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

Abstract Heterocyclic phenothiazine has been proven effective in designing organic hole-transporting materials for perovskite solar cells (PSCs) as it has excellent electron-donating property, 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 phenothiazine based 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 doesn’t alter the absorption, emission and energy levels of phenothiazine based HTMs. However, it can effectively facilitate the dispersion of lithium salt additive to form a uniform thin film, which is generally achieved by additional additive of 4-tert-butylpyridine (tBP). This strategy remarkably improves the film quality and stability of spin-coated HTM for application in PSCs. As a result, PSCs based on tBP 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 phenothiazine based HTMs for PSCs.