ITIC surface modification to achieve synergistic electron transport layer enhancement for planar-type perovskite solar cells with efficiency exceeding 20%

The electron transport layer (ETL), which also serves as the hole-blocking layer, is a key component in planar perovskite solar cells (PSCs). The commonly used ETL is an anatase-TiO2 (an-TiO2) film due to its excellent optical transmittance, chemical stability and semi-conducting characteristics. Nevertheless, its rough surface and plenty of surface defects often lead to a substandard perovskite film and large J-V hysteresis. Herein, a novel low-trap-density ETL is developed by surface modification of the an-TiO2 film using small-molecular ITIC. As a result, the device efficiency has been dramatically increased from 17.12% to 20.08%, entering the league of the highest planar-type perovskite cells. Moreover, the J-V hysteresis has been significantly reduced. Further investigation shows that the ITIC smoothens the TiO2 surface, passivates defects or dangling bands parasitizing the TiO2 surface, and optimizes the device band alignment. In addition, it is demonstrated that the thin ITIC promotes the formation of high quality, uniform perovskite films with better surface coverage and large grain size, implying that there is a synergistic effect between the low-trap-density ITIC and high-mobility TiO2 in improved PSC performance.