Simultaneous Enhancement of Solar Cell Efficiency and Photostability via Chemical Tuning of Electron Donating Units in Diketopyrrolopyrrole-Based Push−Pull Type Polymers Tae In Ryu, † Youngwoon Yoon, § Ji-Hoon Kim, ∥ Do-Hoon Hwang, ∥ Min Jae Ko, § Doh-Kwon Lee, § Jin Young Kim, § Honggon Kim, § Nam-Gyu Park,* ,†,‡ BongSoo Kim,* ,§ and Hae Jung Son* ,§ † School of Chemical Engineering and ‡ Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Korea § Photoelectronic Hybrid Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea ∥ Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Korea * S Supporting Information ABSTRACT: We synthesized a series of push−pull-type copolymers by copolymerizing an electron-deficient diketo- pyrrolopyrrole with three electron-donating benzodithiophene (BDT) moieties. PDPPDTT, which incorporated a dithieno- thiophene (DTT), showed a higher power conversion efficiency (PCE) of 6.11% compared to 3.31% for the BDT- based polymer (PDPPBDT). PDPPDTBDT, which incorpo- rated a dithienobenzodithiophene (DTBDT), also exhibited superior performance, with a PCE of 4.75% although this value was lower than that obtained for PDPPDTT. The presence of the DTT unit in the polymer backbone lowered the energy bandgap of the polymer and induced an optimal morphology in the polymer:PC 71 BM blend film, resulting in higher charge carrier generation. Furthermore, the effectively delocalized frontier orbitals of PDPPDTT enhanced intermolecular interactions between the polymer chains by favoring effective π−π stacking, which facilitated charge carrier transport. By contrast, PDPPDTBDT unexpectedly showed a low-crystallinity thin film despite its backbone planarity, which reduced the performance relative to that of PDPPDTT. Importantly, PDPPDTT exhibited significantly better device stability compared to the other polymers in a light soaking test due to the much higher photochemical stability of PDPPDTT. We demonstrated a systematic approach to simultaneously increasing the photovoltaic performances and device stability, and we explored the basis for the structure−property relationship that accompanied such improvements. ■ INTRODUCTION Over the past decade, bulk heterojunction polymer solar cells (BHJ PSCs) prepared using conjugated polymers as an electron donor and fullerene derivatives (e.g., [6,6]-phenyl C 71 -butyric acid methyl ester, PC 71 BM) as an electron acceptor have been extensively studied, 1 and power conversion efficiencies (PCEs) have reached values of ∼9% for single cells 2 and ∼10% for tandem cells. 3 A lot of conjugated polymers have been developed to achieve high photovoltaic properties, and design strategies relevant to organic photovoltaic devices to achieve a high PCE in BHJ solar cells have been set. Further advancements in the efficiency of a PSC will require developments of new conjugated polymers that satisfy several important criteria, including 4,5 (i) efficient light harvesting to increase the short-circuit current (J sc ), (ii) a lowest unoccupied molecular orbital (LUMO) energy level in the polymer should be at least 0.3 eV higher than that of the acceptor to provide a driving force for efficient charge separation at the polymer/ acceptor interface, (iii) energy offsets between the highest occupied molecular orbital (HOMO) level of the conjugated polymer and the LUMO level of the fullerene derivative should be sufficient to support a high open-circuit voltage (V oc ), (iv) morphology in the BHJ polymer blend should be optimal and include a nanoscale interpenetrating network to achieve a high interfacial area and efficient charge separation and transport, and (v) charge carrier mobilities should be high to increase charge collection and thus J sc by reducing charge recombina- tion. Apart from these points, the polymer must be easily processed and reproducibly synthesized for commercialization of a solar cell device at a low cost. 6 One of the most reliable approaches to prepare conjugated polymers for high-efficiency PSCs has involved synthesizing push−pull-type alternating copolymers composed of electron- donating and electron-accepting conjugated units. In general, aromatic compounds with a high electron density are used as the electron-donating unit, and electron-deficient aromatic compounds functionalized with electron-withdrawing groups are employed as the electron-accepting units. Mono- or oligo- heteroaromatic compounds are typical examples of electron- donating units. 7 4,8-Dialkoxybenzo[1,2-b:4,5-b′]dithiophene Received: June 24, 2014 Revised: August 16, 2014 Published: September 11, 2014 Article pubs.acs.org/Macromolecules © 2014 American Chemical Society 6270 dx.doi.org/10.1021/ma501300a | Macromolecules 2014, 47, 6270−6280