Facile NaF Treatment Achieves 20% Efficient ETL-Free Perovskite Solar Cells

Electron transporting layer (ETL)-free perovskite solar cells (PSCs) exhibit promising progress in photovoltaic devices due to the elimination of the complex and energy-/timeconsuming preparation route of ETLs. However, the performance of ETL-free devices still lags behind conventional devices because of mismatched energy levels and undesired interfacial charge recombination. In this study, we introduce sodium fluoride (NaF) as an interface layer in ETL-free PSCs to align the energy level between the perovskite and the FTO electrode. KPFM measurements clearly show that the NaF layer covers the surface of rough underlying FTO very well. This interface modification reduces the work function of FTO by forming an interfacial dipole layer, leading to band bending at the FTO/perovskite interface, which facilitates an effective electron carrier collection. Besides, the part of Na+ ions is found to be able to migrate into the absorber layer, facilitating enlarged grains and spontaneous passivation of the perovskite layer. As a result, the efficiency of the NaF-treated cell reaches 20%, comparable to those of state-of-the-art ETL-based cells. Moreover, this strategy effectively enhances the operational stability of devices by preserving 94% of the initial efficiency after storage for 500 h under continuous light soaking at 55 degrees C. Overall, these improvements in photovoltaic properties are clear indicators of enhanced interface passivation by NaF-based interface engineering.

Automated summary

In this study, sodium fluoride (NaF) was introduced as an interface layer in ETL-free perovskite solar cells (PSCs) to improve the energy level alignment and reduce charge recombination. The NaF layer effectively covered the rough FTO surface and reduced the work function, facilitating electron carrier collection. Additionally, some Na+ ions migrated into the absorber layer, leading to larger grains and passivation of the perovskite layer. The NaF-treated cells achieved an efficiency of 20% and maintained 94% of the initial efficiency after 500 hours of light soaking at 55 degrees C, indicating enhanced interface passivation.