Mitigating Open-Circuit-Voltage Loss in CsPbI2Br Perovskite Solar Cells Based on CsAc/SnO2 Hybrid Electron Transport Layer

Significant open-circuit-voltage (V_OC) loss is present in all-inorganic perovskite solar cells (e.g., based on CsPbI₂Br), owing to energy level mismatch between the perovskite layer and charge transport layer. Here, cesium acetate (CsAc) was mixed with tin oxide (SnO₂) to form a hybrid electron transport layer that can reduce the energy level mismatch with the CsPbI₂Br perovskite layer. Ultraviolet photoelectron spectroscopy characterization suggested that the conduction band of the electron transport layer was elevated from −4.51 eV for the SnO₂-only material to −3.64 eV for the CsAc/SnO₂ hybrid form. This change due to the introduction of CsAc in the SnO₂ layer to tune its conduction band is most likely attributed to the formation of a Cs⁺–Ac^– dipole layer near the top surface of the CsAc/SnO₂ hybrid electron transport layer. Additionally, the Cs⁺ ions in the top surface of the hybrid CsAc/SnO₂ electron transport layer play a positive role in facilitating the growth of a follow-up CsPbI₂Br perovskite layer with finer grains and a denser morphology. Compared to the SnO₂-only electron transport layer-based device, CsPbI₂Br solar cells based on CsAc/SnO₂ exhibited a significantly improved photovoltaic performance. The V_OC was increased from 1.11 V for the SnO₂-only device to 1.28 V for the CsAc/SnO₂ device, while the power conversion efficiency increased from 10.9% to 14.67%. This work presents a practicable approach through modifying the SnO₂ layer with CsAc to significantly reduce the V_OC loss by 0.17 V, which paves an important road to develop high-V_OC CsPbI₂Br perovskite solar cells.