Heteroatom engineering on spiro-type hole transporting materials for perovskite solar cells

A series of spiro-type hole transporting materials, spiro-OMeTAD, spiro-SMeTAD and spiro-OSMeTAD, with methoxy, methylsulfanyl or half methoxy and half methylsulfanyl terminal groups are designed and prepared. The impact of varied terminal groups on bulk properties, such as photophysical, electrochemical, thermal, hole extraction, and photovoltaic performance in perovskite solar cells is investigated. It is noted that the terminal groups of the hole transporting material with half methoxy and half methylsulfanyl exhibit a better device performance and decreased hysteresis compared with all methoxy or methylsulfanyl counterparts due to better film-forming ability and improved hole extraction capability. Promisingly, the spiro-OSMeTAD also shows comparable performance than high-purity commercial spiro-OMeTAD. Moreover, the highest power conversion efficiency of the optimized device employing spiro-OSMeTAD exceeding 20% has been achieved.