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

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 pi-pi 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 Jsc. 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.