Multi-functional Strategy: Ammonium Citrate-Modified SnO₂ ETL for Efficient and Stable Perovskite Solar Cells

The tin oxide (SnO₂) electron transport layer (ETL) plays a crucial role in perovskite solar cells (PSCs). However, the heterogeneous dispersion of commercial SnO₂ colloidal precursors is far from optimized, resulting in dissatisfied device performance with SnO₂ ETL. Herein, a multifunctional modification material, ammonium citrate (TAC), is used to modify the SnO₂ ETL, bringing four benefits: (1) due to the electrostatic interaction between TAC molecules and SnO₂ colloidal particles, more uniformly dispersed colloidal particles are obtained; (2) the TAC molecules distributed on the surface of SnO₂ provide nucleation sites for the perovskite film growth, promoting the vertical growth of the perovskite crystal; (3) TAC-doped SnO₂ shows higher electron conductivity and better film quality than pristine SnO₂ 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 SnO₂ 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 SnO₂ 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%.