High-Performance Flexible Perovskite Solar Cells with Effective Interfacial Optimization Processed at Low Temperatures

Exploration of low-temperature solution-processing methodologies for fabricating planar perovskite solar cells (PSCs) is important for industrial mass production and helps simplify the manufacture and design of flexible perovskite solar cells. However, the interface between electron-transport layers (ETLs) and perovskite layers is crucial for the development of highly efficient flexible PSCs. We report a drastically improved solar cell efficiency through surface optimization of TiO2 ETLs by using a simple and inexpensive ionic compound that shows high optical transparency and superior electron mobility. Solution-derived TiO2 nanocrystalline films are employed at low temperatures as ETLs through solution processing. The modification of TiO2 with NH4Cl can increase the interactions between the surface and organic-inorganic hybrid perovskites; Cl anions lead to a stronger interfacial coupling between TiO2 and perovskite. Ammonium cations tend to combine with perovskite. Due to this strong combined effect of the ionic compound, the efficiency of PSC from low-temperature solution processing reaches 18.71% on rigid glass/indium tin oxide (ITO) for an improvement of 12.6% over a control device using bare TiO2. Furthermore, the power conversion efficiency (PCE) can reach an efficiency of 17.69% for the ITO/PEN substrates. This work contributes to the evolution of flexible PSCs with simple fabrication and high device performance.