A review on the electroluminescence properties of quantum-dot light-emitting diodes
Graphical abstract
Introduction
The focus in this Review is mainly on quantum dots (QDs) based on II–VI group compounds (such as CdSe, ZnSe, and ZnS) and their application in the electrically-driven devices. Generally, their bandgap can be adjusted by controlling the particle sizes (referred to as quantum confinement effect) and components of the QDs. These inorganic semiconductor QDs are a unique class of photo-electronic materials with the size of a few to tens of nanometer [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]], featuring with narrow full-width at half-maximum emission bandwidth, high color purity, size tunable emission spectra, and excellent photoluminescence (PL) quantum yield. Moreover, QD thin film can be easily formed via a solution processing such as spin coating and inkjet/microcontact printing. Consequently, they have been recognized as one of the most potential candidates as the light receiving and light emitting units in the omnipresent photoelectric devices, such as solar cell, detector, bio-labeling and light emitting diodes [[16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]]. The emission from both optically and electrically excited QDs are widely used in display and lighting due to their high color quality [4,9,[34], [35], [36], [37], [38], [39], [40], [41]]. In addition, Cd-free QDs, such as the InP QDs [[42], [43], [44], [45], [46], [47], [48]], ZnSe QDs [49,50], Cn-In-S QDs [51] and Ag–In–S QDs [52] have attached much attention. Moreover, the quantum-dot light-emitting diodes (QLEDs) based on the Cd-free QDs are also developed rapidly. Especially, the QLEDs with InP/ZnS QDs are recently reported with external quantum efficiency (EQE) of 21.4% [45], which stimulates the enthusiasm to productize the QLEDs for display and lighting applications.
Since they were first described [17], great process has been achieved in device performance including the luminance, efficiency and operation lifetime through material design and device structure engineering. The state-of-the-art QLEDs, especially for the red and green QLEDs, reach high EQE over 20%, operation lifetime of over 1,000,000 h, which have been comparable to or over the organic light emitting diodes. However, the operation lifetime, especially the stability of blue devices, is still a big obstacle for the practical application of the QLEDs. Generally, QLEDs are composed of multiple organic and inorganic semiconducting layers sandwiched between two electrodes with a total thickness of ~100 nm. Every functional layer and their interfaces seriously influence the performance and operation lifetime of QLEDs. Such multiple interfaces increase the complexity to decipher the charge carrier dynamics and device working mechanism. To date, fundamental processes affecting device performance, including carrier transport/recombination and exciton formation remain unclear. A comprehensive understanding on the working mechanisms of QLEDs is needed. So far, the progress of both the QD synthesis and device design were widely reviewed. Therefore, this paper gives review on the representative electroluminescence (EL) properties, including working mechanisms proposed for the QLEDs, QD structure, device optimization, as well as photoelectrical characterization techniques particularly useful for unraveling the carrier dynamic processes in QLEDs rather than a balanced and exhaustive overview of all the methods employed for photoelectrical measurements in devices.
The Review in the following text consists of five sections. First of all, we summarize the device structure and fabrication technologies for QLEDs. Then, the EL mechanisms, exciton formation processes, and non-radiative processes, are presented. The positive and negative ageing phenomena in the devices are discussed in section 4. Section 5 reviews the representative photo-electrical characterization technologies used to reveal the charge carrier dynamics in the QLEDs. Finally, we present the summary and outlook.
Section snippets
Device structure and fabrication technologies
In terms of charge transporting materials, the QLED devices involves three types, all-organic, all-inorganic and hybrid structures over time as shown in Fig. 1. Sometimes, all inorganic QLED is a misnomer because neither the electron transporting materials nor the hole transporting materials are really inorganic materials, even the commonly used ZnO nanoparticles. They generally contain organic ligands at least. Nevertheless, the term “all inorganic QLED” is commonly accepted.
The first QLED
EL processes in QLEDs
Currently, the popular hybrid QLED structure is based on the QDs sandwiched between ETL and hole transport layer (HTL), with a cathode and an anode as the electrical contacts. Basically, the record of device performance is constantly refreshed by device engineering and material optimization. However, the EL mechanism in the QLEDs remains unclear enough so far. Understanding the EL mechanism is necessary to further expand the device applications, even attaining electrically pumped lasers based
Ageing mechanisms of QLEDs
Stability is still the biggest challenge the QLEDs facing before they really enter into the commercialization market. In order to accelerate this progress, it is urgent to realize a comprehensive understanding on the degradation mechanisms of QLEDs. To date, there are different mechanisms to be proposed to describe the degradation in QLEDs, such as Auger induced degradation, creation of quenching sites due to thermal effect and leakage current, and electrochemical reactions, which can lead to
Characterization technologies for QLEDs
To date, much progress has been achieved on the understanding of the EL processes in the QLEDs. However, there are still many unclear mysteries in the QLEDs due to the entangled photo-electric features under electrical excitation, such as the mobility of QDs, charge transport/distribution, exciton formation process, sub-voltage turn-on voltage phenomenon, and so on. The classic and reliable characterization for single QD is no longer applicable for the QDs assembly. Furthermore, there is a huge
Summary and outlook
In the past two decades, great progress has been obtained for the QLEDs, whose performance are approaching or over the organic counterpart. The PL quantum yield of the QDs reaches around 100%, especially the alloyed QDs that is extremely suitable for the efficient QLEDs. A well understanding for the EL and degradation mechanisms is achieving. Both the efficiency and stability of the device have in part been meet the commercialization requirements. All in all, the QLEDs are a very competitive EL
Declaration of competing interest
We declare that there are no conflicts of interests between the authors (Qilin Yuan, Ting Wang, Panlong Yu, Hanzhuang Zhang, Han Zhang, and Wenyu Ji) listed in this article.
Acknowledgements
This work was supported by the program of the National Natural Science Foundation of China (Nos. 11974141 and 12074148), the Fundamental Research Funds for the Central Universities, the Science and Technology Development Project of Jilin Province (20180201057YY).
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