Abstract:

Perovskite solar cells (PSCs) have recently become one of the fastest-growing research fields. Furthermore, the PSCs that have a SnO₂ nanoparticle layer as an electron transport layer (ETL) have received extensive attention. The SnO₂-ETL layer can be prepared at a low temperature, which makes it suitable for flexible device development. However, the energy levels of the SnO₂ layer do not sufficiently match the energy levels of the perovskite light-absorbing layer, which largely affects the charge carrier extraction and reduces the open current–voltage (V_oc) of PSCs. Additionally, the interface between the ETL and perovskite layer always has defects, which cause charge recombination and affect the power conversion efficiency (PCE) of PSCs. Therefore, the interfacial engineering at the SnO₂/perovskite layer is crucial to address these issues. Researchers are looking for suitable passivation materials that could align the energy band and decrease the defect density. Halide materials, such as KCl and NH₄Cl, are promising solutions to solve these problems. However, the preparation process has to be explored, and the mechanism of halide ions at the interface is unclear. This study investigates the effects of SnO₂ surface halogenation on the photovoltaic performance of PSCs in depth. The SnO₂ surface was passivated using tetrabutylammonium chloride (TBAC), tetrabutylammonium bromide (TBAB), and tetrabutylammonium iodide (TBAI) and the concentration gradient of the passivation solution was studied. Extensive characterization of the perovskite layers and PSC devices demonstrated the positive effects of SnO₂ surface halogenation on the SnO₂/perovskite interfacial properties. The causes of improved performance of the interfacial-engineered devices were studied using charge carrier dynamics. Interfacial engineering was further investigated by performing first-principles calculations based on density functional theory (DFT) to determine the energy, structure, charge density, density of states, work function, etc. Experiments and theoretical calculations proved that TBAC could be an optimal passivation material for the SnO₂ surface. Furthermore, the passivation effect became more apparent with the increase in solution concentration. TBAC could promote perovskite crystal growth, decrease defects at the interface, and increase the internal recombination resistance. Consequently, the photovoltaic performance of PSCs was improved. The halide ions on the SnO₂ surface could interact with Sn atoms, which increase the charge density and achieves high-efficiency charge extraction. This work shows the significance of improving the photovoltaic performance of PSCs along with providing physical principles for the interfacial engineering of PSCs toward achieving high efficiency.

Key words: Perovskite solar cell, SnO₂ Surface halogenation, Interface engineering, Density functional theory, Charge transfer kinetics