Pyridine functionalized phenothiazine derivatives as low-cost and stable hole-transporting material for perovskite solar cells

Heterocyclic phenothiazine (PTZ) has been proven effective in designing organic hole-transporting materials for perovskite solar cells (PSCs) as it has excellent electron-donating properties, straightforward synthesis, and feasible structure modifications. However, the resulting photovoltaic performance is still unsatisfactory, and the long-term stability has not been studied. Herein, we report a couple of PTZ-based hole transporting materials (HTMs) (PT-H and PH-HP) to investigate the efficacy of a pyridine functionalization on improving film morphology, hole mobility, and device performance, as well as long-term stability. The covalent incorporation of a pyridine unit in PT-HP does not alters the absorption, emission, and energy levels of PTZ-based HTMs. However, it can effectively facilitate the dispersion of lithium salt additive to form a uniform thin film, which is generally achieved by an additional additive of 4-tert-butylpyridine. This strategy remarkably improves the film quality and stability of spin-coated HTM for application in PSCs. As a result, PSCs based on 4-tert-butylpyridine-free PT-HP achieve a power conversion efficiency of 19.09%, significantly outperforming the reference HTM of PT-H (15.20%). More importantly, the stability of PT-HP based PSCs is successfully enhanced. The work paves the way to develop low-cost, efficient, and stable PTZ-based HTMs for PSCs.