Bulky side-chain density effect on the photophysical, electrochemical and photovoltaic properties of polythiophene derivatives

Low band-gap polythiophene (PT) derivatives, with bulky conjugated side-chains composed of the triphenylamine, thiophene, and vinylene groups (TPATh), are synthesized. The copolymers, synthesized by Grignard metathesis and Stille coupling with different copolymer configurations and side-chain densities, are regioregular-TPATh-PT (rr-TPATh-PT) and random-TPATh-PT (r-TPATh-PT), respectively. The incorporation of bulky conjugated moiety curtails the effective conjugation length in the main chain; thus, low HOMO levels are obtained for the copolymers. Moreover, r-TPATh-PT with less bulky side-chain content exhibits a better conjugation along the polymer backbone than rr-TPATh-PT. Higher absorption intensity in the vision region is observed for r-TPATh-PT in comparison with rr-TPATh-PT. In addition, polymer solar cells (PSCs) are fabricated based on an interpenetrating network of PT derivatives as the electron donor and the fullerene derivatives (PC61BM and PC71BM) as the electron acceptors. Better compatibility is observed for the r-TPATh-PT/PC61BM-blend film as compared to the rr-TPATh-PT/PC61BM-blend film. Higher photovoltaic (PV) performances of the r-TPATh-PT/PC61BM-based PSCs are observed in comparison with the rr-TPATh-PT/PC61BM-based PSCs. The power conversion efficiency (PCE) of the PSC based on the blend of r-TPATh-PT and PC61BM (w/w = 1:1) reaches 0.94% under an illumination of AM 1.5G, 100 mW cm−2, which is almost twice that of the cell based on rr-TPATh-PT. Further improvement of PV performance is achieved for the PSC fabricated from the blend of r-TPATh-PT and fullerene derivative PC71BM (w/w = 1:3), with a short-circuit current of 6.83 mA cm−2, an open-circuit voltage of 0.71 V and a PCE of 1.75%.