Constructing D–A copolymers based on thiophene-fused benzotriazole units containing different alkyl side-chains for non-fullerene polymer solar cells

Abstract

A series of D–A copolymers (P1–P3) based on a thiophene fused benzotriazole (BTAZT) unit and a 4,8-bis(4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDTFT) unit have been designed and synthesized as electron-donating materials for non-fullerene polymer solar cells (PSCs) with ITIC as an electron-accepting material. The fusion of a thiophene ring with a BTAZ unit and introduction of a fluorine atom at the BDTT unit have been concurrently performed to regulate the absorption properties and energy levels, and two different alkyl side-chains have also been introduced to BTAZT and BDTFT units, respectively, to investigate the influence of side-chains on the properties of the polymers as well as the photovoltaic performances of the PSCs. The photovoltaic performance evaluation in combination with morphology characterization indicates that the introduction of a 2-ethylhexyl group simultaneously at BTAZT and BDTFT units is favourable for intermolecular π–π interaction that improves the morphologies and crystalline domain of P1:ITIC blend film, thus enhancing the charge mobility and the short-circuit current density (J_sc). But the large steric hindrance of the 2-butyloctyl group to replace the 2-ethylhexyl group at BDTFT (P2) or BTAZT (P3) deteriorates intermolecular interaction that leads to the decrease of charge mobility and poor J_sc. Thus, P1 based PSCs exhibit the highest power conversion efficiency of 7.14% with the best J_sc (14.11 ± 0.27 mA cm⁻²) and fill factor (63.41 ± 1.46%) among P1–P3 polymers.