22% Record Efficiency in Nanorod Light‐Emitting Diodes Achieved by Gradient Shells

Abstract

The external quantum efficiency (EQE) in light‐emitting diodes (LEDs) based on isotropic quantum dots has approached the theoretical limit of close to 20%. Anisotropic nanorods can break this limit by taking advantage of their directional emission. However, the progress towards higher EQE by using CdSe/CdS nanorods (NRs) faces several challenges, primarily involving the low quantum yield and unbalanced charge injection in devices. Herein, we modified the seeded growth method and obtained anisotropic nanorods with PLQY up to 98% by coating a gradient alloyed CdZnSe shell around conventional spherical CdSe seeds. This intermediate alloyed CdZnSe shell combined with a subsequent rod‐shaped CdZnS/ZnS shell can effectively suppress the electron delocalization in the typical CdSe/CdS nanorods due to their small conduction bandgap offset. Additionally, this alloyed shell can reduce the hole‐injection barrier and create a larger barrier for electron injection, both effects promoting a balanced injection of electrons and holes in LEDs. Hence, we reached LEDs with high brightness (160341 cd/m2) and high efficiency (EQE = 22%, current efficiency = 23.19 cd/A), which are the highest values to date for nanorod LEDs.This article is protected by copyright. All rights reserved

Synthesis and Optical Properties of CdSeTe/CdZnS/ZnS Core/Shell Nanorods

“…The high-quality shell coating improved the PL lifetime from 75 ns for CdSeTe/CdS NRs to 90 ns for CdSeTe/CdZnS, demonstrating the effective defect passivation ( Figure 5 c). These results correspond well with previously published reports [ 29 , 31 , 32 ]. The anisotropy value is increased from 0.203 of CdSeTe/CdS NRs to 0.221 of CdSeTe/CdZnS/ZnS NRs, which is due to the more uniform morphology of CdSeTe/CdZnS/ZnS NRs and the radial distribution of Zn in the CdZnS shell [ 33 ].…”

“…With the shell coating, the intensities of the Cd-3d peaks first increase and then decrease, while those of Se and Te gradually increase, and those of Se-3d and Te-3d continuously decrease. The trends of peak intensities clearly indicate the formation of the core–shell structure of NRs, which are also consistent with our previous report [ 31 , 32 ]. The fitting peaks of Se-3d, Te-3d, and Cd-3d shift to a higher binding energy after the shell growth, which proves the formation of the CdZnS shell.…”

“…In recent years, such core–shell structures have been increasingly explored in NRs [ 29 , 30 ]. Recently, Zeng et al reported CdSe/CdZnS/ZnS NRs and CdSe/CdZnS/CdZnS/ZnS NRs with carefully designed multilayer shell structures, achieving the highest EQE of red NR-LEDs to date [ 31 , 32 , 33 ]. However, reports on the tunability of the emission wavelength of NRs are not as comprehensive as those on QDs.…”