Emulating synaptic functionalities in optoelectronic devices is significant in developing artificial visual-perception systems and neuromorphic photonic computing. Persistent photoconductivity (PPC) in metal oxides provides a facile way to realize the optoelectronic synapses, but the PPC performance is often limited due to oxygen vacancy defects that release excess conduction electrons without external stimuli. In this talk, a high-performance optoelectronic synapse based on the stoichiometry-controlled LaAlO₃/SrTiO₃ (LAO/STO) heterostructure is demonstrated. By increasing La/Al ratio up to 1.057:1, we effectively enhanced the PPC but suppressed the background conductivity at the LAO/STO interface, achieving strong synaptic behaviors. The spectral noise analyses reveal that the synaptic behaviors are attributed to the cation-related point defects and their charge compensation mechanism near the LAO/STO interface. We demonstrate the short-term and long-term plasticity, including the paired-pulse facilitation, in the La-rich LAO/STO device upon exposure to UV light pulses. Beyond the typical oxygen deficiency control, our results show how harnessing the cation stoichiometry can be used to design oxide heterostructures for advanced optoelectronic synapses and neuromorphic applications.