Electron Transport Materials: Evolution and Case Study for High-Efficiency Perovskite Solar Cells

The presence of the electron transport layer (ETL) in perovskite solar cells (PSCs) is critical due to the requirement of enhancing the electron collection selectivity. ETLs are essential for achieving a high open-circuit voltage (VOC), high fill factor (FF), better transport of directional charges, better absorption of incoming light, and thermodynamically competent operation of photogenerated carrier populations. ETLs are sorted as organic, inorganic, or mixed, with different stability, cost effect, and directional charge transport ability. For instance, by using metal oxides as ETLs, power conversion efficiencies (PCEs) higher than 23% are reached for PSCs. Despite the advantages of metal oxide–ETLs and other organic or mixed ETLs, some questions still have to be addressed to achieve better PCEs, like how to passivate or eliminate the surface traps, how to upgrade the comprehension of the heterointerface, and optimization of morphology. Herein, different considerations of ETLs in different physical and environmental conditions, and different deposition methods used, are presented. Finally, the current studies and future challenges are analyzed in the domain of highly efficient PSCs with various ETLs.