Readily Accessible Benzo[d]thiazole Polymers for Nonfullerene Solar Cells with >16% Efficiency and Potential Pitfalls
Liang‐Wen Feng, Jianhua Chen, S. Mukherjee, Vinod K. Sangwan, Wei Huang, Yao Chen, Ding Zheng, Joseph Strzalka, Gang Wang, Mark C. Hersam, Dean M. DeLongchamp, Antonio Facchetti, Tobin J. Marks
Abstract
Here we report facile, high-yield synthetic access to the difluoro BTA building block, 4,7-bis(5-bromo-4-(2-hexyl-decyl)-thiophen-2-yl)-5,6-difluoro-2-(pentadecan-7-yl)-benzo[d]thiazole (BTAT-2f), for use in donor (D)–acceptor 1 (A1)–D–acceptor 2 (A2) polymers [D = bithiophene; A1 = BTA-2f; A2 = benzothiadiazole (BT) derivative] for organic solar cells (OSCs). Fine tuning of polymer optical and electronic properties is achieved by incrementally varying the A2 fluorination level. Bulk-heterojunction (BHJ) PBTATBT-4f:Y6 solar cells deliver a noteworthy power conversion (PCE) efficiency of 16.08% (Voc = 0.81 V; Jsc = 27.25 mAcm–2; FF = 72.70%) without processing additives. In contrast, PBTATBT-2f:Y6 exhibits an irregular morphology and low PCE, ascribable to cocrystal formation-induced recombination, which is unprecedented for nonfullerene (NFA) OSCs. This result should be of guiding significance for future NFA design.