Band Alignment in Two-Dimensional Halide Perovskite Heterostructures: Type I or Type II?

Formation of heterostructures is often inevitable in two-dimensional (2D) halide perovskites and band alignment in 2D perovskite heterostructures is of central importance to their applications. However, controversies abound in literature on the band alignment of the 2D perovskite heterostructures. While external factors have been sought to reconcile the controversies, we show that the 2D perovskite heterostructures are in fact intrinsically prone to band "misalignment", driven by thermal fluctuations. Owing to the "softness" of inorganic layers in the perovskites, electron-phonon coupling at room temperature could be strong enough to override band offsets at zero temperature, leading to oscillatory band alignment between type-I and type-II at 300 K. We further demonstrate that by tuning the inorganic layers, one can increase the band offsets and stabilize the band alignment, paving the way for optoelectronic applications of the 2D perovskite heterostructures.