A solution-processed pillar[5]arene-based small molecule cathode buffer layer for efficient planar perovskite solar cells

A room-temperature solution-processed pillar[5]arene-based small molecule material, termed C3, has been designed, synthesized, and incorporated between a conventional PCBM electron transport layer (ETL) and a metal electrode to function as a single-layer cathode buffer layer (CBL) for efficient planar p-i-n perovskite solar cells (PVSCs). It has been found that C3 has a work function tunneling effect, which can decrease the work function of the Ag electrode; therefore, introduction of C3 successfully enhances the interface contact and reduces the interface barriers, which usually exist between fullerene derivatives and metal electrodes. It was also found that the C3 capping layer could improve the surface quality of PCBM, forming a smooth, dense and pinhole-free morphology with fewer surface defects. Thus, C3 can modify the interface between PCBM and Ag, enhance the diode properties of devices and facilitate electron transport through the devices; therefore, it is a very promising CBL material for PVSCs. A device with a hybrid PCBM ETL and a single cathode buffer layer of C3 exhibited a high power conversion efficiency (PCE) of 17.42% with negligible hysteresis, which was dramatically higher than that of a device based on a pure PCBM ETL. With the major advantages of a low-temperature solution process and interface modification, the excellent PCE of PVSCs on flexible substrates can exceed 13%. These results demonstrate that solution-processed pillar[5]arene-based small molecule materials can serve as high performance CBLs in PVSCs.