Electron Transporting Materials Based on n-Type Polymers in Inverted Perovskite Solar Cells: A Review

Perovskite solar cells (PSCs), despite the stability challenges they face, have made significant progress. It is predicted that this trend will continue due to the progress in composite engineering and the materials that make up the layers. It is expected to continue to achieve high performance and long-term stability in these cells. One of the key components and layers in PSCs is “electron transporting material” (ETM), playing a crucial role in electron transfer, extraction, and cell efficiency. ETMs are categorized into various types based on their constituent materials, including inorganic, organic, and polymer-based ETMs. Polymeric ETMs are of particular importance due to their unique properties and considering the limitations in other types of ETMs. Fullerene derivatives, which are organic ETMs, lack adequate electron mobility and solubility. However, in polymeric ETMs, the ability to connect different functional groups to their central core creates significant changes in the photovoltaic parameters of the cell. By choosing polymeric ETMs, potential advantages such as high electron mobility, easy purification, suitable solubility for processing during layer deposition, and desirable stability can be achieved. Polymeric ETMs with various central cores such as naphthalenediimide (NDI), perylenediimide (PDI), and bithiopheneimide (BTI) have been widely used in reverse PSCs due to their high electron mobility and the absence of additives. Among them, the highest efficiency (20.43%) relates to a cell containing the PDI-core polymer PPDIN6. Additionally, researchers use n-type polymers to modify, create effective interlayer connections, and improve the photovoltaic parameters of reverse PSCs. This review article highlights that the selection of polymeric ETMs along with specific central cores can lead to meaningful changes, resulting in desirable performance and stability in PSCs.