Low-Temperature In Situ Amino Functionalization of TiO2 Nanoparticles Sharpens Electron Management Achieving over 21% Efficient Planar Perovskite Solar Cells

Titanium oxide (TiO2) has been commonly used as an electron transport layer (ETL) of regular-structure perovskite solar cells (PSCs), and so far the reported PSC devices with power conversion efficiencies (PCEs) over 21% are mostly based on mesoporous structures containing an indispensable mesoporous TiO2 layer. However, a high temperature annealing (over 450 degrees C) treatment is mandatory, which is incompatible with low-cost fabrication and flexible devices. Herein, a facile one-step, low-temperature, nonhydrolytic approach to in situ synthesizing amino-functionalized TiO2 nanoparticles (abbreviated as NH2-TiO2 NPs) is developed by chemical bonding of amino (-NH2) groups, via Ti-N bonds, onto the surface of TiO2 NPs. NH2-TiO2 NPs are then incorporated as an efficient ETL in n-i-p planar heterojunction (PHJ) PSCs, affording PCE over 21%. Cs(0.05)FA(0.83)MA(0.12)PbI(2.55)Br(0.45) (abbreviated as CsFAMA) PHJ PSC devices based on NH2-TiO2 ETL exhibit the best PCE of 21.33%, which is significantly higher than that of the devices based on the pristine TiO2 ETL (19.82%) and is close to the record PCE for devices with similar structures and fabrication procedures. Besides, due to the passivation of the surface trap states of perovskite film, the hysteresis of current-voltage response is significantly suppressed, and the ambient stability of devices is improved upon amino functionalization.