Naphthalene Diimide-Modified SnO₂ Enabling Low Temperature Processing for Efficient ITO-Free Flexible Perovskite Solar Cells

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 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 structure, which utilize a metal oxide as an electron transport layer (ETL), necessitate high annealing temperature. 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. Therefore, the development of ETL that can be processed at low temperature is one of the biggest challenges.<br/>In this work, we demonstrated an innovative method of forming high-quality ETL at low temperature for highly efficient flexible PSC. By simply adding a (sulfobetaine-<i>N</i>,<i>N</i>-dimethylamino)propyl naphthalene diimide (NDI-B) into the tin dioxide (SnO₂), we obtained high quality and good coverage of SnO₂ film in spite of low temperature annealing. NDI-B plays a crucial role in ETL/perovskite interface; NDI-B suppresses the interfacial nonradiative recombination between ETL and perovskite by passivating defect centers and modulating the work function difference between ETL and perovskite. Consequently, we have successfully fabricated a low temperature processed flexible PSC and obtained the highest PCE of 17.48 %.