Improved photovoltaic properties of PM6-based terpolymer donors containing benzothiadiazole with a siloxane-terminated side chain

Abstract

A new series of PM6-based terpolymers (PM10Si, PM20Si, and PM30Si) containing 10%, 20% and 30% DTBT (4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole) with siloxane-terminated side chains were designed and synthesized. These terpolymers were used as donor materials with the acceptor Y6 to fabricate polymer solar cells (PSCs), where both the routine blend casting method and the two-step sequential deposition (SD) method were applied to investigate the photovoltaic properties of these terpolymers. The inverted device based on PM20Si with 20% siloxane-terminated side chains on the DTBT unit exhibited better power conversion efficiencies (PCEs) of 14.30% due to having a more suitable phase state and more prominent face-on orientations. With further optimization by the SD method, the device based on PM20Si obtained an excellent efficiency of 15.17% with a short-current density (J_sc) of 26.92 mA cm⁻², and a fill factor (FF) of 70.44%, which could be put down to its more balanced charge mobility (μ_h/μ_e = 1.10). This work demonstrates that controlling the content of DTBT with siloxane-terminated side chains in the terpolymers can supply a positive driving force for phase separation and molecular packing of the active layer, which provides a novel way to improve the photovoltaic performance.