Performance Improvement of Quantum Dot-Light-Emitting Diodes Enabled by an Alloyed ZnMgO Nanoparticle Electron Transport Layer

TLDR

Inorganic ZnO nanoparticles as electron transport layers have rapidly improved the performance of colloidal quantum dot‑light‑emitting diodes, especially for Cd‑based and non‑Cd‑based visible emitters. The study uses three Zn1–xMgxO (x = 0, 0.05, 0.1) nanoparticle ETLs with varying electronic energy levels in solution‑processed I–III–VI QLEDs, and spectroscopic analysis shows that increasing Mg content systematically raises the conduction band minimum toward the vacuum level. Alloyed ZnMgO nanoparticle ETLs substantially improve QLED luminance and efficiency compared to pure ZnO, mainly by lowering the electron injection barrier through an upshifted conduction band minimum.

Abstract

Since the introduction of inorganic ZnO, typically in the form of nanoparticles (NPs), as an electron transport layer (ETL) material, the device performance of electrically driven colloidal quantum dot-light-emitting diodes (QLEDs), in particular, with either Cd-based II–VI or non-Cd-based III–V (e.g., InP) quantum dot (QD) visible-emitters, has been rapidly improved. In the present work, three Zn1–xMgxO (x = 0, 0.05, 0.1) NPs that possess different electronic energy levels are applied as ETLs of solution-processed, multilayered I–III–VI type QLEDs that consist of a Cu–In–S, Cu–In–Ga–S, or Zn–Cu–In–S QD emitting layer (EML) plus a common organic hole transport layer of poly(9-vinlycarbazole). The luminance and efficiency of those QLEDs are found to be strongly dependent on the type of ZnMgO NP ETL, resulting in the substantial improvements by means of alloyed ZnMgO ETL versus pure ZnO one. Ultraviolet photoelectron and absorption spectroscopic measurements on a series of ZnMgO NP films reveal that their conduction band minimum (CBM) levels are systematically closer to the vacuum level with increasing Mg content. Therefore, such beneficial effects of alloyed NPs on QLED performance are primarily ascribed to the reduced electron injection barrier between ETL and QD EML that is enabled by the upshift of their CBM levels.