Origin of extended UV stability of 2D atomic layer titania-based perovskite solar cells unveiled by ultrafast spectroscopy.

The inherent instability of UV-induced degradation in TiO-based perovskite solar cells was largely improved by replacing the anatase-phase compact TiO layer with an atomic sheet transport layer (ASTL) of two-dimensional (2D) TiO. The vital role of microscopic carrier dynamics that govern the UV stability of perovskite solar cells was comprehensively examined in this work by performing time-resolved pump-probe spectroscopy. In conventional perovskite solar cells, the presence of a UV-active oxygen vacancy in compact TiO prohibits current generation by heavily trapping electrons after UV degradation. Conversely, the dominant vacancy type in 2D TiO ASTL is a titanium vacancy, which is a shallow acceptor and is not UV-sensitive. Therefore, it significantly suppresses carrier recombination and extends UV stability in perovskite solar cells with a 2D TiO ASTL. Other carrier dynamics, such as electron diffusion, electron injection, and hot hole transfer processes, were found to be less affected by UV irradiation. Quantitative pump-probe data clearly show a correlation between the carrier dynamics and UV aging of perovskite solar cells, thus providing profound insight into the factors driving UV-induced degradation in perovskite solar cells and the origin of its performance.