Slight Structural Disorder in Bithiophene-based Random Terpolymers with Improved Power Conversion Efficiency for Polymer Solar Cells

A series of random terpolymers P2−P5 were designed and synthesized by randomly embedding 5 mol%, 10 mol%, 15 mol% and 25 mol% feed ratios of low cost 2,2-bithiophene as the third monomer to the famous donor-acceptor (D-A) type copolymer PTB7-Th (P1). All polymers showed similar molecular weight with number-average molecular weight (M_n) and weight-average molecular weight (M_w) in the range of (59−74) and (93−114) kg·mol⁻¹, respectively, to ensure a fair comparison on the structure-property relationships. Compared with the control copolymer PTB7-Th, the random terpolymers exhibited enhanced absorption intensity in a wide range from 400 nm to 650 nm in both solution and film as well as in polymer/PC₇₁BM blends. From grazing incident wide-angle X-ray diffraction (GIWAXS), compared with the regularly alternated copolymer PTB7-Th, the random terpolymers demonstrated mild structural disorder with reduced (100) lamellar stacking and slightly weakened (010) π-π stacking for the polymers as well as slightly reduced PC71BM aggregation in polymer/PC₇₁BM blends. However, the measured hole mobility for terpolymers ((1.20−3.73) × 10⁻⁴ cm²·V⁻¹·s⁻¹) was evaluated to be comparable or even higher than 1.35 × 10⁻⁴ cm²·V⁻¹·s⁻¹ of the alternative copolymer. Enhanced average power conversion efficiency (PCE) from 7.35% to 8.11% and 7.79% to 8.37% was observed in both conventional and inverted device architectures from copolymer P1 to terpolymers P4, while further increasing the 2,2-bithiophene feed ratio decreased the PCE.