Candidate: Dong Seob Chung
Title: Understanding the Influence of ZnO and its Modifications on the Stability of Quantum-dot Light Emitting Devices
Date: June 2, 2023
Time: 3:00 PM
Place: EIT 3142
Supervisor(s): Aziz, Hany
Abstract:
Quantum dots (QDs) have been actively researched as a leading material for use in the next-generation display devices. Thanks to their unique luminescence properties, including nearly 100 % of photoluminescence quantum yield (PLQY), narrow emission spectra, and easy tunability of their emission wavelength, QDs have been receiving enormous attention by the scientific community. Compared to state-of-the-art organic light emitting devices that use organic emitting materials, QDs can offer higher color purity and greatly increase color saturation of display produces. As a result, there have been efforts to incorporate QDs into the display products, particularly in the form of QD-light emitting devices (QLED), which can utilize superior luminescence properties of QDs to provide high luminance with a wide color gamut.
Tremendous efforts have been made to improve QLED performance for last three decades. However, there still remain challenges for their commercialization. Current research progress on QLED realize high device efficiency, exceeding 20 % of EQE in red-, green-, and blue-QLEDs. For commercialization, however, the breakthrough in the QLED stability is required. Although there has been progress and proposed solutions in producing highly stable QLED with certain materials and under specific conditions, solutions remain unclear due to the difficulty of diagnosing the exact causes of device degradation during long-term operation amid numerous external conditions. Therefore, a comprehensive understanding of the degradation mechanisms and of the factors that limit device lifetime is crucial.
The focus of this work is introducing the observation of significant improvement in the QLED stability via new modification methods for ZnO electron transport layer (ETL), and understanding the QLED degradation mechanisms and limited electroluminescence (EL) lifetime caused by ZnO ETL. ZnO and polyethylenimine nanocomposite (ZnO:PEI), carbon tetrafluoride plasma treated ZnO (FZnO), and iodine & ferric chloride incorporated ZnO (I2:ZnO and FeCl3:ZnO) are proposed for new approaches of ZnO modifications to enhance the EL stability of QLED. Various investigation techniques are used to understand mechanisms behind the profound effects from the ZnO modifications on device stability. This study mainly reveals two mechanisms that improve device stability. First, ZnO:PEI and FZnO modify charge distribution and electron concentration across the QLED to reduce accumulated charges at the QDs/hole transport layer (HTL) interface. In QLEDs, in general, due to the large energy barrier between the valence band maximum of CdSe-QDs and the highest occupied molecular orbital (HOMO) energy level of the HTL, holes accumulate at the QDs/HTL interface, leading to device degradation through various processes. Delayed EL measurements, transient PL measurements by applying bias, capacitance-voltage-luminance measurements, and EL characteristics of QLEDs with a luminescent marking layer reveal that the increase in the number of electrons at the QDs/HTL interface as a result of using ZnO:PEI and FZnO ETL leads to a decrease in the number of holes accumulated at the QD/HTL interface, hence the device stability enhancement. Second, incorporating I2 and FeCl3 into ZnO ETL suppresses the formation of positively charged ZnO species during QLED operation. X-ray photoemission spectroscopy (XPS) studies as well as an analysis of changes in the EL and PL characteristics show that electrical aging of QLED leads to an increase in the concentration of ZnO species with higher oxidative states and that a correlation between the magnitude of EL loss and the concentration of these species exists.