Tailoring Band Alignment toward a Type-I to Type-II Transition via Composition Engineering in Layered Perovskite Heterostructures

The efficient separation of charge excitons in type-II heterostructures is vital for the performance of heterostructure-based solar cells. In this work, we systematically investigate the structural and electronic properties of layered perovskite heterostructures, (MX₂)₂(AMTP)₂MX₄ (M = Ge, Sn, Pb; X = Br, Cl), and demonstrate that composition engineering enables a controlled transition from type-I to type-II band alignment. Based on projected band structures and band-decomposed charge-density analyses, we show that the type-II configuration arises from the orbital-level disparities between the constituent layers, which localize the VBM and CBM on different sublayers and thereby induce pronounced spatial separation of electrons and holes at the interface. The plane-averaged charge-density difference further reveals a net electron transfer from the MX₂ layer to the (AMTP)₂MX₄ layer, generating an interfacial built-in electric field that promotes charge separation. Several of the identified type-II heterostructures exhibit suitable direct band gaps and strongly allow optical transitions, indicating promising potential for optoelectronic applications. Our work demonstrates a cation–anion coupled composition-engineering strategy for achieving designer band-alignment transitions, clarifies the electronic-structure origin and interfacial charge-transfer mechanism of type-II perovskite heterojunctions, and provides theoretical guidance for band-structure engineering and material design in two-dimensional perovskite optoelectronic devices.