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

Abstract

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.