Synthesis of Full Color-Emitting and Air-Stable ZnSeTe/ZnSe/ZnS Quantum Dots

Abstract

Next-generation high-performance displays have been developed by using quantum dots with adjustable band gaps because of their superior optical properties and ease of processing in solutions. Cadmium chalcogenide-based red, green, and blue-emitting QD light-emitting diodes (QLED) exhibited high external quantum efficiency (EQE) of more than 20% or near to 20% with high color purity and show sufficient operational lifetime for low luminescent level applications. However, high-performance QLEDs with heavy metal-free QD are urgently needed for more eco-friendly display technology. Sufficient lifetime and EQE that are extremely near to the theoretical maximum were attained in red and green InP-based LEDs. Since it is difficult to cover the blue region with InP QDs, however, an alternative is required. Although ZnSeTe semiconductor QDs recently obtained extremely high photoluminescence quantum yield (PL QY) and EQE at the blue region, the well-known extremely toxic hydrofluoric acid (HF)-based synthesis of ZnSeTe/ZnSe/ZnS QDs makes it difficult to commercialize. In addition, PL QY in ZnSe/ZnSe/ZnSe/ZnS QDs is rapidly reduced under ambient conditions due to fast oxidation, which could be caused by the facile desorption of the surface ligands. Therefore, to obtain air-stable ZnSeTe/ZnSe/ZnSe QDs appropriate for commercial LED devices, a ligand that strongly binds to the QD surface should be employed. We developed a non-toxic synthetic method that fully eliminated toxic HF to synthesize ZnSeTe/ZnSe/ZnS heterostructure QDs appropriate for QLED applications. Instead of HF, excessive metal halides were introduced, and PL QY was shown to be substantially greater. Also importantly, we first demonstrate highly stable ZnSeTe/ZnSe/ZnS QDs in both solution and solid state through a very simple ligand exchange that enables stronger surface binding via thiolate functional groups. Finally, by synthesizing these stable QDs with a broad color spectrum extended from green to red, a new possibility for full-color QD display materials was demonstrated.