Wide bandgap copolymers with asymmetric side chains for efficient organic solar cells

Abstract

Compared to the polymer donor materials with symmetric structures, those with asymmetric structures usually have an increased dipole moment and therefore an enhanced intermolecular interaction which may eventually improve their photovoltaic performance. In this work, two novel wide bandgap donor–acceptor alternating copolymers (POS and PSS) are designed and synthesized by polymerizing benzodithiophene-derived electron-rich units (D) containing symmetric and asymmetric side chains with an electron-deficient unit (A) of 1,3-bis(2-ethylhexyl)-5,7-bis(5-(trimethylstannyl)thiophen-2-yl)benzo[1,2-c:4,5-c′]dithiophene-4,8-dione, respectively. Although both two copolymers possess large optical bandgaps, POS with asymmetric side chains exhibits a smaller π–π stacking distance and a deeper energy level of the highest occupied molecular orbital in comparison with PSS. When blended with the benchmark acceptor of Y6, the best-performing POS:Y6-based device exhibits a higher PCE of 15.02% with an enhanced V_OC of 0.820 V, a J_SC of 26.86 mA cm⁻², and a higher fill factor (FF) of 68.20% in comparison with the PSS:Y6-based OSC which shows a PCE of 13.38% with a V_OC of 0.800 V, a J_SC of 27.26 mA cm⁻², and an FF of 61.06%. The enhanced PCE for the POS:Y6-based device can be attributed to the higher charge carrier mobility, more balanced charge transport, and less charge recombination. This study indicates that the asymmetric side chains on the electron-donating benzodithiophene unit can greatly affect the photovoltaic performance of the resulting D–A copolymers.