Alkali Metal Cations Modulate the Energy Level of SnO2 via Micro-agglomerating and Anchoring for Perovskite Solar Cells.

N-type tin oxide (SnO2) films are commonly used as an electron transport layer (ETL) in perovskite solar cells (PSCs). However, SnO2 films are of poor quality due to facile agglomeration under a low-temperature preparation method. In addition, energy level mismatch between the SnO2 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 SnO2 precursor to solve these problems. First, it is found that the hydrolyzed alkali metal cations tend to change colloid size distribution of SnO2, in which Na-Oxalate with suitable basicity leads to most uniform colloid size distribution and high-quality SnO2-Na films. Second, the electron conductivity is enhanced by slightly agglomerated SnO2-Na, which facilitates the transmission of electrons. Third, alkali metal cations increase the conduction band level of SnO2 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 SnO2-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 SnO2.