Effect of acceptor substituents on photophysical and photovoltaic properties of triphenylamine–carbazole alternating copolymers

Three alternative copolymers of carbazole (Cz) and triphenylamine (TPA) with substituent containing acceptor group of aldehyde (PCzTPA-CHO), mono-cyano (PCzTPA-CN) or di-cyano (PCzTPA-DCN), were designed and synthesized. Through manipulating the acceptor group attached to TPA unit, the electronic properties and energy levels of the copolymers were effectively tuned. The presence of pendant cyano groups leads to a decrease in the LUMO levels, but influences the HOMO levels very little, so that results in a lower bandgap and red-shift of its absorption spectrum. These results suggest a simple and effective approach for tuning the bandgap in a conjugated polymer through modification of the pendant acceptor groups. Their optoelectronic properties are largely determined by the nature of pendant substituent. Polymer solar cell based on PCzTPA-CN as donor and PC70BM as acceptor demonstrates a power conversion efficiency of 0.81% with a high Voc of 0.93 V.