DOI: 10.1002/asia.201403264
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Photochemistry
Organic Donor Materials Based on Bis(arylene ethynylene)s for Bulk Heterojunction Organic Solar Cells with High Voc Values Hongmei Zhan,[a, b] Qian Liu,[b] Fengrong Dai,[b] Cheuk-Lam Ho,*[b] Yingying Fu,[a] Leijiao Li,[c] Li Zhao,[c] Hua Li,[b, d] Zhiyuan Xie,*[a] and Wai-Yeung Wong*[a, b, e]
Abstract: A series of purely organic small molecules 1–4 based on bis(arylene ethynylene)s with various electron-rich aromatic bridges, such as benzene, cyclopentadithiophene (CDT), and diphenyl(p-tolyl)amine, were synthesized by a Sonogashira coupling reaction. Their optical, electronic, and electrochemical properties were fully characterized. The photovoltaic properties of these molecules as donor materials and [6,6]-phenyl-C71-butyric acid methyl ester as an acceptor
material in solution-processed bulk heterojunction devices were studied. Among them, CDT-bridged molecule 2 exhibited the best photovoltaic performance and achieved a power conversion efficiency of 3.28 %. In addition, the photovoltaic efficiencies of these molecules are higher than their corresponding platinum-containing counterparts, probably owing to their stronger light absorption and improved open-circuit voltage.
Introduction
solar cell materials and optimization of the device structure and fabrication technology.[2] A power conversion efficiency (PCE) as high as 8.12 % has been achieved with small molecules based on benzo[1,2-b:4,5-b’]dithiophene and 3-ethylrhodanine units with dioctylterthiophene linkages in solution-processed bulk heterojunction (BHJ) solar cells;[3] this value is similar those of polymer-based devices.[4] To realize low-cost SMBHJ solar cells for commercial applications, their photovoltaic performance should be further improved. To date, in addition to device engineering and morphology control, the photoactive materials, especially electron-donor materials, still play a pivotal role in achieving high efficiency in SMBHJ devices. Therefore, careful molecular design should be carried out to address the collective requirement of SMBHJ solar cells, including broad solar absorption, well-matched energy levels with fullerene derivative acceptors, high charge carrier mobilities, and excellent film formation ability. In our previous work, we reported a series of symmetric platinum(II)–bis(arylene ethynylene) complexes consisting of Pt(PBu3)2 acetylide as the core and D-A-D (D, donor unit; A, acceptor unit) architecture at both sides,[5] in which efficient pelectron delocalization could be established owing to the special electronic properties of the heavy metal platinum.[6] In addition, the triarylamine donor unit with a three-dimensional propeller structure, 3-hexylthiophene, and a bulky Pt(PBu3)2 group could efficiently improve the molecular solubility to help circumvent the poor film formation problem in SMBHJ solar cells.[3, 7] Meanwhile, an efficient intramolecular charge transfer (ICT) interaction between these donor units and electron-accepting benzothiadiazole units enhances the solar absorption abilities. These molecules exhibit wide absorption and higher molar absorption coefficients. Moderate PCEs of 1.45– 2.37 % have been achieved with SMBHJ devices containing
In recent years, solution-processed small-molecule bulk heterojunction (SMBHJ) solar cells have attracted great attention owing to their advantages over their polymer counterparts, including well-defined molecular structures without batch-tobatch variation, relatively simple synthesis, high purity, and reproducible photovoltaic performance.[1] Appreciable solar cell progress has been made with the development of photoactive [a] Dr. H. Zhan, Y. Fu, Prof. Z. Xie, Prof. W.-Y. Wong State Key Laboratory of Polymer Physics and Chemistry Changchun Institute of Applied Chemistry Chinese Academy of Sciences, No. 5625, Renmin Street Changchun 130022 (P. R. China) E-mail: [email protected] [b] Dr. H. Zhan, Dr. Q. Liu, Dr. F. Dai, Dr. C.-L. Ho, Dr. H. Li, Prof. W.-Y. Wong Department of Chemistry, Partner State Key Laboratory of Environmental and Biological Analysis and Institute of Advanced Materials Hong Kong Baptist University Waterloo Road, Kowloon Tong Hong Kong (S.A.R. China) E-mail: [email protected] [c] Dr. L. Li, Dr. L. Zhao State Key Laboratory of Theoretical and Computational Chemistry Institute of Theoretical Chemistry Jilin University, No.2, Liutiao Road Changchun (P.R. China) [d] Dr. H. Li College of Life and Environmental Sciences & Beijing Engineering Research Center of Food Environment and Public Health Minzu University of China, Beijing 100081 (P.R. China) [e] Prof. W.-Y. Wong HKBU Institute of Research and Continuing Education Shenzhen Virtual University Park Shenzhen 518057 (P.R. China) Chem. Asian J. 2015, 00, 0 – 0
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Full Paper Results and Discussion Syntheses and Chemical Characterization The general synthetic routes to compounds 1–4 are depicted in Scheme 1. The terminal acetylenes L1 and L2 were synthesized according to the procedures previously reported in the literature.[4] Owing to the higher reactivity of iodine derivatives as compared to the bromine counterparts in the Sonogashira reaction,[8] we chose the diiodo-substituted aromatic derivatives as the starting materials for the synthesis of the target compounds 1–4. 1,4-Diiodobenzene is commercially available. CDT was alkylated with n-dodecyl bromide by using powdered potassium hydroxide as the base, followed by iodination with NIS in the presence of glacial acetic acid, to give compound CDTC12-I in high yield. Compound DPTA-I was similarly prepared from diphenyl(p-tolyl)amine (DPTA). By using a Sonogashira coupling reaction of terminal acetylene L1 or L2 with the corresponding diiodo-substituted aromatic derivative in a ratio of 2.06:1 and in the presence of a palladium(II)/copper(I) catalyst provides compounds 1–4 in moderate yields. The relatively lower yields of 1 and 4 are ascribed to the formation of 1,3-butadiyne derivatives as a byproduct from the undesired homocoupling reaction of terminal alkynes.[8, 9] Compounds 1–4 exhibit good air stability and solubility in common organic solvents. The proposed formulations of these compounds are all in good agreement with their analytical and spectroscopic (NMR, MS, and FTIR) data.
Figure 1. Structures of metal-free 1–4 and metalated congeners PT1 and PT2.
these platinum-containing small molecules as the electron acceptor blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). In this context, by replacing the central building block Pt(PBu3)2 with various electron-rich organic moieties, such as benzene, cyclopentadithiophene (CDT), and monomethyl-substituted triphenylamine (TPA), we designed and synthesized a series of purely organic small molecules 1–4 (Figure 1). Their photovoltaic performance and other properties were also investigated and compared to explore the influence of chromophore variation and understand better the relationship between molecular structure and photovoltaic performance.
Optical Properties The normalized UV/Vis absorption spectra and photoluminescence (PL) spectra of compounds 1–4 in dichloromethane at
Scheme 1. Synthetic pathways of compounds 1–4. DMSO = dimethyl sulfoxide, THF = tetrahydrofuran, NIS = N-iodosuccinimide.
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Full Paper ically shifted ICT absorption band centered at l = 537 nm with the highest absorption coefficient of 8.49 104 m 1 cm 1, which could be ascribed to the addition of an additional thiophene ring, resulting in the formation of a longer p-conjugation length in 2. The optical band gaps deduced from the absorption onset are in the order of 1 (2.14 eV) 3 (2.13 eV) > 2 (2.06 eV) > 4 (1.99 eV). In addition, in contrast to the ICT absorption band of platinum-containing compounds PT1 (l = 526 nm, e: 5.52 104 m 1 cm 1) and PT2 (l = 561 nm, e: 7.51 104 m 1 cm 1), those of the aromatic ring bridged compounds 1–4 display a slight hypsochromic shift; however, the last series possess a higher absorption ability, as deduced from their higher absorption coefficient. The PL spectra of 1–4 were also investigated and the data are shown in Figure 2. Compounds 1–3 show very similar emission profiles in CH2Cl2 with emission bands at l 669 nm. Compound 4 exhibits a clearly redshifted emission band relative to those of 1–3 with a band wavelength at l = 707 nm. Similar shoulder bands appear at l 788 nm in the PL emission spectra of 1–4, which are due to instrumental artefacts. Furthermore, the measured PL lifetimes for these compounds are very short (ca. 1.76–5.67 ns), which is characteristic of the fluorescent emission, and as evidenced by the reduced quantum yield values, the fluorescence intensity of 1–4 decreases gradually with extension of the p-conjugation length. In addition, it is worth mentioning that organic compounds 1–4 display larger Stoke shifts of 49–70 nm relative to those of platinum-containing counterparts PT1 and PT2 (39 and 36 nm, respectively). These results suggest that 1–4 have larger donor– acceptor charge-transfer character upon excitation,[10] which would facilitate effective photoinduced charge transfer in the energy conversion process.
293 K are presented in Figure 2, and the corresponding optical data are summarized in Table 1. Compounds 1–4 show two absorption bands in the range of l = 274–434 nm, which should
Figure 2. Normalized UV/Vis absorption (a) and emission spectra (b) of compounds 1–4 in CH2Cl2 at 293 K (* indicates an artefact peak).
Table 1. Photophysical data for compounds 1–4 in CH2Cl2 at 293 K.
1 2 3 4
labs [nm] (e 104 m 1 cm 1)
lem [nm]
FF[a] [%]
tF [ns]
310 (5.31), 371 (4.22), 501 (5.63) 313 (4.68), 445 (3.93), 520 (6.47) 311 (5.56), 355 (6.08), 502 (5.60) 310 (5.09), 383 (8.49), 537 (8.49)
669 669 670 707
9.63 3.75 9.35 2.79
5.67 1.76 5.57 2.52
Electrochemical Properties Cyclic voltammetry (CV) was used to investigate the electrochemical properties of compounds 1–4 and evaluate their HOMO and LUMO energy levels, which were calculated from the onset oxidation potential and the onset reduction potential, respectively. The electrochemical data are summarized in Table 2. As shown in Figure 3, all compounds display one quasi-reversible oxidation wave and one irreversible reduction wave. Compounds 1–4 have similar LUMO energy levels (ca. 3.50 eV) because all compounds have the same acceptor unit. The offset of the LUMO levels between 1–4 and PC71BM is slightly higher than the minimum value ( 0.3 eV)[11] and this can induce effective charge separation at the interface of the donor and acceptor (the LUMO level for PC71BM is 4.0 eV).[4e, 12] The HOMO energy levels of 1–4 are 5.33, 5.27, 5.32, and 5.21 eV, respectively, which are highly dependent on the electron-donating unit. Therefore, the stronger electron-donating ability of the CDT moiety and the additional thiophene units will raise the HOMO levels of 2 and 4, respectively. The electrochemical band gaps of 1–4 are estimated to be in the order of 1 (1.83 eV) 3 (1.83 eV) > 2 (1.78 eV) > 4 (1.69 eV), which is consistent with the trend of the corresponding optical band gap.
[a] Quantum yields were measured with an excitation wavelength of l = 488 nm by using rhodamine 6G in ethanol as a reference (FF = 0.95).
be ascribed to the p–p* transition of the p-conjugation system, and another strong absorption band from l = 421 to 664 nm in the long-wavelength region should be assigned to the ICT transition between the donor unit (TPA/thiophene/benzene/CDT) and the electron-deficient benzothiadiazole unit. Compared with benzene-bridged 1 and DPTA-bridged 3, which have very similar ICT absorption maxima at l = 501 and 502 nm, respectively, CDT-bridged 2 exhibits a slight redshift of about 20 nm for the absorption band in the low-energy absorption region with a higher absorption coefficient of 6.47 104 m 1 cm 1 owing to the stronger electron-donating ability of the CDT unit, which enhances the ICT transition between the donor and acceptor units, than the benzene and non-coplanar DPTA unit. Compound 4 shows a more significant bathochromChem. Asian J. 2015, 00, 0 – 0
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Full Paper Table 2. Electrochemical properties of compounds 1–4. Eoxonset [V][a] 1 2 3 4
0.60 0.54 0.59 0.48
EHOMO [eV]
Eredonset [V][b]
5.33 5.27 5.32 5.21
1.23 1.24 1.24 1.21
ELUMO [eV] 3.50 3.49 3.49 3.52
Egec [eV][c]
Egopt [eV][d]
1.83 1.78 1.83 1.69
2.14 2.06 2.13 1.99
[a] Eoxonset : onset oxidation potential versus Ag/AgCl and EHOMO = e(Eoxonset + 4.73) eV. [b] Eredonset : onset reduction potential versus Ag/AgCl and ELUMO = e(Eredonset + 4.73) eV. [c] Egec = ELUMO EHOMO. [d] Egopt : Optical band gap determined from the onset of absorption in CH2Cl2.
Figure 4. DFT-calculated HOMO and LUMO wave functions of the geometryoptimized structures (B3LYP/6-31G*) of compounds 1–4.
Figure 3. Cyclic voltammograms of 1–4 in acetonitrile containing [Bu4N]PF6 at a scan rate of 50 mV s 1.
Theoretical Calculations DFT[13] calculations at the B3LYP/6-31G* level[14] by using the Gaussian 03 (revision C.02) program were used to investigate the optimum geometries and electron–state density distributions of the HOMOs and LUMOs of compounds 1–4, and further evaluate the impact of different p bridges on the molecular architecture, and consequently, on the optoelectronic properties of these molecules. As shown in Figure 4, compounds 1, 2, and 4 adopt a nearly planar geometry owing to the planar central units (benzene/CDT), whereas the molecular conformation of 3 is, to some extent, distorted because the central DPTA unit has a non-coplanar propeller structure. These features will have an important effect on the molecular packing and morphology of the photoactive layer of the SMBHJ devices. In addition, the electron densities of all HOMO wave functions are delocalized over the whole conjugated systems, whereas the electron densities of the LUMO wave functions are mainly distributed on the electron-withdrawing benzothiadiazole unit. Figure 5 shows the theoretical HOMO and LUMO energy values of compounds 1–4. The relative changes in the band gaps on going from 1 to 4 are in agreement with the experimental results.
Figure 5. Theoretical HOMO and LUMO energy levels of compounds 1–4.
sulfonate)(PSS)/1–4:PC71BM/LiF or Ca/Al by using conventional spin-coating solution-processing procedures. Detailed photovoltaic performances of the BHJ device based on the materials is presented in Table 3. When the blend ratio is 1:4 with LiF/Al as the cathode, the thicker blend film for 1 and 4 exhibits better performance; if the blend ratio is 1:3 with Ca/Al as the cathode, the thinner active layer is better for 2 and 3. The current density versus voltage (J–V) curves of 1–4/PC71BM with different blend ratios at various active-layer thicknesses are shown in Figure 6. Compared with 1, which exhibits a PCE of 2.87 % with an open-circuit voltage (Voc) of 0.93 V, a short-circuit current density (Jsc) of 7.04 mA cm 2, and an FF of 0.44, compound 4 shows poorer photovoltaic performance, even if it possesses the best absorption properties in the visible region, which cannot complement the loss of Voc in the energy conversion. The reduced Voc value should be ascribed to the higher HOMO energy level of 4, which results from the addi-
Photovoltaic Performance SMBHJ solar cells were fabricated by using compounds 1–4 as the electron-donor materials and PC71BM as the electron acceptor with a general device structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) (PEDOT):poly(styrene &
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Full Paper cules 1–4 are higher than their corresponding platinum-containing counterparts PT1 and PT2 (2.37 and 2.34 %, respectively), presumably because the former compounds have significantly improved Voc values, which originate from deeper HOMO energy levels, and similar Jsc and slightly improved FF values. The external quantum efficiency (EQE) of the SMBHJ devices based on 1–4 and PC71BM were also measured under monochromatic light and the results are shown in Figure 6. All EQE curves exhibit a broad response in the spectral range of l = 350–700 nm, and the EQE maxima are 53 % at l = 480 nm for 1, 54 % at l = 481 nm for 2, 44 % at l = 495 nm for 3, and 45 % at l = 485 nm for 4; these values indicate that photoelectronic conversion is efficient in these devices. Charge-carrier mobility is a crucial parameter for achieving high efficiency in photovoltaic devices. To ensure effective charge-carrier transport to the electrodes and to diminish photocurrent loss in solar cells, a high hole mobility is required for electron donors. For example, the hole mobilities of 2 and 3 were investigated by using organic field-effect transistors (OFETs) with a bottom-gate top-contact configuration. The measured mobility of 2 with a CDT central unit (4.82 10 5 cm2 V 1 s 1) is higher than that of 3 with a TPA unit (2.12 10 5 cm2 V 1 s 1); this is probably due to the stronger intermolecular interaction of the large and rigid planar CDT unit, which induces the formation of good intermolecular packing. In addition, the higher Jsc values of the photovoltaic devices based on 2 might be attributed to higher hole mobility. Moreover, it is well known that the morphology of the active layer is also a critical factor that controls the performance of SMBHJ solar cells. AFM was used to investigate the morphology of 1–4/PC71BM blend films with various ratios (Figure 7). The root mean square (rms) roughness of the blend films is 0.23 nm for 1/PC71BM, 0.35 nm for 2/PC71BM, 0.36 nm for 3/PC71BM, and 0.24 nm for 4/PC71BM, which indicates that the surfaces of the four blend films are quite smooth and there is good miscibility between the donor materials and PC71BM in the blend films. On the other hand, the four blend films exhibit similar phase images, which suggest similar donor and acceptor distribution conditions. The similar morphology of the four blend films illustrates that the observed PCE values of BHJ devices based on these materials are mainly determined by various properties of the material itself and the device structure.
Table 3. Photovoltaic properties of compounds 1–4. Donor
Film thickness [nm]
Voc [V]
Jsc [mA cm 2]
FF[a]
PCE [%]
1[b]
85 55 90 75 55 80 60 80 55
0.93 0.90 0.94 0.94 0.94 0.96 0.96 0.85 0.83
7.04 4.94 6.28 6.70 7.24 5.09 5.96 6.45 4.83
0.44 0.45 0.46 0.47 0.48 0.42 0.44 0.43 0.44
2.87 1.99 2.72 2.98 3.28 2.06 2.51 2.36 1.77
2[c]
3[c] 4[b]
[a] FF = fill factor. [b] Device structures for 1 and 4: ITO/PEDOT:PSS/ dye:PC71BM (1:4)/LiF/Al. [c] Device structures for 2 and 3: ITO/PEDOT:PSS/ dye:PC71BM (1:3)/Ca/Al.
Figure 6. J–V curves (top) and EQE spectra (bottom) of BHJ solar cells based on compounds 1–4:PC71BM (1:4 or 1:3, w/w) under monochromatic AM 1.5 solar illumination.
tion of the additional thiophene rings, because Voc is generally proportional to the difference in the LUMO energy level of the acceptor material and the HOMO energy level of the donor material. The device based on 2 and PC71BM (1:3, w/w) at a film thickness of 55 nm achieved a Voc of 0.94 V, a Jsc of 7.24 mA cm 2 and FF of 0.48, resulting in a higher PCE of 3.28 % without the use of any additives or post-solvent/thermal treatment.[15] The better photovoltaic properties of 2 than 3 are probably due to its stronger light absorption and regular and planar molecular configuration, and the combined effect results in the higher Jsc value. In addition, it is interesting that the photovoltaic efficiencies of organic bridged small moleChem. Asian J. 2015, 00, 0 – 0
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Conclusion A new series of organic small molecules based on bis(arylene ethynylene) structural motif with various conjugated electronrich aromatic bridges were synthesized by means of a Sonogashira coupling reaction of terminal acetylenes and diiodo-substituted aryl derivatives. The introduction of central aryl bridges instead of the Pt(PBu3)2 unit improved the solar absorption ability and lowered the HOMO energy levels of the donor materials. The lowering of the HOMO energy levels resulted in a higher Voc value. Therefore, molecules 1–4 exhibited better photovoltaic performances than those of their corre5
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Figure 7. AFM height (top) and phase (bottom) images of blend films spin-coated from solutions in chlorobenzene: a,b) 1:PC71BM; c,d) 2:PC71BM; e,f) 3:PC71BM; g,h) 4:PC71BM.
spectrometer. Solution-state PL measurements were obtained by using an LS50B fluorescent spectrometer. The PL spectra were measured in CH2Cl2 with a PTI Fluorescence Master Series QM1 spectrophotometer. For lifetime measurements, the third harmonics l = 355 nm line of a Q-switched Nd:YAG laser was used as the excitation light source. The emission was recorded by using a photomultiplier tube (PMT) and an HP54522A 500 MHz oscilloscope.
sponding platinum-containing counterparts. In addition, a better PCE of 3.28 % was achieved with the 2-based BHJ device than those of other devices; this can mainly be ascribed to its stronger absorption ability and superior molecular geometry. Careful device optimization procedures, including variation of the choice of solvent, thermal annealing, solvent additives, and inverted device structures, should be carried out in the future to improve the photovoltaic performance.
Electrochemical Measurements To calculate the oxidation and reduction potentials and evaluate the HOMO and LUMO energy levels of these conjugated molecules, CV was performed as thin films on a glassy carbon electrode measured in a 0.1 m solution of [Bu4N]PF6 in acetonitrile with a Pt working electrode, Pt wire counter electrode, and a Ag/AgCl reference electrode under a N2 atmosphere at a scan rate of 50 mV s 1. The potential was referenced to the half-wave potential of the ferrocenium/ferrocene couple (E1/2 = 0.07 V in a solution of [Bu4N]PF6 in acetonitrile), which had an absolute energy level of 4.73 eV relative to the vacuum level for calibration. The onset oxidation (Eoxonset) and reduction (Eredonset) potentials were used to determine the HOMO and LUMO energy levels by using the following equations: EHOMO = e(Eoxonset + 4.73) eV and ELUMO = e(Eredonset + 4.73) eV, in which the unit of potential is V versus Ag/Ag + . The difference between the oxidation and reduction potentials gave access to the electrochemical band gap (Egec).
Experimental Section General All reactions, except for the alkylation of CDT, were carried out under a nitrogen atmosphere by using standard Schlenk techniques. Solvents were dried and distilled from appropriate drying agents under an inert atmosphere prior to use. All reagents and chemicals, unless otherwise stated, were purchased from commercial sources and used without further purification. DPTA was prepared according to a method reported in the literature.[16] All reactions were monitored by TLC on Merck precoated glass plates. Flash column chromatography and preparative TLC were carried out by using silica gel from Merck (230–400 mesh).
Physical Measurements 4,4-Didodecyl-cyclopenta[2,1-b:3,4-b’]dithiophene (CDTC12)
NMR spectra were measured in CDCl3 on a Varian Inova 400 MHz FT-NMR spectrometer and chemical shifts are quoted relative to tetramethylsilane (TMS) for 1H and 13C. IR spectra were recorded on the Nicolet Magna 550 Series II FTIR spectrometer as KBr pellets for solid-state spectroscopy. MALDI-TOF spectra were obtained by means of an Autoflex Bruker MALDI-TOF mass spectrometer. UV/ Vis spectroscopy was performed with a Hewlett Packard 8453
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Dodecyl bromide (4.7 g, 18.9 mmol) in DMSO (50 mL) and a catalytic amount of potassium iodide (50 mg) were added to a solution of CDT (1.63 g, 9.14 mmol) in DMSO (50 mL). The mixture was purged with argon for 10 min followed by the slow addition of solid KOH (2.0 g, 35.7 mmol). The dark green mixture was stirred in the dark under argon at room temperature for 72 h. The mixture
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Full Paper was then poured into deionized water (300 mL) and the organic phase was extracted with hexanes (5 100 mL). The organic phases were collected and washed with brine (300 mL), a saturated solution of ammonium chloride (100 mL), and deionized water (300 mL). The organic phase was dried over magnesium sulfate, filtered, and concentrated to give the crude product as a yellow oil. Purification by flash chromatography with hexanes and drying under high vacuum with agitation for 48 h gave a pure product as a colorless oil (3.8 g, 82 %). 1H NMR (500 MHz, CDCl3): d = 7.14 (d, J = 4.80 Hz, 2 H; Th), 6.92 (d, J = 4.75 Hz, 2 H; Th), 1.83–1.79 (m, 4 H; C12H25), 1.27–1.13 (m, 36 H; C12H25), 0.92–0.90 (m, 4 H; C12H25), 0.87 ppm (t, J = 6.45 Hz, 6 H; C12H25).
Compound 2 Yield 79 %; dark red solid; 1H NMR (400 MHz, CDCl3): d = 7.95 (s, 2 H; Ar), 7.90 (d, J = 7.52 Hz, 2 H; Ar), 7.85 (d, J = 8.76 Hz, 4 H; Ar), 7.69 (d, J = 7.60 Hz, 2 H; Ar), 7.16–7.07 (m, 22 H; Ar), 2.83 (t, J = 7.48 Hz, 4 H; C6H13), 2.34 (s, 12 H; CH3), 1.87–1.83 (m, 4 H; C12H25), 1.79–1.76 (m, 4 H; C6H13), 1.44–1.35 (m, 12 H; C6H13), 1.27–1.16 (m, 40 H; C12H25), 0.91 (t, J = 7.00 Hz, 6 H; C6H13), 0.86 ppm (t, J = 7.04 Hz, 6 H; C12H25); 13C NMR (100 MHz, CDCl3): d = 158.8, 153.9, 152.8, 148.8, 148.6, 144.9, 139.7, 138.3, 133.2, 132.8, 130.0, 129.8, 129.6, 128.4, 126.9, 126.1, 125.9, 125.2, 124.9, 123.5, 121.5, 119.2 (Ar), 91.5, 88.0 (CC), 54.1 (quat C), 37.8, 31.9, 31.7, 30.3, 30.1, 29.9, 29.7, 29.6, 29.44, 29.35, 29.0, 24.6, 22.69, 22.66, 20.9, 14.2, 14.1 ppm (C6H13 + C12H25 + CH3); HRMS (MALDI-TOF): m/z calcd for C109H120N6S6 [M] + : 1705.7924; found: 1705.7956; elemental analysis calcd (%) for C109H120N6S6 : C 76.71, H 7.09, N 4.92; found: C 76.80, H 7.22, N 5.05.
CDTC12-I Compound CDTC12 (0.59 g, 1.16 mmol) was dissolved in a mixture of THF/HOAc (4:1, 20 mL). NIS (0.54 g, 1.20 mmol) was added in small portions to the solution at 0 8C. Then the reaction mixture was gradually warmed to room temperature and stirred overnight in the dark. Completion of the reaction was determined by TLC analysis, and the volatile compounds were removed under reduced pressure. The residue was purified by column chromatography on silica gel eluted with hexane to give CDTC12-I as a pale yellow solid (0.76 g, 88 %). 1H NMR (400 MHz, CDCl3): d = 7.08 (s, 2 H; Th), 1.76– 1.72 (m, 4 H; C12H25), 1.34–1.16 (m, 36 H; C12H25), 0.89–0.85 ppm (m, 10 H; C12H25).
Compound 3 Yield 71 %; dark red solid; 1H NMR (400 MHz, CDCl3): d = 7.92 (s, 2 H; Ar), 7.86 (d, J = 7.52 Hz, 2 H; Ar), 7.83 (d, J = 8.80 Hz, 4 H; Ar), 7.65 (d, J = 7.56 Hz, 2 H; Ar), 7.39 (d, J = 8.72 Hz, 4 H; Ar), 7.15–7.04 (m, 28 H; Ar), 2.82 (t, J = 7.52 Hz, 4 H; C6H13), 2.35 (s, 3 H; CH3), 2.33 (s, 12 H; CH3), 1.77–1.74 (m, 4 H; C6H13), 1.45–1.33 (m, 12 H; C6H13), 0.89 ppm (t, J = 7.12 Hz, 6 H; C6H13); 13C NMR (100 MHz, CDCl3): d = 154.0, 152.8, 148.5, 148.4, 147.4, 144.9, 144.0, 139.0, 134.4, 133.1, 132.7, 132.4, 130.3, 130.0, 129.8, 129.7, 128.4, 126.9, 126.0, 125.8, 125.2, 125.0, 123.0, 121.6, 119.9, 116.8 (Ar), 97.2, 82.3 (CC), 31.7, 30.3, 29.9, 29.1, 22.7, 21.0, 20.9, 14.2 ppm (C6H13 + CH3); HRMS (MALDI-TOF): m/z calcd for C95H83N7S4 [M] + : 1450.5618; found: 1450.5641; elemental analysis calcd (%) for C95H83N7S4 : C 78.64, H 5.77, N 6.76; found: C 78.76, H 5.85, N 6.88.
DPTA-I By using a similar method to that reported for CDTC12-I, compound DPTA-I was obtained as a pale yellow solid (85 %). 1H NMR (400 MHz, CDCl3): d = 7.58 (d, J = 8.76 Hz, 4 H; Ph), 7.08 (d, J = 8.24 Hz, 2 H; Ph), 6.96 (d, J = 8.20 Hz, 2 H; Ph), 6.79 (d, J = 8.64 Hz, 4 H; Ph), 1.55 ppm (s, 3 H; CH3).
Compound 4
Under a nitrogen atmosphere, a catalytic amount of Pd(OAc)2 (8 %), PPh3 (24 %), and CuI (8 mol %) were added to a solution of each of the aromatic diiodide compounds in a mixture of triethylamine and CH2Cl2 (1:1, v/v). After the solution was stirred at RT for 30 min, each corresponding ethynyl ligand (L1 or L2; 2.06 equiv) was added to the reaction solution. Then the reaction mixture was stirred overnight at RT. Completion of the reaction was determined by TLC. The volatile compounds were removed under reduced pressure. The residue was purified by column chromatography on silica gel with an appropriate ratio of CH2Cl2/hexane as the eluent to obtain pure compounds 1–4 in moderate yields of 57–79 %.
Yield 64 %; dark red solid; 1H NMR (400 MHz, CDCl3): d = 8.13 (d, J = 3.88 Hz, 2 H; Ar), 7.95 (s, 2 H; Ar), 7.86 (s, 4 H; Ar), 7.53 (d, J = 8.24 Hz, 8 H; Ar), 7.31 (d, J = 3.88 Hz, 2 H; Ar), 7.10 (d, J = 8.32 Hz, 8 H; Ar), 7.04 (d, J = 8.16 Hz, 12 H; Ar), 2.85 (t, J = 7.76 Hz, 4 H; C6H13), 2.34 (s, 12 H; CH3), 1.80–1.76 (m, 4 H; C6H13), 1.46–1.25 (m, 12 H; C6H13), 0.91 ppm (t, J = 7.00 Hz, 6 H; C6H13); 13C NMR (100 MHz, CDCl3): d = 152.5, 152.4, 149.1, 148.0, 146.0, 144.9, 139.6, 137.3, 133.0, 131.1, 130.0, 128.9, 128.4, 127.0, 126.5, 126.2, 125.6, 124.9, 124.8, 124.7, 123.0, 122.1, 119.4 (Ar), 97.1, 84.9 (CC), 31.7, 30.3, 29.7, 29.1, 22.7, 20.9, 14.2 ppm (C6H13 + CH3); HRMS (MALDI-TOF): m/z calcd for C90H76N6S6 [M] + : 1433.4480; found: 1433.4470; elemental analysis calcd (%) for C90H76N6S6 : C 75.38, H 5.34, N 5.86; found: C 75.32, H 5.52, N 5.65.
Compound 1
Solar Cell Fabrication and Characterization
General Procedure for the Synthesis of Compounds 1–4
1
Yield 57 %; red solid; H NMR (400 MHz, CDCl3): d = 7.94 (s, 2 H; Ar), 7.89 (d, J = 7.52 Hz, 2 H; Ar), 7.83 (d, J = 8.76 Hz, 4 H; Ar), 7.67 (d, J = 7.56 Hz, 2 H; Ar), 7.51 (s, 4 H; Ar), 7.15–7.07 (m, 20 H; Ar), 2.85 (t, J = 7.56 Hz, 4 H; C6H13), 2.34 (s, 12 H; CH3), 1.79–1.72 (m, 4 H; C6H13), 1.47–1.35 (m, 12 H; C6H13), 0.91 ppm (t, J = 7.04 Hz, 6 H; C6H13); 13 C NMR (100 MHz, CDCl3): d = 153.9, 152.7, 149.1, 148.6, 144.9, 139.7, 133.1, 132.9, 131.1, 130.0, 129.8, 129.5, 128.3, 126.8, 125.9, 125.2, 124.8, 123.0, 121.5, 119.2 (Ar), 96.9, 84.9 (CC), 31.7, 30.3, 29.9, 29.0, 22.7, 20.9, 14.2 ppm (C6H13 + CH3); HRMS (MALDI-TOF): m/z calcd for C82H72N6S4 [M] + : 1268.4696; found: 1268.4702; elemental analysis calcd (%) for C82H72N6S4 : C 77.56, H 5.72, N 6.62; found: C 77.68, H 5.98, N 6.80. Chem. Asian J. 2015, 00, 0 – 0
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The device structure was ITO/PEDOT:PSS/1–4:PC71BM (1:4 or 1:3, w/w)/LiF or Ca/Al (LiF for 1 and 4; Ca for 2 and 3). ITO glass substrates (10 W per square) were cleaned by sonication in toluene, acetone, ethanol, and deionized water, dried in an oven, and then cleaned with UV ozone for 300 s. PEDOT:PSS (Baytron P AI 4083) was spin-coated onto the precleaned ITO substrate to form a 40 nm thick layer, followed by drying at 120 8C for 30 min in air. Then, the substrates were transferred into a glove box filled with nitrogen. The prepared solution of 1–4/PC71BM in chlorobenzene was spin-coated on top of the PEDOT:PSS layer. Finally, the samples were transferred into an evaporator in which 1 nm LiF or 25 nm Ca and 100 nm Al were thermally deposited under vacuum at
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[3]
[4]
Acknowledgements W.-Y.W. thanks the National Natural Science Foundation of China (project number 91333206), the Science, Technology and Innovation Committee of Shenzhen Municipality (JCYJ20120829154440583), Hong Kong Baptist University (FRG2/12-13/083 and FRG1/13-14/053), and Hong Kong Research Grants Council (HKBU203011) for financial support. The work described herein was partially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (project no. T23-713/11). The work was also supported by the Partner State Key Laboratory of Environmental and Biological Analysis (SKLP-14-15-P011) and Strategic Development Fund of HKBU. The project was also supported by Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. H.L. thanks the 111 Project and Beijing Engineering Research Center of Food Environment and Public Health from the Minzu University of China (nos. B08044 and 10301-01404026) for financial support.
[5] [6]
[7] [8] [9]
[10]
Keywords: conjugation · donor–acceptor systems electrochemistry · organic solar cells · photochemistry
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FULL PAPER Photochemistry Hongmei Zhan, Qian Liu, Fengrong Dai, Cheuk-Lam Ho,* Yingying Fu, Leijiao Li, Li Zhao, Hua Li, Zhiyuan Xie,* Wai-Yeung Wong* Bridge repairs: A series of bis(arylene ethynylene)s with tunable electron-rich bridging groups were used to prepare solution-processed bulk heterojunction
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organic solar cells, which achieved the best power conversion efficiency of 3.28 % with a higher Voc of 0.94 V (see figure).
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Photochemistry
Organic Donor Materials Based on Bis(arylene ethynylene)s for Bulk Heterojunction Organic Solar Cells with High Voc Values Hongmei Zhan,[a, b] Qian Liu,[b] Fengrong Dai,[b] Cheuk-Lam Ho,*[b] Yingying Fu,[a] Leijiao Li,[c] Li Zhao,[c] Hua Li,[b, d] Zhiyuan Xie,*[a] and Wai-Yeung Wong*[a, b, e]
Abstract: A series of purely organic small molecules 1–4 based on bis(arylene ethynylene)s with various electron-rich aromatic bridges, such as benzene, cyclopentadithiophene (CDT), and diphenyl(p-tolyl)amine, were synthesized by a Sonogashira coupling reaction. Their optical, electronic, and electrochemical properties were fully characterized. The photovoltaic properties of these molecules as donor materials and [6,6]-phenyl-C71-butyric acid methyl ester as an acceptor
material in solution-processed bulk heterojunction devices were studied. Among them, CDT-bridged molecule 2 exhibited the best photovoltaic performance and achieved a power conversion efficiency of 3.28 %. In addition, the photovoltaic efficiencies of these molecules are higher than their corresponding platinum-containing counterparts, probably owing to their stronger light absorption and improved open-circuit voltage.
Introduction
solar cell materials and optimization of the device structure and fabrication technology.[2] A power conversion efficiency (PCE) as high as 8.12 % has been achieved with small molecules based on benzo[1,2-b:4,5-b’]dithiophene and 3-ethylrhodanine units with dioctylterthiophene linkages in solution-processed bulk heterojunction (BHJ) solar cells;[3] this value is similar those of polymer-based devices.[4] To realize low-cost SMBHJ solar cells for commercial applications, their photovoltaic performance should be further improved. To date, in addition to device engineering and morphology control, the photoactive materials, especially electron-donor materials, still play a pivotal role in achieving high efficiency in SMBHJ devices. Therefore, careful molecular design should be carried out to address the collective requirement of SMBHJ solar cells, including broad solar absorption, well-matched energy levels with fullerene derivative acceptors, high charge carrier mobilities, and excellent film formation ability. In our previous work, we reported a series of symmetric platinum(II)–bis(arylene ethynylene) complexes consisting of Pt(PBu3)2 acetylide as the core and D-A-D (D, donor unit; A, acceptor unit) architecture at both sides,[5] in which efficient pelectron delocalization could be established owing to the special electronic properties of the heavy metal platinum.[6] In addition, the triarylamine donor unit with a three-dimensional propeller structure, 3-hexylthiophene, and a bulky Pt(PBu3)2 group could efficiently improve the molecular solubility to help circumvent the poor film formation problem in SMBHJ solar cells.[3, 7] Meanwhile, an efficient intramolecular charge transfer (ICT) interaction between these donor units and electron-accepting benzothiadiazole units enhances the solar absorption abilities. These molecules exhibit wide absorption and higher molar absorption coefficients. Moderate PCEs of 1.45– 2.37 % have been achieved with SMBHJ devices containing
In recent years, solution-processed small-molecule bulk heterojunction (SMBHJ) solar cells have attracted great attention owing to their advantages over their polymer counterparts, including well-defined molecular structures without batch-tobatch variation, relatively simple synthesis, high purity, and reproducible photovoltaic performance.[1] Appreciable solar cell progress has been made with the development of photoactive [a] Dr. H. Zhan, Y. Fu, Prof. Z. Xie, Prof. W.-Y. Wong State Key Laboratory of Polymer Physics and Chemistry Changchun Institute of Applied Chemistry Chinese Academy of Sciences, No. 5625, Renmin Street Changchun 130022 (P. R. China) E-mail: [email protected] [b] Dr. H. Zhan, Dr. Q. Liu, Dr. F. Dai, Dr. C.-L. Ho, Dr. H. Li, Prof. W.-Y. Wong Department of Chemistry, Partner State Key Laboratory of Environmental and Biological Analysis and Institute of Advanced Materials Hong Kong Baptist University Waterloo Road, Kowloon Tong Hong Kong (S.A.R. China) E-mail: [email protected] [c] Dr. L. Li, Dr. L. Zhao State Key Laboratory of Theoretical and Computational Chemistry Institute of Theoretical Chemistry Jilin University, No.2, Liutiao Road Changchun (P.R. China) [d] Dr. H. Li College of Life and Environmental Sciences & Beijing Engineering Research Center of Food Environment and Public Health Minzu University of China, Beijing 100081 (P.R. China) [e] Prof. W.-Y. Wong HKBU Institute of Research and Continuing Education Shenzhen Virtual University Park Shenzhen 518057 (P.R. China) Chem. Asian J. 2015, 00, 0 – 0
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Full Paper Results and Discussion Syntheses and Chemical Characterization The general synthetic routes to compounds 1–4 are depicted in Scheme 1. The terminal acetylenes L1 and L2 were synthesized according to the procedures previously reported in the literature.[4] Owing to the higher reactivity of iodine derivatives as compared to the bromine counterparts in the Sonogashira reaction,[8] we chose the diiodo-substituted aromatic derivatives as the starting materials for the synthesis of the target compounds 1–4. 1,4-Diiodobenzene is commercially available. CDT was alkylated with n-dodecyl bromide by using powdered potassium hydroxide as the base, followed by iodination with NIS in the presence of glacial acetic acid, to give compound CDTC12-I in high yield. Compound DPTA-I was similarly prepared from diphenyl(p-tolyl)amine (DPTA). By using a Sonogashira coupling reaction of terminal acetylene L1 or L2 with the corresponding diiodo-substituted aromatic derivative in a ratio of 2.06:1 and in the presence of a palladium(II)/copper(I) catalyst provides compounds 1–4 in moderate yields. The relatively lower yields of 1 and 4 are ascribed to the formation of 1,3-butadiyne derivatives as a byproduct from the undesired homocoupling reaction of terminal alkynes.[8, 9] Compounds 1–4 exhibit good air stability and solubility in common organic solvents. The proposed formulations of these compounds are all in good agreement with their analytical and spectroscopic (NMR, MS, and FTIR) data.
Figure 1. Structures of metal-free 1–4 and metalated congeners PT1 and PT2.
these platinum-containing small molecules as the electron acceptor blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). In this context, by replacing the central building block Pt(PBu3)2 with various electron-rich organic moieties, such as benzene, cyclopentadithiophene (CDT), and monomethyl-substituted triphenylamine (TPA), we designed and synthesized a series of purely organic small molecules 1–4 (Figure 1). Their photovoltaic performance and other properties were also investigated and compared to explore the influence of chromophore variation and understand better the relationship between molecular structure and photovoltaic performance.
Optical Properties The normalized UV/Vis absorption spectra and photoluminescence (PL) spectra of compounds 1–4 in dichloromethane at
Scheme 1. Synthetic pathways of compounds 1–4. DMSO = dimethyl sulfoxide, THF = tetrahydrofuran, NIS = N-iodosuccinimide.
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Full Paper ically shifted ICT absorption band centered at l = 537 nm with the highest absorption coefficient of 8.49 104 m 1 cm 1, which could be ascribed to the addition of an additional thiophene ring, resulting in the formation of a longer p-conjugation length in 2. The optical band gaps deduced from the absorption onset are in the order of 1 (2.14 eV) 3 (2.13 eV) > 2 (2.06 eV) > 4 (1.99 eV). In addition, in contrast to the ICT absorption band of platinum-containing compounds PT1 (l = 526 nm, e: 5.52 104 m 1 cm 1) and PT2 (l = 561 nm, e: 7.51 104 m 1 cm 1), those of the aromatic ring bridged compounds 1–4 display a slight hypsochromic shift; however, the last series possess a higher absorption ability, as deduced from their higher absorption coefficient. The PL spectra of 1–4 were also investigated and the data are shown in Figure 2. Compounds 1–3 show very similar emission profiles in CH2Cl2 with emission bands at l 669 nm. Compound 4 exhibits a clearly redshifted emission band relative to those of 1–3 with a band wavelength at l = 707 nm. Similar shoulder bands appear at l 788 nm in the PL emission spectra of 1–4, which are due to instrumental artefacts. Furthermore, the measured PL lifetimes for these compounds are very short (ca. 1.76–5.67 ns), which is characteristic of the fluorescent emission, and as evidenced by the reduced quantum yield values, the fluorescence intensity of 1–4 decreases gradually with extension of the p-conjugation length. In addition, it is worth mentioning that organic compounds 1–4 display larger Stoke shifts of 49–70 nm relative to those of platinum-containing counterparts PT1 and PT2 (39 and 36 nm, respectively). These results suggest that 1–4 have larger donor– acceptor charge-transfer character upon excitation,[10] which would facilitate effective photoinduced charge transfer in the energy conversion process.
293 K are presented in Figure 2, and the corresponding optical data are summarized in Table 1. Compounds 1–4 show two absorption bands in the range of l = 274–434 nm, which should
Figure 2. Normalized UV/Vis absorption (a) and emission spectra (b) of compounds 1–4 in CH2Cl2 at 293 K (* indicates an artefact peak).
Table 1. Photophysical data for compounds 1–4 in CH2Cl2 at 293 K.
1 2 3 4
labs [nm] (e 104 m 1 cm 1)
lem [nm]
FF[a] [%]
tF [ns]
310 (5.31), 371 (4.22), 501 (5.63) 313 (4.68), 445 (3.93), 520 (6.47) 311 (5.56), 355 (6.08), 502 (5.60) 310 (5.09), 383 (8.49), 537 (8.49)
669 669 670 707
9.63 3.75 9.35 2.79
5.67 1.76 5.57 2.52
Electrochemical Properties Cyclic voltammetry (CV) was used to investigate the electrochemical properties of compounds 1–4 and evaluate their HOMO and LUMO energy levels, which were calculated from the onset oxidation potential and the onset reduction potential, respectively. The electrochemical data are summarized in Table 2. As shown in Figure 3, all compounds display one quasi-reversible oxidation wave and one irreversible reduction wave. Compounds 1–4 have similar LUMO energy levels (ca. 3.50 eV) because all compounds have the same acceptor unit. The offset of the LUMO levels between 1–4 and PC71BM is slightly higher than the minimum value ( 0.3 eV)[11] and this can induce effective charge separation at the interface of the donor and acceptor (the LUMO level for PC71BM is 4.0 eV).[4e, 12] The HOMO energy levels of 1–4 are 5.33, 5.27, 5.32, and 5.21 eV, respectively, which are highly dependent on the electron-donating unit. Therefore, the stronger electron-donating ability of the CDT moiety and the additional thiophene units will raise the HOMO levels of 2 and 4, respectively. The electrochemical band gaps of 1–4 are estimated to be in the order of 1 (1.83 eV) 3 (1.83 eV) > 2 (1.78 eV) > 4 (1.69 eV), which is consistent with the trend of the corresponding optical band gap.
[a] Quantum yields were measured with an excitation wavelength of l = 488 nm by using rhodamine 6G in ethanol as a reference (FF = 0.95).
be ascribed to the p–p* transition of the p-conjugation system, and another strong absorption band from l = 421 to 664 nm in the long-wavelength region should be assigned to the ICT transition between the donor unit (TPA/thiophene/benzene/CDT) and the electron-deficient benzothiadiazole unit. Compared with benzene-bridged 1 and DPTA-bridged 3, which have very similar ICT absorption maxima at l = 501 and 502 nm, respectively, CDT-bridged 2 exhibits a slight redshift of about 20 nm for the absorption band in the low-energy absorption region with a higher absorption coefficient of 6.47 104 m 1 cm 1 owing to the stronger electron-donating ability of the CDT unit, which enhances the ICT transition between the donor and acceptor units, than the benzene and non-coplanar DPTA unit. Compound 4 shows a more significant bathochromChem. Asian J. 2015, 00, 0 – 0
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Full Paper Table 2. Electrochemical properties of compounds 1–4. Eoxonset [V][a] 1 2 3 4
0.60 0.54 0.59 0.48
EHOMO [eV]
Eredonset [V][b]
5.33 5.27 5.32 5.21
1.23 1.24 1.24 1.21
ELUMO [eV] 3.50 3.49 3.49 3.52
Egec [eV][c]
Egopt [eV][d]
1.83 1.78 1.83 1.69
2.14 2.06 2.13 1.99
[a] Eoxonset : onset oxidation potential versus Ag/AgCl and EHOMO = e(Eoxonset + 4.73) eV. [b] Eredonset : onset reduction potential versus Ag/AgCl and ELUMO = e(Eredonset + 4.73) eV. [c] Egec = ELUMO EHOMO. [d] Egopt : Optical band gap determined from the onset of absorption in CH2Cl2.
Figure 4. DFT-calculated HOMO and LUMO wave functions of the geometryoptimized structures (B3LYP/6-31G*) of compounds 1–4.
Figure 3. Cyclic voltammograms of 1–4 in acetonitrile containing [Bu4N]PF6 at a scan rate of 50 mV s 1.
Theoretical Calculations DFT[13] calculations at the B3LYP/6-31G* level[14] by using the Gaussian 03 (revision C.02) program were used to investigate the optimum geometries and electron–state density distributions of the HOMOs and LUMOs of compounds 1–4, and further evaluate the impact of different p bridges on the molecular architecture, and consequently, on the optoelectronic properties of these molecules. As shown in Figure 4, compounds 1, 2, and 4 adopt a nearly planar geometry owing to the planar central units (benzene/CDT), whereas the molecular conformation of 3 is, to some extent, distorted because the central DPTA unit has a non-coplanar propeller structure. These features will have an important effect on the molecular packing and morphology of the photoactive layer of the SMBHJ devices. In addition, the electron densities of all HOMO wave functions are delocalized over the whole conjugated systems, whereas the electron densities of the LUMO wave functions are mainly distributed on the electron-withdrawing benzothiadiazole unit. Figure 5 shows the theoretical HOMO and LUMO energy values of compounds 1–4. The relative changes in the band gaps on going from 1 to 4 are in agreement with the experimental results.
Figure 5. Theoretical HOMO and LUMO energy levels of compounds 1–4.
sulfonate)(PSS)/1–4:PC71BM/LiF or Ca/Al by using conventional spin-coating solution-processing procedures. Detailed photovoltaic performances of the BHJ device based on the materials is presented in Table 3. When the blend ratio is 1:4 with LiF/Al as the cathode, the thicker blend film for 1 and 4 exhibits better performance; if the blend ratio is 1:3 with Ca/Al as the cathode, the thinner active layer is better for 2 and 3. The current density versus voltage (J–V) curves of 1–4/PC71BM with different blend ratios at various active-layer thicknesses are shown in Figure 6. Compared with 1, which exhibits a PCE of 2.87 % with an open-circuit voltage (Voc) of 0.93 V, a short-circuit current density (Jsc) of 7.04 mA cm 2, and an FF of 0.44, compound 4 shows poorer photovoltaic performance, even if it possesses the best absorption properties in the visible region, which cannot complement the loss of Voc in the energy conversion. The reduced Voc value should be ascribed to the higher HOMO energy level of 4, which results from the addi-
Photovoltaic Performance SMBHJ solar cells were fabricated by using compounds 1–4 as the electron-donor materials and PC71BM as the electron acceptor with a general device structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) (PEDOT):poly(styrene &
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Full Paper cules 1–4 are higher than their corresponding platinum-containing counterparts PT1 and PT2 (2.37 and 2.34 %, respectively), presumably because the former compounds have significantly improved Voc values, which originate from deeper HOMO energy levels, and similar Jsc and slightly improved FF values. The external quantum efficiency (EQE) of the SMBHJ devices based on 1–4 and PC71BM were also measured under monochromatic light and the results are shown in Figure 6. All EQE curves exhibit a broad response in the spectral range of l = 350–700 nm, and the EQE maxima are 53 % at l = 480 nm for 1, 54 % at l = 481 nm for 2, 44 % at l = 495 nm for 3, and 45 % at l = 485 nm for 4; these values indicate that photoelectronic conversion is efficient in these devices. Charge-carrier mobility is a crucial parameter for achieving high efficiency in photovoltaic devices. To ensure effective charge-carrier transport to the electrodes and to diminish photocurrent loss in solar cells, a high hole mobility is required for electron donors. For example, the hole mobilities of 2 and 3 were investigated by using organic field-effect transistors (OFETs) with a bottom-gate top-contact configuration. The measured mobility of 2 with a CDT central unit (4.82 10 5 cm2 V 1 s 1) is higher than that of 3 with a TPA unit (2.12 10 5 cm2 V 1 s 1); this is probably due to the stronger intermolecular interaction of the large and rigid planar CDT unit, which induces the formation of good intermolecular packing. In addition, the higher Jsc values of the photovoltaic devices based on 2 might be attributed to higher hole mobility. Moreover, it is well known that the morphology of the active layer is also a critical factor that controls the performance of SMBHJ solar cells. AFM was used to investigate the morphology of 1–4/PC71BM blend films with various ratios (Figure 7). The root mean square (rms) roughness of the blend films is 0.23 nm for 1/PC71BM, 0.35 nm for 2/PC71BM, 0.36 nm for 3/PC71BM, and 0.24 nm for 4/PC71BM, which indicates that the surfaces of the four blend films are quite smooth and there is good miscibility between the donor materials and PC71BM in the blend films. On the other hand, the four blend films exhibit similar phase images, which suggest similar donor and acceptor distribution conditions. The similar morphology of the four blend films illustrates that the observed PCE values of BHJ devices based on these materials are mainly determined by various properties of the material itself and the device structure.
Table 3. Photovoltaic properties of compounds 1–4. Donor
Film thickness [nm]
Voc [V]
Jsc [mA cm 2]
FF[a]
PCE [%]
1[b]
85 55 90 75 55 80 60 80 55
0.93 0.90 0.94 0.94 0.94 0.96 0.96 0.85 0.83
7.04 4.94 6.28 6.70 7.24 5.09 5.96 6.45 4.83
0.44 0.45 0.46 0.47 0.48 0.42 0.44 0.43 0.44
2.87 1.99 2.72 2.98 3.28 2.06 2.51 2.36 1.77
2[c]
3[c] 4[b]
[a] FF = fill factor. [b] Device structures for 1 and 4: ITO/PEDOT:PSS/ dye:PC71BM (1:4)/LiF/Al. [c] Device structures for 2 and 3: ITO/PEDOT:PSS/ dye:PC71BM (1:3)/Ca/Al.
Figure 6. J–V curves (top) and EQE spectra (bottom) of BHJ solar cells based on compounds 1–4:PC71BM (1:4 or 1:3, w/w) under monochromatic AM 1.5 solar illumination.
tion of the additional thiophene rings, because Voc is generally proportional to the difference in the LUMO energy level of the acceptor material and the HOMO energy level of the donor material. The device based on 2 and PC71BM (1:3, w/w) at a film thickness of 55 nm achieved a Voc of 0.94 V, a Jsc of 7.24 mA cm 2 and FF of 0.48, resulting in a higher PCE of 3.28 % without the use of any additives or post-solvent/thermal treatment.[15] The better photovoltaic properties of 2 than 3 are probably due to its stronger light absorption and regular and planar molecular configuration, and the combined effect results in the higher Jsc value. In addition, it is interesting that the photovoltaic efficiencies of organic bridged small moleChem. Asian J. 2015, 00, 0 – 0
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Conclusion A new series of organic small molecules based on bis(arylene ethynylene) structural motif with various conjugated electronrich aromatic bridges were synthesized by means of a Sonogashira coupling reaction of terminal acetylenes and diiodo-substituted aryl derivatives. The introduction of central aryl bridges instead of the Pt(PBu3)2 unit improved the solar absorption ability and lowered the HOMO energy levels of the donor materials. The lowering of the HOMO energy levels resulted in a higher Voc value. Therefore, molecules 1–4 exhibited better photovoltaic performances than those of their corre5
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Figure 7. AFM height (top) and phase (bottom) images of blend films spin-coated from solutions in chlorobenzene: a,b) 1:PC71BM; c,d) 2:PC71BM; e,f) 3:PC71BM; g,h) 4:PC71BM.
spectrometer. Solution-state PL measurements were obtained by using an LS50B fluorescent spectrometer. The PL spectra were measured in CH2Cl2 with a PTI Fluorescence Master Series QM1 spectrophotometer. For lifetime measurements, the third harmonics l = 355 nm line of a Q-switched Nd:YAG laser was used as the excitation light source. The emission was recorded by using a photomultiplier tube (PMT) and an HP54522A 500 MHz oscilloscope.
sponding platinum-containing counterparts. In addition, a better PCE of 3.28 % was achieved with the 2-based BHJ device than those of other devices; this can mainly be ascribed to its stronger absorption ability and superior molecular geometry. Careful device optimization procedures, including variation of the choice of solvent, thermal annealing, solvent additives, and inverted device structures, should be carried out in the future to improve the photovoltaic performance.
Electrochemical Measurements To calculate the oxidation and reduction potentials and evaluate the HOMO and LUMO energy levels of these conjugated molecules, CV was performed as thin films on a glassy carbon electrode measured in a 0.1 m solution of [Bu4N]PF6 in acetonitrile with a Pt working electrode, Pt wire counter electrode, and a Ag/AgCl reference electrode under a N2 atmosphere at a scan rate of 50 mV s 1. The potential was referenced to the half-wave potential of the ferrocenium/ferrocene couple (E1/2 = 0.07 V in a solution of [Bu4N]PF6 in acetonitrile), which had an absolute energy level of 4.73 eV relative to the vacuum level for calibration. The onset oxidation (Eoxonset) and reduction (Eredonset) potentials were used to determine the HOMO and LUMO energy levels by using the following equations: EHOMO = e(Eoxonset + 4.73) eV and ELUMO = e(Eredonset + 4.73) eV, in which the unit of potential is V versus Ag/Ag + . The difference between the oxidation and reduction potentials gave access to the electrochemical band gap (Egec).
Experimental Section General All reactions, except for the alkylation of CDT, were carried out under a nitrogen atmosphere by using standard Schlenk techniques. Solvents were dried and distilled from appropriate drying agents under an inert atmosphere prior to use. All reagents and chemicals, unless otherwise stated, were purchased from commercial sources and used without further purification. DPTA was prepared according to a method reported in the literature.[16] All reactions were monitored by TLC on Merck precoated glass plates. Flash column chromatography and preparative TLC were carried out by using silica gel from Merck (230–400 mesh).
Physical Measurements 4,4-Didodecyl-cyclopenta[2,1-b:3,4-b’]dithiophene (CDTC12)
NMR spectra were measured in CDCl3 on a Varian Inova 400 MHz FT-NMR spectrometer and chemical shifts are quoted relative to tetramethylsilane (TMS) for 1H and 13C. IR spectra were recorded on the Nicolet Magna 550 Series II FTIR spectrometer as KBr pellets for solid-state spectroscopy. MALDI-TOF spectra were obtained by means of an Autoflex Bruker MALDI-TOF mass spectrometer. UV/ Vis spectroscopy was performed with a Hewlett Packard 8453
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Dodecyl bromide (4.7 g, 18.9 mmol) in DMSO (50 mL) and a catalytic amount of potassium iodide (50 mg) were added to a solution of CDT (1.63 g, 9.14 mmol) in DMSO (50 mL). The mixture was purged with argon for 10 min followed by the slow addition of solid KOH (2.0 g, 35.7 mmol). The dark green mixture was stirred in the dark under argon at room temperature for 72 h. The mixture
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Full Paper was then poured into deionized water (300 mL) and the organic phase was extracted with hexanes (5 100 mL). The organic phases were collected and washed with brine (300 mL), a saturated solution of ammonium chloride (100 mL), and deionized water (300 mL). The organic phase was dried over magnesium sulfate, filtered, and concentrated to give the crude product as a yellow oil. Purification by flash chromatography with hexanes and drying under high vacuum with agitation for 48 h gave a pure product as a colorless oil (3.8 g, 82 %). 1H NMR (500 MHz, CDCl3): d = 7.14 (d, J = 4.80 Hz, 2 H; Th), 6.92 (d, J = 4.75 Hz, 2 H; Th), 1.83–1.79 (m, 4 H; C12H25), 1.27–1.13 (m, 36 H; C12H25), 0.92–0.90 (m, 4 H; C12H25), 0.87 ppm (t, J = 6.45 Hz, 6 H; C12H25).
Compound 2 Yield 79 %; dark red solid; 1H NMR (400 MHz, CDCl3): d = 7.95 (s, 2 H; Ar), 7.90 (d, J = 7.52 Hz, 2 H; Ar), 7.85 (d, J = 8.76 Hz, 4 H; Ar), 7.69 (d, J = 7.60 Hz, 2 H; Ar), 7.16–7.07 (m, 22 H; Ar), 2.83 (t, J = 7.48 Hz, 4 H; C6H13), 2.34 (s, 12 H; CH3), 1.87–1.83 (m, 4 H; C12H25), 1.79–1.76 (m, 4 H; C6H13), 1.44–1.35 (m, 12 H; C6H13), 1.27–1.16 (m, 40 H; C12H25), 0.91 (t, J = 7.00 Hz, 6 H; C6H13), 0.86 ppm (t, J = 7.04 Hz, 6 H; C12H25); 13C NMR (100 MHz, CDCl3): d = 158.8, 153.9, 152.8, 148.8, 148.6, 144.9, 139.7, 138.3, 133.2, 132.8, 130.0, 129.8, 129.6, 128.4, 126.9, 126.1, 125.9, 125.2, 124.9, 123.5, 121.5, 119.2 (Ar), 91.5, 88.0 (CC), 54.1 (quat C), 37.8, 31.9, 31.7, 30.3, 30.1, 29.9, 29.7, 29.6, 29.44, 29.35, 29.0, 24.6, 22.69, 22.66, 20.9, 14.2, 14.1 ppm (C6H13 + C12H25 + CH3); HRMS (MALDI-TOF): m/z calcd for C109H120N6S6 [M] + : 1705.7924; found: 1705.7956; elemental analysis calcd (%) for C109H120N6S6 : C 76.71, H 7.09, N 4.92; found: C 76.80, H 7.22, N 5.05.
CDTC12-I Compound CDTC12 (0.59 g, 1.16 mmol) was dissolved in a mixture of THF/HOAc (4:1, 20 mL). NIS (0.54 g, 1.20 mmol) was added in small portions to the solution at 0 8C. Then the reaction mixture was gradually warmed to room temperature and stirred overnight in the dark. Completion of the reaction was determined by TLC analysis, and the volatile compounds were removed under reduced pressure. The residue was purified by column chromatography on silica gel eluted with hexane to give CDTC12-I as a pale yellow solid (0.76 g, 88 %). 1H NMR (400 MHz, CDCl3): d = 7.08 (s, 2 H; Th), 1.76– 1.72 (m, 4 H; C12H25), 1.34–1.16 (m, 36 H; C12H25), 0.89–0.85 ppm (m, 10 H; C12H25).
Compound 3 Yield 71 %; dark red solid; 1H NMR (400 MHz, CDCl3): d = 7.92 (s, 2 H; Ar), 7.86 (d, J = 7.52 Hz, 2 H; Ar), 7.83 (d, J = 8.80 Hz, 4 H; Ar), 7.65 (d, J = 7.56 Hz, 2 H; Ar), 7.39 (d, J = 8.72 Hz, 4 H; Ar), 7.15–7.04 (m, 28 H; Ar), 2.82 (t, J = 7.52 Hz, 4 H; C6H13), 2.35 (s, 3 H; CH3), 2.33 (s, 12 H; CH3), 1.77–1.74 (m, 4 H; C6H13), 1.45–1.33 (m, 12 H; C6H13), 0.89 ppm (t, J = 7.12 Hz, 6 H; C6H13); 13C NMR (100 MHz, CDCl3): d = 154.0, 152.8, 148.5, 148.4, 147.4, 144.9, 144.0, 139.0, 134.4, 133.1, 132.7, 132.4, 130.3, 130.0, 129.8, 129.7, 128.4, 126.9, 126.0, 125.8, 125.2, 125.0, 123.0, 121.6, 119.9, 116.8 (Ar), 97.2, 82.3 (CC), 31.7, 30.3, 29.9, 29.1, 22.7, 21.0, 20.9, 14.2 ppm (C6H13 + CH3); HRMS (MALDI-TOF): m/z calcd for C95H83N7S4 [M] + : 1450.5618; found: 1450.5641; elemental analysis calcd (%) for C95H83N7S4 : C 78.64, H 5.77, N 6.76; found: C 78.76, H 5.85, N 6.88.
DPTA-I By using a similar method to that reported for CDTC12-I, compound DPTA-I was obtained as a pale yellow solid (85 %). 1H NMR (400 MHz, CDCl3): d = 7.58 (d, J = 8.76 Hz, 4 H; Ph), 7.08 (d, J = 8.24 Hz, 2 H; Ph), 6.96 (d, J = 8.20 Hz, 2 H; Ph), 6.79 (d, J = 8.64 Hz, 4 H; Ph), 1.55 ppm (s, 3 H; CH3).
Compound 4
Under a nitrogen atmosphere, a catalytic amount of Pd(OAc)2 (8 %), PPh3 (24 %), and CuI (8 mol %) were added to a solution of each of the aromatic diiodide compounds in a mixture of triethylamine and CH2Cl2 (1:1, v/v). After the solution was stirred at RT for 30 min, each corresponding ethynyl ligand (L1 or L2; 2.06 equiv) was added to the reaction solution. Then the reaction mixture was stirred overnight at RT. Completion of the reaction was determined by TLC. The volatile compounds were removed under reduced pressure. The residue was purified by column chromatography on silica gel with an appropriate ratio of CH2Cl2/hexane as the eluent to obtain pure compounds 1–4 in moderate yields of 57–79 %.
Yield 64 %; dark red solid; 1H NMR (400 MHz, CDCl3): d = 8.13 (d, J = 3.88 Hz, 2 H; Ar), 7.95 (s, 2 H; Ar), 7.86 (s, 4 H; Ar), 7.53 (d, J = 8.24 Hz, 8 H; Ar), 7.31 (d, J = 3.88 Hz, 2 H; Ar), 7.10 (d, J = 8.32 Hz, 8 H; Ar), 7.04 (d, J = 8.16 Hz, 12 H; Ar), 2.85 (t, J = 7.76 Hz, 4 H; C6H13), 2.34 (s, 12 H; CH3), 1.80–1.76 (m, 4 H; C6H13), 1.46–1.25 (m, 12 H; C6H13), 0.91 ppm (t, J = 7.00 Hz, 6 H; C6H13); 13C NMR (100 MHz, CDCl3): d = 152.5, 152.4, 149.1, 148.0, 146.0, 144.9, 139.6, 137.3, 133.0, 131.1, 130.0, 128.9, 128.4, 127.0, 126.5, 126.2, 125.6, 124.9, 124.8, 124.7, 123.0, 122.1, 119.4 (Ar), 97.1, 84.9 (CC), 31.7, 30.3, 29.7, 29.1, 22.7, 20.9, 14.2 ppm (C6H13 + CH3); HRMS (MALDI-TOF): m/z calcd for C90H76N6S6 [M] + : 1433.4480; found: 1433.4470; elemental analysis calcd (%) for C90H76N6S6 : C 75.38, H 5.34, N 5.86; found: C 75.32, H 5.52, N 5.65.
Compound 1
Solar Cell Fabrication and Characterization
General Procedure for the Synthesis of Compounds 1–4
1
Yield 57 %; red solid; H NMR (400 MHz, CDCl3): d = 7.94 (s, 2 H; Ar), 7.89 (d, J = 7.52 Hz, 2 H; Ar), 7.83 (d, J = 8.76 Hz, 4 H; Ar), 7.67 (d, J = 7.56 Hz, 2 H; Ar), 7.51 (s, 4 H; Ar), 7.15–7.07 (m, 20 H; Ar), 2.85 (t, J = 7.56 Hz, 4 H; C6H13), 2.34 (s, 12 H; CH3), 1.79–1.72 (m, 4 H; C6H13), 1.47–1.35 (m, 12 H; C6H13), 0.91 ppm (t, J = 7.04 Hz, 6 H; C6H13); 13 C NMR (100 MHz, CDCl3): d = 153.9, 152.7, 149.1, 148.6, 144.9, 139.7, 133.1, 132.9, 131.1, 130.0, 129.8, 129.5, 128.3, 126.8, 125.9, 125.2, 124.8, 123.0, 121.5, 119.2 (Ar), 96.9, 84.9 (CC), 31.7, 30.3, 29.9, 29.0, 22.7, 20.9, 14.2 ppm (C6H13 + CH3); HRMS (MALDI-TOF): m/z calcd for C82H72N6S4 [M] + : 1268.4696; found: 1268.4702; elemental analysis calcd (%) for C82H72N6S4 : C 77.56, H 5.72, N 6.62; found: C 77.68, H 5.98, N 6.80. Chem. Asian J. 2015, 00, 0 – 0
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The device structure was ITO/PEDOT:PSS/1–4:PC71BM (1:4 or 1:3, w/w)/LiF or Ca/Al (LiF for 1 and 4; Ca for 2 and 3). ITO glass substrates (10 W per square) were cleaned by sonication in toluene, acetone, ethanol, and deionized water, dried in an oven, and then cleaned with UV ozone for 300 s. PEDOT:PSS (Baytron P AI 4083) was spin-coated onto the precleaned ITO substrate to form a 40 nm thick layer, followed by drying at 120 8C for 30 min in air. Then, the substrates were transferred into a glove box filled with nitrogen. The prepared solution of 1–4/PC71BM in chlorobenzene was spin-coated on top of the PEDOT:PSS layer. Finally, the samples were transferred into an evaporator in which 1 nm LiF or 25 nm Ca and 100 nm Al were thermally deposited under vacuum at
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Full Paper 10 6 torr. The effective area was 0.12 cm2. The devices were encapsulated in the glove box and measured in air. Current–voltage characteristics of solar cells were measured by using a Keithley 236 source meter under AM 1.5G illumination at 100 mW cm 2 from a solar simulator (Oriel, 91160A-1000). The EQE was measured at a chopping frequency of 275 Hz with a lock-in amplifier (Stanford, SR830) during illumination with the monochromatic light from a xenon lamp. The AFM measurements were performed on an SPA300HV instrument with an SPI3800 controller (Seiko Instruments). The images were recorded in tapping mode.
[3]
[4]
Acknowledgements W.-Y.W. thanks the National Natural Science Foundation of China (project number 91333206), the Science, Technology and Innovation Committee of Shenzhen Municipality (JCYJ20120829154440583), Hong Kong Baptist University (FRG2/12-13/083 and FRG1/13-14/053), and Hong Kong Research Grants Council (HKBU203011) for financial support. The work described herein was partially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (project no. T23-713/11). The work was also supported by the Partner State Key Laboratory of Environmental and Biological Analysis (SKLP-14-15-P011) and Strategic Development Fund of HKBU. The project was also supported by Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. H.L. thanks the 111 Project and Beijing Engineering Research Center of Food Environment and Public Health from the Minzu University of China (nos. B08044 and 10301-01404026) for financial support.
[5] [6]
[7] [8] [9]
[10]
Keywords: conjugation · donor–acceptor systems electrochemistry · organic solar cells · photochemistry
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Received: November 5, 2014 Published online on && &&, 0000
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Full Paper
FULL PAPER Photochemistry Hongmei Zhan, Qian Liu, Fengrong Dai, Cheuk-Lam Ho,* Yingying Fu, Leijiao Li, Li Zhao, Hua Li, Zhiyuan Xie,* Wai-Yeung Wong* Bridge repairs: A series of bis(arylene ethynylene)s with tunable electron-rich bridging groups were used to prepare solution-processed bulk heterojunction
Chem. Asian J. 2015, 00, 0 – 0
organic solar cells, which achieved the best power conversion efficiency of 3.28 % with a higher Voc of 0.94 V (see figure).
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These are not the final page numbers! ÞÞ
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&& – && Organic Donor Materials Based on Bis(arylene ethynylene)s for Bulk Heterojunction Organic Solar Cells with High Voc Values
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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