An In Situ Ion Exchange-Driven Gradient Heterojunction between MAPbI ₃ Single-Crystal Films and CsPbBr ₃ Quantum Dots for a Photodetector

To address the critical demand for high-sensitivity and low power photodetection under low light conditions, we designed and fabricated a novel Type-II heterojunction (HJ) photodetector (PD). By spin-coating CsPbBr 3 quantum dots (QDs) onto the surface of a two-dimensional MAPbI 3 single-crystal thin film (SCF), an in situ Br/I ion exchange reaction was triggered, enabling the formation of a MAPbI 3 /CsPbBr x I 3– x /β-CsPbI 3 multilayered gradient HJ. It indicates that this gradient HJ establishes an advantageous energy band alignment, which significantly facilitates the separation and transport of photogenerated carriers while effectively suppressing nonradiative recombination at the interface. The optimized PD exhibits a remarkably low dark current at 10 –12 A and achieves an external quantum efficiency (EQE) of 241% under 5 V bias in weak light. These demonstrate that an ion exchange-induced gradient HJ architecture provides a robust and feasible strategy for the development of next-generation high-performance, low-power optoelectronic devices.