Heteroatom effect on linear-shaped dopant-free hole transporting materials for perovskite solar cells

A series of linear-shaped D-pi-D and D-A-D hole transporting materials (HTMs) with different heteroatom-containing bridges, such as furan, thiophene, selenophene, 2,1,3-benzoxadiazole, 2,1,3-benzothiadiazole and 2,1,3-benzoselenadiazole, are designed, prepared and implemented in perovskite solar cells (PSCs). The effect of the variation heteroatom bridge on the photophysical, electrochemical, and the photovoltaic properties in devices are systematically investigated. D-pi-D HTMs exhibit gradually bathochromic spectrum responding along with increasing atomic numbers (from O, S to Se). However, D-A-D molecules exhibit no regular variation trend of photophysical and electrochemical properties. When used in PSCs, the photoelectric conversion efficiency of the devices gradually decreased along with increasing atomic numbers whether D-pi-D or D-A-D HTMs. Unexpectedly, Se-containing molecules show the best hole extraction ability according to photoluminescence study. The relatively low conductivity of Se-based HTMs, resulted from gradually increased dihedral angles between the bridge and the adjacent triphenylamine side arms, may mainly limited the device performance. The results would give some insights for designing novel efficient HTMs for PSCs.