Bulk and surface defect manipulation of the ZnO ETL for all-inorganic CsPbBr₃ perovskite solar cells

The electron transport layer (ETL) in traditional CsPbBr₃ perovskite solar cells (PSCs) without a hole transport layer (HTL) presents the capability to transport electrons and block hole transport, which radically affects the photovoltaic performance of PSCs. However, ZnO ETL prepared using the classic sol-gel method exhibits obvious drawbacks, such as serious interfacial recombination reactions, inducement of oxygen vacancies (V_O) and zinc interstitials (Zn_i). Herein, we demonstrate that alkali metal chloride (e.g. KCl), serving as the passivating agent for the surface and bulk phase, can promote surface modification and doping in the ZnO ETL, respectively. Experimental results show that the interaction between K⁺ and Zn²⁺, and the occupation of V_O by Cl^-, suppresses the internal defect states of the ZnO films, which enhances the crystal coordination between ZnO and CsPbBr₃ and improves the film morphology and the quality of the upper perovskite (PVK) films. Experimental PSCs based on the doping approach achieved the highest power conversion efficiency (PCE) of 9.22%, which ranks the highest PCE of the (FTO/ITO)/ZnO/CsPbBr₃/carbon structure. Moreover, the unpackaged devices of the two experimental PSCs could maintain 97.15% and 74.76% of the original PCE after being exposed for 28 days in the ambient environment, demonstrating the powerful effect of KCl on the regulation of surface and bulk phase defects in the ZnO ETL.