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

Significant open-circuit-voltage (VOC) 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 VOC 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 VOC loss by 0.17 V, which paves an important road to develop high-VOC CsPbI2Br perovskite solar cells.