Donor–Acceptor–Donor Type Cyclopenta[2,1-b;3,4-b′]dithiophene Derivatives as a New Class of Hole Transporting Materials for Highly Efficient and Stable Perovskite Solar Cells

Three new donor–acceptor–donor type (D–A–D) hole-transporting materials (HTMs), YC-1–YC-3, based on the 4-dicyanomethylene-4H-cyclopenta[2,1-b;3,4-b′]dithiophene (DiCN-CPDT) core structure endowed with two arylamino-based units as peripheral groups were designed, synthesized, and applied in perovskite solar cells (PSCs). Hole mobility, steady-state photoluminescence, thin-film surface morphology on top of the perovskite layer, and photovoltaic performance for the YC series were systematically investigated and compared with those of Spiro-OMeTAD. It was found that YC-1 exhibited more efficient hole transport and extraction characteristics at the perovskite/HTM interface. Meanwhile, the film of YC-1 showed a homogeneous and dense capping layer coverage on the perovskite layer without any pinholes, leading to the improvement of the fill factor and open circuit voltage. The PSC device based on YC-1 as a HTM exhibited a high power conversion efficiency (PCE) of 18.03%, which is comparable to that of the device based on the benchmark Spiro-OMeTAD (18.14%), and also a better long-term stability with 85% of the initial efficiency retained in excess of 500 h under the condition of 30% relative humidity, presumably due to the hydrophobic nature of the material. This work demonstrates that the dicyanomethylene-CPDT-based derivatives are promising HTMs for efficient and stable PSCs.