Recent Progress in Hybrid Perovskite Solar Cells Based on p -Type Small Molecules as Hole Transporting Materials

: Organic-inorganic perovskite solar cells (PSCs) have become one of the most promising solar cells, as the power conversion efficiency (PCE) has increased from less than 5% in 2009 to certi fi ed values of over 22%. In the typical PSC device architecture, hole transport materials that can effectively extract and transmit holes from the active layer to the counter electrode (HTMs) are indispensable. The well-known small molecule 2,2',7,7'-tetrakis-( N , N -di-4-methoxy-phenyl amino)-9,9'-spirobifluorene possible routes for further HTM design, and provide some conclusions.