Recent progress in spiro-type hole transport materials for efficient and stable perovskite solar cells

Hole transport materials (HTMs), as an important part of n–i–p perovskite solar cells (PSCs), are one of the main bottlenecks to further improving the efficiency and stability of devices. Since the introduction of Spiro-OMeTAD, it has shown remarkable power conversion efficiency (PCE) in n–i–p PSCs due to its high film quality and matched energy level. However, the high cost, low carrier mobility and poor stability of Spiro-OMeTAD greatly limit the practical application and commercialization of PSCs. Therefore, a large number of HTMs have been reported to replace the traditional Spiro-OMeTAD. Among them, spiro-type HTMs are one class of the most promising competitors with photovoltaic performance comparable to that of Spiro-OMeTAD. But few reviews have summarized spiro-type HTMs. In this review, we summarize efficient spiro-type HTMs reported in the last 8 years and discuss their structure–property relationships and photovoltaic performance. Also, we conclude the design points of spiro-type HTMs. It is hoped that this review can provide guidance for developing low-cost and efficient HTMs.