraine dye SQ5 (λmaxabs = 679 nm, when adsorbed on TiO2) incorporating two carboxylic acid attaching groups, which yielded an IPCE of 73% and a PCE value of 3.7%.[61d] Nazeeruddin and co-workers proposed several basic requirements to guide the molecular engineering of efficient squaraine sensitizers:[61g] (i) the excited-state redox potential of a dye should match the energy of the CB of the TiO2 electrode (i.e., the LUMO level of the dye must be higher than the CB of the TiO2 electrode). (ii) Light excitation should be associated with vectorial electron flow from the light-harvesting moiety (chromophore) of the sensitizer towards the TiO2 surface, providing for efficient electron transfer from the excited dye to the CB of the TiO2 electrode. (iii) Strong conjugation across the chromophores and anchoring groups is required for good electronic coupling between the LUMO of the dye and the CB of the TiO2 electrode. In order to satisfy these requirements, they designed and developed the novel unsymmetrical squaraine sensitizer SQ6 with a carboxylic acid group directly attached to the chromophore. The DSSC sensitized with the unsymmetrical squaraine SQ6 exhibits a PCE value of 4.5%, which is the highest conversion efficiency so far reported for squarainebased DSSCs. The IPCE was over 85%. They concluded that the high efficiency of SQ6 was attributable to two particular

Chem. Rec. 2014, ••, ••–••



O HOOC

O

COOH N+ (CH2)3 H3C

O-

N (CH2)3 CH3

SC1 O

R1 N+ 2 R

N+ (CH2)2 OOC +HNEt3

N (CH2)2

OSQ-5

COO+HNEt3

O

R1 N 2 R

COOH N

O-

O-

SQ-1: R1=CH3, R2= CH2CH2CH2COOH

N+ C8H17

SQ-2: R1=CH2CH2CH2CH3, R2= CH2CH2CH2COOH 1

SQ-6

2

SQ-3: R =CH3, R = CH2COOH SQ-4: R1=CH2CH2CH2CH3, R2= CH2COOH Fig. 9. Chemical structures of squaraine dyes (SQ1-SQ6).

molecular design features. Firstly, the carboxylic acid group is part of the conjugated π-system of the dye and provides strong electronic coupling to the CB of the TiO2. Secondly, the asymmetry created by the octyl chain prevents surface aggregation and limits self-quenching of the excited state. Ionic dyes, such as hemicyanine, merocyanine, squarylium, and cyanine dyes can be regarded as promising sensitizers for DSSCs, because of their tunable absorption in the red to NIR region of the solar spectrum as well as their high absorption coefficient. It is known that some of these have a strong tendency to self associate in solution or at a solid-liquid interface because of strong intermolecular van der Waals forces between the molecules.[62] The aggregates exhibit distinct changes in their absorption compared to monomeric species, and three different aggregation patterns of the dyes have been proposed: red-shifted J-aggregates, blue shifted H-aggregates, and both red and blue shifted herringbone aggregates. It was reported that a broadening of absorption by controlled aggregation can be important to obtain high IPCE in DSSCs.[63] However, these dyes showed rather moderate PCEs in DSSCs because of the formation of aggregates upon adsorption on the TiO2 surface. In addition, cis-trans photoisomerization is one of the major decay pathways for this dye.[64] 3.1.3 DSSCs Based on Other Dyes Huang and co-workers reported that the maximum IPCE for hemicyanine dyes HMC1 and HMC2 (Figure 10) can be increased to near unity by pretreatment of the TiO2 films with hydrochloric acid.[65] The binding of these dyes onto the TiO2 electrode takes place through weak electrostatic interactions between SO3− groups and Ti4+ ions. It was reprted that treatment of the TiO2 surface with HCl increases the number of

Chem. Rec. 2014, ••, ••–••

active sites for dye adsorption. Additionally, the potential of the TiO2 was shifted in the positive direction because of the adsorption of protons, which can increase the driving force for electron injection and reduce back electron transfer.[66] Dyes HMC1 and HCM2 showed PCEs of 3.1 and 1.3%, respectively, on untreated TiO2, while the PCE increased to 5.1 and 4.8%, respectively, upon treatment with HCl. The acid treatment reduced the open-circuit voltage as a consequence of the smaller difference between the flat-band potential of TiO2 and the I 3− I − redox couple, which was compensated by the remarkable enhancement in the short-circuit photocurrent. Hemicyanine dyes HMC3 and HMC4, comprising a naphthothiazolium acceptor and a dialkylamino phenyl donor unit, showed PCEs of 4.0 and 6.3%, respectively, under 80 mW cm−2 illumination.[67] In comparison to dye HMC3, hemicyanine HMC4 contains an additional hydroxy group on the donor unit, which can form hydrogen bonds with the oxygen atom of the TiO2 nanoparticles. This additional anchoring in cooperation with the RSO3− groups increases the contact area between the dye and semiconductor, which eventually led to a decreased number of adsorption sites on the TiO2 surface. Alternatively, the dual binding mode in HMC4 largely enhanced the electron injection from the excited state of the dye to the TiO2 conduction band and increased the overall efficiency. The cell efficiency was reduced to 4.6% when the naphthothiazolium unit was replaced by a benzothiazolium group (in HMC5).[68] The lower efficiency for HMC5 could be due to its higher LUMO level resulting in worse electron injection efficiency. Tian and co-workers found that D-π-A-based hemicyanine dye HMC6 generated a high Jsc value of 13.8 mA/cm2, while the cell efficiency was only 2.1% because of its relatively low Voc value of 0.36 V and low fill factor of



THE CHEMICAL RECORD

H3C

H3C

N

S

H3C N

H3C

N

H3C

+N

S

H3C +N

HMC1

+N

HMC2

SO3-

HMC3 SO3-

SO3-

H3C N

S

H3C

+N

H3C

H3C

N OH

S

COOH

N +N

OH

+N I- C2H5

CH2COO-

HMC4

SO3-

HMC5

HMC6

HOOC

H3C N

N

HOOC HMC7

+N I- C2H5

+N I- C2H5

HOOC HMC8

Fig. 10. Chemical structure of hemicyanine dyes (HMC1- HMC8).

0.41.[69] Hemicyanine dyes HMC7 and HMC8, in which one or two propionic acid groups were attached to the donor part of the molecule, respectively, showed efficiencies of 4.4 and 4.9%, with short circuit current densities of 14.5 and 21.4 mA/cm2, respectively, under irradiation with white light from a xenon lamp (90 mW/cm2).[70] The higher efficiency for HMC8 bearing two carboxylic acid groups was ascribed to a broader absorption of the dye and an increased adsorption on the TiO2 electrode compared to dye HMC7, which contains one carboxylic acid group. Although the adsorption of dye HMC8 was three times larger than that of HMC7, the Jsc value increased by only around 50%. This result was explained by the attachment of carboxylic acid groups to the donor side of the molecule, reducing the charge separation. Arakawa and co-workers studied a series of benzothiazole merocyanines with different alkyl chain lengths and found that the conversion efficiency and the IPCE value increased with an increasing length of the alkyl side chain attached to the benzothiazole ring and with a decreasing number of methylene units between the carboxylic acid group and the dye chromophores.[71] The dye with the longest alkyl side chain was the best sensitizer, generating a PCE of 4.5%. Squarylium dyes (chemical structures shown in Figure 11) have also attracted interest in recent years for use in solar cells. The presence of the rigid squaric acid ring in the methine chain afforded the dyes with high stability towards cis-trans photoisomerization and resulted in intense absorption in the red/near-infrared region. Sayama et al. studied the photovoltaic


properties of S1, which showed a sharp absorption at around 630 nm in ethanol. DSSCs with S1 resulted in an efficiency of 2.4% and a maximum IPCE of 37%.[72] Das and co-workers synthesized symmetrical dye S2 and unsymmetrical dye S3 as sensitizers for DSSCs.[73] The absorption of the unsymmetrical sensitizer was broad and blue-shifted by about 20 nm compared to that of the symmetrical counterpart. DSSCs with the unsymmetrical dye S3 produced a higher photocurrent, thus resulting in a higher efficiency of 2.1% compared to that of symmetrical dye having efficiency of 1%. This effect was ascribed to the unidirectional flow of electrons, which resulted in favorable charge separation and electron injection into the TiO2. In a different approach, Wang and co-workers developed squarylium dyes S4 and S5 with terminal dialkylanilino groups.[74] DSSC devices based on these sensitizers showed that the introduction of a short acetic acid anchoring group to dye S5 resulted in an improved photosensitization compared to S4 with a butyric acid side chain. For the former, a Jsc value of 10.3 mA/cm2 and an overall efficiency of 3.4% were determined. A slightly better overall efficiency of 3.9% and a maximum IPCE of about 65% at 650 nm were obtained with symmetrical squarylium dye S6. Grätzel and co-workers synthesized squarylium dye S7 with triethylammonium propionate anchoring groups.[75] Upon adsorption on a TiO2 electrode, the absorption of the dye was broadened and red-shifted with respect to that of the indole analogue S2. The DSSC showed an increased efficiency of 3.7% in the presence of chenodeoxycholic acid (CDCA) and

Chem. Rec. 2014, ••, ••–••



O

CH2CH2COOC4H9 N

_

COOH

_

O

N +N

O +N CH2CH2COOC4H9

O

HOOC

S2

S1 O_

C4H9

_

O

N+

C8H17 N C8H17

C4H9 N

+N

COOH

(H2C)3

(CH2)3

O

COOH

O

HOOC

S3

S4

_

O C4H9 +N

HOOC

C4H9 N

COOH

O

HOOC

O O-

+N C4H9

S5 O

HOOC

-O

O-

O

O-Et3NH+

O

+N C2H5

O-Et3NH+

COOH

S6

N N+

N C4H9

O

N C2H5

COOH

S8

S7

S9

Fig. 11. Chemical structures of squarylium dyes S1–S9.

a higher Voc value was obtained with tert-butylpyridine (TBP) in the electrolyte. CDCA was used to prevent aggregation of the dye on the surface. More recently, the asymmetric squaraine dye S8 was synthesized in which the carboxylic acid group is directly attached to the chromophores.[76] In an optimized cell with CDCA used as a coadsorbant, a high IPCE of 85% and a Jsc value of 10.5 mA/cm2 was obtained, which yielded an overall efficiency of 4.5%. This high overall performance was attributed to two factors: firstly, the direct attachment of the carboxylic acid group to the conjugated π-system, which provides strong electronic coupling to the conduction band of TiO2, and secondly, that the asymmetry created by the octyl side chain prevents surface aggregation and minimizes self-quenching of the excited state. Recently Grätzel et al.[77] synthesized three unsymmetrical squaraine dyes, containing a bulky spirobifluorene or hexyloxyphenyl unit and squaraine core with an electron

Chem. Rec. 2014, ••, ••–••

donor. These sensitizers, when anchored onto a TiO2 surface, exhibit decreased aggregation as well as enhanced unidirectional flow of electrons. An optimized DSSC with unsymmetrical squaraine dye with hexyloxyphenyl unit gave a short-circuit photocurrent density (Jsc) of 12.82 mA/cm2, an open-circuit voltage (Voc) of 0.54 V and a fill factor (FF) of 0.75, corresponding to an overall conversion efficiency of 5.20%. Three new unsymmetrical squaraine dyes bearing direct ring carboxy functionalized indole as an anchoring moiety with varying donor groups with extended π-conjugation have been successfully synthesized and utilized for dye sensitized solar cell fabrication by Pandey et al. Among them the dye S9 (Figure 11) gave a photoconversion efficiency of 3.3% (Jsc = 7.22 mA/cm2, Voc = 0.63 V and FF = 0.72), mainly harvesting photons in the far-red region between 500 and 700 nm.[78]



THE CHEMICAL RECORD

Fig. 12. Chemical structure of CDPT based dye and HY113.

The best selection for the π-conjugated bridge is the unit containing thiophene units such as oligothiophenes, fused thiophene, or alkylene dioxythiophenes because of their excellent charge transport properties. A fused-ring analogue of 3-alkylthiophene has an absolute planar structure and has been applied widely in synthesizing organic semiconducting materials. Organic dyes using the alkyl-functionalized dithiophene CPDT as the conjugated bridge (Figure 12) showed an extremely high molar absorption coefficient and a high power conversion efficiency of 8.95% in liquid cell and 6% in a solid-state DSSC.[79] Gao et al. have designed a series of metal free dyes with different π-conjugated length to connect bisphenyl triphenylamine (BPTPA) and cyanoacrylic acid.[80] These dyes showed an absorption band extending up to 700 nm. These dyes were employed as sensitizers along with the cobalt(II/III) redox electrolyte and the high PCE of 9.24% (Jsc = 16.27 mA/cm2, Voc = 0.83 V and FF = 0.67) for the metal free organic dye shown below. Sun et al. have synthesized a NIR dye HY113 (Figure 12) with lateral anchoring group and showed a PCE of 5.1% (Jsc = 13.35 mA/cm2, Voc = 0.52 V and FF = 0.73).[81] The design of a conjugated bridge using low band gap building blocks plus long alkyl chains has become popular. Clearly, the low band gap building blocks could extend the spectral response region. On the other side, the introduction of alkyl chains would not only make the dyes more soluble and reduce aggregation in devices, but also suppress charge recombination in DSSCs. The chemical structutres of this type of dye are shown in Figure 13. Jiang, Pei and co-workers syntheised a dye1 which showed absorption in a wide range of solar spectrum.[82] The absorption onset of dye1 was redshifted from about 710 nm in solution to about 800 nm in the solid state. The absorption features in thin films covered nearly the whole visible range, namely, from 300 nm to 800 nm. Such a broad absorption is greatly beneficial to the improvement of photocurrent density and PCE. The devices fabricated from dye1 showed PCE as high as 6.04% (Jsc = 16.46 mA/cm2, Voc = 0.545 V and FF = 0.67). A similar phenomenon was observed by Zhou and co-workers.[83] They introduced a


thieno[3,4-b]pyrazine derivative unit into the conjugated backbone of dye2 and dye3 to tune the absorption spectra. The extended conjugation of the additional acceptor from thieno[3,4-b]pyrazine (42, λmax = 596 nm) to acenaphtho [1,2-b] thieno-[3,4-e] pyrazine dye2 (λmax = 625 nm) bathochromically shifted the absorption peak significantly. They compared the absorption spectra of the terthiophene based dyes and proposed that increasing the thiophene number is not an effective way to bathochromically shift the maximum absorption of the sensitizers significantly (e.g.λmax > 600 nm). It is impressive that the IPCE onsets are located at around 900 nm for the two dyes, which is comparable to that for the black dye. However, dye2 and dye3 exhibit low Voc values (below 500 mV), which partially compensate for the positive effect of the near infrared (NIR) IPCE response, resulting in moderate PCEs of 5.3% (Jsc = 16.46 mA/cm2, Voc = 0.48 and FF = 0.68) and 3.66% (Jsc = 11.29 mA/cm2, Voc = 0.47 V and FF = 0.69), respectively. To obtain panchromatic fluorene based triarylamine dyes, dye4, dye5, dye6 and dye7 were synthesized (Figure 13), in which the unsymmetrical squaraine unit was incorporated as the conjugation bridge.[84, 85] All dyes showed a broad absorption that extended throughout the visible and near infrared region. Unfortunately, cells based on these dyes suffered from low Voc values, and therefore exhibited moderate efficiencies (5.20–6.29%) relative to the aforementioned fluorene based triarylamine dyes. In contrast, the introduction of low-band gap chromophores such as the benzothiadiazole unit in the bridging framework seems to be a good choice. With this molecular design, power conversion efficiencies of 7.51% and 8.19% were achieved with dye4 and dye5 based DSSCs, respectively.[86] Recently Grätzel et al. have synthesized a series of new pyrido[3,4-b]pyrazine based organic sensitizers (dye8-dye12; Figure 14) containing different donors and spacers and employed them in DSSCs and investigated the the impact of the different substitutions on the PP-core molecule, as well as the utilization of different π-spacers, and various donor moieties on the performance of DSSCs.[87] The photovoltaic

Chem. Rec. 2014, ••, ••–••



Fig. 13. Chemical structures of dye1-dye7 dyes.

experiments show that the methoxyphenyl-substituted PP unit can improve the Jsc and Voc of cells, while the use of phenyl π-spacers rather than thiophene or furan moieties results mainly in an increased Voc. Moreover, the introduction of octyloxydiphenyl-amine groups as electron donors resulted in improved Jsc and Voc, contributing to the highest PCE (7.12%)

Chem. Rec. 2014, ••, ••–••

for dye11 based DSSC. A PCE of 6.20% has been achieved for the dye11 sensitized DSSC in conjunction with an ionic-liquid electrolyte-based DSSC. The work on diketopyrrolopyrrole (DPP) based metal free organic dyes as sensitizers was first published in 2010, comparing the phenyl-DPP and thienyl-DPP bridges.[88] Later on, the



THE CHEMICAL RECORD

Fig. 14. Chemical structures of dye8 to dye12.

Fig. 15. Chemical structures of DPP1, DPP2 and DPP3.

Grätzel group investigated the development of DPP based sensitizers for high PCE DSSCs.[89] The chemical structures of some dyes are shown in Figure 15. Meanwhile, S. Qu et al. showed an explicit red shift in the absorption band owing to replacement of a strong donor such as indoline instead of triphenylamine (TPA) and achieved 7.4%.[90] Another approach combining the DPP and porphyrin chromophores yielded panchromatic sensitizers[91,92], and achieved 7.7% very recently. Grätzel et al. have synthesized a series of D-π-A sensitizers comprising the DPP π-bridge chromophore and used as


them for both I 3− I − and Co[(bpy)]3+/2+ redox shuttle DSSC devices. In order to improve upon the reference dye, DPP1, an attempt to balance aggregation, red light response, and electron withdrawing strength on the anchor was made.[92,93] Selective fluorine incorporation on the benchmark sensitizer DPP1, to yield DPP3, improved the PCE with the formally negatively charged I 3− I − based electrolyte, while hindering performance with the formally 3+/2+cobalt-based redox shuttle. Incorporation of a pyridine heterocycle into the DPP chromophore (DPP2) increased spectral breadth, but decreased all output characteristics with both electrolyte systems. These results are understood through detailed electrochemical, photophysical, and computational studies. The benchmark DPP-based sensitizer, DPP1, yielded improved performance with the cobalt-based electrolyte (PCE = 8.74%), compared to our previously reported I 3− I − based electrolyte; however, DPP12 employing the fluorophenylcyanoacrylic acid anchor achieved higher performance (PCE = 8.01%) than DPP7 (PCE = 7.67%) with the I 3− I − based electrolyte and lower performance than DPP1 with the cobalt-based electrolyte. A preliminary stability test with DPP2 conducted under successive sunlight soaking at 60° C showed that the efficiency remained at 93% of the initial value without a drop of the photocurrent after 1000 hr.[94]

Chem. Rec. 2014, ••, ••–••



4 BHJ Solar Cells Based on NIR Absorbing Conjugated Polymers and Small Molecules The vision of organic solar cells is that of a low cost solar energy conversion that provides lightweight, flexible solar cells that are easily incorporated into the existing infrastructure. Organic solar cells (OSCs) have been the subjects of growing research interest over the past quarter century, and now are developed to the point where they are on the verge of introduction into the market. Compared to the inorganic semiconductors, organic materials generally have the advantage of very high absorption coefficient in thin films than silicon. These high absorption coefficients allow the use of very thin films (generally between 50 nm and 150 nm), which will absorb a large fraction of the incident light. In addition, organic semiconducting materials also have much lower density than inorganic semiconductors. The combination of high absorption coefficient and low density leads to extremely small masses of active materials used in organic solar cells, which promises lightweight and potentially inexpensive devices. Organic solar cells based on an active layer of a blend of conjugated polymer as donor and fullerene derivatives as acceptor have attracted more and more attention in recent years because of their potential as a low cost alternative to silicon solar cells. In organic semiconductors, light absorption leads to the creation of excitons, i.e. strong coulombically bound electron-hole pairs, though the promotion of an electron from the HOMO to LUMO level of the molecules[95] as shown in Figure 16. In order to produce the free charges, the

excitons must be dissociated at the donor–acceptor interface, the driving force for this process being provided by the energy offset between the LUMO level of the donor and acceptor molecules and finally then be transported through the donor phase (holes) and the acceptor phase (electrons). Consequently, the four basic steps occurring in the solar cells based on organic semiconductors are: the light absorption/exciton generation, exciton diffusion towards the donor–acceptor interface, charge transfer/exciton dissociation, and charge collection of the separated electrons and holes at the electrodes (Figure 17). It is this necessity of having two distinct and interacting species that provides the necessary driving force of the exciton dissociation. Despite this common attribute, many different types of organic solar cells exist, which can be grouped in two general categories distinguished by the architecture of the active layer, with either a donor–acceptor bilayer or a bicontinuous donor–acceptor composite, known as bulk heterojunction (BHJ). The configurations of bilayer and bulk heterojunction organic solar cells are shown in Figure 18. The first organic bilayer solar cell, consisting of vacuum deposited layers of copper phthalocyanine and a perylene tetracarboxylic derivative, was published by Tang, in 1986.[28] The maximum PCE that has been reached with molecular bilayer (vacuum deposited) devices is 4.2% using copper phthalocyanine as donor and C60 as the acceptor.[96] By adding a mixed donor acceptor interlayer in this system the PCE has been improved to 5%.[97] The highest PCE for a polymerpolymer (solution processed) bilayer solar cell is 2% and was achieved using poly[3-(4-octyl)-phenylthiophene] as the donor

Fig. 16. Difference between the organic and inorganic semicondutors with respect to light absorption.

Chem. Rec. 2014, ••, ••–••



THE CHEMICAL RECORD

Fig. 17. Different processes i.e photon absorption, exciton dissociation, charge transport and charge collection, occurring in organic photovoltaic devices.

Fig. 18. Different types of organic photovoltaic devices i.e. bi-layer and bulk heterojunction types.

and cyano-PPV (PPV = poly-para-phenylenevinylene) as the acceptor.[98] In general, the performance of the bilayer devices based on organic semiconducting materials is limited by the short exciton diffusion length in the organic materials. If the exciton is not created within a very short distance (generally 15–20 nm) of the donor–acceptor interface it will simply decay


back to the ground state and not contribute to the photocurrent.[99] Despite the high absorption coefficients of most of the organic materials, more than a few nanometers of the materials are necessary to absorb a sufficient fraction of incident light for practical function, and this inherently limits the PCE of the bilayer organic solar cells.

Chem. Rec. 2014, ••, ••–••



The problem of short exciton diffusion length can largely be solved using the donor–acceptor BHJ approach introduced in the mid-1990s.[100] Conceptually, a BHJ is a bicontinuous donor–acceptor blend with domain sizes on the order of the exciton diffusion length, such that all excitons created upon the absorption of light by the active layer can reach the interface and dissociate into free charge carriers, rendering the entire volume of the active layer effective for light absorption. Organic solar cells based on the concept of BHJ configuration where an active layer comprises of a blend of p-type (donor) and n-type (acceptor) material represents the most useful strategy to maximize the internal donor–acceptor interfacial area, allowing for efficient charge separation. BHJ devices also have the advantage that the active layer is solution processed in one single step at room temperature, which is especially effective when at least one component has a high molecular weight. At the laboratory level, spin coating is the primary processing method used to make thin films; however, solution processing can be easily adapted for large area devices and is suitable for low cost mass production.[101] Different combinations of donors and acceptors have been studied for the BHJ organic solar cells. The highest PCEs in the range of 1.8–2.3% have been reported for polymer: polymer BHJ solar cells.[101] Combination of organic and inorganic semiconducting components for BHJ active layers have given efficiencies of 2.6%[102] using P3HT and CdSe tetrapods whereas a low band gap polymer with CdSe nanocrystals gave a PCE of 3.13%.[103] So far, the most successful and extensively studied combination in BHJ solar cells is that of conjugated polymer donors and fullerene derivatives acceptors. Through the creation of novel donor and acceptor materials and innovations of device fabrication technology, OSCs based on regioregular poly(3hexylthiophene) (P3HT) as donor components in the BHJ active layer reached PCE values over 6%.[104] However, the relatively large band gap of P3HT (1.9–2.0 eV) limits the absorption of NIR light harvesting efficiency and results in a lower PCE. The solar spectrum covers a very broad wavelength range from ultraviolet to near IR regions. Theoretically the ideal band gap for the conjugated polymer for PSC should be around in range of 1.5–1.7 eV. However, most of the conjugated polymers such as polythiophenes (Pths) and poly(phenylene vinylene)s (PPVs) exhibit an optical band gap larger than or around 1.95 eV and can harvest only the visible light. This mismatch of the absorption of the active layer used for solar cells to the solar spectrum significantly limits the device performance of organic solar cells. To achieve a high PCE for organic solar cells one of the most critical challenges at the molecular level is to develop an ideal conjugated polymer or small molecule donor that simultaneously possesses (i) sufficient solubility to guarantee solution processability and miscibility with acceptor counterpart material, (ii) low band gap

Chem. Rec. 2014, ••, ••–••

for strong and broad absorption spectrum extending to the NIR region of solar spectrum to capture more solar photons, and (iii) high hole mobility for efficient charge transport. One of the successful and elegant approaches to design a low band gap conjugated polymer or small molecules is the incorporation of strong electron donating and electron accepting moieties (push–pull or D-A) along the conjugated backbone.[105] The alternating donor and acceptor arrangement causes the hybridization of the HOMO of the donor with the LUMO of the acceptor and results in the reduction of band gap. Additionally, the incorporation of donor and acceptor segments enhances the intramolecular charge transfer (ICT), which in turns also improves the charge carrier mobility. Various electron-deficient units, e.g., electron-deficient heterocycles,[106,107] perylene diimides,[108] cyanovinylenes,[109] and several electron-rich units, e.g., thiophene, pyrrole, fluorene and carbazole, have been used to construct low band gap polymers. However, of those with NIR absorption[107d,111] only a few exhibit PCE beyond 1%.[110,111e–f ] The PCE of BHJ solar cells has been increased from 2.5% in 2001[112] to nearly 8.0– 8.3% in 2010.[113]

5 Low Band Gap Copolymer Based BHJ Solar Cells Conjugated polymers based on fluorenes contain the structure of para-polarized benzene, where each adjacent pair of phenylene groups are tied together in a coplanar fashion by methylene bridges to form a fluorene unit (chemical structures shown in Figure 19). The first breakthrough in fluorene-based polymer solar cells was demonstrated by Andersson et al.[114] They reported an alternating polyfluorene copolymer, poly[2,7-(9-2′ethylhexyl)-9-hexylfluorene]-alt-5,5-(4′,7′) di-2thienyl-2′1′3′ benzothiadiazole] (PF1) with extended absorption. Films of the polymer show the longest wavelength absorption maximum at approximately 545 nm. In polymer PF1, fluorene groups with two different side chains were introduced in this polymer to enhance its solubility. The two thiophene units together with the benzothiadiazole group formed a donor–acceptor copolymer and improved the optical properties of the polymer. Blended with PCBM, the polymer PF1 solar cell showed a PCE of 2.5%. With different side chains in the 9-position of the fluorene moiety, polymers PF1- PF4 were compared in terms of photovoltaic performance.[115] The side chains influence the packing of the main chains as well as the morphology of the active layer and consequently produce different photovoltaic properties. The octyl substituted polymer exhibited the best power conversion efficiency of 2.6%.[116] The same authors have reported additional polyfluorene copolymers PF5-PF9 with stronger acceptor or donor moieties in order to enhance the absorption of the polymers.[117] The



THE CHEMICAL RECORD

R R'

N

S

R R'

N

S

S

S S

S

n N

PF1 R= hexyl, R'= 2-ethylhexyl PF2 R= R' = hexyl PF3 R= R' = octyl PF4 R= R' = dodecyl R' R R

O

O

n

N

R'

PF5 R= octyl, R'= H PF6 R= octyl, R'= 2-ethylhexyloxy PF7 R= octyl, R'= octyl

S S O N

O

N

R R n

S S N

R'

N

n

R'

PF8 R= octyl, R'= 2-ethylhexyloxy

R' PF9 R= octyl, R'= octyloxy

R'

Fig. 19. Chemical structures of polyfluorene copolymers (PF1-PF9).

absorption spectrum of polymer PF5 shows a broad absorption from 300 to 850 nm with two local maxima at 380 and 615 nm. Unlike PF5, compound PF6 contains two branched alkoxy side chains on the repeating unit. These chains improve the solubility, resulting in a soluble, high-molecular-weight polymer. The absorption spectrum of PF6 demonstrates similar absorption bands to that of PF5, but both peaks are bathochromically shifted. Two broad peaks occur at 430 and 660 nm, respectively. Polymer PF7 has two linear octyl side chains, which also increase the solubility of the polymers and induce a bathochromically shifted absorption with an absorption maximum at 660 nm. Devices based on polymers PF5, PF6, and PF7, blended with PCBM, were fabricated and characterized. The solar cells of polymer PF6 exhibited a top efficiency of 2.2%.[118] Inganäs and co-workers synthesized fluorene-, thiophene-, and quinoxaline-containing copolymers PF8 and PF9.[117f,g] Both polymers showed broad absorption and emission bands. Solar cells based on blends of PF9 and PCBM had a very high efficiency of 3.7%. Polycarbazoles (i.e. polyvinylcarbazoles or PVKs) were first reported as photoconductive materials in photocopiers. Since then, different families of poly(carbazole)s have been reported in the literature such as poly(3,6-carbazole)s, poly(1,8carbazole)s, and poly(2,7-carbazole)s. This class of materials was found to exhibit interesting features that make these polymers attractive for photovoltaic applications. Indeed, poly(2,7carbazole)s possess low HOMO energy levels which lead to air stable materials and to high open circuit voltage (Voc). Upon structure modifications, those materials can be easily fine tuned


to match the optimal solar spectra emission. They can also exhibit good hole mobility values. Also, the 2,7-carbazole ring is fully aromatic, which provides a better chemical stability. Based on the discovery that including oligothiophenes or S,Sdioxide thiophene into the polycarbazole backbone can enhance the absorption via the donor–acceptor effect and, hence, the photovoltaic performance enhanced, Leclerc et al. developed more polycarbazoles by incorporating other stronger acceptor groups into the polymer chains.[119] Polymers PC1PC9 (chemical strucutures shown in Figure 20) was synthesized under this concept design and exhibited two broad absorption peaks in the region between 300 and 700 nm.[119a] Among all these polymers, polymer PC3 showed the highest hole mobility of up to 0.001 cm2/Vs and an on/off current ratio of 3 × 104 in OFETs. The best photovoltaic result was presented for polymer PC3 with a power conversion efficiency of 4.6%.[119b,d] The high Jsc and FF values in polymer PC3based solar cells are attributed to the good hole mobility resulting from the higher structural organization. With the solar cell structure of ITO/PEDOT:PSS/PC3:PC70BM/TiOx/Al, a new record power conversion efficiency of 6.1%, certified by NREL was achieved under AM 1.5 irradiation.[120a] It is important to note that the internal quantum efficiency (IQE) reached nearly 100%, indicating that almost every photon absorbed in the active layer leads to a pair of charge carriers, collected at their respective electrodes. Very recently, a remarkable PCE of 7.1% has been reached for PC3.[120b] To further push the absorption of carbazole-containing copolymers into the near-infrared region, as described for the

Chem. Rec. 2014, ••, ••–••



Acceptor

N C8H17

R1

R1

S S

N R2

N

N

N

N

S

N

S

n

C8H17

N

N

S

n

N

N S PC7 R1 = methyl, R2 = 2-decyltetradecyl N PC8 R1 = methyl

Acceptor

=

PC9 R1 = H

N PC1 N

S

N

PC2

N

O

N

PC3

N

N PC4

O

O

N

N PC5

PC6

R2=

N

O Fig. 20. Chemical structure of polycarbazoles based copolymer (PC1-PC9).

polyfluorene copolymers, 6,7-diphenyl-4,9-bis(thiophene-2yl)[1,2,5]thiadiazolo[3,4-g]quinoxaline repeat units were also introduced into polycarbazoles.[121] In the film, the absorption maxima of polymers PC7-PC9 are 773, 772, and 867 nm, respectively. In particular, polymer PC9 absorbs light up to 1200 nm and displays an optical energy gap of 1.1 eV. However, the strong absorption in the visible and near-infrared region of these three polymers did not help them to achieve reasonable photovoltaic performance in solar cell devices. Using a 1:1 blend of PC9 and PCBM as the active layer yields devices with a Voc of 0.41 V, and Jsc of 5.16 mA cm−2, a FF of 0.29, and a power conversion efficiency of 0.61%. The relatively small energy difference between the HOMO level of polymer PC9 (4.8 eV) and the LUMO level of PCBM (3.8– 4.3 eV) resulted in the low open circuit voltage. The LUMO values of these three polymers are located at about −3.8 eV, which is very close to the LUMO level of PCBM. This suggests a weak driving force for electron transfer from polymers to PCBM; therefore, to obtain higher power efficiency, another acceptor with a lower lying LUMO level has to be used. As an alternative to using benzothiadiazole and its derivatives as acceptor moiety in low-band-gap polymers, recently diketopyrrolopyrrole[122] (DPP) was introduced into carbazolecontaining copolymers. The chemical structures of some of the copolymers based on DPP are shown in Figure 21. The planar

Chem. Rec. 2014, ••, ••–••

pyrrolo[3,4-c] pyrrole-1,4-(2H,5H)-dione or diketopyrrole acceptor building block has been used extensively to combine with other donor units such as fluorene, carbozole, phenylene, thiophene, dibenzosilole, benzodithiophene, dithienosilole, and dithienopyrrole to form various D-A polymers. Tan et al. reported DPP- based D-A copolymers PDBT-coTT and PDQT, which showed high hole mobility close to 1.0 cm2/Vs. Fused aromatic rings such as thienothiophene, benzodithiophene and dithienpthiophene based polymer semiconductors have shown high performance in organic field effect transistor devices, achieving high mobilities ranging from 0.2 to 0.7 cm2/ Vs. Incorporation of a conjugated fused aromatic system is expected to enhance intermolecular interactions through π-π stacking and may reduce the barrier for charge carrier hopping. Winnewisser et al. reported a low-band-gap donor acceptor copolymer (poly[3,6-bis(4′-dodecyl[2,2′]bithiophenyl-5yl)-2,5-bis(2-hexyldecyl)-2,5-dihydropyrrolo[3,4]pyrrole-1,4dione]) containing thiophene (electron-rich unit) and DDP (electron-deficient) units that exhibited excellent ambipolar charge transport properties.[123] Meanwhile, Janssen and co-workers used a similar polymer (PDP1) for solar cell purposes where the polymer was processed from a mixture of chloroform and ortho-dichlorobenzene and blended with [70]PCBM.[124] Under this procedure, the power conversion of the solar cells reached 4.0%. Based on this promising result,



THE CHEMICAL RECORD

C12H25 N

S S

O S

O

N

n

S

N

C8H17 N O

S

PDP1

O

C12H25

C8H17

PDP2

N

S S

N

C8H17

O C8H17

O

O

N O

n

S

N C8H17

C8H17

S

N

S

N

n

n

PDP3 PDP4

S

S

N

S

O

O

S

N O

S

N

n

N

O

S O

O

N

S

n

PDP6

PDP5

C10H21 C8H17

O S

N S

S

O

O

O

O

N S n

N

S

S

n

PDP7

N

C10H21

O

C8H17

PDPP-TNT

Fig. 21. Chemical structures of diketopyrrolopyrrole (DPP) based copolymers.

polymer PDP2 was then designed and synthesized.[125] Polymer PDP2 shows the combination of a high glass transition temperature, good solubility, relatively high molecular weight, and air stability. Preliminary results on the photovoltaic device based on the PDP2:PCBM bulk heterojunction stated a power conversion efficiency of 1.6%. Recently, Hashimoto et al. optimized the structure of compound PDP3 by varying the alkyl substituents to obtain the new polymer PDP3. Together with PDP3, they reported two other DPP-based copolymers where the carbazole unit is replaced by fluorene


(for PDP4) and dithieno[3,2-b:2′,3′-d]pyrrole (for PDP5).[126] From the photovoltaic results of all these three polymers, it is clear that carbazole-based polymer PDP3 delivers the best performance, which is due to the high current (5.35 mA cm−2). However, in direct comparison with polymers PDP3-PDP5, the solar cells based on PDP1 and PCBM showed the best performance with a short current of 9.4 mA/cm2, an open circuit voltage of 0.63 V, and a fill factor of 0.54, resulting in a power conversion efficiency of 3.2% in this study, when the device is fabricated via the chloroform and o-dichlorobenzene

Chem. Rec. 2014, ••, ••–••



C6H13 C6H13

R

R

N S

N S

N

S N

N S

S

N

S

n

S

N

S

S

n

n R= n-C8H17, PDTPBT-C8 n-C6H13, PDTPBT-C6 n-C5H11, PDTPBT-C5

C6H13

C6H13

PCPBTD

a-PTPTBT

N

S

N S N

S

S

S

S S

S N

S

N

N n

PFPBTD

n

PCPBBT N

S

N S n

S PFTBBT

N

S

N

Fig. 22. Chemical structures of some low band gap copolymers having D-A configuration.

solution.[124] Interestingly, the absorption spectrum of PDP1 in chloroform is dominated by an absorption band at 650 nm. In o-dichlorobenzene, however, the polymer has a strong tendency to aggregate (forming a suspension), even at low concentrations. This can be exemplified by a significant shift of the onset of absorption from 720 to 860 nm and the appearance of a vibronic fine structure. Therefore, the photovoltaic performance of PDP1 can be largely improved by changing the casting solvent from chloroform to o-dichlorobenzene and further to the mixture thereof, yielding overall efficiencies of 1.1, 2.9, and 3.2%, respectively. The results indicated that polymers PDP2-PDP5 still have the possibility to show improved device results if a favorable solvent can be found. Yang and co-workers reported the family of alternating copolymers based on the DPP as a central core unit with an overall power conversion efficiency of 4.45%.[127] Patil and Sharma[128] have synthesized diketopyrrolopyrrolebased copolymers PDP6 and PDP7 and used these as a donor for bulk heterojunction photovoltaic devices. The photophysical properties of these polymers showed absorption in the range 500–600 nm with a maximum peak around 563 nm, while PDP7 showed broadband absorption in the range 620–800 nm with a peak around 656 nm. The power conversion efficiencies (PCE) of the polymer solar cells based on these copolymers and PCBM were 0.68% (as cast PDP6: PCBM), 1.51% (annealed PDP6:PCBM), 1.57% (as cast PDP7:PCBM), and 2.78% (annealed PDP7:PCBM), under illumination of AM 1.5 (100 mW/cm2). The higher PCE for PDP7 based polymer solar cells has been attributed to the low band gap of this copolymer as compared to PDP6, which

Chem. Rec. 2014, ••, ••–••

increases the numbers of photogenerated excitons and corresponding photocurrent of the device. Recently Sonar et al. reported a PCE of 4.7% for the D-A conjugated polymer composed of naphthalene as a donor and DPP as an acceptor.[129] Wang et al. synthesized three low band gap conjugated polymers, i.e., PDTPBT-C8, PDTPBT-C6 and PDTPBTC5 (Figure 22), which consist of alternating N-alkyl dithieno [3,2-b:20,30-d]pyrrole and 2,1,3-benzothiadiazole units and carry 1-octylnonyl, 1-hexylheptyl and 1-pentylhexyl as side chains, respectively, and the optical band is around 1.42 eV.[130] These polymers showed strong absorption in the wavelength range of 600–900 nm with enhanced absorption coefficient as the length of alkyl chain decreases. Bulk heterojunction photovoltaic solar cells (PSCs) were fabricated based on the blend of the polymers and PCBM with a weight ratio of 1:3. The device performance is dramatically improved as the length of the side chain decreases, due to enhanced film absorption coefficient and improved film morphology. With the polymer PDTPBT-C5, which carries the shortest alkyl chain, a PCE of up to 2.80% has been achieved. The studies on the alternating thiophene/phenylene/ thiophene (TPT) based copolymers have shown efficient organic photovoltaic characteristics.[131] The chemical structures of this type of copolymers are shown in Figure 22. These polymers exhibit better harvesting solar flux with deeper HOMO levels that gives impressive Voc (>0.8 V) and PCE up to 4.3%. These previous polymers were synthesized from dibromine TPT monomer and the structures were limited to a random feature. Ting et al. synthesized an alternating polymer



THE CHEMICAL RECORD

C12H25

C12H25

S

C12H25 N S

Si S

n SDTBT

N O

Si

N

C6H13

C12H25 N

S

S

S n

SDTBO

C6H13 N O

Si

N

S

SDTBO1

n O O C6H13 C6H13

Fig. 23. Chemical structures of Si-bridged bithiophene copolymers.

(a-PTPTBT) that consists of TPT as the electron-donating unit and 2,1,3-benzothiadiazole (BT) as the electron-accepting moiety and used it as an electron donor along with PC70BM as electron acceptor in the BHJ active layer for a photovoltaic device, and achieved a PCE of 6.4%.[132] It was reported that a copolymer alternated with electrondeficient benzothiadiazole (BTD) and electron-donating cyclopentadithiophene exhibited hole mobilities of up to 1.4 cm2/Vs in OFET devices.[133] Benzo[1,2-c:4,5c0]bis[1,2,5]thiadiazole (BBT) units are known for being very strong acceptors due to their hypervalent sulfur atoms which can be stabilized in quinoidal structures in a conjugated backbone.[5] In addition, they are expected to have short intermolecular S•••N contacts between the thiadiazole rings that result in the rigid planar geometry of the molecules.[134] Thus BBT derivatives can be utilized for lowering band gaps in π-conjugated polymers. Prasad et al. have synthesized new D-A alternating copolymer PCPBBT and PFTBBT containing BBT molecules, and PCPBTD and PFTBTD containing BTD molecules and found their electrochemical band gap varies from 1.01 eV (PCPBBT) to 2.05 eV (PFTBTD).[135] Bazan et al. reported the synthesis and application of two narrow-band-gap π-conjugated polymers, namely, poly[(4,4-didodecyldithieno[3,2 -b:2′,3′-d ]silole)-2,6-diyl-altand (2,1,3-benzothiadiazole)-4,7-diyl] (SDTBT)[136] poly[(4,4-didodecyldithieno [3,2-b:2′,3′-d ]silole)-2,6-diyl-alt(2,1,3-benzooxadiazole)-4,7-diyl] (SDTBO; Figure 23). Both polymers contain an electron-rich Si-bridged bithiophene (SDT)[137] donor unit and an electron-deficient benzothiadiazole (BT) or benzooxadiazole (BO) unit. These materials exhibit broad optical absorption spectra and favorable frontier molecular orbital levels for compatibility in polymer/fullerene BHJ devices. Solar cells fabricated with either polymer and PC71BM gave PCEs above 5%. While these results are encouraging, a limiting factor of both derivatives is their limited solubilities in organic solvents. They have modified the SDTBO copolymer, i.e. SDTBO1, by increasing the alkyl side chain on both donor and acceptor units and found that increasing the number of alkyl side chains helps in breaking up the intermolecular aggregates and increases the solubility in organic solvents.[138] The BHJ organic solar cell using SDTBO1:PC71BM blend (cast from chlorobenzene) shows


PCE of 1.6%, which has been further improved up to 3.7% when 4% of di-iodooctane (DIO) is added to the chlorobenzene (CB). A series of main chain donor–acceptor low-band-gap conjugated polymers (P1-P3) were designed (Figure 24), synthesized by Song et al, and used for the fabrication of polymer solar cells.[139] The absorption spectra of low-band-gap conjugated polymers were tuned by the ratio of three copolymerization monomers. The polymers in films exhibited broad absorption ranging from 300 to 1000 nm with optical band gaps of around 1.40 eV. All of the polymers have been investigated as an electron donor in photovoltaic cells blending with PCBM ([6,6]-phenyl C61-butyric acid methyl ester) as an electron acceptor and PCEs of 1.32–1.8% have been obtained. As for P1, PCE increases from 1.67 to 2.44% after adding 1,8diiodooctance as an additive. The higher PCEs are probably due to better phase separation of blend films. Poly(thienylene vinylene) derivatives (PTVs; chemical structures are shown in Figure 24) possess a broad absorption covering the whole visible region and higher hole mobility,[140] which is attractive for the application as donor materials in PSCs. However, the PCE of the PSCs based on PTVs with alkyl chain as donor is only ca. 0.2–0.3%,[141] which is probably due to the non-luminescent nature of the polymers.[142] Recently, He et al. synthesized a PTV derivative with D–A structure, (PTBTV; see Figure 21), which showed broad absorption and weak photoluminescence (PL). The PSC based on PTBTV as the donor demonstrated a PCE of 0.51%,[143] which is obviously an improvement in comparison with that of the common PTVs. More interestingly Huo et al. synthesized poly(3-carboxylated thienylenevinylene) (P3CTV) with an electron-withdrawing carboxylate substituent, which possesses a lower HOMO energy level and weak PL. The PCE of the PSCs based on P3CTV as the donor reached 2.01%, which is one order increase compared to the PSCs based on the common PTVs.[144] These results indicate that the photovoltaic performance of the PTV derivatives could be greatly improved by appropriate molecular structure modification. In order to further explore, new PTV derivatives, a narrow band gap conjugated polymer poly(2-(5-(5,6-bis(octyloxy)-4-(thiophen2yl)benzo[c])[1,2,5] thiadiazol-7-yl)thiophen-2-yl)-vinylene), POTBTV, was synthesized for application as donor material in

Chem. Rec. 2014, ••, ••–••



C8H17 C8H17

C8H17

C8H17O

OC8H17 S

S

C8H17

C8H17

N

S S

m

N

n

N S

C8H17 N

N

S

N

P1= (m:n=3:1) P2= (m:n=1:1) P3 (m:n=1:3) C6H13

C6H13 S

S N

S

n

C8H17O

OC8H17

S

S N

N

PTBTV

S

n

N

POTBTV

Fig. 24. Chemical structures of more copolymers.

polymer solar cells (PSCs).[145] The absorption edge of the POTBTV film is at 750 nm, indicating a narrow band gap of 1.65 eV. The HOMO and LUMO energy levels of POTBTV are −4.97 eV and −2.99 eV, respectively. The PCE of the PSC based on POTBTV as the donor PC70BM as the acceptor reached 1.53% with a short circuit current density of 6.83 mA cm−2, an open circuit voltage of 0.6V and a fill factor of 0.374 under the illumination of AM1.5, 100 mW cm−2, which is among the highest PCE values for PSCs based on PTV derivatives. Thieno[3,4-b]thiophene(TT)-benzo[1,2-b:4,5-b0]dithiophene (BDT)-based LBG polymers (PBDTTTs) are one kind of star photovoltaic polymer donor materials in PSCs, and several promising results have been reported in the past few years.[146,4,6,7] The chemical structures of some of the low band gap copolymers for this series are shown in Figure 25. In this series of materials, the HOMOs of PBDTTTs have been tuned effectively by introducing electron-withdrawing functional groups on the TT unit and hence the Voc of the PSC based on PBDTTTs has been successfully improved from 0.6 V to 0.78 V.[129] For example, the HOMO level of PBDTTTs can be lowered from −5.01 eV (for PBDTTT-E) to −5.12 eV (for PBDTTT-C) by replacing the carboxylate group with the carbonyl group,[147] and when a fluorine atom was introduced, the HOMO of PBDTTT-CF was further lowered to −5.22 eV. Although the introduction of fluorine reduced the HOMO level of PBDTTTs effectively, the synthesis routes of the fluorine substituted derivatives are quite tedious and also their HOMO levels are still not deep enough to get high Voc materials. Thus it would be useful and also necessary to find another effective way to lower the HOMO level of TT-based LBG polymers. Hou et al. have synthesized a planar benzodithiophene with lower HOMO which was copolymerized with the thieno[3,4-b] thiophene unit to obtain a new low band gap polymer of PBDPTT-C, which exhibited a higher

Chem. Rec. 2014, ••, ••–••

open circuit voltage (Voc) of 0.8 V and a promising efficiency of 5.2%.[148] When PBDTTT-CF is blended with PC71BM, a PCE of 7.73% has been achieved.[147b] Yang et al. have reported a new benzo [1,2-b:4,5-b′] dithiophene-containing polymer PBDTTBT having an optical band gap of 1.75 eV that exhibited a PCE of up to 5.66% when blended with PC70BM.[147] The higher Voc of 0.92 V from PBDTTBT based device originates from the lower HOMO level of the polymer and the higher IQE (above 80%) and the EQE response value beyond 50% in a wide spectral range are the main contributions to high Jsc. Recenlty Hsu et al. have designed and synthesized a new high mobility low-band-gap polymer (optical band gap 1.64 eV), PBDT-DODTBT, based on benzodithiophene and 5,6-bis(octyloxy)-4,7-di(thiophen-2yl)benzothiadiazole. Preliminary studies of the copolymer showed a hole mobility as high as 7.15 × 10−3 cm2/Vs from the SCLC model.[147c] Broad and good absorption, and high hole mobility combined with a nanometer-scale morphology, leads to a polymer with promising photovoltaic properties. Initial bulk heterojunction solar cells based on the blends of PBDTDODTBT with PC71BM had a high PCE up to 4%. Recently, two DPP-based copolymers (PCPDT-PDPP band gap 1.76 eV and PDTP-PDPP band gap 1.69 eV) were designed by molecular engineering by Chen et al.[149] Using a less conjugated phenyl group to replace the thiophene group as the D-A linking bridge, material band gap is enlarged by over 0.5 eV, which results in a considerable Voc and thus a better balance between Voc and Jsc. Without any interface engineering or post-treatment, they obtained a promising PCE of over 2% with PC71CM as electron acceptor, under AM1.5 illumination. Early reports on poly(thieno[3,4-b]thiophene) by Sotzing showed a very low band gap material (1.2 eV) with good electrical properties.[150] The chemical structures of these polymers are shown in Figure 26. A few years later, Yang’s group reported the first poly-(thieno[3,4-b]thiophene) derivative



THE CHEMICAL RECORD

O

R2 PBDTTT-E R1= -C8H17-iso; R2= -OC8H17; X=H

R1 O

X

O

S PBDTTT-C R1= -C8H17-iso; R2= -C7H15; X=H

S

S n

S

S

S R1 O

PBDTTT-EF R1= -C8H17-iso; R2= -OC8H17; X=F S

PBDTTT-CF R1= -C8H17-iso; R2= -C7H15; X=F

S

PBDTTTs

S

n

PBDPTT-C

n-C8H17 S

n-C8H17

N

S

N

S n

S

S

S

n-C8H17 S PBDTTBT

n-C8H17

S

N

O

N

S

S

S

n

N

O

O

N

PCPDT-PDPP

n

N

O PDTP-PDPP

Fig. 25. D-A copolymer based BDT-TT based unit: PBDTTTs, PBDPTT-C, PBDTTBT, PCPDT-PDPP and PDTP-PDPP.

R O

R1 O O

R2

S

R

O

S S

S

S

S S

n

O

O

O

S

R

F

S

O

O

O C8H17

R

S

R O PT4, R= 2-ethylhexyl

n

S

C7H15

F

O

S

S

S S

n


PT2, X= F, R1= n-octyl R2= 2-ethylhexyloxy R O

S

S

R2

n

PT1, R= C8H17

R

X

S

S

S

O

O

S


n

S

S

n


Fig. 26. Copolymers with different donor and acceptor groups.


Chem. Rec. 2014, ••, ••–••



(polymer PT1) having band gap 1.3 eV used in a near infrared photodetector.[151] It has been shown that the utilization of a fused thiophene ring such as the thienothiophene ring can stabilize the quinoid structure of the backbone and can therefore reduce the band gap of the system. They also used an ester group at the 2-position to stabilize the electron-rich thienothiophene ring and to further increase the solubility of the polymer. The BHJ polymer solar cells based on this polymer exhibited a PCE about 1% at 850 nm. Later, Yu et al. reported PT2, nother thieno[3,4-b]thiophene based copolymer with band gap of 1.58 eV, incorporating a benzodithiophene unit.[152] Photovoltaic properties of PT2 were first tested using an ITO/PEDOT: PSS/ polymer:PC60BM/Ca/Al configuration. A very interesting PCE of 4.7% was reported. Moreover, switching to PC70BM, which is known to have a higher absorption coefficient in the visible region, gave a better power conversion efficiency. Indeed, a PCE of 5.6%, a Voc of 0.56 V, a FF of 0.63, and a Jsc of 15.6 mA/cm2 were obtained. The optimal band gap, the rigidity, and planarity of the backbone, and the network morphology can explain those high efficiencies. Since the fluorine atom is a strong electron-withdrawing substituent, the introduction of fluorine into the conjugated backbone would lower both the LUMO and HOMO energy levels of the conjugated polymers, as demonstrated by Heeger and Bredas in a theoretical study of poly(phenylene vinylene) having various substituents.[153] To further study the impact of the side chains on the electronic properties, the solubility, and the morphology of the blend, Yu et al. reported a series of five new copolymers using alkyl side chains such as n-octyl, n-octyloxy, or 2-ethylhexyloxy on the benzodithiophene unit and n-octyl, 2-butyloctyl, 2-ethylhexyl, or n-dodecyl on the thieno[3,4-b]thiophene moiety.[154] It is important to note that PT3 has a fluorine atom at the 3-position. The fluorine atom is relatively small and has a strong electronegativity. Thus, the introduction of the fluorine should have an effect on the electronic properties by lowering both the HOMO and LUMO energy levels. Besides the use of different alkyl of alkyloxy side chains, optical band gaps of the material remained approximately the same for all polymers (1.6 eV). BHJ devices were made using PC60BM with an active area of 9.5 mm2. Polymer PT3 shows the highest Voc (0.74 V) and the best PCE (6.1%).

PT3 was reported by Li’s group and shows good performance in solar cells.[155] The ester group at the 2-position on the thienothiophene ring was replaced by a ketone group, which decreases the HOMO and LUMO energy levels and thereby increases the Voc in the BHJ solar cells. The best cell with polymer PT3 along with PC70BM revealed a very high PCE of 6.6% with a Voc of 0.70 V, a FF of 0.64, and a Jsc of 14.7 mA/cm2. Polymer PT3 shows good solubility in organic solvents owing to the branched side chains on both the ester group and the benzodithiophene moiety.[156] In BHJ solar cells, while using PC70BM as the acceptor and chlorobenzene (CB) as the solvent, a PCE of 3.9% was obtained. By adding processing additives such as 3% DIO (di-iodooctane), the efficiency greatly improved and reached 7.4%. This dramatic increase of the PCE is mainly caused by the change of the morphology in the film. Indeed, films prepared using CB and DIO are more uniform, which will increase the exciton migration in the donor/acceptor interphase. In the same series, recent studies provided polymers PT4 and PT5[157] and photovoltaic characterization led to a PCE of 5.1% for polymer PT4 (with the ester group at the 2-position of the thienothiophene ring). BHJ devices tested with polymer PT5, which possesses a ketone group and a fluorine atom on the thieno[3,4-b]thiophene ring, reached a PCE of 7.7%. This value remains the best reported so far for a fully characterized polymer solar cell. High Voc of 0.76 V, Jsc of 15.2 and a FF of 0.67 were also obtained. Interestingly, Solarmer recently reported a NREL-certified PCE of 8.1–8.3% for, most probably, a similar copolymer. Recently, You et al. have synthesized a new copolymer with fluorinated benzothiadiazole as structural unit (Figure 27), i.e. PBnDTDTffBT having an optical band gap of 1.7 eV.[158] The overall efficiency of the PBnDTDTffBT:PCBM BHJ device achieved 7.2%, which is greater than that for its nonfluorinated copolymer counterpart, PBnDTDTBT. This efficiency is among the highest obtained for polymer/PC61BM BHJ solar cells, and shows a great potential for the DTffBT unit and the incorporation of fluorine atoms in creating high performance materials for BHJ solar cells. Chen et al.,[159] have demonstrated that stronger electron donating ability of the dithienosilole (DTS) units widens the

Fig. 27. Chemical structure of PBnDT-DTBT and PBnDT-DTffBT copolymers.

Chem. Rec. 2014, ••, ••–••



THE CHEMICAL RECORD

C6H13 C8H17

S

S

N

O

C8H17

C8H17

S

O S

S S

S

O N R PSi1, R= 2-ethylhexyl PSi2, R= dodecyl

PDTTTPD

Sn

S

Si

C12H25

N

S PSi3

S

C12H25

N

n

n

S O

n

C8H17

C12H25

Si

C8H17

Si

C12H25

N

S N S

Br S

S

S PSi4

n

Fig. 28. Chemical structures of PDTTTPD, PSi1, PSi2, Psi3 and PSi4.

absorption spectrum of the resulting copolymers PSi1 and PSi2 towards the NIR region, and improves the light harvesting properties of thieno[3,4-c]pyrrole-4,6-dione (TPD) based D–A copolymers without significant effect on Voc compared to PBDTTPD. The branched side chains on TPD units improve the compatibility of PC71BM into the blend film, resulting in higher PCEs in BHJ solar cells (4.4% 3.4% with PSi1 and PSi2, respectively; Figure 28). The best device after process optimization shows up to 4.4% PCE. Bazan et al. have used the narrow-band-gap conjugated copolymer, poly[(4,4-didodecyldithieno[3,2-b:2′,3′-d]silole)2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl], as obtained immediately after polymerization and after treatment with chain-end capping thiophene reagents, namely PSi3 and PSi4 (Figure 28), respectively, to gauge the impact of reactive end groups on the performances of BHJ solar cells.[160] End-capping with thiophene improves performance by making the devices less sensitive to active layer thickness and to thermal degradation. The PCE of the devices based on PSi3:PC71BM and PSi4:PC71BM were 4.2% and 4.7%, respectively. A series of carbazole-based narrow-band gap polymers with two-dimensional donor-π-bridge-acceptor (D-π-A) structures were synthesized and characterized for use in polymer bulk heterojunction solar cells by Huang et al.[161] The resulting polymers have good solubility in common organic solvents and excellent thermal properties. The effects of the alkyl side chains and different dye contents on optical properties, electronic structures, charge-transporting ability, and device performance of these polymers were investigated. By blending these polymers as light-harvesting electron donors with (6,6)-phenyl-C71butyric acid methyl ester (PC71BM) electron acceptors in bulk heterojunction solar cells, high power conversion efficiency, as high as 4.47%, could be achieved. Chen et al. developed a new conjugated polymer, PDTTTPD (Figure 28), comprising 2,5-di-(thiophen-2-


yl)thieno[3,2-b] thiophene (DTT) and thieno[3,4-c]pyrrole4,6-dione (TPD) electron donor and acceptor units, respectively, which exhibits high crystallinity and excellent thermal stability.[162] A device incorporating PDTTTPD and [6,6]phenyl-C71-butyric acid methyl ester (1 : 1, w/w) exhibited a power conversion efficiency of 5.1%. Recently, Ouyang et al.[163] have reported the synthesis of copolymers of electron donating ethynyl.uorene and electronwithdrawing 3,6-dithiophen-2-yl-2,5-dihydropyrrolo[3,4c]pyyrole-1,4-dione having optical band gap of about 1.85 eV. Polymer PVs were fabricated using these polymers as donors and PCBM as acceptor. A PCE of 2.25% was achieved on polymer PVs under AM 1.5G illumination (100 mW cm−2). Yu et al. have synthesized a series of fluorinated polythienothiophene-co-benzodithiophenes (PTBFs) and carried out the characterization of their physical properties, especially their performance in solar cells. Fluorination of the polymer backbone lowered both the HOMO and LUMO energy levels and simultaneously widened the energy band gap of the polymer (0.1–0.2 eV). Incorporation of fluorine into the various positions of the polymer backbone significantly affected the solar cell power conversion efficiency from 2.3% to 7.2%.[164] A series of D-A copolymers PFTTQx, PFTTPz, PCzTTQx, and PCzTTPz (chemical structures shown in Figure 29) have been successfully developed by Cao et al.[165] The copolymers based on thiophene-substituted quinoxaline acceptors PFTTQx and PCzTTQx show red-shifted absorption spectra compared to previously reported benzenesubstituted quinoxaline copolymers. Moreover, the D-A copolymers of PFTTPz and PCzTTPz based on the polycyclic aromatic ring exhibit significant red shift absorption with a smaller band gap of 1.68 and 1.66 eV, respectively. The mobilities of PFTTPz and PCzTTPz are much higher than their analogous copolymers PFTTQx and PCzTTQx due to the enlarged fused planar aromatic ring among them. BHJ PSCs based on the blend of the copolymers with PC71BM show promising performance with overall efficencies of more than 2%. Despite their much higher mobility and more red-shifted absorbance, it was found that both PFTTPz and PCzTTPz exhibited lower PCEs compared to PFTTQx and PCzTTQx due to their poor solubility, which results in non-ideal phase separation. Among all the copolymers, PFTTQx shows the best performance with a PCE of 4.4%, a Voc of 0.90 V, a Jsc of 7.4 mA cm−2, and a FF of 0.59. Hsu et al. have successfully synthesized four D-A copolymers, poly(carbazole-dicyclopentathiophene-alt-benzothiadiazole) (PCDCTBT-C8), poly(carbazoledicyclopentathiophene-alt-dithienylbenzothiadiazole) (PCDCTDTBT-C8), poly(carbazole-dicyclopentathiophene-alt-dithienyldiketopyrrolopyrrole) (PCDCTDTDPP-C8), and ploy(carbazoledicyclopentathiophene-alt-quinoxaline) (PCDCTQX-C8;

Chem. Rec. 2014, ••, ••–••



Fig. 29. Chemical structures of PFTTQx, PFTTPz, PCzTTQx and PCzTTPz copolymers.

Fig. 30. Chemical structures of PCDCTBT-C8, PCDCTDTBT-C8, PCDCTDTDDP-C8 and PCECTQX-C8.

Figure 30)[166] having optical band gaps of 1.64 eV, 1.66 eV, 1.37 eV and 1.72 eV, resepectively. Compared to PCDCTBT using 4-(2-ethylhexoxy)phenyl moieties as the side chains, PCDCTBTC8 exhibits a broadening and bathochromic shift absorption spectrum as well as smaller optical band gap due to the enhanced intermolecular interaction in the solid state. Through such a simple and straightforward engineering of molecular structures, the device based on the PCDCTBTC8:PC71BM (1:3 in wt%) blend achieved a Voc of 0.74 V, a Jsc of 10.3 mA/cm2, a FF of 0.60, delivering an impressive PCE of 4.6%. The corresponding PCDCTBT-C8:PC71BM blend also showed a high hole mobility of 1.2 × 10−3 cm2/Vs, leading to the high current density and fill factor. The improved performance is associated with the modification of the aliphatic side chains on the CDCT structure to optimize the interchain interactions for enhanced charge transporting.

Chem. Rec. 2014, ••, ••–••

Recently, conjugated polymers based on the indacenodithiophene (IDT) unit have exhibited more research interest with promising performance in PSCs,[167] because the IDT unit can enhance the co-planarity of the polymer backbone with a reduced energetic disorder of the polymer. For example, Ting and co-workers have reported a PCE of 6.1% from the alternating polymer of IDT and benzothiadiazole (BT) units.[167c] More importantly, the IDT-based polymers show high and stable field-effect hole mobilities. A hole mobility of as high as ∼1 cm2/Vs from the analogous polymer of IDT and BT units has been reported by Zhang et al.[167d] Quinoxaline (Qx) has been widely implemented as an electron acceptor co-monomer of low-band gap polymers in PSCs because of their high electron affinity attributed to symmetric unsaturated amine nitrogen (C=N) structures.[168] The impressive performance of quinoxaline-based polymers has



THE CHEMICAL RECORD

Fig. 31. Chemical structures of PIDT-diphQ and PIDT-phanQ copolymers.

shown its potential for achieving high performance in PSCs.[169–171] 2,3-Diphenylquinoxaline, which possesses two separated phenyl rings, is one of the commonly investigated quinoxaline derivatives because of its facile synthesis and versatility. Research efforts focusing on tuning the solubility, band gap, and energy levels of 2,3-diphenylquinoxaline-based polymers have resulted in PCEs of ≤6%.[5, 6] However, most of these polymers possess large band-gaps (>1.9 eV) and low charge carrier mobilities (∼1 × 10−5 cm2/Vs). Moreover, two separated phenyl rings on the 2,3-diphenylquinoxaline could induce some steric hindrance to interrupt intermolecular stacking between polymer chains. If two phenyl rings could be connected by a single bond between the ortho positions, it will significantly increase the planarity of quinoxaline and facilitate both intermolecular packing and charge transport. Moreover, the extended π-conjugation of the fused qunioxaline (named as phenanthrenequnioxaline) will function as a stronger electron acceptor, leading to lower band gap in the corresponding polymer. Jen et al.[172] have combined IDT and two quinoxaline derivatives to form new polymers (PIDT-diphQ and PIDT-phanQ (Figure 31) with optical band gap of 1.81 eV and 1.67 eV, respectively). Due to the enhanced planarity of phenanthrenequnioxaline, the PIDTphanQ/ PC71BM-based BHJ device exhibits an improved PCE of 6.24% compared to the PCE of 5.69% in the PIDT-diphQ/ PC71BM-based device. In comparison with Qx, the pyrazino[2,3-g]quinoxaline (PQx) unit is relatively more electron-deficient and more rigid. Therefore, replacing Qx units in conjugated polymers with PQx moieties can give rise to the corresponding polymers with lower band gap, higher ionization potential (IP) and more rigid structures. In addition, the introduction of a planar π-π system in the polymer backbone allows the polymer to form longrange intermolecular π-π stacking arrangements, which can lead to a high charge carrier mobility. A series of low band gap copolymers consisting of electron-accepting pyrazino[2,3g]quinoxaline (PQx) and an electron-donating indolo[3,2b]carbazole and thiophene units (PPQx1-PPQx4; Figure 32) have been designed and synthesized by Stille coupling polymerization.[173] Their optical and electrical properties could also


be facilely fine-modulated for photovoltaic application by adjusting the donor/acceptor ratios, and the absorption can be enhanced by increasing the content of PQx units, which led to enhanced absorption. The best performance was achieved using PPQx3/[70]PCBM blend (1:3) with Jsc = 9.55 mA/cm2, Voc = 0.81 V, FF = 0.42, and PCE = 3.24%, which is the highest efficiency for the PQx and indolo[3,2-b]carbazole based devices. These results also indicate that the efficient photovoltaic materials with suitable electronic and optical properties can be achieved by just fine-tuning the ratios of the strong electron-deficient acceptors and large-π-planar donors. Andersson et al. synthesized three polymers bearing a common carbazole-thiophene- quinoxaline-thiophene backbone, but different side chains, in order to investigate the effect of side chains on their photovoltaic performance.[174] Their photophysical, electrochemical, and photovoltaic properties were investigated and compared. The polymer PPQx7 (Figure 32), with the largest amount of side chains, showed the highest power conversion efficiency of 3.7% with an opencircuit voltage (Voc) of 0.92 V. Recently Sharma et al. have synthesized a new alternating copolymer comprising benzo[1,2-b;4,5,b′]dithiophene (BDT) derivative and 4,9-bis-(5-bromothiophene-2-yl)-6,7-di-(2ethylhexyl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline (DTQx) derivative electron donating and electron withdrawing units, respectively, which posseses a low optical band gap of 1.38 eV. They used a donor component along with PC70BM as acceptor for the BHJ polymer solar cells and after optimization a PCE of 5.12% was achieved with Jsc of 11.47 mA/cm2, Voc of 0.72 V and FF of 0.62.[175] Tao et al. have investigated the morphology and device performance relation of BHJ solar cells based on a new lowband gap polymer with a backbone of alternating thieno[3,4b]thiophene and benzodithiophene.[176] By adding a suitable amount of DIO additive to the host solvent (chlorofoorm in this study), the PCE was improved by a factor of more than three times from 1.45% to 4.83%. The enhancement in PCE is attributed to the improvement in the nanoscale morphology of the BHJ active layer.

Chem. Rec. 2014, ••, ••–••



C6H13 N

C6H13

N

C6H13

S

S S N

C6H13

N

n

S

m

N

N

C6H13 PPQx1 (m:n=0.9:0.1), PPQx2 (m:n=0.8:0.2) PPQx3 (m:n=0.7:0.3), PPQx4 (m:n=0.6:0.4)

OC8H17

C8H17O

C8H17

C8H17

N

N

C8H17

N

N

N

N S

S

S

n

S

n

PPQx6

PPQx5 OC8H17

C8H17O C8H17

C8H17

N

N

N S

S

n

PPQx7

Fig. 32. Chemical structures of various copolymers with pyrazino [2,3-g]quinoxaline electron acceptor unit.

Fig. 33. Chemical structure of POD2T-DTBT copolymer.

Recently, Chen et al. have reported the synthesis and characterization D-A copolymer POD2T-DTBT (Figure 33) having an optical band gap 1.59 eV, which is a candidate for high performance OTFTs and OPVs.[177] Field effect transistors made from POD2TDTBT were shown here to exhibit hole carrier mobilities of 0.13–0.20 cm2/Vs and, when applied in solar cells, blends of POD2T-DTBT and PC71BM have yielded promising PCEs of 5.83–6.26%. Bronstein et al.[178] report the synthesis of two novel thieno[3,2-b]thiophene diketopyrrolopyrrole containing D-A copolymers having optical band gaps of 1.38 eV and 1.28 for PDDP1 and PDDP2, respectively (Figure 34) and with a

Chem. Rec. 2014, ••, ••–••

maximum field effect hole mobility of 1.95 cm2/Vs, which is the highest mobility from a polymer-based OFET reported to date. Bulk-heterojunction solar cells comprising PDDP1 and PC71BM gave a power conversion efficiency of 5.4%. The tetrathienoanthracene moiety with two-dimensional (2D) extended π-conjugated structure was chosen as a building block for new polymers,[179] which favors stronger co-facial π-π stacking. A series of new polymers (PTAT-1, PTAT-2, PTAT-3 and PTAT-4; Figure 35) were synthesized with thieno[3,4-b]thiophene as comonomer. The results demonstrate that tetrathienoanthracene is a promising building block for the development of semiconducting polymers with high photovoltaic efficiency. A series of semiconducting copolymers (PTAT-1, PTAT-2, PTAT-3 and PTAT-4) containing extended π-conjugated tetrathienoanthracene units have been synthesized by Yu et al.[180] It was shown that the extended conjugation system enhanced the π-π stacking in the polymer/ PC61BM blend films and facilitated the charge transport in heterojunction solar cell devices. After structural fine-tuning, the polymer with bulky 2-butyloctyl side chains (PTAT-3) exhibited a PCE of 5.6% when it was blended with PC61BM. The power conversion efficiencies of bulk heterojunction (BHJ) solar cells can be increased from 5 to 6.5% by incorporating an ultrathin conjugated polyelectrolyte (CPE) layer



THE CHEMICAL RECORD

Fig. 34. Chemical structures of PDDP1 and PDDP2.

R1 S S S

S S

n F

S

COOEH PTAT-X PTAT-1: R1= 2-ethylhexyl PTAT-2: R1= 2-decyltetradecyl PTAT-3: R1= 2-butyloctyl PTAT-4: R1= 2-ethyldodecyl R1

Fig. 35. Chemical structures of PTAT-1, PTAT-2, PTAT-3 and PTAT-4 copolymers.

between the active layer and the metal cathode. Poly[N-9″heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′benzothiadiazole)] (PCDTBT) and PC71BM were chosen for the photoactive layer. CPEs with cationic polythiophenes, in both homopolymer and block copolymer configurations, were used to improve the electronic characteristics.[181] New copolymer semiconductors based on thiazolothiazole and dithienosilole moieties and different side chains (PSOTT, PSEHTT and PSOxTT (Figure 36) with low band gap of 1.79 eV, 1.82 eV and 1.73 eV, respectively) have been synthesized by Jenekhe et al.[181] and were found to have promising charge transport and photovoltaic properties. High performance (5.0% PCE) BHJ solar cells were developed from PSEHTT with branched alkyl side chains and perpendicular orientation of π-π stacking to the substrate. In contrast, BHJ solar cells based on PSOxTT and PSOTT with linear side chains and π-stacking that is parallel to the substrate had average efficiencies of 2.1–4.1%. Wei et al. prepared a new intramolecular charge transfer (ICT) conjugated polymer, PBDTBO, featuring alternating


rigid, coplanar, electron-rich benzo[1,2-b:4,5-b′]dithiophene BDT units and soluble, electron-deficient 5,6bis(octyloxy)benzo[c][1,2,5]-oxadiazole BO units as the D and A units, respectively.[182] A PSC device incorporating PBDTBO and PC61BM (blend weight ratio, 1:1), prepared without requiring special treatment, exhibited a high opencircuit voltage (0.86 V) and a high solar energy PCE (5.7%). Recently Hou et al. report the new copolymer of BDT and TT with sulfonyl substituent, PBDTTT-S (Figure 36). The PBDTTT-S fillm shows a broad absorption with an absorption edge at 750 nm and a lower HOMO energy level at −5.12 eV.[183] The PSC based on PBDTTT-S as donor and PC71BM as acceptor exhibits high Voc of 0.76 V and PCE of 6.22%. Obviously, the sulfonyl group is a promising candidate as a strong electronwithdrawing group applied to high-efficiency PSCs. Two new soluble alternating alkyl-substituted benzo[1,2b:4,5-b0]dithiophene and ketone-substituted thieno[3,4b]thiophene copolymers were synthesized and characterized by Tao et al.[184] They found that grafting 3-butyloctyl side chains to the benzo[1,2-b:4,5-b0]dithiophene unit at C4 and C8 afforded the resulting polymer (PBDTKT1; Figure 36) with a high hole mobility (∼10−2 cm2/Vs) and a low-lying HOMO energy level (5.22 eV). The bulk heterojunction solar cells using PBDTKT1 as the electron donor demonstrated a high power conversion efficiency of 4.8% even with PC61BM as the electron acceptor. The introduction of an electron-withdrawing fluorine atom into the thieno[3,4-b]thiophene unit at the C3 position (PBDTKT2; Figure 36) lowers the HOMO energy level and consequently improves the open circuit voltage from 0.78 to 0.86 V. These values are about 0.1 V higher than those reported for their analogues based on alkoxy-substituted benzo[1,2b:4,5-b0]dithiophene, resulting in a power conversion efficiency of about 3.9%. This work demonstrates that the replacement of the alkoxy chains on the benzo[1,2-b:4,5b0]dithiophene unit with less electron-donating alkyl chains is able to lower the HOMO energy levels of this class of polymers without increasing their band gap energy.

Chem. Rec. 2014, ••, ••–••



R

S

[PSOTT] R= R' = S

N

N

S

S

S

S n

Si

R

R'

[PSEHTT] R= R' =

R' [PSOxTT]

OEH

O

R=

OC8H17

C8H17O

S

R=

S

S S

n N

OEH

O

N

PBDTBO R O S O

C4H9 C7H15

C5H11

O

X OR

S S

S

S

S

S S

S n

OR PBDTTT-S R= 2-ethylhexyl

n

X = H (PBDTKT1) X = F (PBDTKT2) C4H9

C5H11

Fig. 36. Chemical structures of PSOTT, PSEHTT, PBDTBO, PBDTTT-S, PBDTKT1 and PBDTKT2.

Fig. 37. Chemical structures of PDT-Si-TDP and PDT-Ge-TDP.

Reynolds et al. have reported the synthesis and bulk heterojunction photovoltaic performance of the first dithienogermole (DT-Ge)-containing conjugated polymer (PDT-Ge-TDP), dithienosilinale (DT-Si; Figure 37).[185] The use of this heterocycle in a donor–acceptor copolymer using 1,3-dibromo-n-octyl-thienopyrrolodione (TPD) as acceptor unit displays light absorption extending to 735 nm (with band gap 1.69 eV), and a higher HOMO level than the analogous copolymer containing the commonly utilized dithienosilole

Chem. Rec. 2014, ••, ••–••

(PDT-Si-TDP) heterocycle. When PDT-Ge-TPD:PC70BM blends are utilized in inverted bulk heterojunction solar cells, the cells display average power conversion efficiencies of 7.3%, compared to 6.6% for the PDT-Si-TPD containing cells prepared under identical conditions. The performance enhancement is a result of a higher short-circuits current and fill factor in the PDT-Ge-TDP-containing cells, which comes at the cost of a slightly lower open circuit voltage than for the PDT-SiTPD based cells. Recently Thompson et al. have synthesized a family of novel semirandom P3HT-DPP copolymers containing different contents (5–15%) of the DPP acceptor unit.[186] The chemical structures of these copolymers are shown in Figure 38. These polymers combine broad absorption profiles, high absorption coefficients, high hole mobilities and semicrystalline structures similar to P3HT. In BHJ solar cells with PC61BM, the polymers show effective film formation with optimized polymer:fullerene ratios that vary based on the content of DPP in the polymer backbone, and efficiencies of nearly 5.0% are observed for P3HT-DPP-10% in a 1:1.3 polymer:fullerene ratio. A broad photocurrent response, representative of the polymer absorption profile confirm that semirandom donor–acceptor copolymers are an effective platform for improving light harvesting in BHJ solar cells. A series of alternating copolymers of cyclopenta[2,1-b;3,4b′]dithiophene (PCPDT) and thieno[3,4-c]pyrrole-4,6-dione



THE CHEMICAL RECORD

Fig. 38. Chemical structure of P3HT–DPP based copolymers.

Fig. 39. Chemical structures of various PCPDTTPDs.

(TPD) have been prepared and characterized for polymer solar cell (PSC) applications by Ding et al.[187] Different alkyl side chains, including butyl (Bu), hexyl (He), octyl (Oc), and 2-ethylhexyl (EH), are introduced to the TPD unit in order to adjust the packing of the polymer chain in the solid state, while the hexyl side chain on the PCPDT unit remains unchanged. The chemical structures of these copolymers are shown in Figure 39. These polymers in this series have a narrow band gap (1.57–1.61 eV) and a broad light absorption. The different alkyl chains on the TPD unit not only significantly influence the solubility and chain packing, but also fine tune the energy levels of the polymers. The polymers with Oc or EH group have lower HOMO and LUMO energy levels, resulting in higher open circuit voltages (Voc) of the PSC devices. Power conversion efficiencies (PCEs) up to 5.5% and 6.4% are obtained from the devices of the Oc substituted polymer (PCPDTTPD-Oc) with PC61BM and PC71BM, respectively. This side chain effect on the PSC performance is related to the formation of a fine bulk heterojunction structure of polymer and PCBM domains. Recently, You et al. have reported two new polymers incorporating benzodithiophene (BnDT) as the donor and either benzotriazole (HTAZ) or its fluorinated analogue (FTAZ) as the acceptor (Figure 40).[188] Both polymers show an optical gap of 2.0 eV, which is even slightly larger than that of


P3HT (1.9 eV). However, the photovoltaic performance of PBnDT-HTAZ is on par with that of P3HT, with an overall efficiency of 4.3% at an active layer thickness of 230 nm. More impressive results come from the PBnDT-FTAZ:PC61BMbased BHJ cells, which show a Voc of 0.79 V, a Jsc of 12. 45 mA/cm2, and a very notable FF of 72.2%, leading to a highest overall efficiency of 7.1% with an active layer thickness of 250 nm. Although the two fluorine atoms have a minimal effect on the optical and electrochemical properties of the polymer, they have a profound effect on the hole mobility of the polymer and thus the photovoltaic performance. A new alternating copolymer of dithienosilole and thienopyrrole-4,6-dione (PDTSTPD; Figure 40) have recently been developed by Tao et al.,[189] which possesses both a low optical band gap (1.73 eV) and a deep highest occupied molecular orbital energy level (5.57 eV). The introduction of branched alkyl chains to the dithienosilole unit was found to be critical for the improvement of the polymer solubility. When blended with PC71BM, PDTSTPD exhibited a power conversion efficiency of 7.3%[190] on the photovoltaic devices with an active area of 1 cm2. Mikroyannidis et al.[191] have synthesized two novel low band gap soluble copolymers, PV1 and PV2. PV1 (Figure 41) consisted of alternating dihexyloxyphenylene and α-[4-(diphenylamino)phenyl]methylene]-4-nitro-benzeneacetonitrile, whereas PV2 consisted of alternating dihexyloxyphenylene and α,α′-[(1,4-phenylene)dimethylidyne] bis(αZ, α′Z)-4-nitrobenzeneacetonitrile. These copolymers showed broad absorption curves with long-wavelength absorption maxima around 620 nm and optical bands of 1.68 and 1.64 eV for PV1 and PV2, respectively. Both PV1 and PV2 were blended with PCBM to study the photovoltaic response of BHJ solar cells. The HOMO and LUMO levels of both PV1 and PV2 are well aligned with those of PCBM acceptor. This allows efficient photoinduced charge transfer and high open circuit voltage, leading to an overall PCE of 3.15% and 2.60% for the as-cast PV1:PCBM- and PV2:PCBM-based devices, respectively. The PCE of the devices has been further improved up to 4.06% and 3.35% for the devices based on thermally annealed PV1:PCBM and PV2:PCBM blends, respectively. Copolymer PB (Figure 41) has the donor–acceptor (D-A) structure with the dihexyloxyphenylenevinylene as the D unit, and the BF2-azopyrrole complex as the A unit was developed by Mikroyannidis et al.[192] The hexyloxy side chains of PB enhanced the solubility of the copolymer, which was used for bulk heterojunction solar cells. A thin film of PB showed a broad absorption band with a long wave absorption maximum at 511 nm and an optical band gap of 1.63 eV. The overall PCE for the BHJ PV devices was 3.54%. Sharma et al. have demonstrated an approach for enhancing the PV performance of the polymer solar cells based on a D-A phenylenevinylene copolymer PV3:PCBM blend, by

Chem. Rec. 2014, ••, ••–••



Fig. 40. Chemical structures of PBnDT-HTAZ, PBnDT-FTAZ and PDTSTPD copolymers.

NO 2

OC6H13

NO 2

NC

N C6H13O

OC6H13

n

OC6H 13

NC n C6H13O S

NC

C6H13O n

CN

NO 2

O 2N

P V1

PV 2

P V3

NC NO2 N

OC6H13

OC6H 13

N N

N

N

S

F

n

C6H13O

N

B F

n

C6H13O

PB

PV4

Fig. 41. Chemical structures of PV1, PV2, PV3, PV4 and PB copolymers.

treating the active layer with dichlorobenzene (DCB) vapor and subsequently thermal annealing.[193] Compared to the device (without solvent annealing) that is subjected only to thermal annealing, the DCB vapor treatment can induce copolymer PV3 self-organizing into an ordered structure, resulting in enhanced absorption in the visible region and hole mobility. The subsequent thermal annealing of the device at 120 °C makes PCBM molecules diffuse into aggregates, which when combined with the ordered copolymer phase form bicontinuous pathways in the entire active layer for efficient

Chem. Rec. 2014, ••, ••–••

charge separation and transport, resulting in overall PCE ofabout 3.7%. Recently, Mikroyannidis et al. have synthesized a novel low band gap phenylenevinylene copolymer, PV4, which contains alternative phenylenevinylene and thiophene along the backbone and a pyrrole side group (Figure 41).[193] The latter is attached to the thiophene ring through the nitrogen and carries cyanovinylene 4-nitrophenyl substituents to broaden the absorption spectrum. The dihexyloxyphenylene, thiophene, and pyrrole groups behave as electron donors, while the



THE CHEMICAL RECORD

C10H21

C10H21

C10H21

C 10H21 S N

S

S C6H13

S

S S

S

S N

S C10H21

S

n

C6H13 S n

S

S

N

N S C10H21

C10H21

N

C6H 13

C6H13

S

N

C 10H21 PBDT-DTNT

PBDT-DTBT

Fig. 42. Chemical structures of PBDT-DTBT and PBDT-DTNT copolymers.

Fig. 43. Chemical structures of PBT-0F, PBT-1F, PBT-2F and PBT-3F.

cyanovinylene 4-nitrophenyl group behaves as an electronwithdrawing moiety. The band gaps estimated from optical absorption and cyclic voltammetry measurements are 1.65 and 1.70 eV, respectively. The PSC based on the PV4 modified PCBM, i.e. F blend cast from CN/THF and subsequently thermally annealed, shows a PCE of 4.14%. The higher PCE for the devices cast from mixed solvents and further improvement with thermal annealing has been attributed to the increase in the crystallite size of PV4, which facilitates charge transport in the device. Cao et al. have reported the use of naphtho[1,2-c:5,6c]bis[1,2,5]- thiadiazole (NT) as a new acceptor unit in D-A conjugated polymers (PBDT-DTNT) for high-performance PSC. Comparing the conjugated polymer with BT, i.e. PBDT-DTBT, PBDT-DTNT (Figure 42) has an enlarged planar aromatic structure containing two fused 1,2,5thiadiazole rings, which may facilitate the interchain packing of the resulting polymer and may further enhance the carrier mobility. In addition, the electron-withdrawing capability of NT is slightly stronger than that of BT, which will lower the resulting polymer band gap, resulting in more efficient solar energy harvesting while still maintaining enough driving force


for the charge separation between the polymer and PC61BM. Consequently, compared with the analogous BT-based polymer, the NT-based polymer exhibited a red shifted absorbance, greatly enhanced hole mobility, and much improved PSC performance, with a PCE of 6.0%.[194] It has been observed that the introduction of an F atom into the different positions resulted in a distinct influence on the photovoltaic performance.[195] Hou et al. have recently synthesized a polymer backbone with D-A structure based on 4,8-bis(thiophene-2-yl)-benzo[1,2-b:4,5-b]dithiophene (BDT-T) and TT with thiophenes as a π-bridge, which is ben-eficial for improving the FF of the corresponding device.[196] This backbone, named as PBT, was selected as the model structure, and fluorine atoms were added onto the D units or/and A units of the backbone. Accordingly, a series of D–A polymers, PBT-0F, PBT-1F, PBT-2F and PBT-3F were synthesized (Figure 43). Among these copolymers, PBT-3F based solar cells showed the highest PCE of 8.6% with Jsc of 15.2 mA/ cm2, Voc of 0.78V and FF of 0.724. Therefore, introducing fluorine atoms onto the appropriate positions of the donor units in D–A polymers will be a promising method to effectively tune their molecular energy levels of for applications in PSCs.

Chem. Rec. 2014, ••, ••–••



Fig. 44. Chemical structures of PBDT4, PBDT-TTT, PBDT-BTT and PBDT-TT1.

Wang et al. have described the synthesis and characterization of four polymers based on BDT and isoindigo with zero, one, two, and three thiophene spacer groups, namely PBDT-I, PBDT-TIT, PBDT-BTI, and PBDT-TTI (Figure 44).[197] They have demonstrated that the use of bithiophene as a spacer unit improves the geometry of the polymer chain, making it become planar, and hence potentially enhanced π–π stacking occurs. As a result of the favorable interaction of the polymer chains, enhanced absorption coefficient, and optimal morphology, PBDT-BTI, which possesses bithiophene as a spacer, revealed a high current and fill factor, leading to a PCE of 7.3% (Jsc = 14.96 mA/cm2, Voc = 0.72 V and FF = 0.68) in devices, making this polymer the best performing isoindigo-based material in PSCs. Moreover, PBDT-BTI could still maintain an efficiency of over 6% with the active layer thickness of 270 nm, which makes it a promising candidate for a material in printed PSCs. Thus, the use of thiophene spacers in D–A polymers could be an important design strategy to produce high-performance polymers for solar cells. Heeger et al. have reported results obtained from another low band gap naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole-based polymer (NT; Figure 45),[198] which also yields much lower solar cell performance (PCE = 3.6%) when blended with PC70BM than with PC60BM (PCE = 5.8%). Interestingly, these solar cells exhibit the opposite trend when 1,8diiodooctane (DIO) is used as a processing additive in the blend solutions. NT:PC70BM solar cells then outperform NT:PC60BM solar cells (PCE = 6.4% vs. PCE = 6.1%). Recently, Jiang et al. have successfully designed and synthesized a series of new fluorinated benzothiadiazole-based conjugated copolymers: PBDTTEH-DTHBTff, PBDTTEHDTEHBTff, and PBDTHDO-DTHBTff (Figure 46).[199] The

Chem. Rec. 2014, ••, ••–••

Fig. 45. Chemical structure of NT polymer.

power conversion efficiencies of 4.46, 6.20, and 8.30% were achieved for PBDTTEH-DTHBTff, PBDTTEH-DTEHBTff, and PBDTHDO-DTHBTff based devices within ∼100 nm thickness active layers without any processing additives or posttreatments, respectively. The PCE of 8.30% for PBDTHDODTHBTff is the highest value for the reported traditional single-junction polymer solar cells via a simple fabrication architecture without any additives or post-treatments. In addition, it is noteworthy that PBDTHDO-DTHBTff also allows making highly efficient polymer solar cells with high PCEs of 7.27 and 6.56% under the same conditions for ca. 200 and ca. 300 nm thickness active layers, respectively. Excellent photoelectric properties and good solubility make polymer PBDTHDO-DTHBTff become an alternative material for highperformance polymer solar cells.



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Fig. 46. Chemcal structures of PBDTTEH-DTH BTff, PBDTTEH-DTEH BTff and PBDTHDO-DTH BTff.

6 BHJ Photovoltaic Devices Based on Small Molecules as Donor Bulk heterojunction OSCs utilizing solution-processable small molecules as donors and fullerene as acceptors have made their debut.[200] Solution-processable small molecules are attractive for OSCs as they are easier to synthesize and purify, offer easier processing compared with polymers, and show, in general, higher charge carrier mobilities.[201] The power conversion efficiency (PCE) of solution processable small molecules as donor have steadily improved over the past decade from about 0.03% to over 4.4%[203] due to considerable efforts toward the development of new molecules. Particularly, significant progress has been made in the synthesis and processing of donor–acceptor (D–A) small molecules. Theoretical and experimental advances in OSCs have demonstrated that such D–A small molecules have great potential to be one class of promising materials for OSCs.[180] The D–A small molecules were considered as efficient donor materials for OSCs because of their several key advantages: (i) the D–A small molecules are usually easier to be purified and synthesized; (ii) the absorption spectrum of the D–A molecules can be extended towards longer wavelength by intramolecular charge transfer (ICT) transition between donor and acceptor inside molecules[204], (iii) the energy level can be easily controlled by introducing various electron-donating or accepting groups into the molecules. The electron-donating molecules in the active layer of BHJ OSCs should have the following features: (1) the molecule should have broad absorption (e.g. 400–800 nm), which can efficiently harvest a large amount of energy from the solar spectrum; (2) the molecule must be air stable, i.e. the HOMO level must be lower than the air oxidation threshold (i.e. −5.27 eV),[205] which ensures a relatively high Voc; (3) the LUMO level of the molecule should be different by 0.3 eV


from that of the electron-accepting molecule in the active layer, for decreasing the energy loss during the electron transfer between donor and acceptor inside the active layers;[206] (4) the molecule should have a balanced mobility between the hole mobility and the electron mobility, which can decrease the carrier recombination at the interface of the donor and acceptor;[207] (5) the molecule should have good miscibility with an acceptor so as to form an interpenetrating network between the donor and acceptor, which can ensure maximum exciton dissociation at the interface between the donor and acceptor and increase the Jsc of OSCs. In other words, the ideal electrondonating molecule should show broad absorption, appropriate HOMO and LUMO levels, appropriate and balanced carrier mobilities and better miscibility with the acceptor. Triphenylamine (TPA)-containing molecules have attracted special research interests for the solution-processable organic optoelectronic molecules, because of their good solution processability benefiting from the three-dimensional propeller structure of TPA. The chemical structures of some small molecules are shown in Figure 47. The TPA-containing D-A structured molecules were designed to take advantage of (1) the higher hole-transporting mobility of TPA,[208] (2) the extended absorption spectrum of the D-A structure toward longer wavelength by an intramolecular charge transfer (ICT),[209] and (3) the higher oxidation potential of the D-A molecules for a higher open circuit voltage of the OSCs with the molecules as donor.[210] Among the molecules, a star-shaped molecule with TPA as core and three benzothiadiazole-hexylthiophene (BTHT) arms, S(TPA-BT-4HT), displayed high photovoltaic performance: the PCE of the OSC based on S(TPA-BT-HT)/ PC70BM reached 2.39% under the illumination of AM.1.5, 100 mW/cm2,[211] which is higher than the device based on L(TPA-BT). Li and co-workers synthesized a solution processable star-shaped organic molecule S(TPA-BT) with

Chem. Rec. 2014, ••, ••–••



CN CN S NC

CN

O N

N N

DADP

TDCV-TPA

S

S

CN

CN CN

NC C6H13 S N S N

N

N

N

S N

S N N N S N

S

S

C6H13

S(TPA-BT-4HT) C6H13 N S N N

N S N S(TPA-BT) N

N

N

S

N

N N L(TPA-BT)

Fig. 47. Chemical structures of various solution processable DADP, TDCA-TPA, S(TPA-BT-4HT), S(TPA-BT) and L(TPA-BT) small molecules.

three D–π–A–π–D structured branches based on triphenylamine (TPA) as core and donor units (D), benzothiadiazole (BT) as acceptor units (A), and the ethylene double bonds as π-connections between them, for application in OSCs.[212] S(TPA-BT) film shows a broader and stronger absorption band in the range of 440–670 nm, a lower band gap of 1.86 eV, a higher hole mobility of 4.71 × 10−5 cm2/Vs. The PCE of an OSC based on a blend of S(TPA-BT) and PCBM

Chem. Rec. 2014, ••, ••–••

(1:3, w/w) reached 1.33% under A.M. 1.5 illumination, 100 mW/cm2. In further work, they have synthesized a new solution processable D-A structured molecule, B(TPA-BTHT),[213] with the propeller-shaped TPA as core and two BT-HT arms. B(TPA-BT-HT), which shows a broad absorption in the wavelength range from 300 to 600 nm. The OSC device based on B(TPA-BT-HT) as donor and PC70BM as acceptor displayed a power conversion efficiency of 1.96%.



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Recently, Zhan and co-workers[214] have reported the synthesis and characterization of a new 3D, star-shaped, D–A–D organic small molecule with TPA as the core and donor unit, benzothiadiazole as the bridge and acceptor unit, and oligothiophene as the arm and donor unit S(TPA-BT-HTT), where S is star, BT is benzothiadiazole, and HTT is hexylterthiophene. Solution processed OSCs based on a blend of S(TPA-BT-HTT) and PC71BM afforded a PCE as high as 4.3% without any post-treatments, e.g., thermal annealing, solvent annealing, or additive addition. Tian et al.[215] have synthesized a series of novel solution processible small molecules (2TAPM, 4TAPM and 2BTAM; Figure 48) consisting of an electron-accepting unit (2-pyran4-ylidenemalononitrile, PM) and an electron-donating unit

NC

CN S

S

O N

N 2TAPM NC

CN S

S

O N

N 4TAPM

S

S

NC

CN S S

S S

O N

N 2BTAM

Fig. 48. Chemical structures of 2TAPM, 4TAPM and 2BTAM small molecules.

(triphenylamine and different thiophene units). 2-Pyran-4ylidenemalononitrile is a strong electron-accepting group which can increase the electron affinity and reduce the band gap of the conjugated system when combined with strong electron-donating moieties. Triphenylamine was introduced into the molecule in order to increase the electron-donating ability and high dimensionality. The bulk heterojunction photovoltaic devices were fabricated by using small molecules (2TAPM or 4TAPM) as donor and PC71BM as acceptor. A high short-circuit current density of 7.86 mA/cm2 and PCE of 2.47% were achieved for 4TAPM:PC71BM under simulated air mass 1.5 global (AM 1.5 G) irradiation (100 mW/cm2). Mikroyannidis and co-workers[216] synthesized two new soluble vinylene compounds, TPA-TNP and BTDTNP (Figure 49), which contained triphenylamine and benzothiadiazole segments, respectively, and terminal p-nitrophenyl units. Their absorption was broad and extended up to about 800 nm with optical band gaps of 1.65–1.67 eV. The presence of the electron-withdrawing nitro groups on the terminal phenyl rings broadened the absorption of these compounds. The photovoltaic properties of these compounds blended with PCBM were investigated and the power conversion efficiency is 1.13% and 1.32% for TPA-TNP and BTD-TNP, respectively. The device with thermally annealed BTDTNP shows power conversion efficiency of about 2.42%. Dicyanovinyl (DCN) is a well-known acceptor group in the organic optoelectronic materials. Roncali et al.[204] synthesized the star-shaped D-π-A molecules with TPA as core and donor unit, DCN as end group and acceptor unit, and thiophene as the π-bridge. The molecules showed promising photovoltaic performance with a PCE of 1.17% for the bilayer OSC based on the molecules as donor and C60 as acceptor.[205] Recently Li et al. designed and synthesized two new starshaped D-π-A molecules with TPA as core and donor unit, DCN as end group and acceptor unit, and 4,4′-dihexyl-2, 2′-bithiophene or 4,4′-dihexyl-2,2′-bithiophene vinylene as π-bridge, i.e. S(TPA-bT-DCN) and S(TPA-bTV-DCN) (Figure 50) having optical band gaps of 1.83 eV and 1.65 eV, respectively. The photovoltaic performance of the OSC device based on a blend of S(TPA-bTV-DCN) and PC70BM (1:2, w/w) exhibited a PCE of 3.0%.[217]

Fig. 49. Chemical structures of TPA-TNP and BTD-TNP small molecules.


Chem. Rec. 2014, ••, ••–••



Fig. 50. Chemical structures of small molecules S(TPA-bT-DCN) and S(TPA-bTV-DCN).

Unsubstituted oligothiophenes possess electron-donating and hole-transporting properties. Increasing oligothiophene length provides stronger light absorption and promotes better π-π stacking/aggregation of the oligothiophenes and hence charge carrier transport. The combination of electron-donating oligothiophenes units with strongly electron-withdrawing groups results in the emergence of a new band assigned to an ICT transition that gives rise to the extension of the absorption spectrum of the resulting molecules. In the past few years, a series of conjugated D–A small molecules consisting of electron-withdrawing groups coupled to different oligothiophene segments were conceptually designed and synthesized to provide new electron donor materials for application in OSCs. Wong et al. have reported a series of oligothiophene based D–A small molecules in which oligothiophenes are asymmetrically end-capped with diphenylaminofluorenyl and dicyanovinyl groups.[218] This class of oligothiophene derivatives offers a sufficient introduction of dicyanovinyl or tricyanovinyl acceptors that the optical band gap of oligothiophene thin films are significantly reduced to 1.93–1.46 eV, and the light absorption and intermolecular π–π stacking in the solid state are greatly enhanced upon extension of oligothiophene length. For example, the highest PCE of BHJ OSCs based on a blend of PhN-OFOT(4)-DCN (Figure 51) and PCBM is 2.7%. Another effective new class of diketopyrrolopyrrole based oligothiophene D–A small molecules was reported by Nguyen et al. The optical and electronic properties and molecular packing of the materials can be controlled by changing the substituents and/or the number of thiophene units. The

Chem. Rec. 2014, ••, ••–••

electrochemical band gaps of diketopyrrolopyrrole-based oligothiophenes derivatives (SMDPPEH; Figure 51) ranged from 1.80 eV to 2.25 eV and the highest PCE reached 3.0%.[219] Roncali et al. synthesized unsymmetrical triphenylamine-oligothiophene hybrid conjugated systems and used these materials as electron donor materials for solutionprocessed BHJ organic solar cells and achieved a PCE about 2.02 %.[220] One interesting result was recently obtained by Liu et al. by terminating an oligothiophene with dicyanovinyl moieties. The terminal electron-withdrawing groups narrowed the band gap of the heptathiophene, extending the absorption to 750 nm. This broad absorption, coupled with a relatively high hole mobility of 1.5 × 10−4 cm2/Vs, allowed this system to produce an unusually large current. The optimized device produced a Jsc of 12.4 mA/cm2, Voc of 0.88 V, FF of 0.4, and PCE of 3.7%.[221] Nguyen et al. have reported a new class of D–A small molecules based on self-assembled diketopyrrolopyrrole (DPP) derivatives. The chemical structures of some of the small molecules based on DPP are shown in Figure 52. These molecules contain the DPP chromophore as core unit, thiophene units on the 2nd and 5th positions and various alkyl chains on the N,N positions of the lactam rings to control the π-stacking and physical properties, including solubility, crystallization, energy levels and self-assembly processes. They found that highly ordered nanostructures are formed after thermal annealing, and the morphology is related to the molecular structure of the individual units. The DPP core includes a planar conjugated bicyclic structure, which leads to strong π-π interactions, with



THE CHEMICAL RECORD

O

Bu

N S

S C6H13

S

S

S

S

N

C6H13

CN H

PhN-OFOT(4)-DCN

O

C8H17

S S

O

S

S

O

N S

S

S

C8H17

S N

O

C8H17

O

DPP(TBFu)2

QTF O

O R

R N

O

S

S

S

S

N

R N

S

S R

4

S

O

C8H17

S

NC

N

SMDPPEH

S

Bu

O R

N

O

R

O

NDT(TDPP)2 Fig. 51. Chemical structures of some small molecules: SMDPPEH, PhN-OFOT (4)-DCN and QTF, DPP(TBFu)2 and NDT(TDPP)2.

polar carbonyl groups capable of hydrogen bonding. The self-assembly of DPP moiety substituted with a variety of neutral hydrocarbon chains and the changing numbers of oligothiophenes demonstrate that the optical and electronic properties and molecular packing of the materials can be controlled by changing the substituents and/or the conjugation lengths. More appropriate for incorporation into devices is the excellent tendency to self-assemble into well-organized and persistent domains. Two oligothiophenes containing ethylhexyl substituents on the 2nd and 5th positions were incorporated into diketopyrrolopyrrole (SMDPPEH), which is a new oligothiophene-DPP system for application in solar cells. The morphology of SMDPPEH:PCBM blend film is very smooth and formed fiber-like structures, which indicated the high degree of ordering due to the self-organization of SMDPPEH. The blends also exhibit balanced electron and hole mobilities. The device shows a Jsc of 9.2 mA/cm2, Voc of 0.75 V and PCE of 3.0%. In order to further improve the PCE value of DPP derivatives, Nguyen et al. aimed to increase the Voc. The LUMO level of the acceptor should be at least 0.3 eV lower than that of the donor to drive charge separation after exciton


formation. An offset greater than 0.3 eV will result in energy loss during electron transfer. Based on these considerations, they envisioned using a fused benzofuran system to replace the terminal bithiophene units. The fused system maintains a highly conjugated structure while the electronegative oxygen atom stabilizes the HOMO of the molecule. They found that solution processed films of the benzofuran-substituted DPP(TBFu)2 (Figure 51) have good absorption properties and frontier energy levels that are appropriately aligned with those of PC71BM in BHJ OSCs. DPP(TBFu)2:PC71BM mixtures form good quality films and can self-assemble into BHJ morphologies with bicontinuous networks of donor and acceptor rich domains after annealing. Annealed DPP(TBFu)2:PC71BM devices yield PCEs of up to 4.4% with Voc of 0.9 V.[202j] Frechet et al. have designed and synthesized a series of crystalline platinum-acetylide oligomers containing athienylbenzothiadiazole-thienyl core and oligothiophenes alkynyl ligands.[222] The absorption spectra of these oligomers are significantly broadened by tuning the length of the oligothiophene alkynyl ligands, thus enhancing the overlap of

Chem. Rec. 2014, ••, ••–••



OC6H13 R O2N CN

NC NO2

R C6H13O T,A

T: R=

N

S

A: R=

S

NC

N

N

NO2 N

CN

NC NO2

N

S

S

O2N CN

M2 NC

NC

NO2

N=N B: X= H B6: X= C6H13

N X

S

S

M1

(H3C)2N

N

S

S

O2N

S

N=N B, B6

NO2 N C6H13 C

NO2 O2N NC

NC

NO2

CN

O2N

S

S CN

N

N

N

R N PH, FL

SM

NC

CN OC6H13 O2N

C6H13

C6H13

NO2

PH: R= FL: R=

C6H13O Fig. 52. Chemical structures of various small molecules used as donor for BHJ solution processable solar cells.

the absorption of these materials with the solar spectrum. In addition to the light-absorbing properties, incorporation of the oligothiophene “arms” enables the solid state intermolecular packing of these platinum-acetylide oligomers via edge-to-face interactions. The use of these molecules as electron donor was demonstrated in BHJ devices with either PC61BM or PC71BM as electron acceptors in a simple ITO/PEDOT:PSS/blend/Al device structure. Solution-processed photovoltaic cells show bicontinuous morphology, which is optimal for charge separation and high PCEs up to 3%. Mikroyannidis et al. have synthesized two novel soluble compounds T and A (Figure 52) that contain a central

Chem. Rec. 2014, ••, ••–••

dihexyloxy-p-phenylenevinylene unit, intermediate moieties of thiophene or anthracene, respectively, and terminal cyanovinylene nitrophenyls.[223] They possess the donor– acceptor (D-A) architecture, specifically, their central dialkoxyphenylene and intermediate thiophene or anthracene units behaved as electron-donating segments, while the terminal cyanovinylene nitrophenyls acted as electron acceptors. Their absorption was broad and extended up to about 750 nm with the longer-wavelength maximum around 640 nm and an optical band gap of 1.70 eV. When compounds A and T were blended with PCBM, the PCE dramatically increased up to 1.66% and 1.36% for devices with A:PCBM and T:PCBM,



THE CHEMICAL RECORD

respectively. The efficiencies of the devices were further enhanced upon thermal annealing up to 2.49% and 2.33% for devices based on A:PCBM and T:PCBM, respectively. We have recently synthesized two novel conjugated low band gap small molecules (SMs), M1 and M2, containing benzobisthiadiazole and thienothiadiazole central units, (chemical structures shown in Figure 52), respectively.[224] Both SMs carried terminal cyanovinylene 4-nitrophenyl at both sides, which were connected to the central unit with a thiophene ring. The longwavelength absorption band was located at 591–643 nm and the optical band gap was 1.62–1.63 eV, which is lower than that of P3HT. These two SMs were investigated as electron donor materials along with PCBM or F as electron acceptors for fabrication of BHJ organic photovoltaic devices. The power conversion efficiency (PCE) for M1:PCBM, M1:F, M2:PCBM and M2:F was 1.05%, 2.02%, 1.23% and 2.72%, respectively. We have also fabricated devices with M2:F cast film from mixed solvents. The PCE for the BHJ devices with the as-cast and thermally annealed M2:F (mixed solvents) is 3.34% and 3.65%, respectively. In continuation of our work we have designed three new soluble small molecules (B, B6, and C; chemical structures shown in Figure 52) with a low band gap based on 2-styryl-5phenylazo-pyrrole. Molecules B and B6 contained pyrrole and N-hexylpyrrole, respectively, as the central unit, which was connected to N,N-dimethylphenyl-4-azo on one side of the pyrrole molecule.[225] Molecule A contained N-hexylpyrrole as the central unit, which was connected to anthracenyl-9-azo on one side of the pyrrole molecule. The other side of the pyrrole molecule was connected to cyanovinylene 4-nitrophenyl for all molecules. The long-wavelength absorption maximum of the molecules was located at 601–637 nm, and their optical band gap was 1.62–1.67 eV. The photovoltaic properties have been investigated using blends of B, B6, or A with PCBM, and it was found that the device based on C:PCBM had a higher power conversion efficiency (2.06%) than the devices based on B:PCBM (1.33%) and B6:PCBM (1.36%). This has been attributed to the higher hole mobility, the lower band gap of A relative to that of B or B6, and the higher energy difference between the LUMO of C and PCBM. The effect of solvent annealing and thermalsolvent annealing on the photovoltaic response of the device based on the C:PCBM blend has been investigated, and it was found that the devices based on solvent-treated and subsequent thermally annealed blends have PCEs of 2.56 and 2.83%, respectively. The increase in the PCE has been attributed to the enhanced crystallinity of the blend and the improvement in the charge transport due to a reduction in the difference between the electron and hole mobility in the blend. A novel small molecule (SM) with a low-band-gap based on acenaphthoquinoxaline was synthesized and characterized. It was soluble in polar solvents such as N,N-


dimethylformamide and dimethylacetamide.[226] SM (chemical structure shown in Figure 52) showed broad absorption curves in both solution and thin films with a long-wavelength maximum at 642 nm. The thin film absorption onset was located at 783 nm, which corresponds to an optical band gap of 1.59 eV. SM was blended with PCBM to study the donor– acceptor interactions in the blended film morphology and the photovoltaic response of the bulk heterojunction devices. The cyclic voltammetry measurements of the materials revealed that the HOMO and LUMO levels of SM are well aligned with those of PCBM, allowing efficient photoinduced charge transfer and suitable open circuit voltage, leading to overall power conversion efficiencies of approximately 2.21 and 3.23% for devices with the as-cast and thermally annealed blended layer, respectively. Starting from triphenylamine, two low band gap small molecules, PH and FL, based on phenylenevinylene and fluorenevinylene, respectively, were synthesized (chemical structures shown in Figure 52).[227] Their long-wavelength absorption maximum was at 605–643 nm with optical band gap of 1.64–1.66 eV and deeper HOMO level. The photovoltaic properties have been investigated using the bulk heterojunction active layer of PH or FL with PCBM. The device based on FL:PCBM displayed higher power conversion efficiency (1.42%) than the device based on PH:PCBM (1.02%). We have used a modified PCBM, i.e. F as electron acceptor along with FL as electron donor, to increase the light harvesting in the wavelength region below 500 nm, and the PCE is about 4.38% when the BHJ (FL:F blend) device was spin casted from mixed 1-chloronaphthalene/o-dichlorobenzene solvents. The improved PCE has been attributed to the increased light absorption and higher hole mobility in the active layer, which resulted in more balanced charge transport. Marks et al. reported the synthesis of a naphtho[2,3-b′; 6,7-b′]dithiophene (NDT) based donor molecule. When NDT(TDPP)2 (TDP-thiophene capped diketopyrrolopyrrole; Figure 53) is combined with PC60CM, a PCE of 4.06% is achieved.[228] Recently, Chen et al. have designed and synthesized a series of thiophene-based small molecules end-capped with electron-withdrawing alkyl cyanoacetate groups (DCAE7T, DCAO7T and DCAEH7T, and investigated the correlation between these different end groups and their BHJ device performance.[229] The optical band gap of DCAE7T, DCAO7T and DCAEH7T in thin films was estimated from the onset of the absorption spectra and found to be 1.73, 1.74 and 1.75 eV, respectively. Using the simple solution spincoating fabrication process, DCAO7T/PCBM based OSCs exhibit a PCE as high as 5.08% without any special treatment. Recently Heeger et al, achieved a record PCE of 6.7% for a new small molecule DTS (PTTh)n as donor and PC70BM acceptor. A variety of other materials have appeared in the literature over the past few years incorporating a diverse assortment

Chem. Rec. 2014, ••, ••–••



C6H13 S

O

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O

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O SQ4 R=H; SQ5 R=CH3; SQ6 R=C2H5

Fig. 53. Chemical structures of BODIPY, MD1, MD2, MD3, SQ1a, SQ1b, SQ2, SQ3 SQ4, SQ5 and SQ6.

of structures, including several well-known classes of dye molecules such as borondipyrromethene (BODIPY),[230] merocyanine,[231] squaraine,[232] isoindigo,[233] and [234] phthalocyanine. In 2009 Roncali and co-workers have fabricated a photovoltaic device by combining two of these BODIPY dyes with complementary absorption characteristics and blending with PCBM and achieved a PCE of 1.7%.[230b] Considering that the phenyl group on BODIPY is not coplanar, bithiophene was incorporated on the first BODIPY derivative (Figure 53) to give structure BODIPY, resulting in increased hole mobility and broadened absorption while retaining the electronic and optical properties of BODIPY.[230c] The crystallinity was slightly improved and the hole mobility increased from

Chem. Rec. 2014, ••, ••–••

5 × 10−5 cm2/Vs to 9.7 × 10−5 cm2/Vs, leading to a PCE of 2.2%. Merocyanine (MC) dyes represent a traditional class of chromophores with a general structural feature consisting of an electron-donating and an electron-accepting moiety which are connected by a polymethine chain (Figure 53). The absorption wavelengths as well as band gaps of these chromophores are easily tunable by variation of the chain length and the donor and acceptor groups. Meerholz has prepared a series of merocyanine dye based D–A small molecules.[231a] These D–A small molecules offer sufficiently large variability in the position of the HOMO/LUMO levels, and high absorption coefficients. The MC family is a successful demonstration of the D–A structure small molecules and the PCE of a MC-based



THE CHEMICAL RECORD

(MD1) solar cell was 1.74%. After identifying a promising structure through the analysis of seven different MC homologues, they modified the structure by covalently bonding the alkyl chain on pyrrole moiety to the adjacent benzene ring (MD2), in order to control molecular packing in the solid state by rigidification. Interestingly, while the original compound showed a drop in hole mobility upon mixing with PCBM by two orders of magnitude, the hole mobility of MD2 decreased only slightly upon blending with PCBM. Increased crystallinity, as observed by thin film X-ray diffraction (XRD), enhanced charge transport and photocurrent improved the PCE from 1.54% to 2.59%.[231b] Recenlty, the same group has reported the synthesis and characterization of the optical, electrochemical, and photovoltaic properties of a series of new merocyanine (MD3) dyes containing the strong electron acceptor moiety 2-oxo-5-dicyanomethylene-pyrrolidine, which imparts absorption of the MC dyes in the NIR. Application of the present merocyanine chromophores in solution processed solar cells based on blends with PC71BM yielded devices with PCEs of up to 1%.[235] Squaraine dyes have been the subject of many recent investigations for small molecule BHJs because of their unique photochemical/photophysical properties and also that they strongly absorb the light over a broad range from 500 nm to 900 nm. Pagani and co-workers synthesized two new squaraine dyes, substituted at the pyrrolic nitrogen n-hexyl (SQ1a) or n-hexenyl (SQ1b) chains (Figure 53) and reported Jsc of 5.70 mA/cm2, Voc of 0.62 V, FF of 0.35, and PCE of 1.24% from device based on SQ1a squarine dye, processed in the air.[232a] Later on, the authors reported PCEs of up to 2.05% using the same conjugated structure with hexenyl solublizing groups (SQ1b).[232b] Wurthner and co-workers reported a high Jsc of up to 12.6 mA/cm2 from their squaraine-based small molecule (SQ2) with PCBM.[232c] Crystallinity was increased by replacing the ketone on the squarine moiety with dicyanovinyl group (SQ2), resulting in a hole mobility of up to 1.3 × 10−3 cm2/Vs and a high FF of 0.47. Wei et al.[232d] also recently reported a squaraine (SQ3; Figure 53) SMBHJ using PCBM as an acceptor. These devices achieved a Jsc of 8.85 mA/cm2, Voc of 0.89 V, FF of 0.35, and PCE of 2.7%. The PCE was improved to 2.9% by adding an evaporated BCP layer at the cathode. In this study, bilayer control devices using the same donor material with evaporated C60 layers were found to have a significantly higher PCE of 4.1%, largely due to a higher FF of 0.54. Forrest et.al have reported a 5.7% PCE based on OSC fabricated with 2,4-bis[4-N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) as donor and C60 as electron acceptor.[232e] New low-band-gap small molecules based on a squaraine (SQ) chromophore, bis[4-(2,6-di-tert-butyl)vinyl-pyrylium] squaraine (SQ4), bis[2,6-di-tert-butyl-4-(prop-1-en-2yl)pyrylium]squaraine (SQ5) and bis[4-(but-1-en-2-yl)-2,6-di-


tert-butylpyrylium]squaraine (SQ6), as shown in Figure 53, were synthesized and used as electron donors along with PC70BM for their application in solution processed organic bulk heterojunction (OBHJ) solar cells by Bhanuprakash et al.[236] These SQ dyes show the characteristic strong narrow long wavelength absorption band typical of SQ compounds having an absorption peak around 720, 765 and 758 nm (1.72, 1.62 and 1.63 eV) for SQ4, SQ5 and SQ6, respectively. The respective redshifts of 45 and 38 nm in MeTBU-SQ and EtTBU-SQ compared to the parent TBU-SQ is attributed to the presence of charge-donating methyl and ethyl groups at the vinylic position exerting a positive inductive effect. The optical energy band gaps estimated from the onset absorption wavelengths are 1.64, 1.52 and 1.54 eV for SQ4, SQ5 and EtTBUSQ6, respectively, which are lower than the conjugated polymers. The PCE of the BHJ devices based on SQ4:PC70BM, SQ5:PC70BM and SQ6:PC70BM are 1.71%, 2.15% and 1.89%, respectively, for the blends cast from CF. The higher value of PCE for the SQ5:PC70BM blend is attributed to the enhanced values of Jsc and Voc. The improved crystallinity of SQ5:PC70BM blend film cast from DIO–THF led to higher PCE (2.73%). The PCE has been further enhanced up to 3.14% for the device based on thermally annealed blend SQ4:PC70BM cast from DIO–THF solvent. The improved PCE of the BHJPV devices based on SQ5:PC70BM blend cast from DIO–CF solvent and thermally annealed has been attributed to the improved charge transport in the devices due to the higher hole mobility. Recent development of DPP containing donor molecules is mainly focused on extending molecular conjugated backbone to bis-DPPs,[237–240] following the rationale that an increase in conjugation into bis-DPP is expected to lower the band gap, increase the absorption, and enhance the charge carrier mobility. Marks et al. reported a donor molecule having naphthodithiophene central group flanked with two DPP units.[241] Using PC61BM as acceptor, the optimized solar cells exhibited PCE up to 4.06%. Nguyen et al. synthesized three novel molecular architectures with the DPP choromophores (tri-DPP) in conjugated backbone.[242] A key feature of this molecular architecture is the use of a DPP-phenyl group as the central moiety commonly used in bis-DPPs. The two tri-DPP molecules differ only in the presence of either 2-ethylhexyl (DPP1) or n-hexyl (DPP2) alkyl substituents. The incorporation of an extra electron deficient DPP unit imparts this series of tri-DPPs with low lying HOMO energy levels. Moreover, the resultant phenylthiophene linkages induce a slight twist to the conjgated backbone’s conformation, which further deepens the HOMO energy levels and improves the molecule’s solubility in organic solvents.[243,244] This transition from bis-DPP to tri-DPP demonstrates a useful approach to synthesize high-performance donor molecules by extending the conjugation length and the push-pull characteristics of the

Chem. Rec. 2014, ••, ••–••



Fig. 54. Chemical structures of different small molecules based on DPP.

molecule. The chemical structures of some the DPP based small molecules DPP1-DPP6 are shown in Figure 54. The HOMO energy levels for DPP1 and DPP2 were about –5.4 eV and –5.2 eV, respectively. The deeper HOMO energy level for DPP5 may be probably due to its more steric

Chem. Rec. 2014, ••, ••–••

2–ethylhexyl substitutions on the central DPP unit, resulting in a more twisted conformation in the solid state, as well as lower tendency of crystallization in DPP5 films. When blended with PC71BM, DPP1 and DPP2 lead to solar cell devices with optimized PCEs up to 4.8% (Jsc = 8.98 mA/cm2,



THE CHEMICAL RECORD

Voc = 0.89 V, FF = 0.61) and 5.5% (Jsc = 10.40 mA/cm2, Voc = 0.86 V and FF = 0.62) respectively. Russell et al. have designed a new small molecule DPP3 having D-A-D-A′ type structure, where A/A′ and D correspond to electron deficient acceptor and electron rich donor, respectively.[245] The D-A-D core consists of a central DPP (acceptor) unit flanked by two 3,3″-dioctyl-2,2:5′2″terthiophene (donor) units. Octylcyanoacetate units end-cap the molecule to increase intramolecular charge transfer, and thus narrow the optical band gap (1.41 eV), lower the HOMO energy level (−5.17 eV) and also improve the stability. They have investaged the role of solvent additive on the photovoltaic properties of the devices based on DPP3:PC71BM blend and found the highest PCE of 4.73% (Jsc = 13.6 mA/cm2, Voc = 0.72 V and FF = 0.476) when 3% of DIO was used as solvent additive. Zhan et al. have designed and synthesized a linear pushpull small molecule DPP4 based on 5-alkylthiophene-2-yl substituted BDT as core and DPP as arms.[246] DPP4 shows excellent solution processability and thermal stability, broad absorption spectra with very high extinction coefficient and low band gap (1.65 eV), a deep lying HOMO energy level (−5.23 eV) and high hole mobility (1.6 × 10−3 cm2/Vs estimated from FET). The films of DPP4:PC61BM (1:1w/w) blend exhibit relatively smooth surface and nanoscale crystalline domains. The BHJ OPVs based on DPP4:PC61BM blend without post-thermal treatment afford a PCE of 4.09% (Jsc = 10.59 mA/cm2, Voc = 0.89 V and FF = 0.434). After thermal annealing at 110 °C for 10 min, BHJ devices based on DPP4:PC61BM have shown an improved PCE up to 5.79 % (Jsc = 11.97 mA/cm2, Voc = 0.84 V and FF = 0.576). The flat polyarelene moieties were not derivatized with alkyl chains, and led the authors to introduce relatively long carbon chains onto the DPP unit to achieve sufficient solubility. In this context triazatruxene (TAT), consisting of three fused, carbazole modules of C3 symmetry, is a particularly interesting chemical unit. It can be adequately functionalized and easily linked to photoactive fragments.[247] In addition, electron donating TAT units provide both a good π-π stacking ability due to the perfectly flat conjugated core and high solubility due to alkylation of the indole moieties. Ziessel et al. have synthesized triazatruxene-diketopyrrolopyrrole dumbbellshaped molecules, DPP5 and DPP6, using TAT as flat end capper units and DPP as the central unit.[248] The DPP5 and DPP6 showed indential optical band gaps and HOMO energy levels of 1.69 eV and −5.31 eV, respectively. Both the small molecules were used as donor along with PC71BM as acceptor for BHJ solar cells, and achieved a PCE of 5.3% (Jsc = 14.6 mA/cm2, Voc = 0.63 V and FF = 0.58) and 4.4% (Jsc = 13.1 mA/cm2, Voc = 0.65 V and FF = 0.52) for thermally annealed DPP5:PC71BM and DPP6:PC71BM, respectively. The slightly lower PCE of DPP6:PC71BM based device results


from the high series resistance and consequently lower FF, which could be caused by the lower out-of-plane charge carrier mobility in the DPP6:PC71BM blend. Yao et al.[249] recently synthesized four solution processed C2- or C3-symmetric small molecules based on a phenyl core with conjugated thiophene (T), DPP, and BDT attached at the ortho, para, and meta positions on the core, denoted as DPP7, DPP8, DPP9, DPP10 and DPP11 (Figure 55). C3-symmetric DPP7 and DPP9 showed stronger light harvesting ability than their C2-symmetric counterparts DPP10 and DPP11. Moreover, higher conjugated DPP9 and DPP11, which have the two-dimensional benzothiophene (2D-BDT) as bridge, exhibit stronger light harvesting ability and higher hole mobilites than less conjugated 3D-T-P and 2D-T-P. Among all these small molecules, DPP9 as donor and PC71BM as acceptor showed the highest PCE of 3.60 % with Jsc of 7.69 mA/cm2, Voc of 0.77 V and FF of 0.59. Yao et al. synthesized A-D-A small molecule DPP12 based on two-dimensional benzodithiophene (BDT) and DPP (Figure 55), which shows broad absorption spectra and exhibits HOMO and LUMO levels of −5.15 eV and −3.44 eV, respectively.[250] They have showen that the the device based on DPP12:PC71BM active layer processed from o-DCB parent solvent with 0.7% DIO solvent additive exhibited the PCE of 5.29% with Jsc of 11.86 mA/cm2, Voc of 0.72 V and FF of 0.62. Chen et al. synthesized a DPP derivative end-capped with an ethyl thiophene-2-carboxylate moiety, named as DPP13 (Figure 55), which posseses an optical band gap of 1.65 eV and HOMO energy level of −5.33 eV.[251] The deep HOMO level is due to the electronegativity of the ester group and conjugation effect of the thiophene ring. When DPP13 is used as the electron donor to blend with PC71BM for solution processable OSCs, a power conversion efficiency of 4.02% combined with an open-circuit voltage (Voc) as high as 0.94 V, Jsc of 8.55 mA/ cm2 and FF of 0.50 and a broad photovoltaic response range extending to around 750 nm were obtained. Adachi et al. have designed and synthesized simple DPP based small molecules DPP14 and DPP15 (Figure 55), since the incorporation of multiple flexible chains into the rigid π-conjugated DPP core can provide highly light absorbing donor materials forming liquid crystalline ordering.[252] Both DPP14 and DPP15 exhibited a low optical band gap of 1.80 eV and deep HOMO energy level around −5.20 eV. They have investigated the effect of LC properties on photovoltaic performance of the BHJ devices based on DPP18:PC71BM and DPP19:PC71BM films, in which the molecular packing and thinfilm morphologies are strongly affected by the length of the terminal alkyl chains. The Jsc and FF have increased significantly for the DPP18-based devices through the LC organization process, and the best-performing device has generated a high PCE of 4.3% with a Voc of 0.93 V,

Chem. Rec. 2014, ••, ••–••



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O S

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DPP14 (R=n-C6H13) DPP13

DPP15 (R=n-C12H25)

Fig. 55. Chemical structures of more DPP small molecules.

Jsc of 8.4 mA/cm2, and FF of 0.55 without use of any additives. Jo et al. have synthesized four different DPP based small molecules denoted as DPP16, DPP17, DPP18 and DPP19 with A-D-A structure (Figure 56), where A is thiophene

Chem. Rec. 2014, ••, ••–••

capped DPP and the electron donating unit D was systematically varied with different electron donating strength (thiophene vs. phenylene vs. thienothiophene vs. naphthalene) and different molecular planarity (bithiophene vs. thienothiophene and biphenylene vs. naphthalene).[253] Since



THE CHEMICAL RECORD

O

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Aromatic donor

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N R

N

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Fused

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DPP20 R= 2-hexyldecyl (HD) DPP21 R=2-octyldodecyl (OD)

O

DPP22 R=2-octyldodecyl (OD)

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NO2

N S N

N

N

N

S O2N

O

O

O

O

O

S

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CN

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DPP23 NH DPP24

DPP25

Fig. 56. Chemical structures of some more DPP based small molecules.

the electron donating capabilities of biphenylene and naphthalene are weaker than bithiophene and thienothiophene, respectively, DPP28 and DPP29 exhibit deeped HOMO levels than DPP16 and DPP17. As a consequence devices based on DPP18 and DPP19 exhibt higher Voc than the device fabricated from DPP16 and DPP17. The introduction of fused rings (thienothiophene and naphthalene) in the small molecules lowers the band gap and enhances the hole mobility due to high crystallinity derived from the planar structure of the fused aromatic ring. As the result, DPP17 and DPP18 showed higher Jsc than DPP16 and DPP18. The BHJ device based on DPP19 as donor and PC71BM showed a PCE of 4.4% with Jsc of 9.5 mA/cm2, Voc of 0.87 V and FF of 0.53. To further increase the PCE of the DPP based device, Park et al. inserted acetylene linkages between pyrene and DPP and synthesized three DPP based SMs, i.e. DPP20 and DPP21


with acetylene linkage and DPP22 without acetylene linkage (Figure 56).[254] The acetylene incorporation reduces the HOMO-LUMO gap through the extended conjugation and also increases the ionization potential of the DPPs based on the relatively larger electron withdrawing characteristics of sp-hybridization over sp2-hybridzation. Among these DPPs, DPP24 showed the highest PCE of 3.15%. Sharma et al. have designed a solution processed DPP based SM containing DPP as the central core and cyanovinylene 4-nitrophenyl at the terminal ends, i.e. DPP23 (Figure 56), and used it as donor along with PCBM and modified PCBM as electron acceptor.[255] The optimized photovoltaic device showed a PCE of 3.34% with Jsc of 7.7 mA/cm2, Voc of 0.94 V and FF of 0.46. Sharma et al. further developed two simple D–A–D structured small molecules based on diketopyrrolopyrrole as central

Chem. Rec. 2014, ••, ••–••



Fig. 57. Chemical structure of N(Ph-2T-DCN-Me)3, N(Ph-2T-DCN-Et)3, N(Ph-2T-DCN-Hex)3 and N(Ph-2T-DCN-Dodec)3.

acceptor unit and different terminal units (indole and 2,4,6triisopropylphenyl for DPP24 and DPP25,respectively; Figure 56) with broad absorption bands and suitable energy levels.[256] DPP24 and DPP25 exhibited optical band gaps of about 1.76 eV and 1.89 eV, respectively. The organic BHJ solar cells based on DPP24:PC70BM and DPP25:PC70BM cast from chloroform solution exhibit PCEs of about 3.26 and 2.42%, respectively. The higher PCE for DPP24:PC70BM has been attributed to the broader band absorption, low optical band gap and higher hole mobility for DPP24 as compared to that for DPP25. The PCE of the device based on DPP24:PC70BM has been enhanced up to 4.96%, when the device was annealed at 110 °C for 10 min. This increase in the PCE has been attributed to mainly higher Jsc and FF. We have demonstrated that the morphological changes upon the thermal annealing can increase the local electrical polarizations of the BHJ active layer in the device, which enhances the

Chem. Rec. 2014, ••, ••–••

optical absorption coefficient, charge generation and their transportation, leading to the increase in both Jsc and FF. Recently Min et al. synthesized four TPA unit based D-π-A star molecules N(Ph-2T-DCN-Me)3, N(Ph-2T-DCNEt)3, N(Ph-2T-DCN-Hex)3, and N(Ph-2T-DCN-Dodec)3 with methyl, ethyl, hexyl and dodecyl as alkyl chains (Figure 57).[257] They have investigated the structure property relationships with various lengths of alkyl end groups (methyl, ethyl, hexyl and dodecyl). All the molecules exhibit good thermal stability, similar absorption in solution, and low and similar HOMO energy levels. However, N(Ph-2T-DCNMe)3, which has methyl as the end groups, showed broadened absorption in the film and higher hole and electron mobilities as well as longer electron lifetime in devices compared to the other three small molecules. The PCE of solution processed organic BHJ solar cells based on N(Ph-2T-DCNMe)3:PC71BM (1:2 wt%) gave up to 4.76% with Jsc of



THE CHEMICAL RECORD

Fig. 58. Chemical structures of Cz-TBT-CAC8.

Fig. 59. Chemical structures of TBDTCNR and TBDTZN small molecules.

8.67 mA/cm2, Voc of 0.98 V and FF of 0.56, without any post-treatment, indicating that introduction of methyl end groups instead of longer alkyl chains may reduce streric hindinace and torsional interactions of the molecule, thereby enhancing the intermolecular interactions, charge separation and transportation, charge carrier lifetime and consequently improving the photovoltaic performance. Wang et al. synthesized A′-A-D-A-A′ type low-band-gap small molecule Cz-TBT-CAC8 (Figure 58) with 2,7-carbazole as the center donor (D), benzothiadiazole as an electron acceptor (A) and alkyl cyanoacetate as another electron acceptor (A′).[258] This small molecule possesses a low lying HOMO level at 5.30 eV and an optical band gap of 1.73 eV. The solar cell based on Cz-TBT-CAC8:PC61BM blend film spin-coated from chlorobenzene solution exhibits a PCE of 2.87% with a high Voc of 1.05 V. After adding 0.2% DIO into the chlorobenzene solution as an additive, the PCE was further improved to 3.95% with a Voc of 1.03 V, a Jsc of 7.32 mA/cm2 and a FF of 0.52. Chu et al. have strategically designed and convergently synthesized two novel, symmetrical, and linear A−D−A-type π-conjugated donor molecules (TBDTCNR, TBDTCN), each containing a planar electron-rich 2-octylthiene-5-ylsubstituted benzodithiophene (TBDT) unit as the core, flanked by octylthiophene units and end-capped with electrondeficient cyanoacetate (CNR) or dicyanovinyl (CN) units (Figure 59).[259] These TBDT-based species possessed deep HOMO energy levels and provided devices exhibiting good values of Voc. TBDTCNR showed not only good packing in the solid state but also superior charge transport properties and


favorable nanoscale morphology relative to TBDTCN and thus produced higher PCE values. Among tested systems, a photovoltaic device containing a blend of TBDTCNR and PC61BM at a weight ratio of 1:0.4 provided the highest PCE (5.42%), with high values of Voc (0.90 V) and Jsc (9.08 mAcm−2) and a notable FF (66%). This performance is comparable with that reported recently for BDT-containing π-conjugated small molecules. Chen et al. synthesized three small molecules named DR3TBDTT, DR3TBDTT-HD, and DR3TBD2T (Figure 60) with a benzo[1,2-b:4,5-b′]dithiophene (BDT) unit as the central building block and terminal rhodanine units linked by alkyl-substituted terthiophene-based π-conjugated spacers to guarantee good solubility and also to form an effectively long conjugated acceptor−donor−acceptor (A−D−A) backbone structure with strong intramolecular charge transfer and broad absorption.[260] The film spectra exhibit broad absorption over the range from 300 to 800 nm and have two strong absorption peaks, one of which is a vibronic shoulder, indicating of effective π−π spacking between the molecule backbones. The optical band gaps were estimated to be 1.72, 1.76, and 1.76 eV for DR3TBDTT, DR3TBDTT-HD, and DR3TBDT2T, respectively. Before the addition of PDMS, DR3TBDTT and DR3TBDT2T with larger conjugation in the orthogonal direction demonstrated high PCEs of 7.51% (Jsc = 13.15 mA/cm2, Voc = 0.91 V, FF = 0.628) and 7.58% (Jsc = 11.87 mA/cm2, Voc = 0.90 and FF = 0.704), respectively. However, replacing the 2-ethylhexyl substituents in DR3TBDTT with the bulkier 2-hexyldecyl groups in DR3TBDTT-HD resulted in a lower PCE of 6.32%

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Fig. 60. Chemical structures of DR3TBDTT-HD, DR3TBDTT and DR3TBDT2T small molecules.

Fig. 61. Chemical structures of D1, DO1, D2 and DO2 small molecules.

(Jsc = 12.36 mA/cm2, Voc = 0.96 V and FF = 0.533). This lower PCE for DR3TBDTT-HD corresponds its lower FF relative to the other three molecules, which is believed to be due to the lower mobility and relatively poor morphology. The especially high Voc of 0.96 V for DR3TBDTT-HD with longalkyl-chain substituents on the thiophene units at the BDT 4and 8-positions might be caused by the weak intermolecular interactions due to the bulk effect of long alkyl chains. After addition of PDMS, the PCEs of the four molecules all increased. Notably, PCEs of 8.12% (Jsc = 13.17 mA/cm2, Voc = 0.93 V and FF = 0.663) and 8.02% (Jsc = 12.09 mA/ cm2, Voc = 0.92 V and FF = 0.721) were achieved for DR3TBDTT and DR3TBDT2T, respectively. Chen et al.[261] introduced the bithienyl-substituted benzodithiophene BDTT unit into solution processable small molecules and synthesized two solution processable A-D-A structured organic molecules with BDTT as central building block and donor unit, indenedione (ID) as acceptor unit

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and end groups, and thiophene (T) or bithiophene (BT) as π-bridges, D1 and D2. Two corresponding molecules with

alkoxy side chains on BDT, DO1, and DO2 were also synthesized for comparison (Figure 61). The four compounds possess broad absorption covering the wavelength range 450−740 nm and relatively lower HOMO energy levels from −5.16 to about −5.19 eV. D2 and DO2 with bithiophene π-bridges demonstrate stronger absorbance and higher hole mobilities than the compounds with thiophene π-bridges. The PCE values of the OSCs based on the organic compounds/PC70BM (1.5:1, w/w) are 6.75% (Jsc = 11.05 mA/cm2, Voc = 0.92 V and FF = 0.664) for D2, 5.67% (Jsc = 10.07 mA/cm2, Voc = 1.03 V and FF = 0.482) for D1, 5.11% (Jsc = 8.58 mA/cm2, Voc = 0.92 V and FF = 0.65) for DO2, and 4.15% (Jsc = 9.47 mA/cm2, Voc = 0.91 V and FF = 0.482) for DO1. The results indicate that the photovoltaic performance of the molecules with bithienyl conjugated side chains is better than that of the corresponding molecules with alkoxy side chains on the BDT



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Fig. 62. Chemical structures of small molecules with DTS central unit.

unit, and the molecules with bithiophene π-bridges show better photovoltaic performance than that of the corresponding molecules with thiophene π-bridges. Bazan et al. have reported planar small molecule DTS1 with dithlenosilone (DTS) as central core that shows an optical band gap of 1.73 eV and HOMO energy level about −4.95 eV.[262] The chemical structures of some of the small molecules based on DTS central unit are shown in Figure 62. The optimized organic solar cell with BHJ active layer of DTS1:PCBM gave a PCE of 5.84% with a Voc of 0.80 V, Jsc of 11.51 mA/cm2 and FF of 0.64. Bazan and co-workers reported a series of DTS-based small molecules DTS1 and DTS2 having optical band gap of 1.50 eV and 1.55 eV, respectively, and HOMO energy levels of −5.20 eV and −5.12 eV, respectively, as donors for solution processed SM-OPV devices.[263, 264] Devices fabricated from the blend solutions with DTS1:PC71BM (7:3 w/w) showed a PCE of 4.52%, with Jsc of 12.5 mA/cm2, Voc of 0.80 V,and FF of 0.452. When 0.25% (v/v) DIO (1,8-diiodooctane) was added during the filmforming process, a PCE of 6.7% was achieved.[263] The improvement of PCE was ascribed to the formation of more preferable domain size as a result of the addition of DIO. In their following work, 5-fluorobenzo[c][1,2,5] thiadiazole instead of [1,2,5]thiadiazolo[3,4-c]pyridine was used as the acceptor unit and molecule DTS2 was synthesized. A BHJ device with DTS2 and PC71BM as the active layer and annealed at 130°C gave a PCE of 5.8% with Voc of 0.82 V, Jsc of 10.8 mA/cm2, and FF of 0.65. When 0.4 v/v% DIO as asolvent additive was incorporated, an improved PCE of 7.0% (Voc = 0.809 V, Jsc = 12.8 mA/cm2, and FF = 0.68) was achieved. Recently, a PCE of over 8% has been achieved with DTS2 and PC71BM as the active layer through device optimization.[264]


Heeger et al. have used two similar structure versions of a molecular donor, in which two terminal hexyl-subsituted bithiophene units are connected to a central DTS unit through electron deficient thiadiazolopyridine units and which differ only in the position of pyridyl N atoms. They explored these molecules to study the interplay of crystallization and vertical phase segregation as a result of annealing.[265] The difference in position of the pyridal N atom which points away (distal configuration; compound DTS3) or towards (proximal configuration; compound DTS4) from the DTS core modifies the aggregation/molecular packing in the solid state, resulting in differences in the phase segregation and formation of crystalline domains. When DTS3 or DTS4 was blended with PC61BM and followed by thermal annealing at 100 °C, PCEs of 4.5% and 5.7% were attained respectively. In this study, we make two findings: (i) the Voc of DTS4:PC61BM is significantly higher (ca. 70–100 mV) than DTS3:PC61BM devices at various annealing conditions despite similarity in orbital energy levels. A decrease in Voc (ca. 13–15%) after heat treatment was also observed in both molecular BHJ systems, which is related to vertical phase separation. (ii) Higher crystallinity of donor molecule DTS4 has a greater impact on cell performance despite a competing effect of undesired donor/ aluminum cathode interface, resulting in its superior performance to the DTS3:PC61BM device. The higher PCE might be expected by tuning the surface composition to allow electrode selectivity and reduce recombination at the donor/ cathode interface and thus increase Voc. Bazan et al. have introduced the new molecular donor, 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene2,6-diyl)bis(6-fluoro4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo [c][1,2,5]thiadiazole), p-DTS(FBTTh2)2 (DTS5), to control chemical processes at interfaces.[266, 267] In this small molecule,

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the carbon fluorinated bond in the electronic backbone provides an electron withdrawing functionality without introducing basic heteroatoms, rendering it inert towards the acidity of PEDOT:PSS. It was used as a donor along with PC71BM as activel layer in BHJ solar cells, the as-cast active layer from choloform gives as relatively low PCE of 1.8%; thermal annealing leads to a simulateneous increase in both Jsc, Voc and FF, resulting in an overall increase in PCE up to 5.6%. Furthermore, when the active layer was cast from solvent additive (0.4% DIO), the PCE was improved up to 7%.[268,269] The drastic changes in the device performance associated with different processing conditions are likely due to the variations in the resultant morphology. They have investigated the changes in the morphologies of the active layers under different conditions and found that after the thermal annealing the crystal structure of the molecule remains flat, and π-stacks with a relatively large overlapping area between adjacent molecules. They proposed that the overlap of the conjugated backbones enables more efficient charge transport, explaining the increase in performance upon crystal growth. The saturated photocurrent in the annealed device is significantly lower than in the as-cast devices due to a reduction in charge generation, likely from a loss in interfacial area. Addition of DIO to the blend solution also results in crystallization of p-DTS(FBTTh2)2 concomitant with an increase in PCE to 7.0%. Under ideal solvent additive conditions, the films show an inter-connected network of crystalline p-DTS(FBTTh2)2 domains with a characteristic size of 30 nm. The result is a high saturated photocurrent at very low effective voltages, explaining both the high FF as well as the extremely high internal quantum efficiency at short circuit, reaching over 90%. This demonstrates both efficient charge generation and collection. The same research group has demonstrated that the FF of the organic solar cell based on the above BHJ is sensitive to the thickness of a calcium layer in between the active layer and Al cathode; for 20 nm Ca thickness, the FF is 0.73, resulting in an overall PCE of 8.01%.[270] Recently, same group has reported a PCE of 9.02% when a thin film of barium (Ba) of 10 nm thickness was inserted in between the active layer and Al cathode.[271] They attributed this improvement in the device performance to a decrease in the series resistance and an increase in the shunt resistance. Intensity dependence of current-voltage char-acteristics shows that suppression of the trap assisted recombination at short circuit leads to increased charge collection. An increase in the built-in potential (Vbi) after insertion of the Ba layer due to the lower work function of Ba (2.7 eV), compared to Al (4.3 eV), further assists faster sweep-out. Ba shows excellent hole-blocking even at high baises and outperforms all the other reported cathode interlayers in improving the device fill factor. They have also reported that when a thin film of solution processed ZnO layer was inserted in between the active layer and Al cathode, a PCE

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of about 8.90% is achieved with Jsc of 15.5 mA/cm2, Voc of 0.80 V and FF of 0.724.[272] The optical spacer improves the charge collection, improves the hole-blocking at the cathode, and simultaneously reduces the charge recombination.

7 Conclusions In summary, two important classes of solar cells, i.e. DSSCs and BHJs, based on different types of metal free dyes and conjugated donor–acceptor (D-A) copolymers and small molecules have been presented. The power conversion efficiency of DSSCs based on metal free dyes has reached about 11% (porphyrin dyes). The PCE for the BHJ has been reported at about 12%[273] and 6.7%[274], 8.90 %[272] for conjugated D-A copolymers and small molecules, respectively. During the last decade, much effort has been made on the development of various types of organic dye sensitizers for DSSCs, and there has been a gradual accumulation of information about the relationship between the chemical structures and photovoltaic performances of DSSCs. DSSCs based on organic dyes have reached η values as high as 9%, comparable to those of Ru complexes (η = 10–11%). The η values of DSSCs based on the organic dyes reached up to 9.5% (indolines), 11.1% (porphyrins). Out of these organic dye sensitizers, D-π-A dyes possessing broad and intense absorption spectral features in the visible region have exhibited especially excellent performances and are regarded as one of the most promising classes of organic sensitizers. Recently, NIR dye sensitizers providing good absorption in the red/NIR region of the solar spectrum have attracted increasing interest. After years of intensive interdisciplinary research, some basic rules have become clear for designing an efficient donor material for BHJ solar cell applications. First, the material should possess a relatively low optical band gap (1.2–1.9 eV) and have strong optical absorption in the solar spectral range extending up to NIR (1000 nm). Secondly, the material should have a low-lying HOMO energy level to offer high open circuit voltage (Voc). Thirdly, the energy offset between the LUMO energy levels of the donor material and acceptor (not necessarily a fullerene derivative) should be well controlled to be just large enough to provide a driving force for efficient charge separation and not to cause too much energy loss. Fourthly, the material should have a good hole mobility for efficient charge transport. Finally, the material should own good solubility in organic solvents for solution processing. If one follows these designing rules and obtains an optimized and stable morphology, a PCE of 10% should be reached. The organic compounds for efficient organic solar cells should satisfy several requirements: (i) the materials should exhibit intense and broad absorption in the visible and near infrared regions for efficient light harvesting; (ii) the energy levels of the donor and acceptor



THE CHEMICAL RECORD

materials should be matched to achieve the efficient charge separation and a large Voc; (iii) the charge carriers should exhibit high mobility to facilitate efficient charge transport and extraction. The appropriate HOMO and LUMO levels, strong absorption coefficients and broad absorption spectra as well as high mobility of organic semiconductors lend credence to optimistic predictions that PCEs of up to 8–10% can be achieved in a single-active layer device and up to 15% in a tandem device in the near future.

Acknowledgements SPS thanks XII FY CSIR-INTELCOAT (CSC0114) for financial support.

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