A Type I Heterointerface between Amorphous PbI₂ Overlayers on Crystalline CsPbI₃

PbI2, an insulating byproduct formed from lead halide perovskite decomposition, is thought to pose issues for implementation of these materials in photovoltaic applications. Here, we examine the electronic properties of amorphous PbI2 overlayers on the CsPbI3– model perovskite. Using a combination of molecular dynamics and density functional theory, we report that α-PbI2 does not introduce midgap states between the CsPbI3 band extrema; instead, it brackets the frontier bands of the bulk CsPbI3 lattice forming a type I heterointerface. While this overlayer may be detrimental to solar devices, it offers a unique opportunity to form heterogeneous nanocrystals, akin to strategies employed for other binary semiconductors (e.g., PbSe nanocrystals coated in wider gap PbS).