Novel inorganic electron transport layers for planar perovskite solar cells: Progress and prospective

Perovskite solar cells (PSCs) have emerged as a promising class of photovoltaic devices since they combine the benefits of high efficiency beyond 20%, low material cost, as well as easy and scalable processing. The appropriate choice of the electron transport layer (ETL) in these devices is one crucial aspect for achieving high efficient PSCs. The conventional ETL TiO₂ is not the best choice due to its relatively low conductivity and problematic photocatalytic activity. Therefore, novel ETLs have attained increasing attention and are making rapid progress and with it the further development and optimization of planar PSCs has been promoted. In this review, we start by introducing the essential functions of ETLs in planar PSCs. Next, we give an extensive description of novel ETL materials, looking at both crystalline and amorphous systems. Their emergence, development, and accompanying optimization strategies will be discussed. Additionally, we provide a brief discussion about the correlation between materials, fabrication methods, and interface related issues. In the end, we propose some prospective research subjects that will be relevant for the further development of novel ETLs.