Naphthalene Diimide‐Modified SnO ₂ Enabling Low‐Temperature Processing for Efficient ITO‐Free Flexible Perovskite Solar Cells

Abstract A low‐cost and indium‐tin‐oxide (ITO)‐free electrode‐based flexible perovskite solar cell (PSC) that can be fabricated by roll‐to‐roll processing shall be developed for successful commercialization. High processing temperatures present a challenge for the PSC fabrication on flexible substrates. The most efficient planar n–i–p PSC structures, which utilize a metal oxide as an electron transport layer (ETL), necessitate high annealing temperatures. In addition, the device performance deteriorates owing to the migration of halogen ions, which causes the oxidation of the metal electrodes. These drawbacks conflict with the development of highly efficient flexible PSCs fabricated on ITO‐free transparent electrodes. Herein, an efficient ETL material that enables low‐temperature processing is presented. Tin dioxide (SnO ₂ ) is modified by (sulfobetaine‐ N , N ‐dimethylamino)propyl naphthalene diimide (NDI‐B) and used as an ETL. The NDI‐B effectively reduces the interfacial nonradiative recombination between the ETL and perovskite and suppresses the ion migration by passivating oxygen‐vacancy defects in SnO ₂ and strongly interacting with halogen ions, respectively. Based on the NDI‐B‐blended SnO ₂ ETL, a record PCE of 17.48% is achieved in the ITO‐free flexible PSC fabricated at low temperature.