Unraveling the Role of Perovskite in Buried Interface Passivation

Interfaces in perovskite solar cells play a crucial role in their overall performance, and therefore, detailed fundamental studies are needed for a better understanding. In the case of the classical n-i-p architecture, TiO₂ is one of the most used electron-selective layers and can induce chemical reactions that influence the performance of the overall device stack. The interfacial properties at the TiO₂/perovskite interface are often neglected, owing to the difficulty in accessing this interface. Here, we use X-rays of variable energies to study the interface of (compact and mesoporous) TiO₂/perovskite in such a n-i-p architecture. The X-ray photoelectron spectroscopy and X-ray absorption spectroscopy methods show that the defect states present in the TiO₂ layer are passivated by a chemical interaction of the perovskite precursor solution during the formation of the perovskite layer and form an organic layer at the interface. Such passivation of intrinsic defects in TiO₂ removes charge recombination centers and shifts the bands upward. Therefore, interface defect passivation by oxidation of Ti³⁺ states, the organic cation layer, and an upward band bending at the TiO₂/perovskite interface explain the origin of an improved electron extraction and hole-blocking nature of TiO₂ in the n-i-p perovskite solar cells.