Ambipolar Photoresponse of CsPbX₃‑ZnO (X = Cl, Br, and I) Heterojunctions

All-inorganic perovskite CsPbX₃ (X = Cl, Br, and I) quantum dots (QDs) are emerging as attractive materials for photodetectors due to their prominent positive photoresponse. In this work, ambipolar photodetectors were constructed based on heterojunctions of CsPbX₃ QDs and ZnO nanocrystals (NCs). The transition between the positive photoconductivity (PPC) and negative photoconductivity (NPC) was successfully controlled not only by the drain-source voltage (V_DS) but also by the excitation wavelength and CsPbX₃ crystal size. A high responsivity (R), on–off ratio (η), and detectivity (D*) were achieved for NPC (R = 1046 mA/W, η = 281, and D* = 1.65 × 10¹¹ Jones), with PPC (R = 146 mA/W, η = 64, and D* = 1.54 × 10¹¹ Jones) simultaneously revealed in the same device. A general mechanism combining the type-II heterojunction, the component and size-related band structure, and most critically the in-band trap states of the perovskites was established for understanding the unique ambipolar property. It exhibits application potential in developing functional photodetectors, memristors, and optical logic devices.