Multi-functional Strategy: Ammonium Citrate-Modified SnO ETL for Efficient and Stable Perovskite Solar Cells.

The tin oxide (SnO) electron transport layer (ETL) plays a crucial role in perovskite solar cells (PSCs). However, the heterogeneous dispersion of commercial SnO colloidal precursors is far from optimized, resulting in dissatisfied device performance with SnO ETL. Herein, a multifunctional modification material, ammonium citrate (TAC), is used to modify the SnO ETL, bringing four benefits: (1) due to the electrostatic interaction between TAC molecules and SnO colloidal particles, more uniformly dispersed colloidal particles are obtained; (2) the TAC molecules distributed on the surface of SnO provide nucleation sites for the perovskite film growth, promoting the vertical growth of the perovskite crystal; (3) TAC-doped SnO shows higher electron conductivity and better film quality than pristine SnO while offering better energy-level alignment with the perovskite layer; and (4) TAC has functional groups of C═O and N-H containing lone pair electrons, which can passivate the defects on the surface of SnO and perovskite films through chemical bonding and inhibit the device hysteresis. In the end, the device based on TAC-doped ETL achieved an increased power conversion efficiency (PCE) of 21.58 from 19.75% of the reference without such treatment. Meanwhile, the PSCs using the TAC-doped SnO as the ETL maintained 88% of their initial PCE after being stored for about 1000 h under dark conditions and controlled RH of 10-25%.