Alternative Electron Transport Layer Based on Al-Doped ZnO and SnO₂ for Perovskite Solar Cells: Impact on Microstructure and Stability

The stability of perovskite solar cells (PSCs) is currently a hot topic, but the investigation as well as the understanding of the degradation mechanisms remain incomplete. We present the intrinsic degradation mechanisms of a traditional monocation perovskite in photovoltaic devices performed with various electron transport layers (ETLs). The monocation perovskite material is a Cl-doped CH3NH3PbI3 system, known to provide a favorable morphology leading to improved efficiency. With the long-term view of developing low-temperature processes for PSCs, two emerging ETLs compatible with TiO2 substitution were chosen in order to study both the initial perovskite state and performance, along with their stability after aging. Aluminum-doped zinc oxide (AZO) and tin oxide (SnO2) were thus selected and placed as the ETL in a planar NIP solar cell architecture, leading to different n-type substrates that can imply deviations in the formation and/or degradation of the perovskite layer. As a result, the overall performance and stability for the designed devices were strongly impacted using AZO as compared to SnO2. A detailed investigation using complementary characterization techniques helped in the understanding of the initial compositions and morphologies of the perovskite according to the underlying ETL layer used and their unalike evolution during mild aging conditions (inert atmosphere, dark, 35 °C). Infrared spectroscopy, X-ray diffraction, UV–visible absorption, photoluminescence, scanning electron microscopy, and current–voltage characteristics brought a new understanding of the local degradation mechanisms and their consequences on the macroscopic functional properties of PSC devices. Two different degradation mechanisms specific to the ETL have been distinguished. The ETL nature controls the perovskite microstructure and thereby the performance and stability of the complete device.