Alkali Metal Cations Modulate the Energy Level of SnO 2 via Micro-agglomerating and Anchoring for Perovskite Solar Cells.

N-type tin oxide (SnO 2 ) films are commonly used as an electron transport layer (ETL) in perovskite solar cells (PSCs). However, SnO 2 films are of poor quality due to facile agglomeration under a low-temperature preparation method. In addition, energy level mismatch between the SnO 2 and perovskite (PVK) layer as well as interfacial charge recombination would cause open-circuit voltage loss. In this work, alkali metal oxalates (M-Oxalate, M = Li, Na, and K) are doped into the SnO 2 precursor to solve these problems. First, it is found that the hydrolyzed alkali metal cations tend to change colloid size distribution of SnO 2 , in which Na-Oxalate with suitable basicity leads to most uniform colloid size distribution and high-quality SnO 2 -Na films. Second, the electron conductivity is enhanced by slightly agglomerated SnO 2 -Na, which facilitates the transmission of electrons. Third, alkali metal cations increase the conduction band level of SnO 2 in the sequence of K + , Na + , and Li + to promote band alignment between ETLs and perovskite. Based on the optimized film quality and energy states of SnO 2 -Na, the best PSC efficiency of 20.78% is achieved with a significantly enhanced open-circuit voltage of 1.10 V. This work highlights the function of alkali metal salts on the colloid particle distribution and energy level modulation of SnO 2 .