Efficient quantum-dot light-emitting diodes featuring the interfacial carrier relaxation and exciton recycling

Highlights

• New liquid-solid-solution processed magnesium-doped zinc oxide nanocrystals are synthesized for electron transport.

• Quantum-dot light-emitting diodes with an EQE of 22.3% and slight efficiency roll-off are achieved.

• The boost in the efficiency arises from a unique interfacial carrier relaxation governing abnormal exciton recycling.

• The EQE of 24% EQE is achieved for flexible devices due to light scattering of silver nanowires-based electrodes.

Abstract

Tremendous achievements of colloidal quantum-dot (QD) light-emitting diodes (QLEDs) in both efficiency and lifetime have been witnessed in the past decade. However, multiple interfacial losses such as defect-induced exciton quenching and multicarrier Auger recombination can severely suppress the device performance of QLEDs. Here, we present the efficient QLEDs by adopting the new liquid-solid-solution processed magnesium-doped zinc oxide nanocrystals (ZMO-LSS) as an electron-transport layer. A magic carrier relaxation dynamics is demonstrated at the QD emitter/ZMO-LSS interface for abnormal exciton recycling and enhanced radiative recombination, which arises from the subtle intragap band coupling of surface trap states as directly clarified by the prolonged electroluminescence decays of devices. Red-emitting QLEDs on rigid glass achieve a maximum external quantum efficiency (EQE) of 22.3% with virtual droop-free over a wide range of brightness from 10,000 to 200,000 cd m⁻². By further combining a silver nanowires-based composited electrode on a plastic substrate, a substantial boost in EQE up to 24.0% is realized for flexible devices. The present results provide an in-depth study on interfacial recombination and convey a clear picture of constructing ZMO-LSS for efficient QLEDs and related optoelectronic devices.

Introduction

Colloidal quantum-dot (QD) light-emitting diodes (QLEDs) have been intensively investigated as next-generation display technologies due to their unique properties, such as tunable color emission, narrow spectral bandwidth, high luminescence efficiency, and solution processability for ease of mass production [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]]. With the rapid developments of colloidal QDs [[2], [3], [4], [5]] and the optimized device architectures [[6], [7], [8], [9], [10]], the device performance of QLEDs has been significantly improved, achieving the impressive champion external quantum efficiencies (EQEs) of 28.7%, 23.9%, and 19.8% for red, green, and blue emissions, respectively [[11], [12], [13]].

Efficient and bright QLEDs are typically constructed by sandwiching the QD monolayer or multilayer between hole-transport layers and electron-transport layers (HTL and ETL) for efficient excitonic confinement. Zinc oxide (ZnO) nanocrystals (NCs) or their derivatives are commonly used as an ETL because of their high electron mobility, superior optical properties, and tunable energy levels. However, it is well known that the use of a ZnO NC-based ETL in QLEDs usually causes excess electron injection, leading to the electron accumulation at the QDs/ETL interface, and thus, the QD charging. The efficiency of QLEDs is thus impeded by surface defect-caused nonradiative recombination and multicarrier Auger recombination loss for the QD monolayer in contact with the ETL [14,15]. Inserting an ultrathin electron-blocking layer (EBL) is a common strategy to prevent the exciton quenching of QDs by balancing the injection of holes and electrons [6,7]. However, the device performance is rather sensitive to the control over the EBL’s thickness due to its insulating nature [3]. For this reason, one key challenge for achieving high-performance QLEDs relies on the realization of insulating-layer-free devices with balanced carrier injections. In recent years, tremendous efforts have been devoted to addressing this issue by incorporating stoichiometric lithium (Li), magnesium (Mg), or aluminum (Al) dopant in ZnO lattices, which can subtly tune the electronic structures and electrical conductivity of ZnO NCs for suppressing the interfacial quenching [9,13,16].

For the synthesis of ZnO NCs and their derivatives, the traditional sol-gel precipitation (SOL) method is widely implemented due to its convenience and high repeatability [2]. However, several shortcomings, including intricate coordination reaction, slow nucleation, and ligand-prohibited growth, are usually involved during the hydrolysis reaction process, leading to the difficulty in precisely controlling the crystallization process, as well as trap densities of the products under the ambient conditions [17]. For practical applications, the fabrication of ZnO NC-based ETLs should also be fully compatible with flexible plastic substrates that require low-temperature processing for flexible displays and lighting sources [18]. Moreover, it is recognized that the surface defects and interior traps of ZnO NCs and their derivatives may cause severe exciton quenching, as well as carrier trapping/scattering by the number of nonradiative recombination sites [14]. It is considered that the surface-related trap states of ZnO NCs with extremely tiny sizes can participate in multiple carrier scattering or relaxing phenomena. However, the direct physical process remains unclear.

Herein, we demonstrate an innovative synthetic method for producing defect-controllable Mg-alloyed ZnO (ZMO) NCs (denoted as ZMO-LSS) with high uniformity and reproducibility through the liquid-solid-solution (LSS) reaction [19]. When employed as an ETL in QLEDs, this subtly designed ZMO-LSS features obvious carrier relaxation dynamics at the QD emitter/ETL interface for abnormal exciton recycling and enhanced radiative recombination. A unique carrier relaxation governing is directly verified by the prolonged decays of electroluminescence (EL) spectra. Red-emitting QLEDs, with an inverted structure, achieve a high EQE of 22.3%, and a virtually droop-free efficiency curve is realized over a wide range of luminance up to 200,000 cd m⁻². By integrating a silver nanowire (AgNW)-derived transparent electrode, the maximum EQE can be further improved to 24.0% for flexible QLEDs on plastic substrates due to the intense light-scattering effect induced by randomly distributed AgNWs.