Thermally Stable All-Polymer Solar Cells with High Tolerance on Blend Ratios
Yannan Zhang, Yalong Xu, Michael J. Ford, Fangchao Li, Jianxia Sun, Xufeng Ling, Yongjie Wang, Jinan Gu, Jianyu Yuan,* and Wanli Ma*
1Institute of Functional Nano & Soft Materials (FUNSOM) Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices Joint International Research Laboratory of Carbon-Based Functional Materials and Devices Soochow University 199 Ren-Ai Road, Suzhou Industrial Park, Suzhou, Jiangsu 215123, P. R. China
2Center of Polymers and Organic Solids Materials Department University of California Santa Barbara, CA 93106, USA
Tuning the blend composition is an essential step to optimize the power conversion effciency (PCE) of organic bulk heterojunction (BHJ) solar cells. PCEs from devices of unoptimized donor: acceptor (D:A) weight ratio are generally significantly lower than optimized devices. Here, two high-performance organic nonfullerene BHJ blends PBDB-T:ITIC and PBDB-T:N2200 are adopted to investigate the effect of blend ratio on device performance. It is found that the PCEs of polymer-polymer (PBDB-T:N2200) blend are more tolerant to composition changes, relative to polymer-molecule (PBDB-T:ITIC) devices. In both systems, short-circuit current density (Jsc) is tracked closely with PCE, indicating that exciton dissociation and transport strongly inﬂuence PCEs. With dilute acceptor concentrations, polymer-polymer blends maintain high electron mobility relative to the polymer-molecule blends, which explains the dramatic difference in PCEs between them as a function of D:A blend ratio. In addition, polymer-polymer solar cells, especially at high D:A blend ratio, are stable (less than 5% relative loss) over 70 d under continuous heating at 80 °C in a glovebox without encapsulation. This work demonstrates that all-polymersolar cells show advantage in operational lifetime under thermal stress and blend-ratio resilience, which indicates their high potential for designing of stable and scalable solar cells.