Defect‐Induced Persistent Photocurrent in Cu ₂ SnS ₃ Nanocrystals: Insights from High Resolution Transmission Electron Microscopic and Surface Passivation

Persistent photocurrent (PPC) has significant advantages and is used in technologies like photodetectors, optical memory devices, neural networks, and anticounterfeiting systems. Many materials, including II-VI and III-V semiconductors and halide perovskites, are known to show PPC. Quaternary chalcogenides like Cu₂ZnSn(S/Se)₄ (CZTS) and CuInGa(S/Se)₂ (CIGS) also exhibit PPC due to antisite defects. However, some ternary chalcogenides, which are impurity phases of these quaternary materials, are easier to synthesize in a pure phase. These ternary chalcogenides, such as Cu₂SnS₃ (CTS), are gaining attention as lead-free alternatives due to their nontoxicity, stability, and simple synthesis methods. In this study, PPC in Cu₂SnS₃ nanoparticles is reported. High-resolution transmission electron microscopy revealed stacking faults in CTS nanoparticles that directly cause the PPC. These defects within the bandgap region trap the photoexcited charge carriers, resulting in delayed photoresponses. The effect of ligand exchange of the CTS nanoparticles on their PPC behavior is further studied. Photoconductivity is enhanced by 10³ times due to the ligand exchange. Additionally, the delay time is slightly affected by different passivating ligands, suggesting that surface passivation can partially control the defect states.