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.