Investigating the Effect of ZnO Modifications on the Electroluminescence Stability of Quantum-dot Light Emitting Devices and the Underlying Factors
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Date
2023-08-18
Authors
Chung, Dong Seob
Journal Title
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Publisher
University of Waterloo
Abstract
Quantum dots (QDs) have garnered significant attention as promising materials for next-generation flat panel displays due to their unique luminescence properties. These properties, such as an impressive photoluminescence quantum yield (PLQY) of nearly 100%, narrow emission spectra, and the ability to finely adjust emission wavelengths, have captured the interest of the scientific community. In comparison to state-of-the-art organic light emitting devices (OLEDs) that rely on organic emitting materials, QDs offer the potential to deliver displays with heightened color purity and significantly enhanced color saturation. This has led to endeavors to incorporate QDs into display technologies, initially starting with conventional liquid-crystal displays (LCDs) and OLED displays.
Notably, Samsung has introduced commercial displays that leverage QDs in color conversion layers for both LCDs and OLED displays. These implementations have contributed to improvements in color purity and color gamut. However, these devices have yet to fully harness the complete advantages of QDs. As a result, current research endeavors are centered around incorporating QDs into the emission layer of self-emissive devices, allowing them to harness their electroluminescence (EL) properties beyond their function as color conversion layers. Unlike LCDs and OLEDs, where a considerable portion of light output is absorbed by color conversion layers, QDs integrated into the emission layer of LEDs (referred to as QLEDs) offer the potential for enhanced luminance, broader color gamut, and heightened color purity due to their sharper emission spectra, all without requiring additional conversion layers.
Over the last decades, substantial efforts have been dedicated to improving QLED performance. Recent advances have resulted in device efficiencies exceeding 20% in red, green, and blue QLEDs. However, significant challenges still stand in the way of their commercialization. The critical breakthrough needed for QLED commercialization revolves around enhancing their stability. While progress has been made and potential solutions have been proposed for achieving highly stable QLEDs using specific materials and conditions, challenges persist due to the complexity of pinpointing the exact causes of device degradation under diverse external conditions during prolonged operation. Thus, a comprehensive understanding of degradation mechanisms and the factors that limit device lifespan remains imperative.
The central focus of this thesis is twofold: first, to highlight a notable enhancement in QLED stability through novel modification techniques applied to the ZnO electron transport layer (ETL); and second, to unravel the degradation mechanisms intrinsic to QLEDs and the constraints imposed by the ZnO ETL. The overarching goal of this research effort can be summarized in two main objectives:
A. Explores novel approaches for altering the ZnO to improve device stability. This involves two primary approaches: (ⅰ) incorporating polymer additives into ZnO to modulate charge distribution within the QLED, and (ⅱ) using chemical treatments, involving plasma and wet processes, to manipulate stoichiometry and defect distribution in the ZnO, addressing issues related to oxygen vacancies and charge trap states.
B. Explores how the introduced modifications to the ZnO layer influence device stability. This entails examining the effects of different additives within the ZnO ETL on overall device performance. Various investigative techniques are employed, encompassing aspects such as charge distribution estimation, analysis of charge carrier accumulation and annihilation within the QLED, probing recombination centers, measurement of exciton lifetime, and evaluation of the chemical and electronic states of materials.
This research centers on ZnO modifications achieved through the integration of a ZnO-polyethylenimine nanocomposite (ZnO:PEI), ZnO treated with carbon tetrafluoride plasma (FZnO), and ZnO infused with iodine and ferric chloride (I2:ZnO and FeCl3:ZnO). These modifications represent innovative pathways to bolster the electroluminescence stability of QLEDs. Through extensive investigation, two primary mechanisms responsible for enhancing device stability emerge:
1. ZnO:PEI and FZnO modifications impact charge distribution and electron concentration across the QLED, mitigating the accumulation of holes at the interface of QDs and the hole transport layer (HTL). The typical scenario in QLEDs involves holes accumulating at this interface, which can lead to device degradation. Experimental measurements, including delayed electroluminescence and transient photoluminescence measurements under bias, capacitance-voltage-luminance analyses, and electroluminescence characteristics assessments involving luminescent marking layers, collectively demonstrate that ZnO:PEI and FZnO treatments lead to an increased presence of electrons at the QDs/HTL interface, thus decreasing hole accumulation and consequently enhancing device stability.
2. The incorporation of I2 and FeCl3 into the ZnO ETL suppresses the formation of positively charged ZnO species during QLED operation. This is corroborated through X-ray photoemission spectroscopy (XPS) studies and an analysis of changes in electroluminescence and photoluminescence characteristics. These investigations reveal that extended QLED operation leads to a higher concentration of ZnO species with oxidative states, and a direct correlation between the magnitude of electroluminescence loss and the concentration of these species is established. Ultimately, the application of halide treatment to ZnO materials demonstrates its potential for enhancing the stability of green, blue, and red QLEDs alike.
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Keywords
quantum dot light emitting device, electroluminescence stability, zinc oxide nanoparticles, luminescence of quantum dot, electron transport material