Phenothiazine Functionalized Multifunctional A-π-D-π-D-π-A-Type Hole-Transporting Materials via Sequential C-H Arylation Approach for Efficient and Stable Perovskite Solar Cells

Three phenothiazine-based A-π-D-π-D-π-A-type small molecules containing various terminal acceptor units, which act as Lewis base blocks, have been synthesized via an efficient and step-economical, direct C-H arylation strategy in the aim toward the development of hole-transporting materials (HTMs) with multifunctional features (such as efficient hole extraction layer, trap passivation layer, and hydrophobic protective layer) for perovskite solar cells (PrSCs). Optical-electrochemical correlation and density functional theory studies reveal that dicyanovinylene acceptor in SGT-421 downshifted the highest occupied molecular orbital (HOMO) level (-5.41 eV), which is more proximal to the valence band (-5.43 eV) of the perovskite, whereas N-methyl rhodanine in SGT-420 and 1,3-indanedione (IND) in SGT-422 destabilized the HOMO, leading to an increased interfacial energy-level offset. SGT-421 exhibits superior properties in terms of a sufficiently low-lying HOMO level and favorable energy-level alignment, intrinsic hole mobility, interfacial hole transfer, hydrophobicity, and trap passivation ability over spiro-OMeTAD as a benchmark small-molecule HTM. As envisaged in the design concept, SGT-421-based PrSC not only yields a comparable efficiency of 17.3% to the state-of-art of spiro-OMeTAD (18%), but also demonstrates the enhanced long-term stability compared to the spiro-OMeTAD because of its multifunctional features. More importantly, the synthetic cost of SGT-421 is estimated to be 2.15 times lower than that of spiro-OMeTAD. The proposed design strategy and the study of acceptor-property relationship of HTMs would provide valuable insights into and guidelines for the development of new low-cost and efficient multifunctional HTMs toward the realization of efficient and long-term stable PrSCs.