TiO2/perovskite interface with oxyacid salt for efficient carbon-based CsPbI2Br solar cell: The chemical adsorption induced surface electron transfer modulates energy level

Abstract

The photovoltaic performance of carbon-based CsPbI₂Br solar cells is considerably limited by the quality of electron transporting layer (ETL)/perovskite interface. Specifically, the ETL/perovskite interface contains defects such as iodine vacancies. Meantime, the Fermi energy (E_F) level of ETL is generally much higher than ones of perovskite and transparent conductive oxide (TCO) electrode. The above two issues together contribute to serious charge carrier recombination in devices. Oxyacid salts have been developed to be promising interfacial materials for ETL/perovskite interface, owing to their effective defect passivation and energy level tuning. However, the structure-functionality relationship of these materials has not been well established due to the unclear correlation between interfacial interaction and energy level tuning. Herein, through using CdSO₄ as modifier for TiO₂ ETL and controlling the thickness, we investigate the interfacial interaction and successfully correlates it to energy level tuning. It is shown that the chemically adsorbed ${\text{SO}}_{\text{4}}^{\text{2-}}$ withdraws electrons from TiO₂. This interaction results in a downshifted E_F level for TiO₂ and sets up a surface dipole moment which upshifts the energy band of perovskite. As a result, we obtain improved energy level alignment at both TCO/ETL interface and ETL/perovskite interface. The above effects lead to enhanced electron transport and mitigated charge carrier loss. Therefore, the power conversion efficiency of carbon-based CsPbI₂Br solar cell is increased from 13.61 % to 15.09 %. Equally noteworthy is the remarkably enhanced environmental stability exhibited by these devices.