Electron Transport Bilayer Design for Perovskite-Based Tandem Solar Cells Using Thermal Evaporation

The development of high-performance perovskite solar cells (PSCs) and perovskite-based tandem solar cells (PTSCs) has attracted significant attention from researchers owing to their simple and cost-effective fabrication process. However, careful consideration is required regarding the selection of charge-transport layers, which play a key role in improving the efficiency and stability of devices. In p–i–n (inverted) PSCs, fullerene and its derivatives are widely employed as electron-transport layers (ETLs). Among them, fullerene is compatible with low-temperature and vacuum-based solar cell fabrication processes. However, the single layer of fullerene tends to form nonuniform layers due to its aggregation propensity. Furthermore, it struggles to withstand the damage that occurs during the formation of transparent conductive layers in tandem configurations. To overcome the limitation of fullerene, employing bilayer ETL has emerged as a promising strategy. Therefore, in this study, the evaporation processes for ETLs in inverted PSCs are focused on, emphasizing the importance of bilayer ETL architectures in enhancing the efficiency and durability of PTSCs. The discussion highlights material options for bilayer ETLs and examines optimal thickness parameters to achieve superior efficiency. By providing a comprehensive understanding of bilayer ETLs, this review aims to guide future advancements in the PSCs and PTSCs technologies.