Revisiting carbazole-based polymer donors for efficient and thermally stable polymer solar cells: structural utility of coplanar π-bridged spacers

Abstract

For the realization of highly efficient and thermally stable polymer solar cells (PSCs), we develop a new series of polymer donors (P_Ds) containing carbazole (Cz)-based units, N-dodecyl-carbazole[3,4-c:5,6-c]bis[1,2,5]-thiadiazole (CBT). The coplanar penta-fused-ring structures of the CBT units coupled with π-conjugated bridges (thiophene (T) or thienothiophene (TT)) exhibit highly planar P_D conformations that are non-covalently locked by secondary interactions. As a result, the series of P_Ds (i.e., P1, P2, and P3) exhibit superior electrical and photovoltaic properties. In particular, the TT–π-bridged P_Ds, P2 and P3, achieve high power conversion efficiencies (PCEs) of 13.17 and 14.58% when paired with Y6 acceptor, respectively, which outperform the T–π-bridged P_D, P1 (10.78%). This result is attributed to the compact intermolecular packing, enhanced crystallinity, and extended conjugation of the P_Ds featuring TT. Furthermore, an even higher PCE of 15.54% is achieved by a P3-based ternary (P3:Y6:PC₇₁BM) blend system. This PCE represents the highest among the PCEs of previously reported PSCs featuring Cz-based P_Ds. In addition, the P3:Y6 blend shows excellent thermal stability by maintaining 90% of its initial PCE after 144 h at 120 °C. Therefore, this study provides important molecular design rules for developing Cz-based P_Ds and realizing high-performance and thermally stable PSCs.