Stability of InP/ZnSe/ZnS Quantum Dots in Light-Emitting Diodes: Role of Shell Thickness and Surface Chemistry

In typical III-V colloidal I $n$ P quantum dots (QDs), an inorganic Z $n$ S outermost shell is used to provide stability when overcoated onto the I $n$ P core. However, we found a faster photo-degradation of InP/ZnSe/ZnS QDs with a thicker ZnS shell than that with a thin ZnS shell when 1-octanethiol was applied as a sulfur source to form ZnS outmost shell. Herein, we demonstrate that 1-octanethiol induces the form of weakly-bound carboxylate ligand via proton transfer on the QD surface, resulting in a faster degradation at UV light even though a thicker Z $n$ S shell was formed onto InP/ZnSe QDs. Detailed insight into surface chemistry was obtained from proton nuclear magnetic resonance spectroscopy and thermogravimetric analysis-mass spectrometry. However, the lifetimes of the electroluminescence devices fabricated from InP/ZnSe/ZnS QDs with a thick or a thin ZnS shell show surprisingly the opposite result to the material stability of QDs, where the QD light-emitting diodes (QD-LEDs) with a thick Z $n$ S shelled QDs maintained its luminance more stable than that with a thin Z $n$ S shelled QDs. In this study, we elucidate the degradation mechanism of the QDs and the QD light-emitting diodes based on our results and discuss why the material stability of QDs is different from the lifetime of QD-LEDs. As a result, we believe this better understanding of the relation between QDs and QD devices paves the way to industrially feasible QD-LEDs.