The sustained photoconductivity of ZnO offers the potential for developing ZnO-based optoelectronic synaptic devices. However, the decay dynamics of sustained photoconductivity and the retention characteristics of the synaptic response are significantly influenced by interface recombination, defect state occupancy, and carrier transport at the heterojunction interfaces. Consequently, designing the interface structure is an effective strategy to regulate the optoelectronic synaptic performance of ZnO-based heterojunction devices. In this paper, Cu/CuO/Al₂O₃/ZnO/ITO multilayer heterojunction device is fabricated on glass substrates using magnetron sputtering. The device performance is modulated by varying the sputtering time of the Al₂O₃ interlayer (0 min, 1 min, and 3 min) and by introducing post-annealing treatment in an Ar atmosphere. The structure and electrical properties are characterized using SEM, EDS, XRD, absorption spectroscopy, current-voltage measurements, and time-dependent photoresponse. The results indicate that the device exhibits a distinct multilayer structure, and the Al₂O₃ interlayer produces pronounced modulation of the dark-current characteristics. After annealing, the dark-state transport behavior of the devices becomes more closely approximated by Ohmic conduction, while distinguishable electrical differences are retained among devices with different interlayer conditions. Under 365 nm ultraviolet light irradiation, all devices display typical persistent photoconductivity with a retention-and-decay response after the removal of UV excitation. Both the photocurrent amplitude and residual-current retention increase with increasing light power density. Compared to the device without an Al₂O₃ interlayer, the device with a 3 minute Al₂O₃ interlayer sputtering time demonstrates stronger residual-current retention and slower photocurrent decay.
Under pulsed light stimulation, all three devices can achieve adjustable excitatory postsynaptic currents, transition from short-term memory to long-term memory, frequency-dependent plasticity, and paired-pulse facilitation. Double-exponential process fitting reveals that a rapid relaxation process on the second scale and a slow relaxation process on the order of 103 s jointly contribute to the photoresponse decay. These results suggest that the Al₂O₃ interlayer plays an important role in coordinating interfacial carrier transport, persistent-photoconductivity retention, and synaptic relaxation dynamics. This work provides useful guidance for the interface-structure design and dynamic regulation of ZnO-based optoelectronic synaptic devices.