© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Letian Dou, Wei-Hsuan Chang, Jing Gao, Chun-Chao Chen, Jingbi You, and Yang Yang* A Selenium-Substituted Low-Bandgap Polymer with Versatile Photovoltaic Applications L. Dou, [+] W.-H. Chang, [+] J. Gao, C.-C. Chen, Dr. J. You, Prof. Y. Yang Department of Materials Science and Engineering University of California Los Angeles, Los Angeles, CA 90095, USA E-mail: yangy@ucla.edu L. Dou, W.-H. Chang, Prof. Y. Yang California Nano Systems Institute University of California Los Angeles, Los Angeles, CA 90095, USA [+] These authors contributed equally to this work. DOI: 10.1002/adma.201203827 Organic photovoltaic (OPV) devices provide an opportunity to utilize the solar energy efficiently while maintaining low cost. [1] To harvest a greater part of the solar spectrum, lowering the energy bandgap of the active material is a major task for materials scientists. The design and synthesis of low-bandgap (LBG) conjugated polymers for use as electron donor materials for bulk heterojuction (BHJ) polymer solar cell (PSC) appli- cations have attracted remarkable attention during the last decade. [2] The reasons for pursuing LBG polymers include: 1) The Shockley-Quiesser equation indicates a bandgap of around 1.4 eV is ideal for a single junction solar cell device. [3] 2) Tandem PSCs require an active material with a bandgap less than 1.5 eV together with a wide bandgap (WBG) material having a bandgap around 1.9 eV. [4] 3) Some specific applications such as visibly-transparent PSCs need an active material to be more absorbing of near-infrared (NIR) and ultra-violet (UV) light and less absorbing of visible light. [5] In order to realize these goals, several synthetic strategies have been proven to be very effective in terms of narrowing the bandgap of organic polymeric mate- rials. [2] However, a small bandgap does not directly guarantee high power conversion efficiency (PCE) of the solar cell devices. Proper alignment of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are also critical for efficient charge transfer to the electron acceptor material (for example, [6,6]-phenyl-C 71 -butyric acid methyl ester [PC 71 BM]) and to ensure a large open circuit voltage ( V OC ) of the device; High charge carrier mobility as well as favorable morphology when blended with the acceptor material are required as well to enhance device’s short circuit current ( J SC ) and fill factor ( FF). [1,2] So far, PCEs of over 7% for single junction devices have been achieved using carefully- designed active materials with bandgaps of 1.8 to 1.6 eV (note: mid-bandgap polymer, or MBG polymer). [6] Regardless of great efforts having been made, there is still a lack of high perform- ance polymers with bandgaps less than 1.5 eV that can compete with the state-of-art MBG polymers such as the thionothiophene (TT) and benzodithiophene (BDT) based PTB7 and PBDTTT- CF (Eg = 1.6 eV). [6a,6b,7] Recently, we have demonstrated a series of LBG polymers (Eg < 1.5 eV) based on alternating diketopyrrolopyrrole (DPP) and thienylbenzodithiophene (BDTT) units. When the BDTT unit is copolymerized with the furan-containing DPP unit (FDPP), the resulting polymer (PBDTT-FDPP, Eg = 1.51 eV) gives a PCE ∼ 5% in a single junction solar cell. [8] By switching the furan to a thi- ophene moiety, PBDTT-DPP (Eg = 1.46 eV) shows increased J SC and FF, and this resulted in a higher PCE of 6.5%. [4g,8] The suc- cessful application of PBDTT-DPP in tandem PSCs has led to a National Renewable Energy Laboratory certified PCE of 8.6% and its application in visibly-transparent PSCs has lead to ∼4% PCE with over 60% transparency in the visible region. [4g,5f ] Neverthe- less, the efficiencies were limited mainly by the relatively narrow absorption range (up to 850 nm) and low external quantum efficiency (EQE, <50%) in the NIR region. Further lowering the bandgap to harvest more photons in the NIR part of the solar radiation, as well as increasing the charge carrier mobility of the LBG polymers is desired to reach higher efficiency in both types of devices. Very recently, it has been found that changing the sulfur atom on the thiophene moiety of the DPP unit to a sele- nium atom to form the selenophene-based DPP (SeDPP) unit in the conjugated polymer backbone can decrease the bandgap and enhance the charge transport properties in organic field effect transistor (FET) devices. [9,10] However, the photovoltaic perform- ance of the SeDPP-based LBG polymer (PSeDPPDTT) was lower than its thiophene counterpart mainly due to its higher HOMO level and thus lower V OC of the device. [10a] Similarly, early efforts on changing the sulfur atoms on poly(3-hexylthiophene) (P3HT) to selenium atoms to form poly(3-hexylselenophene) (P3HS) gave lower bandgap and lower PCE. [10b,10c] On the other hand, Yu et al. recently reported a Se-substituted PTB8 polymer based on TT and BDT units, which showed similar V OC , FF, higher J SC , and thus higher PCE than its thiophene counterpart. [11] Based on the above contradictory results, more investigation of the effects of Se-substitution is definitely needed. Herein, we show that the reduction of the bandgap and the enhancement of the charge transport properties of a LBG polymer (PBDTT-DPP) can be accomplished simultaneously by substi- tuting the sulfur atoms on the DPP unit with selenium atoms. The newly designed polymer, poly{2,6′-4,8-di(5-ethylhexylthienyl) benzo[1,2-b;3,4-b]dithiophene-alt-2,5-bis(2-butyloctyl)-3,6- bis(selenophene-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione} (PBDTT- SeDPP, Eg = 1.38 eV), shows excellent photovoltaic performance in single junction devices with PCEs over 7% and photo-response up to 900 nm. Tandem polymer solar cells and visibly-transparent solar cells based on PBDTT-SeDPP are also demonstrated with a 9.5% and 4.5% PCE, which are more than 10% enhancement over those based on PBDTT-DPP. Adv. Mater. 2012, DOI: 10.1002/adma.201203827