Improved photovoltaic properties of copolymer donors by regulating alkyl and alkylsilyl side chains

Abstract

By changing the ratio of alkyl and tripropylsilyl side chain of the donor unit, a series of copolymer donor materials, named PBT1, PBT10Si, PBT20Si, PBT30Si and PBDB-Si, are synthesized and applied in polymer solar cells (PSCs). All copolymers exhibit complementary absorption spectra and matched energy levels with small molecular acceptor Y6. Grazing incidence wide angle X-ray scattering (GIWAXS) measurements demonstrate that all copolymer pure films and blend films show distinct face-on orientations. Whereas, the blend film based on PBT20Si:Y6 manifests appropriate phase separation and less bimolecular recombination, which is in favor of effective exciton disintegration and charge transfer. As a result, the PBT20Si-based device exhibits a relatively high hole mobility (μh) value of 4.99 × 10−4 cm2 V−1 s−1, and obtains the highest power conversion efficiency (PCE) of 13.06%. These results indicate that the moderate introduction of tripropylsilyl side chains into polymer donors can improve the molecular aggregation and suppress bimolecular recombination of the photoactive layer, which is an effective strategy worthy of further research to promote the device performances.