Towards scalability: progress in metal oxide charge transport layers for large-area perovskite solar cells

Metal oxides are emerging as prominent materials for the charge transport layer (CTL) because of their low fabrication cost, solution processability, robust material stability, and flexibility in material modification. In the construction of perovskite solar cells (PSCs), the CTL plays a pivotal role by facilitating the extraction of charge carriers photogenerated from the perovskite layer. For large-area PSCs, there is a need for scalable fabrication to achieve uniform and optimized CTLs. While PSCs have already demonstrated power conversion efficiencies (PCE) exceeding 26% at a laboratory scale, scaling up their production remains a significant challenge, thus impeding their commercial viability. To date, several researchers have made strides in developing scalable PSCs using both solution processes and vacuum deposition techniques. However, there is a pressing need for more intensive research and development. The primary goal is to fabricate CTLs that are not only efficient but also well-defined, uniform, homogeneous, and minimally defective over large areas. Therefore, in this review, we discuss the most promising metal oxide CTLs for large-area PSCs, emphasizing SnO₂, TiO₂, and NiO_x, and highlight their material processing methods. Additionally, we explore emerging strategies such as bilayer techniques, interfacial modification, and additive engineering, which aim to address and mitigate the macro/micro defects that invariably arise during the up-scaling process.