Synthesis and photovoltaic properties of copolymers of carbazole and thiophene with conjugated side chain containing acceptor end groups

Abstract

Four alternating copolymers of carbazole (Cz) and thiophene (Th) with thienylene-vinylene (TV) conjugated side chains containing different acceptor end groups of aldehyde (PCzTh–TVCHO), mono-cyano (PCzTh–TVCN), di-cyano (PCzTh–TVDCN), and 1,3-diethyl-2-thiobarbituric acid (PCzTh–TVDT), have been designed and synthesized. The structures and properties of the main chain donor–side chain acceptor D–A copolymers were fully characterized. Through changing the acceptor group attached to the TV conjugated side chain on thiophene unit, the electronic properties and energy levels of the copolymers were effectively tuned. The effect of substituent on the electronic structures of the copolymers was also studied by molecular simulation. These results indicate that it is a simple and effective approach to tune the bandgap in a conjugated polymer by attaching an acceptor end group on the conjugated side chains. The polymers were used as donor in polymer solar cells. The device based on PCzTh–TVDCN/PC₇₀BM demonstrates a power conversion efficiency of 2.16% with a high V_oc of 1.03 V under the illumination of AM1.5G, 100 mW cm⁻².