New TIPS-substituted benzo[1,2-b:4,5-b′]dithiophene-based copolymers for application in polymer solar cells

Abstract

Novel triisopropylsilylethynyl (TIPS)-substituted benzodithiophene-based copolymers, poly[4,8-bis(triisopropylsilylethynyl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4,6-(2-ethylhexyl-thieno[3,4-b]thiophene-2-carboxylate)] (P1), poly[4,8-bis(triisopropylsilylethynyl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-[4,6-{(1-thieno[3,4-b]thiophen-2-yl)-2-ethylhexan-1-one}] (P2), and poly[4,8-bis(triisopropylsilylethynyl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4,6-(2-ethylhexyl(3-fluorothieno[3,4-b]thiophene)-2-carboxylate)] (P3), were designed and synthesized for use in polymer solar cells (PSCs). We describe the effects of the different acceptor segment side groups on the optical, electrochemical, field-effect hole mobility, and photovoltaic characteristics of the resulting TIPS-based copolymers. The side groups in the copolymers were found to significantly influence the carrier mobilities and photovoltaic properties of the copolymers. The field-effect mobilities of the holes varied from 9 × 10⁻⁵ cm² V⁻¹ s⁻¹ in P2 to 3 × 10⁻³ cm² V⁻¹ s⁻¹ in P1. Under optimized conditions, the TIPS-based polymers showed power conversion efficiencies (PCEs) for the PSCs in the range of 3.16–5.76%. Among the TIPS-based copolymers studied here, P1 showed the best photovoltaic performance, with an open-circuit voltage (V_oc) of 0.82 V, a short-circuit current density (J_sc) of 12.75 mA cm⁻², a fill factor (FF) of 0.55, and a power-conversion efficiency of 5.76% using a P1:PC₇₁BM blend film as the active layer under AM 1.5G irradiation (100 mW cm⁻²).