Molten Li Salt Modified Buried Interface Releases Strain and Realizes High Performance n‐<i>i</i>‐p Type CsPbI₃ Perovskite Solar Cells

Inorganic CsPbI₃ perovskite solar cells (pero-SCs) have attracted great attention in photovoltaic research in recent years. Thermal annealing treatment is important for device optimization of the pero-SCs, nevertheless, traditional annealing process leads to residual strain owing to the difference of thermal expansion coefficients between perovskite active layer and SnO₂ electron transporting layer (ETL) in n-i-p pero-SCs. Additionally, abundant defects and mismatched energy levels restrict the application of SnO₂ as ETL in the CsPbI₃ n-i-p pero-SCs. Here, by applying a molten mixture of Li salts during the fabrication of the ETL, the residual strain is eliminated and the energy level alignment is optimized. The molten salt (MS) treatment improves the quality of the SnO₂ ETL, facilitates better crystallization of the perovskite active layer, and reduces lattice strain. As a result, the n-i-p pero-SCs based on CsPbI₃ with SnO₂ ETL show a notable increase in power conversion efficiency, from 17.46% for the device without the MS treatment to 20.49% for that with the MS treatment, and enhanced stability of the pero-SCs. This approach addresses key challenges in the pero-SCs, demonstrating that optimizing the interface between the ETL and perovskite active layer can greatly improve the photovoltaic performance of the CsPbI₃-based n-i-p pero-SCs.