© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim COMMUNICATION 1 wileyonlinelibrary.com www.MaterialsViews.com www.advenergymat.de Side-Chain Tunability via Triple Component Random Copolymerization for Better Photovoltaic Polymers Wei-Hsuan Chang, Jing Gao, Letian Dou, Chun-Chao Chen, Yongsheng Liu, and Yang Yang* W.-H. Chang, J. Gao, L. Dou, C.-C. Chen, Dr. Y. Liu, Prof. Y. Yang Department of Materials Science and Engineering University of California Los Angeles Los Angeles, CA 90095, USA E-mail: yangy@ucla.edu W.-H. Chang, L. Dou, Dr. Y. Liu, Prof. Y. Yang California Nano Systems Institute University of California Los Angeles Los Angeles, CA 90095, USA DOI: 10.1002/aenm.201300864 Organic polymer solar cells (PSCs) provide an opportunity to efficiently generate energy from sunlight at moderate cost. [1] Currently led by a binary bulk heterojuction (BHJ) device, the key core of this solution-processable technology is a mixed layer of p-type polymeric semiconductor and n-type material typically based on fullerene derivatives such as [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM). Because of the many studies in this field over the past decade, the power conversion efficiency (PCE) of single junction PSCs has improved significantly and has gradually approached the 10% milestone. [1a,b,d,e,g] From the material standpoint, these improvements are mainly based on tailoring the bandgap ( E g ) of conjugated polymers, where the ease of doing so is afforded by constructing an electron donor– acceptor alternating copolymer. [2] Via bandgap lowering, more photons are absorbed and thus higher short-circuit current ( J SC ) can be achieved. Simultaneously, careful control over the energy level alignment between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of p-type and n-type materials, respectively, can help in realizing a large open-circuit voltage ( V OC ). [2] Using these strat- egies, several successful mid- or low-bandgap polymers ( E g of 1.8–1.4 eV) were made and were demonstrated to have 5–9% PCE; these include PTB7, PBDTTT-CF, PBDTTPD, PDTS(G) TPD, PMDPP3T, PBDTTSeDPP, etc. [3,4] These state-of-the-art polymers all exist at an energy level that can collaborate with [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM) or PC 71 BM. Nevertheless, the PCEs reported to date are still much lower than the theoretical limit (largely limited by low J SC and fill factor (FF)) and there is still much room for improvement within the given bandgap. [5] In past few years, although there has been a vast variety of newly designed conjugated building blocks, few investigations have focused on side-chain modification. Side chains are known to affect solu- tion-processed PSC extensively. To ensure a good solubility in common organic solvent (i.e., chlorobenzene (CB), chloroform (CF), etc.), long or bulky aliphatic groups are usually attached onto the conjugated moieties. These groups, on the contrary, act as a blocking layer for a good packing between the polymers and therefore can affect the thin-film morphology and per- formance in electronic devices. Previous efforts to optimize the side-chains’ size and shape often have led to poor solu- bility or poor solar cell performance; only few examples have been successful. [3f,4c,6] In early work by Yang et al., a more balanced charge transport in poly{[2,7-(9-(20-ethylhexyl)-9- hexyl-fluorene])-alt-[5,50-(40,70-di-2-thienyl-20,10,30-benzothi- adiazole)]} (PFDTBT)-based solar cells was achieved by sub- stitution of an unbulky side chain. [6a] Fréchet et al. found that a longer but less bulky side chain increased the ordering in a PBDTTPD:fullerene blend thin film and, thus, a better cell was obtained. [3f ] Later, the same group applied a similar strategy to diketopyrrolopyrrolo (DPP)-based polymers, and a comparable improvement was seen. [4c] Bao et al. tried functionalizing the isoindigo unit with a more soluble siloxane group with the branched point residing away from the polymer backbone. Consequently, charge transport in organic field-effect transis- tors (OFETs) was improved, but the photovoltaic performance was not as good as expected because of morphology issues when blended with PC 71 BM. [6c–d] Therefore, modifying the side chains indeed provides a chance to acquire a more optimized molecular structure, but doing so without giving up the mate- rial’s original properties is still difficult. Recently, triethylene glycol (TEG) side chains have been found to effectively induce the self-assembly of organic semi- conductors while still maintaining good solubility. [7,8] For example, poly(3-hexylthiophene) (P3HT) with TEG substi- tuted for the hexyl groups showed better crystallinity. [7b,c] TEG- functionalized DPP-based conjugated molecule and polymer both showed better molecular stacking and thereby a higher charge carrier mobility in OFET devices. [8] Inspired by these earlier results, we suggest that the strong self-assembling effect awarded by TEG side chain could be helpful to improve the photovoltaic performance in a given polymer system. To realize this goal, we propose that the number of TEG groups should be controlled very carefully to reach the most optimized con- ditions, as the morphology and photovoltaic performance are usually affected by a subtle change in the chemical structure of the materials. [1g,2] Herein, we report a successful and easy way of introducing a certain amount of straight TEGs as side chains into conjugated polymers via a tri-component copolym- erization approach. The concept of triple component random copolymerization has recently been adapted as an effective way to adjust the bandgap and energy levels of the conjugated poly- mers. [9] However, side-chain modification through this kind of approach has not been realized to date. A low-bandgap con- jugated polymer (PBDTT-DPP, or PBD) based on alternating Adv. Energy Mater. 2013, DOI: 10.1002/aenm.201300864