ZnS shelling as the outermost layer on luminescent nanocrystals: a key strategy for quantum dot light-emitting diodes

Highly luminescent nanocrystals (NCs), including semiconductor quantum dots (QDs), have been synthesized as light emitters for various applications during the last three decades. Recently, light-emitting diodes based on NCs or QDs (hereafter commonly called QLEDs) have been fabricated, achieving an external quantum efficiency of more than 20%, closely approaching practical requirements. In the QLED structure, the outermost shell of the luminescent NCs or QDs plays a crucial role, as it needs to simultaneously meet the following requirements: (i) to act as an effective shell to passivate imperfections or traps on the NC surface, while confining injected charge carriers within the emitting QDs/NCs; and (ii) to possess compatibility with both the electron transport layer (ETL) and the hole transport layer (HTL) for efficient injection of electrons and holes into the core NCs/QDs. Additionally, the outermost shell of the NCs/QDs should be environmentally friendly. Practically, the outermost ZnS shell appears to satisfy all the above requirements for various types of emitting NCs/QDs, including II-VI (CdSe, CdTe, ZnSe), III-V (InP, GaP), I-III-VI (CuInS2, AgInS2, AgInSe2), and metal halide perovskites. This has naturally led to numerous studies conducted on different NCs and QDs with the same outermost ZnS shell. Using a ZnS shell is also advantageous for designing uniform QLED structures with different NCs or QDs to emit a variety of wavelengths. In this article, we review the recent development of QLEDs using various NCs or QDs as light emitters, where the spectral emission range can be controlled by simply adjusting the size or composition of the core NCs/QDs. We also discuss why ZnS is suitable for shelling various types of NCs/QDs and how it aligns appropriately with the ETLs and HTLs being studied for facilitating the injection of charge carriers in QLEDs.