A novel π-D1-A-D2 type low bandgap squaraine dye for efficient small molecular organic solar cells

Squaraine dyes have attracted increasing attention as photovoltaic materials for organic solar cells (OSCs) because of their facile and low-cost synthesis, intense and broad absorption in the visible and near-infrared (Vis-NIR) regions as well as colorful films. Herein, we report the synthesis of a novel pi-D-1-A-D-2 type squaraine dye (DTT-USQ-11) with a low optical bandgap of 1.50 eV. In comparison with the classic D-1-A-D-2 type squaraine dye (USQ-11), DTT-USQ-11 demonstrates a lower optical bandgap, deeper highest occupied molecular orbital (HOMO) energy level, and higher hole mobility. Consequently, solution-processed bulk heterojunction small molecular OSC fabricated with the DTT-USQ-11/PC71BM blend film exhibits a much higher power conversion efficiency (PCE: 5.69%) compared with the classical USQ-11/PC71BM system (PCE: 4.15%). This enhanced PCE can be attributed to the simultaneous increase in the open-circuit voltage, short-circuit current density and fill factor of the DTT-USQ-11-based devices.