Article pubs.acs.org/accounts
Oligomer Molecules for Efficient Organic Photovoltaics Yuze Lin†,‡ and Xiaowei Zhan*,† †
Department of Materials Science and Engineering, College of Engineering, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing 100871, China ‡ Department of Chemistry, Capital Normal University, Beijing 100048, China CONSPECTUS: Solar cells, a renewable, clean energy technology that efficiently converts sunlight into electricity, are a promising long-term solution for energy and environmental problems caused by a mass of production and the use of fossil fuels. Solution-processed organic solar cells (OSCs) have attracted much attention in the past few years because of several advantages, including easy fabrication, low cost, lightweight, and flexibility. Now, OSCs exhibit power conversion efficiencies (PCEs) of over 10%. In the early stage of OSCs, vapor-deposited organic dye materials were first used in bilayer heterojunction devices in the 1980s, and then, solution-processed polymers were introduced in bulk heterojunction (BHJ) devices. Relative to polymers, vapordeposited small molecules offer potential advantages, such as a defined molecular structure, definite molecular weight, easy purification, mass-scale production, and good batch-to-batch reproducibility. However, the limited solubility and high crystallinity of vapor-deposited small molecules are unfavorable for use in solution-processed BHJ OSCs. Conversely, polymers have good solution-processing and film-forming properties and are easily processed into flexible devices, whereas their polydispersity of molecular weights and difficulty in purification results in batch to batch variation, which may hamper performance reproducibility and commercialization. Oligomer molecules (OMs) are monodisperse big molecules with intermediate molecular weights (generally in the thousands), and their sizes are between those of small molecules (generally with molecular weights 10000). OMs not only overcome shortcomings of both vapor-deposited small molecules and solutionprocessed polymers, but also combine their advantages, such as defined molecular structure, definite molecular weight, easy purification, mass-scale production, good batch-to-batch reproducibility, good solution processability, and film-forming properties. Therefore, OMs are a good choice for solution-processed reproducible OSCs toward scalable commercialized applications. Considerable efforts have been dedicated to developing new OM electron donors and electron acceptors for OSCs. So far, the highest PCEs of solution-processed OSCs based on OM donors and acceptors are 9−10% and 6−7%, respectively. OM materials have become promising alternatives to polymer and/or fullerene materials for efficient and stable OSCs. In this Account, we present a brief survey of the recent developments in solution-processable OM electron donors and acceptors and their application in OSCs. Rational design of OMs with star- and linear-shaped structures based on triphenylamine, benzodithiophene, and indacenodithiophene units and their impacts on device performance are discussed. Structure−property relationships are also proposed. Furthermore, the remaining challenges and the key research directions in the near future are also addressed. In the next years, an interdisciplinary approach involving novel OM materials, especially electron acceptor materials, accurate morphology optimization, and advanced device technologies will probably bring high-efficiency and stable OSCs to final commercialization.
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INTRODUCTION
Recently, OSCs have exhibited power conversion efficiencies (PCEs) of >10% in single4−6 and tandem devices.7 In the early stages, vapor-deposited organic dye materials were first used in bilayer heterojunction OSCs by Tang in the 1980s,8 and then, solution-processed polymers were introduced in bulk heterojunction (BHJ) OSCs by Heeger2 and Friend9 and co-workers. Relative to their polymer counterparts, vapordeposited small molecules offer potential advantages, such as defined molecular structure, definite molecular weight, easy purification, mass-scale production, and good batch-to-batch
Nowadays, the production and use of fossil fuels has given rise to a mass of environmental problems, furthermore, their stores are diminishing; thus, the need to develop renewable energy sources is urgent. The solar cell, which transforms inexhaustible solar energy into electricity, has been considered one of the green and effective technologies to address energy and environmental issues. Solution-processed organic solar cells (OSCs)1 with heterojunction structures2 have attracted much attention in past years because to certain inherent advantages, including easy fabrication, simple device structure, low cost, lightweight, and capability to be fabricated into flexible devices.3 © 2015 American Chemical Society
Received: August 4, 2015 Published: November 5, 2015 175
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Figure 1. Chemical structures of star-shaped photovoltaic OMs.
reproducibility.10 However, the limited solubility and high crystallinity of vapor-deposited small molecules are unfavorable for use in solution-processed BHJ OSCs. Conversely, polymers have good solution-processing and film-forming properties and are easily processed into flexible devices, whereas their polydispersity of molecular weights and difficulty in purification results in batch to batch variation, which may hamper performance reproducibility and commercialization.1,11 According to the IUPAC definition, an oligomer molecule (OM) is a molecule of intermediate relative molecular mass, the structure of which essentially comprises a small plurality of units derived, actually or conceptually, from molecules of lower relative molecular mass. Oligomer molecules (OMs) are monodisperse big molecules with relatively large molecular weights (generally thousands), and their sizes are between those of small molecules (generally with molecular weights 10000). OMs not only overcome shortcomings of both vapor-deposited small molecules and solution-processed polymers, but also combine their advantages, such as a defined molecular structure, definite molecular weight, easy purification, mass-scale production, good batch-to-batch reproducibility, good solution processability, and film-forming properties. Therefore, OMs are a promising choice for solution-processed reproducible OSCs toward scalable commercialized applications. Recently, great efforts have been dedicated to developing
new OMs for applications in solar cells as active layer materials, including electron donors12,13 and electron acceptors.14 So far, the highest PCEs of solution-processed OSCs based on OM donors and acceptors are up to 9−10%15 and 6.8%,16 respectively. In this Account, we present a brief survey of the recent developments in solution-processable OM electron donors and acceptors, and their application in OSCs. Rational design of OMs with star- and linear-shaped structures based on triphenylamine (TPA), benzodithiophene (BDT), and indacenodithiophene (IDT) units and their impacts on device performance are discussed. Structure−property relationships are also proposed. Furthermore, the remaining challenges and key research directions in the near future are also addressed.
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TPA-BASED STAR-SHAPED OMS The sp3 hybrid orbital of the nitrogen atom leads to the special propeller starburst molecular structure of TPA. As a result, amorphous materials with isotropic optical and charge-transporting properties could be expected when combining TPA with linear π-conjugated systems.17 Roncali18−21 and Li22 and co-workers carried out the initial research on star-shaped TPA hybrid oligothiophenes. In 2006, Roncali and co-workers reported a star-shaped tris[4-(5″-hexyl-5-terthienyl)phenyl]amine (S(TPA-HTT) with TPA as a core and terthiophene as arms.18 Compound S(TPA-HTT) exhibited a good hole 176
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3.60 4.3 0.81 1.20 3.32 4.16 5.00 5.79 4.00 5.29 2.82 0.83 5.32 2.61 3.93 6.31 6.80 0.56 0.52 0.452 0.379 0.336 0.75 49.6 0.576 0.54 0.58 37.6 49.5 49.0 0.668 0.523 0.48 0.591 0.88 0.87 0.99 1.18 0.88 0.88 0.91 0.84 0.90 0.93 0.88 1.17 0.90 0.70 0.90 0.97 0.81 7.30 9.51 1.81 2.68 11.92 6.30 10.52 11.97 8.24 9.80 8.53 1.43 11.55 5.58 8.33 13.55 14.21 (B) (N) (N) (N) 10−4 10−4 10−4 10−4 a
In thin film. bOptical band gap. cMeasured in neat (N) or blend (B) film.
× × × ×
3.0 × 10−4 (B) 536 532 738 722 702 IDT-3T-R IDT-2PDI DC-IDT2T IEIC ITIC
1.88 1.54 1.55 1.57 1.59
1.12 (N) 1.27 × 10−3 (B) 624 640 BDTS-2DPP IDT-2DPP
1.65 1.74
504 538 482 596 536 560 520 626 S(TPA-3T-CA) S(TPA-BT-3T) S(TPA-BBT) S(TPA-DPP) S(TPA-PDI) BDT-3T-CA IDT-3T-CA BDT-2DPP
1.95 1.90 2.18 1.85 1.76 1.87 1.90 1.65
0.01 (N) 1.7 × 10−4 (B) 0.04 (N)
1.5 × 10 (N) 4.9 × 10−4 (N)
3 × 10−5
3.9 3.3 2.1 3.0
2.6 × 10−6 (N) 6.8 × 10−6 (B) (N)
8.16 × 10−5 (B)
−5.19/−3.27 −5.53/−3.83 −5.43−3.85 −5.42/−3.82 −5.48/−3.83
−5.28/−3.49 −5.11/−3.32
S(TPA-3T-CA):PC71BM S(TPA-BT-3T):PC71BM P3HT:S(TPA-BBT) P3HT:S(TPA-DPP) PBDTTT-C-T:S(TPA-PDI) BDT-3T-CA/PC61BM IDT-3T-CA:PC71BM BDT-2DPP:PC61BM BDT-2DPP:IEIC BDTS-2DPP:IEIC IDT-2DPP:PC71BM P3HT:IDT-2DPP IDT-3T-R:PC71BM P3HT:IDT-2PDI PBDTTT-C-T:DC-IDT2T PTB7-Th:IEIC PTB7-Th:ITIC −5.28/−3.43 −5.28/−3.11 −5.48/−3.10 −5.26/−3.26 −5.40/−3.70 −5.20/−2.90 −5.18/−3.29 −5.23/−3.46
ref PCE (%) FF VOC (V) JSC (mA cm−2) active layer HOMO/LUMO (eV) μec (cm2 V−1 s−1) μhc (cm2 V−1 s−1) Egb (eV) λmaxa (nm)
Table 1. Optical and Electronic Properties, Mobilities, and Photovoltaic Properties of OMs 177
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mobility of 0.01 cm2 V−1 s−1, but its narrow absorption led to poor photovoltaic performance (PCE = 0.32%). We incorporated electron-withdrawing alkyl cyanoacetate (CA) end-groups into the TPA-terthiophene hybrid and designed a star-shaped donor-acceptor (D-A) molecule, S(TPA-3T-CA) (Figure 1),23,24 because CA can simultaneously broaden optical absorption due to intramolecular charge-transfer (ICT) and improve solubility. S(TPA-3T-CA) in solution exhibited an ICT absorption maximum at 494 nm, which was red-shifted by 65 nm relative to that of S(TPA-HTT) without the CA acceptor unit. Solution-processed blended film of S(TPA-3TCA):PC71BM (1:2 w/w) exhibited a good interpenetrating network with a relatively smooth surface. The OSCs based on the S(TPA-3T-CA):PC71BM (1:2 w/w) blend without any post-treatment afforded a PCE of 3.60% and a fill factor (FF) of 0.56 (Table 1). The electron-withdrawing benzothiadiazole (BT)25 was also introduced into the TPA-terthiophene hybrid as acceptor bridges and a star-shaped D-A-D OM, S(TPA-BT3T), was synthesized.26 Relative to its counterpart S(TPAHTT) without BT, S(TPA-BT-3T) exhibited broader absorption with a red-shift of 83 nm as a result of ICT. Without any post-treatment, the BHJ OSCs based on S(TPA-BT3T):PC71BM (1:2 w/w) exhibited a short-circuit current (JSC) of 9.51 mA cm−2, open-circuit voltage (VOC) of 0.87 V, FF of 0.52, and PCE of 4.3%. Conversely, the TPA-based OM framework was also applied in the design of electron acceptors. In general, electron acceptors should possess basic properties of n-type (electrontransporting) semiconductors, such as relatively large electron affinity and good inherent electron-transporting ability.27 5,5Bibenzo[c][1,2,5]thiadiazole (BBT), a 5,5-connected BT dimer, has several attractive properties, such as easy molecular structure tailoring and facile electronic structure manipulation.28−31 Relative to BT, BBT has a stronger electronaccepting ability due to two BT units and was therefore applied as a building block for the synthesis of electron acceptors. Starshaped BBT-based OM S(TPA-BBT) was synthesized and used as an electron acceptor.28 The BHJ OSCs based on the poly(3hexylthiophene) (P3HT):S(TPA-BBT) (1:1, w/w) blend gave a PCE of 0.81% with a JSC of 1.81 mA cm−2, VOC of 0.99 V, and FF of 0.452. The low PCE was partly attributed to the weak optical absorption and low electron mobility of S(TPA-BBT). For improving the optical absorption and performance of TPAbased star-shaped electron acceptors, diketopyrrolopyrrole (DPP) dye with strong absorption in the visible region was introduced into star-shaped acceptors.32 Compound S(TPADPP) with TPA as a core and DPP as arms showed a broader absorption with a smaller optical band gap (Eg = 1.85 eV) relative to that of S(TPA-BBT) (Eg = 2.18 eV). The solar cells based on the P3HT:S(TPA-DPP) (1:1, w/w) blend yielded a PCE of 1.20% and a very high VOC of 1.18 V. The VOC of 1.18 V is among the highest values reported for single-junction OSCs. Both S(TPA-BBT) and S(TPA-DPP) have relatively high LUMO energy levels of −3.1 to −3.3 eV, and these LUMO levels mismatch with those of high-performance narrow-band gap donors, such as PBDTTT-C-T,33 PTB7-Th,34 and so forth. For downshifting the energy levels of the star-shaped electron acceptors, perylene diimide (PDI), with a stronger electronwithdrawing property than DPP and BBT units, was used in the molecular design of OM acceptors.35 PDI derivatives are the earliest acceptors investigated in OSCs. Since the first bilayer heterojunction OSCs in 1986, many early studies of OSCs
23 26 28 32 37 39 40 41, 42 51 51 44 44 40 45 46 47 16
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Figure 2. Chemical structures of TPA-, BDT-, and IDT-based OMs.
construction. Because the 2D 5-alkylthiophene-2-yl-substituted BDT unit is one of the most promising blocks for organic photovoltaic materials,38 we inserted it into oligothiophenes end-capped with alkyl-CA.39 In contrast to amorphous nonplanar star-shaped S(TPA-3T-CA), the linear molecule BDT-3T-CA (Figure 2) with conjugated side chains showed strong crystallinity with a high melting point of 255 °C and stronger and red-shifted absorption. The X-ray diffraction pattern of the BDT-3T-CA film clearly exhibited strong reflection peaks (100, 200, 300) at 2θ = 6.0°, 11.9°, and 18.1°, corresponding to d100, d200, and d300-spacing distances of 14.72, 7.45, and 4.90 Å, respectively, implying a highly ordered assembly of this OM in the solid state. This highly ordered thin-film structure led to a relatively high hole mobility of 0.01 cm2 V−1 s−1, which was nearly 1 order of magnitude higher than that of S(TPA-3T-CA) (1.5 × 10−3 cm2 V−1 s−1). The high hole mobility is beneficial to enhacing JSC and FF. Because of different solubilities of BDT-3T-CA (10−4 cm2 V−1 s−1) and balanced hole and electron mobilities. Relative to the PDI unit, 2-(3-oxo-2,3-dihydroinden-1ylidene)malononitrile (INCN) has a smaller size but similar 179
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(Figure 5) were slightly lower than those of BDT-2DPP (LUMO = −3.46 eV, HOMO = −5.23 eV), which can be attributed to the π-acceptor capability of sulfur atoms. Furthermore, linear alkylthio substituents of BDTS-2DPP led to a strong intermolecular interaction, ordered structure, and high hole mobility of up to 1.12 cm2 V−1 s−1. Finally, after optimizing phase separation of active layers using binary solvent, OSCs based on BDTS-2DPP:IEIC (1:1, w/w) blends yielded enhanced performance with a PCE of 5.29%, which is one of the best values reported for solution-processed fullerenefree all-OM OSCs.
electron-withdrawing property. INCN-flanked IDT OM DCIDT2T had a LUMO level of −3.8 eV, which is similar to that of IDT-2PDI.46 The deep LUMO level matched with some narrow-band gap donor materials, such as PBDTTT-C-T and PTB7-Th. OSCs based on blends of PBDTTT-C-T: DCIDT2T (1.2:1, w/w) exhibited a PCE of 3.93%. Alkyl groups on thiophene in IEIC improved solution processability and crystallinity. OSCs based on blends of PTB7-Th:IEIC (1:1.5, w/w), using a PDI derivative as a cathode interlayer, showed very promising PCEs of up to 6.31%.47 Furthermore, a larger IDT analogue, indacenodithieno[3,2-b]thiophene (IDTT) flanked by INCN end groups (ITIC), exhibited even better photovoltaic performance; PTB7-Th:ITIC (1:1.3, w/w)-based solar cells gave a PCE of up to 6.8% with a VOC of 0.81 V, JSC of 14.21 mA cm−2, and FF of 0.591.16 The PCE of 6.8% is the record value reported for solution-processed fullerene-free OSCs based on OM acceptors. The PTB7-Th:ITIC-based OSCs exhibited even better performance than the control devices based on PTB7-Th:PC61BM (PCE = 6.05%) and close to that of PTB7-Th:PC71BM-based devices (PCE = 7.52%) (Figure 4).
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ROLL-COATED LARGE-AREA SOLAR CELLS BASED ON OMS Most reported OSCs were prepared by a combination of spin coating and vacuum evaporation on rigid glass substrates with small area (95%.53 The roll-coated largearea flexible OSCs based on the DPP-based OM:PC61BM blend exhibited a PCE of >1%.54 Furthermore, we fabricated large-area flexible OSCs based on OM acceptors.55,56 Roll-coated devices based on PBDTTT-CT:DC-IDT2T (1:1.25, w/w) exhibited a PCE of up to 1%.56 Compared to PBDTTT-C-T:PC71BM-based control devices, PBDTTT-C-T:DC-IDT2T-based OSCs exhibited better stability (Figure 6). The PCE of the OM acceptor-based device maintained 85% under continuous AM 1.5G illumination for 180 h, whereas the PCE of the fullerene-based device decayed to 50%. The appearance in the optical microscopy images of the PBDTTT-C-T:DC-IDT2T blended film did not change, whereas some black dots (∼10 μm) resulting from fullerene self-aggregation were clearly observed in the PBDTTT-CT:PC71BM blended film after 180 h stability test, which decreased the PCE. Roll-coated large-area flexible OSCs based on PTB7-Th:IEIC (1:1.5, w/w) showed a promising PCE of 2.3%, similar to that of the PTB7-Th:PC61BM control devices (2.4%) and with better stability than the PC61BM-based device.57
Figure 4. J−V curves of devices based on PTB7-Th:ITIC, PC61BM, or PC71BM. Reproduced with permission from ref 16. Copyright (2015) Wiley-VCH Verlag GmbH & Co. KGaA.
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EFFICIENT ALL-OM SOLAR CELLS In the past decade, much of the focus has been on the development of polymer donor:fullerene acceptor-based OSCs, which have seen a dramatic rise in efficiency up to 11%.4 However, both batch to batch variation of polymer materials and morphology instability of fullerene systems hinder the commercial applications of OSCs. For these problems to be solved, fullerene-free OM donor:OM acceptor (all-OM) systems have been investigated. However, most of the all-OM OSCs showed relatively low PCEs of 2−3%,45,48,49 and very few papers reported PCEs of >4%.50 Very recently, fullerene-free BHJ OSCs based on a blend of BDT-2DPP:IEIC (1:1, w/w) showed a promising PCE as high as 4.0%.51 The molecular structure of the BDT-2DPP donor was modified via side-chain engineering by using a linear alkylthio instead of 2-ethylhexyl in BDT-2DPP. The sulfur atom had some π-acceptor capability due to the formation of pπ(C)−dπ(S) orbital overlap, where divalent sulfur accepted π-electron from the p-orbital of the carbon−carbon double bond into its empty 3d orbitals.52 Because of the same molecular main chain, BDTS-2DPP exhibited a similar broad absorption profile to BDT-2DPP. The LUMO (−3.49 eV) and HOMO (−5.28 eV) of BDTS-2DPP
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CONCLUSIONS AND OUTLOOK A number of OM donor materials have been created, such as 3D star-shaped OMs and 2D linear-shaped OMs. Now, the small-size OSCs based on OM donors have exhibited high PCEs of >10%, which is at the same level as that of polymerbased devices. However, larger-area OSCs based on OM donors have very limited reports. Thus, in the next few years, larger area coating technologies should be widely applied in the 180
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Figure 5. (a) Chemical structures and (b) energy levels of BDT-2DPP, BDTS-2DPP, and IEIC; (c) J-V curves of devices with the structure ITO/ PEDOT:PSS/BDT-2DPP or BDTS-2DPP:IEIC (1:1, w/w)/Ca/Al.
provide extended conjugation and effective interchain π−π overlaps, enhancing intermolecular charge transport. Modifying fused-rings using electron-deficient groups, such as imide/ amide, cyano, and their analogues, can lower the LUMO and HOMO energy levels, form push−pull electronic structures and ICT, and extend absorption spectra to capture more solar energy. It is also critical to form a bicontinuous donor/acceptor interpenetrating network with an optimum morphology and to build two distinct highways for transporting free charge carriers, which need moderate compatibility of donor and acceptor materials. Multidimensional structure and/or rigid side-chains out of fused rings can reduce intermolecular interaction and tune phase separation of blended films but are unfavorable for charge transport. Thus, it is a challenge to balance morphology control and charge transport via molecular design. Very few solution processed OM donor/acceptor systems have been explored. In terms of thermodynamics, OM donor and acceptor with matched energy levels can be blended for BHJ OSCs as active layers, and now, one of the limited factors for the development of all-OM OSCs is the blend morphology far away from optimization. It is a challenge to control film morphology, molecular packing, and phase separation. In addition to the device efficiency, the long-term stability is another key point for OSC production. Most of the research work on device stability focused on polymer:fullerene-based OSCs, whereas the stability of OM-based OSCs was rarely investigated. Although OM-based OSCs with better stability than the fullerene-based control device was demonstrated, more efforts should be dedicated to investigating a decay mechanism of OM-based devices and understanding the relationship among molecular structure, morphology, and device lifetime. High stability achievement is a systematic
Figure 6. Stability of roll-coated flexible OSCs based on PBDTTT-CT:DC-IDT2T or PC71BM. Reproduced with permission from ref 56. Copyright (2015) Wiley-VCH Verlag GmbH & Co. KGaA.
preparation of roll-to-roll flexible solar cells based on OM donors. OM electron acceptors are a promising alternative to fullerene derivatives for OSCs, and solution-processed OSCs based on OM electron acceptors yielded PCEs of up to 6.8%. However, continued development of OM electron acceptors still require a better understanding of the relationships between molecular structure, electronic structure, materials microstructure, charge transport, and photovoltaic properties than is currently available. The basic requirements of intrinsic properties necessary for ideal OM acceptor materials include relatively low energy levels, broad absorption, and good inherent electron-transporting ability. Tailoring extended fused-rings with electron-deficient groups is an effective strategy for the design of high-performance OM acceptors. The aromatic fused-ring blocks (e.g., perylene and IDT/IDTT) 181
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(10) Lin, Y.; Li, Y.; Zhan, X. Small Molecule Semiconductors for High-Efficiency Organic Photovoltaics. Chem. Soc. Rev. 2012, 41, 4245−4272. (11) Li, Y. Molecular Design of Photovoltaic Materials for Polymer Solar Cells: Toward Suitable Electronic Energy Levels and Broad Absorption. Acc. Chem. Res. 2012, 45, 723−733. (12) Chen, Y.; Wan, X.; Long, G. High Performance Photovoltaic Applications Using Solution-Processed Small Molecules. Acc. Chem. Res. 2013, 46, 2645−2655. (13) Coughlin, J. E.; Henson, Z. B.; Welch, G. C.; Bazan, G. C. Design and Synthesis of Molecular Donors for Solution-Processed High-Efficiency Organic Solar Cells. Acc. Chem. Res. 2014, 47, 257− 270. (14) Lin, Y.; Zhan, X. Non-Fullerene Acceptors for Organic Photovoltaics: an Emerging Horizon. Mater. Horiz. 2014, 1, 470−488. (15) Kan, B.; Zhang, Q.; Li, M.; Wan, X.; Ni, W.; Long, G.; Wang, Y.; Yang, X.; Feng, H.; Chen, Y. Solution-Processed Organic Solar Cells Based on Dialkylthiol-Substituted Benzodithiophene Unit with Efficiency near 10%. J. Am. Chem. Soc. 2014, 136, 15529−15532. (16) Lin, Y. Z.; Wang, J. Y.; Zhang, Z. G.; Bai, H. T.; Li, Y. F.; Zhu, D. B.; Zhan, X. W. An Electron Acceptor Challenging Fullerenes for Efficient Polymer Solar Cells. Adv. Mater. 2015, 27, 1170−1174. (17) Roncali, J. Molecular Bulk Heterojunctions: An Emerging Approach to Organic Solar Cells. Acc. Chem. Res. 2009, 42, 1719− 1730. (18) Roquet, S.; Cravino, A.; Leriche, P.; Aleveque, O.; Frere, P.; Roncali, J. Triphenylamine-Thienylenevinylene Hybrid Systems with Internal Charge Transfer as Donor Materials for Heterojunction Solar Cells. J. Am. Chem. Soc. 2006, 128, 3459−3466. (19) Cravino, A.; Roquet, S.; Aleveque, O.; Leriche, P.; Frere, P.; Roncali, J. Triphenylamine-Oligothiophene Conjugated Systems as Organic Semiconductors for Opto-Electronics. Chem. Mater. 2006, 18, 2584−2590. (20) Cravino, A.; Leriche, P.; Alévêque, O.; Roquet, S.; Roncali, J. Light-Emitting Organic Solar Cells Based on a 3D Conjugated System with Internal Charge Transfer. Adv. Mater. 2006, 18, 3033−3037. (21) Ripaud, E.; Rousseau, T.; Leriche, P.; Roncali, J. Unsymmetrical Triphenylamine-Oligothiophene Hybrid Conjugated Systems as Donor Materials for High-Voltage Solution-Processed Organic Solar Cells. Adv. Energy Mater. 2011, 1, 540−545. (22) Zhang, J.; Deng, D.; He, C.; He, Y.; Zhang, M.; Zhang, Z.-G.; Zhang, Z.; Li, Y. Solution-Processable Star-Shaped Molecules with Triphenylamine Core and Dicyanovinyl Endgroups for Organic Solar Cells. Chem. Mater. 2011, 23, 817−822. (23) Lin, Y. Z.; Zhang, Z. G.; Bai, H. T.; Li, Y. F.; Zhan, X. W. A StarShaped Oligothiophene End-Capped with Alkyl Cyanoacetate Groups for Solution-Processed Organic Solar Cells. Chem. Commun. 2012, 48, 9655−9657. (24) Lin, Y. Z.; Zhang, Z. G.; Li, Y. F.; Zhu, D. B.; Zhan, X. W. One, Two and Three-Branched Triphenylamine-Oligothiophene Hybrids for Solution-Processed Solar Cells. J. Mater. Chem. A 2013, 1, 5128− 5135. (25) Parker, T. C.; Patel, D. G.; Moudgil, K.; Barlow, S.; Risko, C.; Bredas, J.-L.; Reynolds, J. R.; Marder, S. R. Heteroannulated Acceptors Based on Benzothiadiazole. Mater. Horiz. 2015, 2, 22−36. (26) Shang, H.; Fan, H.; Liu, Y.; Hu, W.; Li, Y.; Zhan, X. A SolutionProcessable Star-Shaped Molecule for High-Performance Organic Solar Cells. Adv. Mater. 2011, 23, 1554−1557. (27) Anthony, J. E.; Facchetti, A.; Heeney, M.; Marder, S. R.; Zhan, X. n-Type Organic Semiconductors in Organic Electronics. Adv. Mater. 2010, 22, 3876−3892. (28) Lin, Y. Z.; Wang, H. F.; Li, Y. F.; Zhu, D. B.; Zhan, X. W. A StarShaped Electron Acceptor Based on 5,5 ′-Bibenzothiadiazole for Solution Processed Solar Cells. J. Mater. Chem. A 2013, 1, 14627− 14632. (29) Liu, Y.; Wang, H. F.; Dong, H. L.; Tan, J. H.; Hu, W. P.; Zhan, X. W. Synthesis of a Conjugated Polymer with Broad Absorption and Its Application in High-Performance Phototransistors. Macromolecules 2012, 45, 1296−1302.
combination of a materials intrinsic properties with judicious device optimization. Finally, OM materials are promising alternatives to polymer and/or fullerene materials for efficient and stable OSCs. In the next few years, an interdisciplinary approach, such as novel OM materials, especially electron acceptor materials, accurate morphology optimization, and advanced device fabrication technologies will probably bring high-efficiency and stable OSCs to final commercialization.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. Biographies Yuze Lin received his Ph.D. degree at the Institute of Chemistry, Chinese Academy of Sciences (ICCAS), in 2014. He is currently doing research in Prof. Xiaowei Zhan’s lab as a visiting scholar. His research interests focus on new materials for energy applications. Xiaowei Zhan received his Ph.D. degree from Zhejiang University in 1998. Dr. Zhan worked at the University of Arizona and Georgia Institute of Technology from 2002 to 2006 as a Research Associate and Research Scientist, respectively. He has been a professor at ICCAS since 2006. He is currently a professor at Peking University. His research interests focus on the development of organic and polymeric materials for organic electronics and photonics.
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ACKNOWLEDGMENTS This work was supported by the 973 Program (2013CB834702) and NSFC (91433114, 51261130582, 21025418, 21504058).
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REFERENCES
(1) Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Synthesis of Conjugated Polymers for Organic Solar Cell Applications. Chem. Rev. 2009, 109, 5868−5923. (2) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells - Enhanced Efficiencies Via a Network of Internal Donor-Acceptor Heterojunctions. Science 1995, 270, 1789−1791. (3) Sondergaard, R.; Hosel, M.; Angmo, D.; Larsen-Olsen, T. T.; Krebs, F. C. Roll-to-Roll Fabrication of Polymer Solar Cells. Mater. Today 2012, 15, 36−49. (4) He, Z. C.; Xiao, B.; Liu, F.; Wu, H. B.; Yang, Y. L.; Xiao, S.; Wang, C.; Russell, T. P.; Cao, Y. Single-Junction Polymer Solar Cells with High Efficiency and Photovoltage. Nat. Photonics 2015, 9, 174− 179. (5) Liu, Y. H.; Zhao, J. B.; Li, Z. K.; Mu, C.; Ma, W.; Hu, H. W.; Jiang, K.; Lin, H. R.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-Efficiency Polymer Solar Cells. Nat. Commun. 2014, 5, 5293. (6) Zhang, J.; Zhang, Y.; Fang, J.; Lu, K.; Wang, Z.; Ma, W.; Wei, Z. Conjugated Polymer−Small Molecule Alloy Leads to High Efficient Ternary Organic Solar Cells. J. Am. Chem. Soc. 2015, 137, 8176−8183. (7) You, J. B.; Dou, L. T.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C. C.; Gao, J.; Li, G.; Yang, Y. A Polymer Tandem Solar Cell with 10.6% Power Conversion Efficiency. Nat. Commun. 2013, 4, 1446. (8) Tang, C. W. Two Layer Organic Photovoltaic Cell. Appl. Phys. Lett. 1986, 48, 183−185. (9) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Efficient Photodiodes from Interpenetrating Polymer Networks. Nature 1995, 376, 498−500. 182
DOI: 10.1021/acs.accounts.5b00363 Acc. Chem. Res. 2016, 49, 175−183
Article
Accounts of Chemical Research
Small Molecule:Perylene Diimide Bulk Heterojunction Solar Cell. Adv. Mater. 2013, 25, 4403−4406. (49) Douglas, J. D.; Chen, M. S.; Niskala, J. R.; Lee, O. P.; Yiu, A. T.; Young, E. P.; Fréchet, J. M. J. Solution-Processed, Molecular Photovoltaics that Exploit Hole Transfer from Non-Fullerene, nType Materials. Adv. Mater. 2014, 26, 4313−4319. (50) Kwon, O. K.; Park, J. H.; Kim, D. W.; Park, S. K.; Park, S. Y. An All-Small-Molecule Organic Solar Cell with High Efficiency Nonfullerene Acceptor. Adv. Mater. 2015, 27, 1951−1956. (51) Lin, Y.; Wang, J.; Li, T.; Wu, Y.; Wang, C.; Han, L.; Yao, Y.; Ma, W.; Zhan, X. High-Performance Monodisperse Photovoltaic Macromolecules with High Crystallinity and Small Phase Separation. Unpublished. (52) Cui, C.; Wong, W.-Y.; Li, Y. Improvement of Open-Circuit Voltage and Photovoltaic Properties of 2D-Conjugated Polymers by Alkylthio Substitution. Energy Environ. Sci. 2014, 7, 2276−2284. (53) Lin, Y. Z.; Dam, H. F.; Andersen, T. R.; Bundgaard, E.; Fu, W. F.; Chen, H. Z.; Krebs, F. C.; Zhan, X. W. Ambient Roll-to-Roll Fabrication of Flexible Solar Cells Based on Small Molecules. J. Mater. Chem. C 2013, 1, 8007−8010. (54) Liu, W. Q.; Liu, S. Y.; Zawacka, N. K.; Andersen, T. R.; Cheng, P.; Fu, L.; Chen, M. R.; Fu, W. F.; Bundgaard, E.; Jorgensen, M.; Zhan, X. W.; Krebs, F. C.; Chen, H. Z. Roll-Coating Fabrication of Flexible Large Area Small Molecule Solar Cells with Power Conversion Efficiency Exceeding 1%. J. Mater. Chem. A 2014, 2, 19809−19814. (55) Liu, W. Q.; Shi, H. Q.; Andersen, T. R.; Zawacka, N. K.; Cheng, P.; Bundgaard, E.; Shi, M. M.; Zhan, X. W.; Krebs, F. C.; Chen, H. Z. Roll-Coating Fabrication f ITO-Free Flexible Solar Cells Based on a Non-Fullerene Small Molecule Acceptor. RSC Adv. 2015, 5, 36001− 36006. (56) Cheng, P.; Bai, H.; Zawacka, N. K.; Andersen, T. R.; Liu, W.; Bundgaard, E.; Jørgensen, M.; Chen, H.; Krebs, F. C.; Zhan, X. RollCoated Fabrication of Fullerene-Free Organic Solar Cells with Improved Stability. Adv. Sci. 2015, 2, 1500096. (57) Liu, K.; Larsen-Olsen, T. T.; Lin, Y.; Beliatis, M.; Bundgaard, E.; Jørgensen, M.; Krebs, F. C.; Zhan, X. Roll-Coating Fabrication of Flexible, Large-Area Organic Solar Cells: Comparison of Fullerene and Fullerene-Free Systems. Unpublished.
(30) Wang, H. F.; Cheng, P.; Liu, Y.; Chen, J. M.; Zhan, X. W.; Hu, W. P.; Shuai, Z. G.; Li, Y. F.; Zhu, D. B. A Conjugated Polymer Based on 5,5 ′-Bibenzo[c][1,2,5]thiadiazole for High-Performance Solar Cells. J. Mater. Chem. 2012, 22, 3432−3439. (31) Wang, H. F.; Fukumatsu, T.; Liu, Y.; Hu, W. P.; Seki, S.; Zhan, X. W. A D-A-D Swivel-Cruciform Oligothiophene Based on 5,5 ′-Bibenzothiadiazole. J. Mater. Chem. C 2013, 1, 414−417. (32) Lin, Y.; Cheng, P.; Li, Y.; Zhan, X. A 3D Star-Shaped NonFullerene Acceptor for Solution-Processed Organic Solar Cells with a High Open-Circuit Voltage of 1.18 V. Chem. Commun. 2012, 48, 4773−4775. (33) Huo, L. J.; Zhang, S. Q.; Guo, X.; Xu, F.; Li, Y. F.; Hou, J. H. Replacing Alkoxy Groups with Alkylthienyl Groups: A Feasible Approach To Improve the Properties of Photovoltaic Polymers. Angew. Chem., Int. Ed. 2011, 50, 9697−9702. (34) Liao, S. H.; Jhuo, H. J.; Cheng, Y. S.; Chen, S. A. Fullerene Derivative-Doped Zinc Oxide Nanofilm as the Cathode of Inverted Polymer Solar Cells with Low-Bandgap Polymer (PTB7-Th) for High Performance. Adv. Mater. 2013, 25, 4766−4771. (35) Zhan, X.; Facchetti, A.; Barlow, S.; Marks, T. J.; Ratner, M. A.; Wasielewski, M. R.; Marder, S. R. Rylene and Related Diimides for Organic Electronics. Adv. Mater. 2011, 23, 268−284. (36) Li, C.; Wonneberger, H. Perylene Imides for Organic Photovoltaics: Yesterday, Today, and Tomorrow. Adv. Mater. 2012, 24, 613−636. (37) Lin, Y.; Wang, Y.; Wang, J.; Hou, J.; Li, Y.; Zhu, D.; Zhan, X. A Star-Shaped Perylene Diimide Electron Acceptor for High-Performance Organic Solar Cells. Adv. Mater. 2014, 26, 5137−5142. (38) Ye, L.; Zhang, S. Q.; Huo, L. J.; Zhang, M. J.; Hou, J. H. Molecular Design toward Highly Efficient Photovoltaic Polymers Based on Two-Dimensional Conjugated Benzodithiophene. Acc. Chem. Res. 2014, 47, 1595−1603. (39) Lin, Y. Z.; Ma, L. C.; Li, Y. F.; Liu, Y. Q.; Zhu, D. B.; Zhan, X. W. Small-Molecule Solar Cells with Fill Factors up to 0.75 via a Layerby-Layer Solution Process. Adv. Energy Mater. 2014, 4, 1300626. (40) Bai, H. T.; Wang, Y. F.; Cheng, P.; Li, Y. F.; Zhu, D. B.; Zhan, X. W. Acceptor-Donor-Acceptor Small Molecules Based on Indacenodithiophene for Efficient Organic Solar Cells. ACS Appl. Mater. Interfaces 2014, 6, 8426−8433. (41) Lin, Y. Z.; Ma, L. C.; Li, Y. F.; Liu, Y. Q.; Zhu, D. B.; Zhan, X. W. A Solution-Processable Small Molecule Based on Benzodithiophene and Diketopyrrolopyrrole for High-Performance Organic Solar Cells. Adv. Energy Mater. 2013, 3, 1166−1170. (42) Huang, J. H.; Zhan, C. L.; Zhang, X.; Zhao, Y.; Lu, Z. H.; Jia, H.; Jiang, B.; Ye, J.; Zhang, S. L.; Tang, A. L.; Liu, Y. Q.; Pei, Q. B.; Yao, J. N. Solution-Processed DPP-Based Small Molecule that Gives High Photovoltaic Efficiency with Judicious Device Optimization. ACS Appl. Mater. Interfaces 2013, 5, 2033−2039. (43) Tang, A. L.; Li, L. J.; Lu, Z. H.; Huang, J. H.; Jia, H.; Zhan, C. L.; Tan, Z. A.; Li, Y. F.; Yao, J. N. Significant Improvement of Photovoltaic Performance by Embedding Thiophene in SolutionProcessed Star-Shaped TPA-DPP Backbone. J. Mater. Chem. A 2013, 1, 5747−5757. (44) Bai, H. T.; Cheng, P.; Wang, Y. F.; Ma, L. C.; Li, Y. F.; Zhu, D. B.; Zhan, X. W. A Bipolar Small Molecule Based on Indacenodithiophene and Diketopyrrolopyrrole for Solution Processed Organic Solar Cells. J. Mater. Chem. A 2014, 2, 778−784. (45) Lin, Y.; Wang, J.; Dai, S.; Li, Y.; Zhu, D.; Zhan, X. A Twisted Dimeric Perylene Diimide Electron Acceptor for Efficient Organic Solar Cells. Adv. Energy Mater. 2014, 4, 1400420. (46) Bai, H.; Wang, Y.; Cheng, P.; Wang, J.; Wu, Y.; Hou, J.; Zhan, X. An Electron Acceptor Based on Indacenodithiophene and 1,1Dicyanomethylene-3-Indanone for Fullerene-Free Organic Solar Cells. J. Mater. Chem. A 2015, 3, 1910−1914. (47) Lin, Y.; Zhang, Z.-G.; Bai, H.; Wang, J.; Yao, Y.; Li, Y.; Zhu, D.; Zhan, X. High-Performance Fullerene-Free Polymer Solar Cells with 6.31% Efficiency. Energy Environ. Sci. 2015, 8, 610−616. (48) Sharenko, A.; Proctor, C. M.; van der Poll, T. S.; Henson, Z. B.; Nguyen, T.-Q.; Bazan, G. C. A High-Performing Solution-Processed 183
DOI: 10.1021/acs.accounts.5b00363 Acc. Chem. Res. 2016, 49, 175−183
