Precisely controlled type-II band alignment for efficient carrier transfer in Ruddlesden–Popper perovskite heterojunctions

The performance of two-dimensional Ruddlesden–Popper perovskites (RPPs) is often limited by disordered phase distributions in solution-processed films, which create unpredictable carrier transfer pathways and obscure structure–property relationships. While phase engineering can globally modulate film properties, the nanoscale carrier dynamics governing device performance remain unresolved. Here, we overcome this limitation by constructing precisely defined RPP heterojunctions via dry-transfer assembly of exfoliated single-crystalline nanoflakes. This platform enables direct investigation of how the interfacial energy landscape governs carrier dynamics. Systematic spectroscopic measurements and computational studies support the designed type-II band alignments and reveal that well-type heterojunctions promote ultrafast interphase carrier transfer and yield substantially higher transfer efficiencies than cascade-type and barrier-type heterojunctions. These precisely constructed heterojunctions establish directional carrier transfer pathways that correlate with improved photodetector performance. Our work establishes a clear relationship between nanoscale energy-landscape design, carrier dynamics, and device performance, providing design principles for high-efficiency RPP optoelectronics through controlled energy landscape engineering.