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Tuning morphology and photovoltaic properties of diketopyrrolopyrrole-based small-molecule solar cells by taloring end-capped aromatic groups† Shanlin Zhang,a Xue Wang,ab Ailing Tang,a Jianhua Huang,a Chuanlang Zhan*a and Jiannian Yao*a In this article, we selected BDT–DPP–BDT (DPP = diketopyrrolopyrrole and BDT = 4,8-di-2-(2-ethylhexyl)thienyl-benzo[1,2-b:4,5-b 0 ]dithiophene) as the model backbone and end-capped it with hydrogen, octyl 2-cyano-3-(thiophen-2-yl)acrylate (CNR), and 2-hexylbithiophene (HTT), respectively, forming three small molecule donors: BDB, CNRBDB and HTTBDB. Introduction of a polar and planar electronwithdrawing unit of CNR to both ends of the BDB backbone enhances the hole mobility from 4.14 104 to 7.75 103 cm2 V1 s1 and raises the fill factor from 27 to 57% when blended with PC71BM. This is associated with the PC71BM phase size decreasing from 70 to 20 nm. When the electrondonating unit of HTT with poorer planarity is linked to both ends of the BDB backbone, both donor and
Received 28th October 2013, Accepted 5th December 2013
acceptor phase sizes are decreased to 20 nm. The short-circuit current density is greatly improved from
DOI: 10.1039/c3cp54548b
4.22 to 9.66 mA cm2, and the fill factor is enhanced to 46%. Overall, this work demonstrates that the
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end-capped aromatic groups play an important role in tuning the phase size and photovoltaic properties of DPP-based small molecule solar cells.
1. Introduction In recent years, intensive research has been focused on solution-processed bulk-heterojunction polymer solar cells (BHJ PSCs), because of their advantages of low cost, lightweight, flexibility, and potential to afford green energy.1–6 Owing to the great efforts devoted to the material design and device optimization, impressive power conversion efficiency (PCE) has recently reported been in the literature.2,7–11 Compared to PSCs, solution-processed small molecule solar cells (SMSCs) have increasingly captured the attention of scientists, benefiting from their well-defined molecular structure, high purity, and reproducible device performance.12–19 To date, only a few backbones affording comparable PCE to their polymer counterparts have been reported, such as oligothiophene derivatives with electron-withdrawing end groups20–22 and dithienosilole (DTS) derivatives with bow-shaped architecture.23–25 a
Beijing National Laboratory of Molecular Science, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing, P. R. China. E-mail: [email protected], [email protected]; Fax: +86-10-82616517 b Key Laboratory of Life-Organic Analysis, QuFu Normal University, QuFu, 273165, P. R. China † Electronic supplementary information (ESI) available: Synthetic details, measurements and instruments, fabrication and characterization of photovoltaic devices, SCLC curves, optical data, 1H NMR, 13C NMR. See DOI: 10.1039/ c3cp54548b
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Understanding the structure–property relationship is urgently required for the rational design of efficient small molecule donors. As known, the molecular configuration of the donor material plays an important role in the photovoltaic properties of SMSC devices because the backbone controls the material’s band gap, energy levels and light harvesting ability.13 To date, a series of donor–acceptor (D–A) typed backbones such as linear-, bow-, star- and X-shaped have been reported13,20,23,26 by covalently combining the electron-donating and electronwithdrawing groups such as benzo[1,2-b:4,5-b 0 ]dithiophene (BDT),21,27 DTS,23,24 triphenylamine (TPA),28 diketopyrrolopyrrole (DPP),27,29 and cyanoacetate.21,30 Alternatively to the backbone engineering, the alkyl-chain engineering can greatly manipulate the solubility, crystallization, intra- and intermolecular forces, and in turn the film-morphology of the blend films, which can effectively improve the photovoltaic efficiency without altering the molecular optoelectronic properties. Recent reports have shown the important influence of alkyl-chain’s length, size, position, branching point, symmetry, chirality and density on the device performance.30a,31 Besides the backbone and alkyl-chain design, modulation of the molecular backbone by end-capping it with electrondonating or -withdrawing aromatic groups is important not only for improving the molecular optoelectronic properties but also to control aggregation of the materials.13,25,32 Detailed knowledge of how the end-capped electron-donating or -withdrawing aromatic
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on the photovoltaic properties, we modified the BDT–DPP–BDT backbone by end-capping it with octyl 2-cyano-3-(thiophen-2-yl)acrylate (CNR)20,30 as an electron-withdrawing unit or 2-hexylbithiophene (HTT)23,24 as an electron-donating unit, forming another two donors, namely CNRBDB and HTTBDT, as shown in Scheme 1. Our results demonstrated that the fill factor and short-circuit current density are both greatly influenced by the end-capped aromatic units.
2. Results and discussion Scheme 1
Chemical structures of BDB, CNRBDB and HTTBDB.
unit translates to material optoelectronic properties is thus helpful for the rational design of the donor materials. Recently, solution-processed DPP-based small molecules have gained considerable attention, because of their efficient light harvesting and excellent hole transproting ability, and easy chemical modifications.13,33 The DPP-based small molecule donors armed by two electron-donating units have been widely studied. The efficiency reported in the literatures, however, distributed in a wide range value of 0.7–4.7% when blended with [6,6]-phenyl-C61 (C71)-butyric acid methyl ester, PC61BM (PC71BM).29c,e,33 To understand the relationship between molecular structure and device performance, we armed the DPP unit using 4,8-di-2-(2-ethylhexyl)-thienyl-benzo[1,2-b:4,5-b 0 ]dithiophene (BDT), forming the modelling BDT–DPP–BDT backbone (Scheme 1). As an electron donating unit, BDT has been flourishing in the past few years for its symmetric and planar conjugated structure, which facilitates the p-electron delocalization when incorporated with the electron-drawing groups such as the DPP unit.27,29c To investigate the effect of the end-capped electron-donating or electron-withdrawing groups on the aggregation and further
2.1.
Synthesis
The synthetic routes to the three molecules are shown in Scheme 2. Compounds 2 and 4 were synthesized according to the literature methods.34 Compound 3 was obtained by the Stille coupling reaction through two steps in a yield of 65 and 55%, respectively. Compound 6 was obtained by Stille coupling reaction as orange oil in 55% yield. Symmetrical coupling with 2 and 7, 4 and 7, or 6 and 7 yielded BDB, HTTBDB, and 8, respectively. We subjected 8 to Knoevenagel condensation, generating CNRBDB.20 All three molecules are fully characterized using H-NMR, C-NMR, TOF-mass and elemental analysis. They are soluble in commonly used organic solvents such as chloroform, toluene and ortho-dichlorobenzene (o-DCB). The good solubility of the synthesized molecules provides goodquality films via the solution-processed method for photovoltaic applications. 2.2.
Theoretical calculations
We investigated the backbone conformations of BDB, CNRBDB and HTTBDB through the density functional theory (DFT) at the B3LYP/6-31G level. Fig. 1 displays their backbone conformations, including the dihedral angles of y1 (between BDT and DPP units), y2 (between BDT and thienyl units) and y3
Scheme 2 Synthetic routes of BDB, CNRBDB and HTTBDB. Reaction conditions: (i) n-BuLi, 78 1C, 1 h; then (CH)3SnCl, rt, 2 h. (ii) Pd(PPh3)4, toluene, 110 1C, 24 h. (iii) Pd(PPh3)4, toluene, 110 1C, 5 h. (iv) CHCl3, piperidine, rt, 36 h. EH = 2-ethylhexyl.
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and polar CNR units may provide stronger intermolecular interactions, whereas introducing the more poorly planar HTT unit may decrease the aggregation ability because of its bigger dihedral angles y2 and y3. These different conformations of the backbone will affect the packing order and film morphology. 2.3.
Fig. 1 Optimal molecular conformations of BDB (a, b), CNRBDB (c, d), and HTTBDB (e, f). For clarity, the alkyl chains are not shown.
(between thienyl and ethenyl units or between thienyl and hexylthienyl units). The y1 for these three molecules are almost identical to each other (10.4, 9.9 and 9.31, respectively), which indicates that the introduction of CNR or HTT units has little influence on the planarity of the BDT–DPP–BDT backbone. However, the dihedral angles of y2 and y3 are very different: for CNRBDB, y2 = 2.21 and y3 = 0.51, while for HTTBDB, y2 = 10.91 and y3 = 11.71. This suggests that end-capping the planar
Fig. 2
Thermal stability
The thermal stability is investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). As shown in Fig. 2a, the decomposition temperature (Td) (5% weight loss) was observed to be 415.1, 390.3, and 420.2 1C for BDB, CNRBDB and HTTBDB, respectively, indicating that all compounds are thermally stable enough for further characterization and device fabrication. Fig. 2b shows their DSC results, in which the melting temperatures (Tm) of BDB, CNRBDB and HTTBDB are 242.5, 315.4 and 238.2 1C, respectively, and their respective crystallizing temperatures (Tc) are of 205.8, 292.6 and 222.9 1C. The crystallization enthalpies are 10.36, 6.24 and 3.22 J g1, respectively, for BDB, CNRTBDB and HTTBDB. Compared to BDB and HTTBDB, the higher Tm and Tc of CNRBDB, in which two electron-accepting units were endcapped, implying a higher degree of crystallization in the solid state, ascribed to its stronger intermolecular interactions induced by the polar and planar CNR groups. Its crystallization enthalpy is lower than the mother backbone of BDB, likely due to the effects from the n-octyl chains. Unlike CNRBDB and BDB, HTTBDB shows a weak and broad crystallization peak with the lowest enthalpy, indicating its decreased aggregation ability, which results from its bigger dihedral angles of y2 and y3. In addition, CNRBDB displays double Tm (305.3 and 315.4 1C) and Tc (292.6 and 259.0 1C), which suggests that there exist two kinds of crystallization states.35 2.4.
Optical properties
Fig. 3 shows the absorption spectra of the three compounds in chloroform solutions (all with c = 1 106 M) and in solid-state films. The relevant optical data are summarized in Table 1. As shown in Fig. 3a, all three molecules display an absorption band emerging at 500–750 nm, which is ascribed to the
TGA (a) and DSC curves (b) of BDB, CNRBDB and HTTBDB obtained at a heating rate of 10 1C min1 under nitrogen atmosphere.
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Fig. 3 UV-vis absorption spectra in the chloroform solutions, all with a concentration of 1 106 M (a), and in the solid-state films (b) of BDB, CNRBDB and HTTBDB.
Table 1
Optoelectronic properties of BDB, CNRBDB and HTTBDB
Compounds
a lsol max (nm)
emaxb (104 M1 cm1)
c lfilm max (nm)
d lfilm edge (nm)
e Eopt (eV) g
Ered (V)
Eox (V)
LUMO (eV)
HOMO (eV)
f Ecv (eV) g
BDB CNRBDB HTTBDB
608, 650 624, 672 623, 668
9.8 9.4 9.5
632, 698 669, 743 656, 728
787 811 800
1.58 1.53 1.55
0.80 0.73 0.82
0.76 0.77 0.73
3.60 3.67 3.58
5.16 5.17 5.13
1.56 1.50 1.55
a The absorption maximum in solution. b Molar extinction coefficient at lmax in solution. c The absorption maximum in film. d The onset of the f film absorption edge. e Optical band gap estimated from the formula of 1240/lfilm edge. Electrochemical band gap estimated from the cyclic voltammetry data.
intramolecular charge transfer (ICT) transition between the acceptor unit (DPP) and donor unit (CNR- or HTT-capped BDT).34 All three molecules exhibit an impressive light-harvesting ability in solution with a maximum molar extinction coefficient (emax) of about 9.5 104 M1 cm1. With respect to BDB, the maximum absorption (lmax) is red-shifted, which is ascribed to the introduction of end-capped conjugated units (CNR or HTT) at the BDB backbone expanding the conjugate length. For BDB, there exist two bands localizing around 350 and 375 nm. When introducing end-capped aromatic groups (CNR or HTT), the 375 nm band is red-shifted to 440 nm for CNRBDB and 419 nm for HTTBDB. Fig. 3b shows the normalized absorption spectra
of the three molecules in solid-state films. In comparison to absorption in the solution, the absorption of the corresponding film is broadened and markedly red-shifted, which is caused by intermolecular p–p stacking. Compared to the corresponding solution spectrum, the absorption edge extends to 787, 811, and 800 nm in the solid-state film, respectively, for BDB, CNRBDB and HTTBDB. The optical bandgap calculated from the film absorption edge is 1.58, 1.53 and 1.55 eV, respectively. 2.5.
Electrochemical properties
Fig. 4a shows a cyclic voltammogram (CV) of the three molecules with Ag/AgCl as reference electrode. The empirical
Fig. 4 Cyclic voltammograms of BDB, CNRBDB and HTTBDB in CHCl3/0.1 M Bu4NBF6 at 100 mV s1 (a) and energy levels of different components in a photovoltaic device (b).
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equation of EHOMO/LUMO = (Eonset(ox/red) + 4.40) eV was applied to estimate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, using the onset of the oxidation/reduction potentials.36 The electrochemical parameters are summarized in Fig. 4b and Table 1. The HOMO energy for BDB, CNRBDB and HTTBDB is 5.16, 5.17 and 5.13 eV, respectively, and the LUMO energy is 3.60, 3.67 and 3.58 eV, respectively. With respect to the mother backbone of BDB, introducing the electronwithdrawing CNR unit onto the electron-donating BDT slightly decreases the LUMO energy (0.07 eV), while introducing the electron-donating HTT group leads to a tiny raise of the LUMO energy, which is in reasonable agreement with the energy levels using DFT calculations, as shown in Table S1 (ESI†). The electrochemical bandgaps (ECV g ) for BDB, CNRBDB and HTTBDB are ca. 1.56, 1.50, and 1.55 eV, respectively, consistent with the optical bandgaps. The HOMO and LUMO energy levels of each compound match well with that of the acceptor material of PC71BM,37 as shown in Fig. 4b, implying that all of them can be applied for the photovoltaic devices. 2.6.
Photovoltaic properties
In order to evaluate the effects of end-capped units on the photovoltaic performances, solar cell devices were fabricated using BDB, CNRBDB or HTTBDB as the donor material and PC71BM as the acceptor material with the traditional architecture of ITO/PEDOT:PSS/blends/Ca/Al. Solar cells were tested under AM 1.5 G illumination with 100 mW cm2 and the active area of the devices was 6 mm2. The blends were prepared from o-DCB solutions with a total concentration of 40 mg ml1. After optimizing the weight ratio of the donor and acceptor (Table S2–S4, ESI†), a donor/PC71BM weight ratio of 1 : 1 showed the best device performance. At the 1 : 1 weight ratio without any additives, the BDB/PC71BM based best device gave an opencircuit voltage (Voc) of 0.78 V, a short circuit current ( Jsc) of 4.22 mA cm2, a fill factor (FF) of 27%, resulting in a PCE of 0.91%. The CNRBDB/PC71BM based best devices showed Voc of 0.75 V, Jsc of 1.64 mA cm2, FF of 52%, and a PCE of 0.64%; the best HTTBDB/ PC71BM device showed a PCE of 1.65% with a Voc of 0.70 V, a Jsc of 6.22 mA cm2 and an FF of 38% (Table S3 and S4, ESI†). Then 1, 8-diiodooctane (DIO) was used as a solvent additive to improve the device performance. For BDB/PC71BM, the addition of DIO did not improve the performance of the devices (Table S2, ESI†). However, for CNRBDB/PC71BM and HTTBDB/PC71BM based devices, using DIO as the solvent additive plays an important role in optimizing the device performance. The best device results are summarized in Table 2. Current density–voltage ( J–V)
Table 2 Photovoltaic performances based on the three molecules blended with PC71BM (D/A ratio = 1 : 1)
DIO Voc Donor molecule (v/v) (V) BDB CNRBDB HTTBDB
Jsc FF PCE (avg) Hole mobility (mA cm2) (%) (%) (cm2 V1 s1)
0 0.78 4.22 0.2% 0.78 3.44 0.2% 0.64 9.66
27 57 46
0.91 (0.86) 4.14 104 1.52 (1.50) 7.75 103 2.85 (2.81) 1.19 103
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Fig. 5
J–V characteristics (a) and EQE spectra of the best devices (b).
curves are shown in Fig. 5a. The CNRBDB/PC71BM based best device with 0.2% DIO (v/v) displayed an increased Jsc from 1.64 to 3.44 mA cm2 and an improved FF from 52 to 57%, resulting in an improved PCE from 0.64 to 1.52%. The HTTBDB/ PC71BM based best device, also with 0.2% DIO as an additive, showed an increased Jsc from 6.22 to 9.66 mA cm2 and an improved FF from 38 to 46%, giving an improved PCE of 2.85%. Comparing with the best devices from the three molecules, introducing the electron-withdrawing CNR groups leads to an increase of the FF (57% vs. 27%), while end-capping the electron-donating units of HTT results in an enhancement of the Jsc (9.66 vs. 4.22 mA cm2) and also the FF (46% vs. 27%). We note that compared to the electron-pulling unit of CNR, the electron-donating unit of HTT gives a higher Jsc (9.66 vs. 3.44 mA cm2). As for Voc, BDB and CNRBDB showed similar values of 0.78 V, while HTTBDB showed a lower value of 0.64 V. As known, Voc can be influenced by several factors, such as the energy level of the intermolecular charge transfer (CT) state mediated by the interactions of HOMO of donor and LUMO of acceptor, the nature of the D/A nanostructure interfaces determining the volume density of the CT states and the coupling between the CT state and the ground state,14,38 and the metal– organic interfacial contacts.32b In view of the similar HOMO
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energy for all three molecules and the same device structure, the different Voc suggests different interfacial structures and/or energy of the CT state in HTTBDB–PC71BM blend films as compared to another two systems. In short, the end-capped units obviously influence the device performance. The external quantum efficiency (EQE) curves of the optimized devices are shown in Fig. 5b. All three curves exhibit a broad response from 350 to 800 nm and are consistent with their respective UV-vis absorption profile of the blended films (Fig. S1, ESI†), indicating the absorbed photons contribute to the photon-to-electron conversion. The maximum monochromatic EQE are 19.1, 21.2, and 42.8% for BDB, CNRBDB, and HTTBDB, respectively (Fig. 5b). It is obvious that HTTBDB shows the highest value (42.8%), giving rise to the highest Jsc of 9.66 mA cm2, which indicates that the photon response is efficient in HTTBDB based devices. The monochromatic EQE of CNRBDB in the range of 575–742 nm is lower than that of BDB, implying weaker light harvest, leading to the lowest Jsc of 3.44 mA cm2. 2.7.
Hole mobility
The hole mobilities of the BDB, CNRBDB and HTTBDB blends all with PC71BM were tested using the space-charge limited current (SCLC) model and with the device configuration of ITO/ PEDOT:PSS/active layer/Au.39 The organic layers of the SCLC devices were prepared under the same conditions as that for constructing the best solar cell devices. Fig. S2 (ESI†) shows the
corresponding data. The hole mobilities of BDB, CNRBDB and HTTBDB were estimated as 4.14 104, 7.75 103 and 1.19 103 cm2 V1 s1, respectively, indicating that introduction of the end-capped units enhances hole mobility. Although CNRBDB shows a higher hole mobility than HTTBDB, as indicated from the film-morphology studies (vide post), CNRBDB yields a much larger donor phase size than HTTBDB (50 nm vs. 20 nm). Combination of the hole mobility and filmmorphology contributes to different photovoltaic properties. 2.8.
Film-morphology
The film-morphology of the best devices was studied by transmission electron microscopy (TEM). As shown in the TEM images (Fig. 6a–c), the bright regions can be attributed to the donors’ domains, while the dark regions can be attributed to the PC71BM domains due to its high electron scattering density.40 From the blend film of BDB–PC71BM, ball-like dark domains with an average diameter of about 70 nm were observed. The excessive phase separation is consistent with the low FF (27%) and a small Jsc (4.22 mA cm2). Unlike the BDB–PC71BM blend film, the CNRBDB–PC71BM system displayed a ribbon pattern with a size of about 50 nm and a decreased dark area with an average diameter of about 20 nm, agreeing with the increase of FF from 27 to 57%. Compared to the CNRBDB–PC71BM system, the HTTBDB–PC71BM blend film showed a typical fibrillar structure, and both the donor and acceptor phase sizes were decreased to 20 nm, which is close to
Fig. 6 TEM (a, b, and c) and AFM phase (d, e, and f) images of the BDB/PC71BM (a and d), CNRBDB/PC71BM (b and e) and HTTBDB/PC71BM (c and f) based best solar cell devices, respectively.
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the exciton diffusion length (typically, 10 nm), contributing to enhancing the exciton diffusion and separation efficiency.41 Therefore, HTTBDB/PC71BM based devices showed the highest Jsc (9.66 mA cm2) and also an improved FF (46%). The morphology study in this work indicates that introducing end-capped aromatic groups (CNR and HTT units) to the BDB backbone is helpful for improved compatibility between donors and PC71BM, which results in control of the film morphology. Atomic force microscopy (AFM) was used to confirm the TEM results. Fig. 6d–f and Fig. S3 (ESI†) showed the AFM phase and height images collected from the best device films. As shown in Fig. S3 (ESI†), the root mean square roughness (RMS) of the blend films was 3.69 nm (BDB–PC71BM), 0.61 nm (CNRBDB–PC71BM), 0.18 nm (HTTBDB–PC71BM), respectively, indicating that HTTBDB and CNRBDB yielded more evenly distributed morphological features than BDB. For the BDB– PC71BM blend film, sphere-like shapes with an average diameter of about 50–100 nm were observed in the dark regions (Fig. 6d), consistent with the ball-like dark domains observed from the TEM image. Compared to the BDB–PC71BM system, the CNRBDB–PC71BM blend film displayed smaller roughness with RMS of 0.61 nm and decreased dark regions with an average diameter of about 20–30 nm (Fig. 6e). HTTBDB– PC71BM blend film showed a very uniform fibrillar structure and exhibited continuous phase separation with a size of 10–20 nm (Fig. 6f), which is well consistent with the TEM pattern discussed above. Overall, the combination of absorption spectra (Fig. 3), hole mobility and film-morphology (Fig. 6) suggests that CNRBDB exhibits broader optical absorption, lower band gap and higher hole mobility than HTTBDB, it however, shows a much larger size of donor phase, likely due to its stronger aggregation ability, as indicated from DSC data (Fig. 2b). Taking all these factors into accounts, film-morphology, e.g. the donor phase size contributes the efficiency difference between the CNRBDB and HTTBDB systems.
3. Conclusions Three small molecule donors of BDB, CNRTDBD and HTTBDD were synthesized by end-capping the electron-withdrawing CNR units or the electron-donating HTT units onto the model BDB backbone. The device performances are finely tuned by the endcapped aromatic groups (CNR and HTT units). The SMSC device made from BDB/PC71BM with a donor–acceptor ratio of 1 : 1 gives rise to a low Jsc of 4.22 mA cm2 and a small FF of 27%, which is associated with excessive phase separation. When introducing the polar and planar units of CNR to both ends of the BDB backbone, the resulted CNRBDB exhibits an increase of hole mobility from 4.14 104 to 7.75 103 cm2 V1 s1 and an obvious enhancement of FF to 57%, which is associated with the PC71BM phase size decreasing from 70 to 20 nm. When end-capping the electron-donating units (HTT) with poorer planarity, HTTBDB exhibits better compatibility between PC71BM, and as a result, both the donor
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and acceptor phase sizes are decreased to 20 nm, yielding an enhanced Jsc and FF, to 9.66 mA cm2 and 46%, respectively. Overall, this work demonstrates that end-capping the electronwithdrawing or -donating unit selectively enhances the fill factor and the short-circuit current density or both, and thus enriches the train of design ideas about efficient small molecule donors.
Acknowledgements This work was financially supported by NSFC (No. 21327805, 91227112 and 21221002), the Chinese Academy of Sciences, Project 973 (2011CB808400).
References 1 O. Morton, Nature, 2006, 443, 19. 2 Z. He, C. Zhong, X. Huang, W.-Y. Wong, H. Wu, L. Chen, S. Su and Y. Cao, Adv. Mater., 2011, 23, 4636. 3 Y. J. Cheng, S. H. Yang and C. S. Hsu, Chem. Rev., 2009, 109, 5868. ¨llen, 4 C. Li, M. Liu, N. G. Pschirer, M. Baumgarten and K. Mu Chem. Rev., 2010, 110, 6817. 5 L. J. A. Koster, S. E. Shaheen and J. C. Hummelen, Adv. Eng. Mater., 2012, 2, 1246. ¨nes, H. Neugebauer and N. S. Sariciftci, Chem. Rev., 6 S. Gu 2007, 107, 1324. 7 H. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu and G. Li, Nat. Photonics, 2009, 3, 649. 8 L. T. Dou, J. B. You, J. Yang, C. C. Chen, Y. J. He, S. Murase, T. Moriarty, K. Emery, G. Li and Y. Yang, Nat. Photonics, 2012, 6, 153. 9 W. Li, A. Furlan, K. H. Hendriks, M. M. Wienk and R. A. J. Janssen, J. Am. Chem. Soc., 2013, 135, 5529. 10 Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray and L. Yu, Adv. Mater., 2010, 22, E135. 11 C. E. Small, S. Chen, J. Subbiah, C. M. Amb, S.-W. Tsang, T.-H. Lai, J. R. Reynolds and F. So, Nat. Photonics, 2012, 6, 115. 12 M. Singh, R. Kurchania, J. A. Mikroyannidis, S. S. Sharma and G. D. Sharma, J. Mater. Chem. A, 2013, 1, 2297. 13 Y. Lin, Y. Li and X. Zhan, Chem. Soc. Rev., 2012, 41, 4245. ¨uerle, Angew. Chem., Int. Ed., 2012, 14 A. Mishra and P. Ba 51, 2020. 15 S. Qu and H. Tian, Chem. Commun., 2012, 48, 3039. 16 J. E. Anthony, Chem. Mater., 2011, 23, 583. 17 Y. Li, Q. Guo, Z. Li, J. Pei and W. Tian, Energy Environ. Sci., 2010, 3, 1427. 18 J. Roncali, Acc. Chem. Res., 2009, 42, 1719. ¨rthner and K. Meerholz, Chem.–Eur. J., 2010, 16, 9366. 19 F. Wu 20 J. Zhou, X. Wan, Y. Liu, Y. Zuo, Z. Li, G. He, G. Long, W. Ni, C. Li, X. Su and Y. Chen, J. Am. Chem. Soc., 2012, 134, 16345. 21 J. Zhou, Y. Zuo, X. Wan, G. Long, Q. Zhang, W. Ni, Y. Liu, Z. Li, G. He, C. Li, B. Kan, M. Li and Y. Chen, J. Am. Chem. Soc., 2013, 135, 8484.
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22 R. Fitzner, E. Mena-Osteritz, A. Mishra, G. Schulz, ¨rner, H. Ziehlke, C. Elschner, E. Reinold, M. Weil, C. Ko ¨uerle, K. Leo, M. Riede, M. Pfeiffer, C. Uhrich and P. Ba J. Am. Chem. Soc., 2012, 134, 11064. 23 Y. M. Sun, G. C. Welch, W. L. Leong, C. J. Takacs, G. C. Bazan and A. J. Heeger, Nat. Mater., 2012, 11, 44. 24 T. S. Van der Poll, J. A. Love, T.-Q. Nguyen and G. C. Bazan, Adv. Mater., 2012, 24, 3646. 25 J. E. Coughlin, Z. B. Henson, G. C. Welch and G. C. Bazan, Acc. Chem. Res., 2014, 47, 257. ¨se, W. J. Mitchell, N. Kopidakis, C. H. Chang, 26 (a) M. E. Ko S. E. Shaheen, K. Kim and G. Rumbles, J. Am. Chem. Soc., 2007, 129, 14257; (b) Z. Lu, C. Li, T. Fang, G. Li and Z. Bo, J. Mater. Chem. A, 2013, 1, 7657. 27 (a) J. Huang, C. Zhan, X. Zhang, Y. Zhao, Z. Lu, H. Jia, B. Jiang, J. Ye, S. Zhang, A. Tang, Y. Liu, Q. Pei and J. Yao, ACS Appl. Mater. Interfaces, 2013, 5, 2033; (b) S. Shen, P. Jiang, C. He, J. Zhang, P. Shen, Y. Zhang, Y. Yi, Z. Zhang, Z. Li and Y. Li, Chem. Mater., 2013, 25, 2274. 28 (a) H. Shang, H. Fan, Y. Liu, W. Hu, Y. Li and X. Zhan, Adv. Mater., 2011, 23, 1554; (b) S. Paek, N. Cho, S. Cho, J. K. Lee and J. Ko, Org. Lett., 2012, 14, 6326; (c) W. L. Rance, B. L. Rupert, W. J. Mitchell, M. E. Kose, D. S. Ginley, S. E. Shaheen, G. Rumbles and N. Kopidakis, J. Phys. Chem. C, 2010, 114, 22269; (d) A. Tang, L. Li, Z. Lu, J. Huang, H. Jia, C. Zhan, Z. Tan, Y. Li and J. Yao, J. Mater. Chem. A, 2013, 1, 5747. 29 (a) S. Loser, C. J. Bruns, H. Miyauchi, R. P. Ortiz, A. Facchetti, S. I. Stupp and T. J. Marks, J. Am. Chem. Soc., 2011, 133, 8142; (b) K. A. Mazzio, M. Yuan, K. Okamoto and C. K. Luscombe, ACS Appl. Mater. Interfaces, 2011, 3, 271; (c) O. P. Lee, A. T. Yiu, P. M. Beaujuge, C. H. Woo, T. W. Holcombe, J. E. Millstone, ´chet, Adv. Mater., 2011, J. D. Douglas, M. S. Chen and J. M. J. Fre 23, 5359; (d) E. Ripaud, D. Demeter, T. Rousseau, E. Boucard´tol, M. Allain, R. Po, P. Leriche and J. Roncali, Dyes Pigm., Ce 2012, 95, 126; (e) J. Huang, H. Jia, L. Li, Z. Lu, W. Zhang, W. He, B. Jiang, A. Tang, Z. Tan, C. Zhan, Y. Li and J. Yao, Phys. Chem. Chem. Phys., 2012, 14, 14238. 30 (a) Y. Liu, X. Wan, F. Wang, J. Zhou, G. Long, J. Tian, J. You, Y. Yang and Y. Chen, Adv. Energy Mater., 2011, 1, 771; (b) D. Ye, X. Li, L. Yan, W. Zhang, Z. Hu, Y. Liang, J. Fang, W.-Y. Wong and X. Wang, J. Mater. Chem. A, 2013, 1, 7622; (c) D. Patra, C.-C. Chiang, W.-A. Chen, K.-H. Wei, M.-C. Wu and C.-W. Chu, J. Mater. Chem. A, 2013, 1, 7767.
This journal is © the Owner Societies 2014
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31 (a) J. W. Jung, J. W. Jo, F. Liu, T. P. Russell and W. H. Jo, Chem. Commun., 2012, 48, 6933; (b) T.-Y. Chu, J. Lu, ´, Y. Zhang, J.-R. Pouliot, J. Zhou, A. Najari, S. Beaupre M. Leclerc and Y. Tao, Adv. Funct. Mater., 2012, 22, 2345; (c) I. Osaka, T. Kakara, N. Takemura, T. Koganezawa and K. Takimiya, J. Am. Chem. Soc., 2013, 135, 8834; (d) B. Wang, J. Zhang, H. L. Tam, B. Wu, W. Zhang, M. S. Chan, F. Pan, G. Yu, F. Zhu and M. S. Wong, Polym. Chem., 2014, 5, 836. 32 (a) L. Dou, J. You, Z. Hong, Z. Xu, G. Li, R. A. Street and Y. Yang, Adv. Mater., 2013, 25, 6642; (b) Y. Li, Acc. Chem. Res., 2012, 45, 723; (c) S. D. Dimitrov and J. R. Durrant, Chem. Mater., 2014, 26, 616. 33 H. Wang, F. Liu, L. Bu, J. Gao, C. Wang, W. Wei and T. P. Russell, Adv. Mater., 2013, 25, 6519. 34 S. Zhang, B. Jiang, C. Zhan, J. Huang, X. Zhang, H. Jia, A. Tang, L. Chen and J. Yao, Chem.–Asian J., 2013, 8, 2407. 35 (a) E. Perez, E. Rinde, A. Bello, R. Benavente and J. M. Perena, Macromolecules, 1992, 25, 605; (b) Z. Bashir, N. Khan and D. M. Price, Thermochim. Acta, 1998, 319, 47. 36 (a) S. Janietz, D. D. C. Bradley, M. Grell, C. Giebeler, M. Inbasekaran and E. P. Woo, Appl. Phys. Lett., 1998, 73, 2453; (b) Y. F. Li, Y. Cao, J. Gao, D. L. Wang, G. Yu and A. J. Heeger, Synth. Met., 1999, 99, 243; (c) J. Huang, Y. Zhao, X. Ding, H. Jia, B. Jiang, Z. Zhang, C. Zhan, S. He, Q. Pei, Y. Li, Y. Liu and J. Yao, Polym. Chem., 2012, 3, 2170. 37 Y. He, H. Chen, J. Hou and Y. Li, J. Am. Chem. Soc., 2010, 132, 5532. 38 (a) X. Zhang, Z. Lu, L. Ye, C. Zhan, J. Hou, S. Zhang, B. Jiang, Y. Zhao, J. Huang, S. Zhang, Y. Liu, Q. Shi, Y. Liu and J. Yao, Adv. Mater., 2013, 25, 5797; (b) T. W. Holocombe, J. E. Norton, J. Rivnay, C. H. Woo, L. Goris, C. Piliego, G. Griffini, ´das, A. Salleo and J. M. J. Fre ´chet, J. Am. A. Sellinger, J.-L. Bre Chem. Soc., 2011, 133, 12106; (c) D. Veldman, S. C. J. Meskers and R. A. J. Janssen, Adv. Mater., 2009, 19, 1939. 39 (a) J. Huang, Y. Zhao, W. He, H. Jia, Z. Lu, B. Jiang, C. Zhan, Q. Pei, Y. Liu and J. Yao, Polym. Chem., 2012, 3, 2832; (b) G. G. Malliaras, J. R. Salem, P. J. Brock and C. Scott, Phys. Rev. B: Condens. Matter, 1998, 58, 13411. 40 X. N. Yang, J. Loos, S. C. Veenstra, W. J. H. Verhees, M. M. Wienk, J. M. Kroon, M. A. J. Michels and R. A. J. Janssen, Nano Lett., 2005, 5, 579. 41 T. M. Clarke and J. R. Durrant, Chem. Rev., 2010, 110, 6736.
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Tuning morphology and photovoltaic properties of diketopyrrolopyrrole-based small-molecule solar cells by taloring end-capped aromatic groups† Shanlin Zhang,a Xue Wang,ab Ailing Tang,a Jianhua Huang,a Chuanlang Zhan*a and Jiannian Yao*a In this article, we selected BDT–DPP–BDT (DPP = diketopyrrolopyrrole and BDT = 4,8-di-2-(2-ethylhexyl)thienyl-benzo[1,2-b:4,5-b 0 ]dithiophene) as the model backbone and end-capped it with hydrogen, octyl 2-cyano-3-(thiophen-2-yl)acrylate (CNR), and 2-hexylbithiophene (HTT), respectively, forming three small molecule donors: BDB, CNRBDB and HTTBDB. Introduction of a polar and planar electronwithdrawing unit of CNR to both ends of the BDB backbone enhances the hole mobility from 4.14 104 to 7.75 103 cm2 V1 s1 and raises the fill factor from 27 to 57% when blended with PC71BM. This is associated with the PC71BM phase size decreasing from 70 to 20 nm. When the electrondonating unit of HTT with poorer planarity is linked to both ends of the BDB backbone, both donor and
Received 28th October 2013, Accepted 5th December 2013
acceptor phase sizes are decreased to 20 nm. The short-circuit current density is greatly improved from
DOI: 10.1039/c3cp54548b
4.22 to 9.66 mA cm2, and the fill factor is enhanced to 46%. Overall, this work demonstrates that the
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end-capped aromatic groups play an important role in tuning the phase size and photovoltaic properties of DPP-based small molecule solar cells.
1. Introduction In recent years, intensive research has been focused on solution-processed bulk-heterojunction polymer solar cells (BHJ PSCs), because of their advantages of low cost, lightweight, flexibility, and potential to afford green energy.1–6 Owing to the great efforts devoted to the material design and device optimization, impressive power conversion efficiency (PCE) has recently reported been in the literature.2,7–11 Compared to PSCs, solution-processed small molecule solar cells (SMSCs) have increasingly captured the attention of scientists, benefiting from their well-defined molecular structure, high purity, and reproducible device performance.12–19 To date, only a few backbones affording comparable PCE to their polymer counterparts have been reported, such as oligothiophene derivatives with electron-withdrawing end groups20–22 and dithienosilole (DTS) derivatives with bow-shaped architecture.23–25 a
Beijing National Laboratory of Molecular Science, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing, P. R. China. E-mail: [email protected], [email protected]; Fax: +86-10-82616517 b Key Laboratory of Life-Organic Analysis, QuFu Normal University, QuFu, 273165, P. R. China † Electronic supplementary information (ESI) available: Synthetic details, measurements and instruments, fabrication and characterization of photovoltaic devices, SCLC curves, optical data, 1H NMR, 13C NMR. See DOI: 10.1039/ c3cp54548b
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Understanding the structure–property relationship is urgently required for the rational design of efficient small molecule donors. As known, the molecular configuration of the donor material plays an important role in the photovoltaic properties of SMSC devices because the backbone controls the material’s band gap, energy levels and light harvesting ability.13 To date, a series of donor–acceptor (D–A) typed backbones such as linear-, bow-, star- and X-shaped have been reported13,20,23,26 by covalently combining the electron-donating and electronwithdrawing groups such as benzo[1,2-b:4,5-b 0 ]dithiophene (BDT),21,27 DTS,23,24 triphenylamine (TPA),28 diketopyrrolopyrrole (DPP),27,29 and cyanoacetate.21,30 Alternatively to the backbone engineering, the alkyl-chain engineering can greatly manipulate the solubility, crystallization, intra- and intermolecular forces, and in turn the film-morphology of the blend films, which can effectively improve the photovoltaic efficiency without altering the molecular optoelectronic properties. Recent reports have shown the important influence of alkyl-chain’s length, size, position, branching point, symmetry, chirality and density on the device performance.30a,31 Besides the backbone and alkyl-chain design, modulation of the molecular backbone by end-capping it with electrondonating or -withdrawing aromatic groups is important not only for improving the molecular optoelectronic properties but also to control aggregation of the materials.13,25,32 Detailed knowledge of how the end-capped electron-donating or -withdrawing aromatic
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on the photovoltaic properties, we modified the BDT–DPP–BDT backbone by end-capping it with octyl 2-cyano-3-(thiophen-2-yl)acrylate (CNR)20,30 as an electron-withdrawing unit or 2-hexylbithiophene (HTT)23,24 as an electron-donating unit, forming another two donors, namely CNRBDB and HTTBDT, as shown in Scheme 1. Our results demonstrated that the fill factor and short-circuit current density are both greatly influenced by the end-capped aromatic units.
2. Results and discussion Scheme 1
Chemical structures of BDB, CNRBDB and HTTBDB.
unit translates to material optoelectronic properties is thus helpful for the rational design of the donor materials. Recently, solution-processed DPP-based small molecules have gained considerable attention, because of their efficient light harvesting and excellent hole transproting ability, and easy chemical modifications.13,33 The DPP-based small molecule donors armed by two electron-donating units have been widely studied. The efficiency reported in the literatures, however, distributed in a wide range value of 0.7–4.7% when blended with [6,6]-phenyl-C61 (C71)-butyric acid methyl ester, PC61BM (PC71BM).29c,e,33 To understand the relationship between molecular structure and device performance, we armed the DPP unit using 4,8-di-2-(2-ethylhexyl)-thienyl-benzo[1,2-b:4,5-b 0 ]dithiophene (BDT), forming the modelling BDT–DPP–BDT backbone (Scheme 1). As an electron donating unit, BDT has been flourishing in the past few years for its symmetric and planar conjugated structure, which facilitates the p-electron delocalization when incorporated with the electron-drawing groups such as the DPP unit.27,29c To investigate the effect of the end-capped electron-donating or electron-withdrawing groups on the aggregation and further
2.1.
Synthesis
The synthetic routes to the three molecules are shown in Scheme 2. Compounds 2 and 4 were synthesized according to the literature methods.34 Compound 3 was obtained by the Stille coupling reaction through two steps in a yield of 65 and 55%, respectively. Compound 6 was obtained by Stille coupling reaction as orange oil in 55% yield. Symmetrical coupling with 2 and 7, 4 and 7, or 6 and 7 yielded BDB, HTTBDB, and 8, respectively. We subjected 8 to Knoevenagel condensation, generating CNRBDB.20 All three molecules are fully characterized using H-NMR, C-NMR, TOF-mass and elemental analysis. They are soluble in commonly used organic solvents such as chloroform, toluene and ortho-dichlorobenzene (o-DCB). The good solubility of the synthesized molecules provides goodquality films via the solution-processed method for photovoltaic applications. 2.2.
Theoretical calculations
We investigated the backbone conformations of BDB, CNRBDB and HTTBDB through the density functional theory (DFT) at the B3LYP/6-31G level. Fig. 1 displays their backbone conformations, including the dihedral angles of y1 (between BDT and DPP units), y2 (between BDT and thienyl units) and y3
Scheme 2 Synthetic routes of BDB, CNRBDB and HTTBDB. Reaction conditions: (i) n-BuLi, 78 1C, 1 h; then (CH)3SnCl, rt, 2 h. (ii) Pd(PPh3)4, toluene, 110 1C, 24 h. (iii) Pd(PPh3)4, toluene, 110 1C, 5 h. (iv) CHCl3, piperidine, rt, 36 h. EH = 2-ethylhexyl.
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and polar CNR units may provide stronger intermolecular interactions, whereas introducing the more poorly planar HTT unit may decrease the aggregation ability because of its bigger dihedral angles y2 and y3. These different conformations of the backbone will affect the packing order and film morphology. 2.3.
Fig. 1 Optimal molecular conformations of BDB (a, b), CNRBDB (c, d), and HTTBDB (e, f). For clarity, the alkyl chains are not shown.
(between thienyl and ethenyl units or between thienyl and hexylthienyl units). The y1 for these three molecules are almost identical to each other (10.4, 9.9 and 9.31, respectively), which indicates that the introduction of CNR or HTT units has little influence on the planarity of the BDT–DPP–BDT backbone. However, the dihedral angles of y2 and y3 are very different: for CNRBDB, y2 = 2.21 and y3 = 0.51, while for HTTBDB, y2 = 10.91 and y3 = 11.71. This suggests that end-capping the planar
Fig. 2
Thermal stability
The thermal stability is investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). As shown in Fig. 2a, the decomposition temperature (Td) (5% weight loss) was observed to be 415.1, 390.3, and 420.2 1C for BDB, CNRBDB and HTTBDB, respectively, indicating that all compounds are thermally stable enough for further characterization and device fabrication. Fig. 2b shows their DSC results, in which the melting temperatures (Tm) of BDB, CNRBDB and HTTBDB are 242.5, 315.4 and 238.2 1C, respectively, and their respective crystallizing temperatures (Tc) are of 205.8, 292.6 and 222.9 1C. The crystallization enthalpies are 10.36, 6.24 and 3.22 J g1, respectively, for BDB, CNRTBDB and HTTBDB. Compared to BDB and HTTBDB, the higher Tm and Tc of CNRBDB, in which two electron-accepting units were endcapped, implying a higher degree of crystallization in the solid state, ascribed to its stronger intermolecular interactions induced by the polar and planar CNR groups. Its crystallization enthalpy is lower than the mother backbone of BDB, likely due to the effects from the n-octyl chains. Unlike CNRBDB and BDB, HTTBDB shows a weak and broad crystallization peak with the lowest enthalpy, indicating its decreased aggregation ability, which results from its bigger dihedral angles of y2 and y3. In addition, CNRBDB displays double Tm (305.3 and 315.4 1C) and Tc (292.6 and 259.0 1C), which suggests that there exist two kinds of crystallization states.35 2.4.
Optical properties
Fig. 3 shows the absorption spectra of the three compounds in chloroform solutions (all with c = 1 106 M) and in solid-state films. The relevant optical data are summarized in Table 1. As shown in Fig. 3a, all three molecules display an absorption band emerging at 500–750 nm, which is ascribed to the
TGA (a) and DSC curves (b) of BDB, CNRBDB and HTTBDB obtained at a heating rate of 10 1C min1 under nitrogen atmosphere.
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Fig. 3 UV-vis absorption spectra in the chloroform solutions, all with a concentration of 1 106 M (a), and in the solid-state films (b) of BDB, CNRBDB and HTTBDB.
Table 1
Optoelectronic properties of BDB, CNRBDB and HTTBDB
Compounds
a lsol max (nm)
emaxb (104 M1 cm1)
c lfilm max (nm)
d lfilm edge (nm)
e Eopt (eV) g
Ered (V)
Eox (V)
LUMO (eV)
HOMO (eV)
f Ecv (eV) g
BDB CNRBDB HTTBDB
608, 650 624, 672 623, 668
9.8 9.4 9.5
632, 698 669, 743 656, 728
787 811 800
1.58 1.53 1.55
0.80 0.73 0.82
0.76 0.77 0.73
3.60 3.67 3.58
5.16 5.17 5.13
1.56 1.50 1.55
a The absorption maximum in solution. b Molar extinction coefficient at lmax in solution. c The absorption maximum in film. d The onset of the f film absorption edge. e Optical band gap estimated from the formula of 1240/lfilm edge. Electrochemical band gap estimated from the cyclic voltammetry data.
intramolecular charge transfer (ICT) transition between the acceptor unit (DPP) and donor unit (CNR- or HTT-capped BDT).34 All three molecules exhibit an impressive light-harvesting ability in solution with a maximum molar extinction coefficient (emax) of about 9.5 104 M1 cm1. With respect to BDB, the maximum absorption (lmax) is red-shifted, which is ascribed to the introduction of end-capped conjugated units (CNR or HTT) at the BDB backbone expanding the conjugate length. For BDB, there exist two bands localizing around 350 and 375 nm. When introducing end-capped aromatic groups (CNR or HTT), the 375 nm band is red-shifted to 440 nm for CNRBDB and 419 nm for HTTBDB. Fig. 3b shows the normalized absorption spectra
of the three molecules in solid-state films. In comparison to absorption in the solution, the absorption of the corresponding film is broadened and markedly red-shifted, which is caused by intermolecular p–p stacking. Compared to the corresponding solution spectrum, the absorption edge extends to 787, 811, and 800 nm in the solid-state film, respectively, for BDB, CNRBDB and HTTBDB. The optical bandgap calculated from the film absorption edge is 1.58, 1.53 and 1.55 eV, respectively. 2.5.
Electrochemical properties
Fig. 4a shows a cyclic voltammogram (CV) of the three molecules with Ag/AgCl as reference electrode. The empirical
Fig. 4 Cyclic voltammograms of BDB, CNRBDB and HTTBDB in CHCl3/0.1 M Bu4NBF6 at 100 mV s1 (a) and energy levels of different components in a photovoltaic device (b).
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equation of EHOMO/LUMO = (Eonset(ox/red) + 4.40) eV was applied to estimate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, using the onset of the oxidation/reduction potentials.36 The electrochemical parameters are summarized in Fig. 4b and Table 1. The HOMO energy for BDB, CNRBDB and HTTBDB is 5.16, 5.17 and 5.13 eV, respectively, and the LUMO energy is 3.60, 3.67 and 3.58 eV, respectively. With respect to the mother backbone of BDB, introducing the electronwithdrawing CNR unit onto the electron-donating BDT slightly decreases the LUMO energy (0.07 eV), while introducing the electron-donating HTT group leads to a tiny raise of the LUMO energy, which is in reasonable agreement with the energy levels using DFT calculations, as shown in Table S1 (ESI†). The electrochemical bandgaps (ECV g ) for BDB, CNRBDB and HTTBDB are ca. 1.56, 1.50, and 1.55 eV, respectively, consistent with the optical bandgaps. The HOMO and LUMO energy levels of each compound match well with that of the acceptor material of PC71BM,37 as shown in Fig. 4b, implying that all of them can be applied for the photovoltaic devices. 2.6.
Photovoltaic properties
In order to evaluate the effects of end-capped units on the photovoltaic performances, solar cell devices were fabricated using BDB, CNRBDB or HTTBDB as the donor material and PC71BM as the acceptor material with the traditional architecture of ITO/PEDOT:PSS/blends/Ca/Al. Solar cells were tested under AM 1.5 G illumination with 100 mW cm2 and the active area of the devices was 6 mm2. The blends were prepared from o-DCB solutions with a total concentration of 40 mg ml1. After optimizing the weight ratio of the donor and acceptor (Table S2–S4, ESI†), a donor/PC71BM weight ratio of 1 : 1 showed the best device performance. At the 1 : 1 weight ratio without any additives, the BDB/PC71BM based best device gave an opencircuit voltage (Voc) of 0.78 V, a short circuit current ( Jsc) of 4.22 mA cm2, a fill factor (FF) of 27%, resulting in a PCE of 0.91%. The CNRBDB/PC71BM based best devices showed Voc of 0.75 V, Jsc of 1.64 mA cm2, FF of 52%, and a PCE of 0.64%; the best HTTBDB/ PC71BM device showed a PCE of 1.65% with a Voc of 0.70 V, a Jsc of 6.22 mA cm2 and an FF of 38% (Table S3 and S4, ESI†). Then 1, 8-diiodooctane (DIO) was used as a solvent additive to improve the device performance. For BDB/PC71BM, the addition of DIO did not improve the performance of the devices (Table S2, ESI†). However, for CNRBDB/PC71BM and HTTBDB/PC71BM based devices, using DIO as the solvent additive plays an important role in optimizing the device performance. The best device results are summarized in Table 2. Current density–voltage ( J–V)
Table 2 Photovoltaic performances based on the three molecules blended with PC71BM (D/A ratio = 1 : 1)
DIO Voc Donor molecule (v/v) (V) BDB CNRBDB HTTBDB
Jsc FF PCE (avg) Hole mobility (mA cm2) (%) (%) (cm2 V1 s1)
0 0.78 4.22 0.2% 0.78 3.44 0.2% 0.64 9.66
27 57 46
0.91 (0.86) 4.14 104 1.52 (1.50) 7.75 103 2.85 (2.81) 1.19 103
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Fig. 5
J–V characteristics (a) and EQE spectra of the best devices (b).
curves are shown in Fig. 5a. The CNRBDB/PC71BM based best device with 0.2% DIO (v/v) displayed an increased Jsc from 1.64 to 3.44 mA cm2 and an improved FF from 52 to 57%, resulting in an improved PCE from 0.64 to 1.52%. The HTTBDB/ PC71BM based best device, also with 0.2% DIO as an additive, showed an increased Jsc from 6.22 to 9.66 mA cm2 and an improved FF from 38 to 46%, giving an improved PCE of 2.85%. Comparing with the best devices from the three molecules, introducing the electron-withdrawing CNR groups leads to an increase of the FF (57% vs. 27%), while end-capping the electron-donating units of HTT results in an enhancement of the Jsc (9.66 vs. 4.22 mA cm2) and also the FF (46% vs. 27%). We note that compared to the electron-pulling unit of CNR, the electron-donating unit of HTT gives a higher Jsc (9.66 vs. 3.44 mA cm2). As for Voc, BDB and CNRBDB showed similar values of 0.78 V, while HTTBDB showed a lower value of 0.64 V. As known, Voc can be influenced by several factors, such as the energy level of the intermolecular charge transfer (CT) state mediated by the interactions of HOMO of donor and LUMO of acceptor, the nature of the D/A nanostructure interfaces determining the volume density of the CT states and the coupling between the CT state and the ground state,14,38 and the metal– organic interfacial contacts.32b In view of the similar HOMO
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energy for all three molecules and the same device structure, the different Voc suggests different interfacial structures and/or energy of the CT state in HTTBDB–PC71BM blend films as compared to another two systems. In short, the end-capped units obviously influence the device performance. The external quantum efficiency (EQE) curves of the optimized devices are shown in Fig. 5b. All three curves exhibit a broad response from 350 to 800 nm and are consistent with their respective UV-vis absorption profile of the blended films (Fig. S1, ESI†), indicating the absorbed photons contribute to the photon-to-electron conversion. The maximum monochromatic EQE are 19.1, 21.2, and 42.8% for BDB, CNRBDB, and HTTBDB, respectively (Fig. 5b). It is obvious that HTTBDB shows the highest value (42.8%), giving rise to the highest Jsc of 9.66 mA cm2, which indicates that the photon response is efficient in HTTBDB based devices. The monochromatic EQE of CNRBDB in the range of 575–742 nm is lower than that of BDB, implying weaker light harvest, leading to the lowest Jsc of 3.44 mA cm2. 2.7.
Hole mobility
The hole mobilities of the BDB, CNRBDB and HTTBDB blends all with PC71BM were tested using the space-charge limited current (SCLC) model and with the device configuration of ITO/ PEDOT:PSS/active layer/Au.39 The organic layers of the SCLC devices were prepared under the same conditions as that for constructing the best solar cell devices. Fig. S2 (ESI†) shows the
corresponding data. The hole mobilities of BDB, CNRBDB and HTTBDB were estimated as 4.14 104, 7.75 103 and 1.19 103 cm2 V1 s1, respectively, indicating that introduction of the end-capped units enhances hole mobility. Although CNRBDB shows a higher hole mobility than HTTBDB, as indicated from the film-morphology studies (vide post), CNRBDB yields a much larger donor phase size than HTTBDB (50 nm vs. 20 nm). Combination of the hole mobility and filmmorphology contributes to different photovoltaic properties. 2.8.
Film-morphology
The film-morphology of the best devices was studied by transmission electron microscopy (TEM). As shown in the TEM images (Fig. 6a–c), the bright regions can be attributed to the donors’ domains, while the dark regions can be attributed to the PC71BM domains due to its high electron scattering density.40 From the blend film of BDB–PC71BM, ball-like dark domains with an average diameter of about 70 nm were observed. The excessive phase separation is consistent with the low FF (27%) and a small Jsc (4.22 mA cm2). Unlike the BDB–PC71BM blend film, the CNRBDB–PC71BM system displayed a ribbon pattern with a size of about 50 nm and a decreased dark area with an average diameter of about 20 nm, agreeing with the increase of FF from 27 to 57%. Compared to the CNRBDB–PC71BM system, the HTTBDB–PC71BM blend film showed a typical fibrillar structure, and both the donor and acceptor phase sizes were decreased to 20 nm, which is close to
Fig. 6 TEM (a, b, and c) and AFM phase (d, e, and f) images of the BDB/PC71BM (a and d), CNRBDB/PC71BM (b and e) and HTTBDB/PC71BM (c and f) based best solar cell devices, respectively.
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the exciton diffusion length (typically, 10 nm), contributing to enhancing the exciton diffusion and separation efficiency.41 Therefore, HTTBDB/PC71BM based devices showed the highest Jsc (9.66 mA cm2) and also an improved FF (46%). The morphology study in this work indicates that introducing end-capped aromatic groups (CNR and HTT units) to the BDB backbone is helpful for improved compatibility between donors and PC71BM, which results in control of the film morphology. Atomic force microscopy (AFM) was used to confirm the TEM results. Fig. 6d–f and Fig. S3 (ESI†) showed the AFM phase and height images collected from the best device films. As shown in Fig. S3 (ESI†), the root mean square roughness (RMS) of the blend films was 3.69 nm (BDB–PC71BM), 0.61 nm (CNRBDB–PC71BM), 0.18 nm (HTTBDB–PC71BM), respectively, indicating that HTTBDB and CNRBDB yielded more evenly distributed morphological features than BDB. For the BDB– PC71BM blend film, sphere-like shapes with an average diameter of about 50–100 nm were observed in the dark regions (Fig. 6d), consistent with the ball-like dark domains observed from the TEM image. Compared to the BDB–PC71BM system, the CNRBDB–PC71BM blend film displayed smaller roughness with RMS of 0.61 nm and decreased dark regions with an average diameter of about 20–30 nm (Fig. 6e). HTTBDB– PC71BM blend film showed a very uniform fibrillar structure and exhibited continuous phase separation with a size of 10–20 nm (Fig. 6f), which is well consistent with the TEM pattern discussed above. Overall, the combination of absorption spectra (Fig. 3), hole mobility and film-morphology (Fig. 6) suggests that CNRBDB exhibits broader optical absorption, lower band gap and higher hole mobility than HTTBDB, it however, shows a much larger size of donor phase, likely due to its stronger aggregation ability, as indicated from DSC data (Fig. 2b). Taking all these factors into accounts, film-morphology, e.g. the donor phase size contributes the efficiency difference between the CNRBDB and HTTBDB systems.
3. Conclusions Three small molecule donors of BDB, CNRTDBD and HTTBDD were synthesized by end-capping the electron-withdrawing CNR units or the electron-donating HTT units onto the model BDB backbone. The device performances are finely tuned by the endcapped aromatic groups (CNR and HTT units). The SMSC device made from BDB/PC71BM with a donor–acceptor ratio of 1 : 1 gives rise to a low Jsc of 4.22 mA cm2 and a small FF of 27%, which is associated with excessive phase separation. When introducing the polar and planar units of CNR to both ends of the BDB backbone, the resulted CNRBDB exhibits an increase of hole mobility from 4.14 104 to 7.75 103 cm2 V1 s1 and an obvious enhancement of FF to 57%, which is associated with the PC71BM phase size decreasing from 70 to 20 nm. When end-capping the electron-donating units (HTT) with poorer planarity, HTTBDB exhibits better compatibility between PC71BM, and as a result, both the donor
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and acceptor phase sizes are decreased to 20 nm, yielding an enhanced Jsc and FF, to 9.66 mA cm2 and 46%, respectively. Overall, this work demonstrates that end-capping the electronwithdrawing or -donating unit selectively enhances the fill factor and the short-circuit current density or both, and thus enriches the train of design ideas about efficient small molecule donors.
Acknowledgements This work was financially supported by NSFC (No. 21327805, 91227112 and 21221002), the Chinese Academy of Sciences, Project 973 (2011CB808400).
References 1 O. Morton, Nature, 2006, 443, 19. 2 Z. He, C. Zhong, X. Huang, W.-Y. Wong, H. Wu, L. Chen, S. Su and Y. Cao, Adv. Mater., 2011, 23, 4636. 3 Y. J. Cheng, S. H. Yang and C. S. Hsu, Chem. Rev., 2009, 109, 5868. ¨llen, 4 C. Li, M. Liu, N. G. Pschirer, M. Baumgarten and K. Mu Chem. Rev., 2010, 110, 6817. 5 L. J. A. Koster, S. E. Shaheen and J. C. Hummelen, Adv. Eng. Mater., 2012, 2, 1246. ¨nes, H. Neugebauer and N. S. Sariciftci, Chem. Rev., 6 S. Gu 2007, 107, 1324. 7 H. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu and G. Li, Nat. Photonics, 2009, 3, 649. 8 L. T. Dou, J. B. You, J. Yang, C. C. Chen, Y. J. He, S. Murase, T. Moriarty, K. Emery, G. Li and Y. Yang, Nat. Photonics, 2012, 6, 153. 9 W. Li, A. Furlan, K. H. Hendriks, M. M. Wienk and R. A. J. Janssen, J. Am. Chem. Soc., 2013, 135, 5529. 10 Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray and L. Yu, Adv. Mater., 2010, 22, E135. 11 C. E. Small, S. Chen, J. Subbiah, C. M. Amb, S.-W. Tsang, T.-H. Lai, J. R. Reynolds and F. So, Nat. Photonics, 2012, 6, 115. 12 M. Singh, R. Kurchania, J. A. Mikroyannidis, S. S. Sharma and G. D. Sharma, J. Mater. Chem. A, 2013, 1, 2297. 13 Y. Lin, Y. Li and X. Zhan, Chem. Soc. Rev., 2012, 41, 4245. ¨uerle, Angew. Chem., Int. Ed., 2012, 14 A. Mishra and P. Ba 51, 2020. 15 S. Qu and H. Tian, Chem. Commun., 2012, 48, 3039. 16 J. E. Anthony, Chem. Mater., 2011, 23, 583. 17 Y. Li, Q. Guo, Z. Li, J. Pei and W. Tian, Energy Environ. Sci., 2010, 3, 1427. 18 J. Roncali, Acc. Chem. Res., 2009, 42, 1719. ¨rthner and K. Meerholz, Chem.–Eur. J., 2010, 16, 9366. 19 F. Wu 20 J. Zhou, X. Wan, Y. Liu, Y. Zuo, Z. Li, G. He, G. Long, W. Ni, C. Li, X. Su and Y. Chen, J. Am. Chem. Soc., 2012, 134, 16345. 21 J. Zhou, Y. Zuo, X. Wan, G. Long, Q. Zhang, W. Ni, Y. Liu, Z. Li, G. He, C. Li, B. Kan, M. Li and Y. Chen, J. Am. Chem. Soc., 2013, 135, 8484.
This journal is © the Owner Societies 2014
View Article Online
Published on 05 December 2013. Downloaded by University of California - Santa Cruz on 24/10/2014 00:42:32.
Paper
22 R. Fitzner, E. Mena-Osteritz, A. Mishra, G. Schulz, ¨rner, H. Ziehlke, C. Elschner, E. Reinold, M. Weil, C. Ko ¨uerle, K. Leo, M. Riede, M. Pfeiffer, C. Uhrich and P. Ba J. Am. Chem. Soc., 2012, 134, 11064. 23 Y. M. Sun, G. C. Welch, W. L. Leong, C. J. Takacs, G. C. Bazan and A. J. Heeger, Nat. Mater., 2012, 11, 44. 24 T. S. Van der Poll, J. A. Love, T.-Q. Nguyen and G. C. Bazan, Adv. Mater., 2012, 24, 3646. 25 J. E. Coughlin, Z. B. Henson, G. C. Welch and G. C. Bazan, Acc. Chem. Res., 2014, 47, 257. ¨se, W. J. Mitchell, N. Kopidakis, C. H. Chang, 26 (a) M. E. Ko S. E. Shaheen, K. Kim and G. Rumbles, J. Am. Chem. Soc., 2007, 129, 14257; (b) Z. Lu, C. Li, T. Fang, G. Li and Z. Bo, J. Mater. Chem. A, 2013, 1, 7657. 27 (a) J. Huang, C. Zhan, X. Zhang, Y. Zhao, Z. Lu, H. Jia, B. Jiang, J. Ye, S. Zhang, A. Tang, Y. Liu, Q. Pei and J. Yao, ACS Appl. Mater. Interfaces, 2013, 5, 2033; (b) S. Shen, P. Jiang, C. He, J. Zhang, P. Shen, Y. Zhang, Y. Yi, Z. Zhang, Z. Li and Y. Li, Chem. Mater., 2013, 25, 2274. 28 (a) H. Shang, H. Fan, Y. Liu, W. Hu, Y. Li and X. Zhan, Adv. Mater., 2011, 23, 1554; (b) S. Paek, N. Cho, S. Cho, J. K. Lee and J. Ko, Org. Lett., 2012, 14, 6326; (c) W. L. Rance, B. L. Rupert, W. J. Mitchell, M. E. Kose, D. S. Ginley, S. E. Shaheen, G. Rumbles and N. Kopidakis, J. Phys. Chem. C, 2010, 114, 22269; (d) A. Tang, L. Li, Z. Lu, J. Huang, H. Jia, C. Zhan, Z. Tan, Y. Li and J. Yao, J. Mater. Chem. A, 2013, 1, 5747. 29 (a) S. Loser, C. J. Bruns, H. Miyauchi, R. P. Ortiz, A. Facchetti, S. I. Stupp and T. J. Marks, J. Am. Chem. Soc., 2011, 133, 8142; (b) K. A. Mazzio, M. Yuan, K. Okamoto and C. K. Luscombe, ACS Appl. Mater. Interfaces, 2011, 3, 271; (c) O. P. Lee, A. T. Yiu, P. M. Beaujuge, C. H. Woo, T. W. Holcombe, J. E. Millstone, ´chet, Adv. Mater., 2011, J. D. Douglas, M. S. Chen and J. M. J. Fre 23, 5359; (d) E. Ripaud, D. Demeter, T. Rousseau, E. Boucard´tol, M. Allain, R. Po, P. Leriche and J. Roncali, Dyes Pigm., Ce 2012, 95, 126; (e) J. Huang, H. Jia, L. Li, Z. Lu, W. Zhang, W. He, B. Jiang, A. Tang, Z. Tan, C. Zhan, Y. Li and J. Yao, Phys. Chem. Chem. Phys., 2012, 14, 14238. 30 (a) Y. Liu, X. Wan, F. Wang, J. Zhou, G. Long, J. Tian, J. You, Y. Yang and Y. Chen, Adv. Energy Mater., 2011, 1, 771; (b) D. Ye, X. Li, L. Yan, W. Zhang, Z. Hu, Y. Liang, J. Fang, W.-Y. Wong and X. Wang, J. Mater. Chem. A, 2013, 1, 7622; (c) D. Patra, C.-C. Chiang, W.-A. Chen, K.-H. Wei, M.-C. Wu and C.-W. Chu, J. Mater. Chem. A, 2013, 1, 7767.
This journal is © the Owner Societies 2014
PCCP
31 (a) J. W. Jung, J. W. Jo, F. Liu, T. P. Russell and W. H. Jo, Chem. Commun., 2012, 48, 6933; (b) T.-Y. Chu, J. Lu, ´, Y. Zhang, J.-R. Pouliot, J. Zhou, A. Najari, S. Beaupre M. Leclerc and Y. Tao, Adv. Funct. Mater., 2012, 22, 2345; (c) I. Osaka, T. Kakara, N. Takemura, T. Koganezawa and K. Takimiya, J. Am. Chem. Soc., 2013, 135, 8834; (d) B. Wang, J. Zhang, H. L. Tam, B. Wu, W. Zhang, M. S. Chan, F. Pan, G. Yu, F. Zhu and M. S. Wong, Polym. Chem., 2014, 5, 836. 32 (a) L. Dou, J. You, Z. Hong, Z. Xu, G. Li, R. A. Street and Y. Yang, Adv. Mater., 2013, 25, 6642; (b) Y. Li, Acc. Chem. Res., 2012, 45, 723; (c) S. D. Dimitrov and J. R. Durrant, Chem. Mater., 2014, 26, 616. 33 H. Wang, F. Liu, L. Bu, J. Gao, C. Wang, W. Wei and T. P. Russell, Adv. Mater., 2013, 25, 6519. 34 S. Zhang, B. Jiang, C. Zhan, J. Huang, X. Zhang, H. Jia, A. Tang, L. Chen and J. Yao, Chem.–Asian J., 2013, 8, 2407. 35 (a) E. Perez, E. Rinde, A. Bello, R. Benavente and J. M. Perena, Macromolecules, 1992, 25, 605; (b) Z. Bashir, N. Khan and D. M. Price, Thermochim. Acta, 1998, 319, 47. 36 (a) S. Janietz, D. D. C. Bradley, M. Grell, C. Giebeler, M. Inbasekaran and E. P. Woo, Appl. Phys. Lett., 1998, 73, 2453; (b) Y. F. Li, Y. Cao, J. Gao, D. L. Wang, G. Yu and A. J. Heeger, Synth. Met., 1999, 99, 243; (c) J. Huang, Y. Zhao, X. Ding, H. Jia, B. Jiang, Z. Zhang, C. Zhan, S. He, Q. Pei, Y. Li, Y. Liu and J. Yao, Polym. Chem., 2012, 3, 2170. 37 Y. He, H. Chen, J. Hou and Y. Li, J. Am. Chem. Soc., 2010, 132, 5532. 38 (a) X. Zhang, Z. Lu, L. Ye, C. Zhan, J. Hou, S. Zhang, B. Jiang, Y. Zhao, J. Huang, S. Zhang, Y. Liu, Q. Shi, Y. Liu and J. Yao, Adv. Mater., 2013, 25, 5797; (b) T. W. Holocombe, J. E. Norton, J. Rivnay, C. H. Woo, L. Goris, C. Piliego, G. Griffini, ´das, A. Salleo and J. M. J. Fre ´chet, J. Am. A. Sellinger, J.-L. Bre Chem. Soc., 2011, 133, 12106; (c) D. Veldman, S. C. J. Meskers and R. A. J. Janssen, Adv. Mater., 2009, 19, 1939. 39 (a) J. Huang, Y. Zhao, W. He, H. Jia, Z. Lu, B. Jiang, C. Zhan, Q. Pei, Y. Liu and J. Yao, Polym. Chem., 2012, 3, 2832; (b) G. G. Malliaras, J. R. Salem, P. J. Brock and C. Scott, Phys. Rev. B: Condens. Matter, 1998, 58, 13411. 40 X. N. Yang, J. Loos, S. C. Veenstra, W. J. H. Verhees, M. M. Wienk, J. M. Kroon, M. A. J. Michels and R. A. J. Janssen, Nano Lett., 2005, 5, 579. 41 T. M. Clarke and J. R. Durrant, Chem. Rev., 2010, 110, 6736.
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