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, TiO 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 TiO/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) TiO/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 TiO 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 TiO removes charge recombination centers and shifts the bands upward. Therefore, interface defect passivation by oxidation of Ti states, the organic cation layer, and an upward band bending at the TiO/perovskite interface explain the origin of an improved electron extraction and hole-blocking nature of TiO in the n-i-p perovskite solar cells.