Yang Lu, Young‐Kwang Jung, Miloš Dubajić, Xinjuan Li, Shabnum Maqbool, Qichun Gu, Xinyu Bai, Yorrick Boeije, Xian Wei Chua, Alessandro J. Mirabelli, Taeheon Kang, Lars Sonneveld, Youcheng Zhang, Thomas A. Selby, Capucine Mamak, Kan Tang, Zhongzheng Yu, Tianjun Liu, Miguel Anaya, Stephen Barlow, Seth R. Marder, Bruno Ehrler, Caterina Ducati, Richard H. Friend, Samuel D. Stranks
Halide perovskites exhibit superior optoelectronic properties but lack precise thickness and band offset control in heterojunctions, which is critical for modular multilayer architectures such as multiple quantum wells. We demonstrate vapor-phase, layer-by-layer heteroepitaxial growth exemplified by CsPbBr 3 deposition on single crystals of PEA 2 PbBr 4 (PEA: 2-phenylethylammonium). Angstrom-level thickness control and subangstrom smooth layers enable quantum-confined photoluminescence of CsPbBr 3 from monolayer, bilayer, and through to bulk. The interfacial structure controls the electronic structure from a Cs‒PEA-terminated interface (type II heterojunction) to a PEA‒PEA-terminated interface (type I heterojunction), with a layer-tunable band offset shift exceeding 0.5 electron volts. Electron transfer from CsPbBr 3 to PEA 2 PbBr 4 for a type II Cs‒PEA heterojunction results in delayed electron-hole recombination beyond 10 microseconds. Precise quantum confinement control and large band offset tunability unlock perovskite heterojunctions as platforms for scalable, superlattice-based optoelectronic applications.