A new unsymmetrical near-IR small molecule with squaraine chromophore for solution processed bulk heterojunction solar cells

A new unsymmetrical low bandgap push-pull squaraine chromophore bis-DMFA-Th-SQ-Th-DCA denoted as JK216D was synthesized and its optical and electrochemical properties (both theoretical and experimental) were investigated. This small molecule having a strong electron withdrawing moiety exhibits a broader absorption band in thin film. The JK216D exhibited suitable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital energy levels, compatible with that of PC71BM, for efficient photoinduced electron transfer. The bulk heterojunction solar cell fabricated with JK216D:PC71BM (optimized weight ratio of 1 : 2 and processed with CB) exhibited a PCE of 3.60% (J(sc) = 9.97 mA cm(-2), V-oc = 0.86 V and FF = 0.42). The PCE has been further improved up to 5.25% (J(sc) = 11.86 mA cm(-2), V-oc = 0.82 V and FF = 0.54), when the optimized JK216D:PC71BM (1 : 2) active layer was processed with the addition of 3 v% 1,8-diiodoctane (DIO) into the CB solution.