Naphthalene diimide based polymer as electron transport layer in inverted perovskite solar cells

The solution-processable electron transport layer (ETL) is essential for the electron transport and extraction of inverted perovskite solar cells (PSCs). A new naphthalene diimide based copolymer with phosphite ester pendants (PFNDI) is developed as an ETL in inverted PSCs. The naphthalene diimide backbone and the phosphite ester pendants endow PFNDI a good electron transport property and a strong binding force on perovskite. By introducing PFNDI on the perovskite interface, we succeed in improving surface morphologies of perovskite film, reducing the trap density and facilitating electron transport properties. The PFNDI-based devices exhibit an optimal power conversion efficiency of 18.25% (with the V_OC = 1.068 V, J_SC = 22.53 mA cm⁻² and FF = 75.85%) and fine long-term stabilities. The un-encapsulated PFNDI-based devices maintain 80% of their initial performance after storage for more than 500 h at 20 °C in air with a relative humidity of 25% and 75% of their initial efficiency after continuously heating for about 300 h. Our work demonstrated that NDI-based polymers with proper pendant groups can be the ideal n-type organic materials for high-performance PSCs.