Publication date: 3rd July 2020
Though significant progress has been made in recent years towards researchers’ understanding of QD chemistry, the performance of QD-based optoelectronic devices remains limited compared to bulk semiconductor-based analogs. The low-level performance of QD materials is commonly attributed to deleterious mid-gap trap states that arise at the QD surface. While surface trap states are known to diminish QD properties, there remains poor control over their complete passivation. The lack of control in mitigating surface trap states stems from a limited understanding of the atomic identity and nature (e.g. oxidation state, bonding environment, and redox potential) of the defect sites themselves. To improve our understanding, we use here PbS QDs as a platform to study the nature of such defects by quantifying changes in surface chemistry upon addition of redox-active chemical probes. We employ complementary NMR, XPS and UV-Vis-NIR absorbance spectroscopies to probe structural and electronic changes at the QD surface upon reacting with the chemical probes. These studies provide important insight into the nature of native and induced redox-active defects by indicating their atomic identity, reactivity, and approximate reduction potential. We further explore the impact of QD size and capping ligand identity on the number and type of defects observed, and find that these factors have measurable effects. Through this work, we gain understanding of the identity and reactivity of surface sites that limit QD-based device success and provide insight into strategies for passivation.
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