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Title:Exploring charge carrier dynamics in quantum dot and double heterojunction nanorod light emitting devices using transient electroluminesence measurements
Author(s):Rogers, Steven
Director of Research:Shim, Moonsub
Doctoral Committee Chair(s):Shim, Moonsub
Doctoral Committee Member(s):Huang, Pinshane; Dillon, Shen; Diao, Ying
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):QLED, CSQD, DHNR, Transient Electroluminescence
Abstract:Semiconductor nanocrystals are an impressive class of materials showing significant promise for light emitting applications due to their size tunable optical band gap, compatibility with scalable bottom-up colloidal synthesis techniques, and ability to be processed using simple solution-based deposition techniques. Light emitting devices (LEDs) fabricated utilizing these materials have already demonstrated competitive performance with their organic based counter-parts (i.e. OLEDs). One of the more promising nanocrystalline systems to emerge in recent years is the double heterojunction nanorod (DHNR) whose morphology and type-II band alignment has helped to realize multi-functional displays which can both emit as well as detect light. In addition to this functionality, the rod structure can enhance light outcoupling efficiency as compared to the core-shell (CSQD) morphologies. Despite these advantages, the DHNR light emitting devices exhibit similar efficiency losses as their CSQD counterparts, the source of which are attributed to the location and concentration of the different charge carriers residing in the emitting layer. There are still open debates in the quantum dot light emitting diode (QD-LED) community regarding the details of the charge carrier populations which are expected to influence efficiency droop at higher current densities as well as the location and formation of the radiative recombination zone. Despite the importance that charge carrier properties play in the theories invoked to describe device performance, little work has been done to explore the dynamics of the carrier populations under operation. The work described herein helps provide the ground work for studying and interpreting these processes by analyzing the transient electroluminescence (TREL) of both CSQD and DHNR-LEDs. In OLED literature, studying the TREL profile has yielded insight into the motion of carriers through the emitting layer, helped to identify sources of charge traps, and understand device degradation mechanisms which limit operation lifetime of the device. We first study the TREL of CSQD-LEDs and are able to provide insights into the evolution of electrons and holes throughout the different phases of the TREL signal. Through this initial work we identify that the large hole injection barrier and low carrier mobility with the CSQD film govern the main features of the TREL signal. We then demonstrate how the DHNR morphology is uniquely suited to address these issues and highlight ways in which the DHNR structure influences charge carrier dynamics within DHNR-LEDs. We finally make concluding remarks regarding future work involving both the TREL measurement technique as well as how the DHNR structure can be further utilized to study more fundamental aspects of charge carrier dynamics within QD-LEDs.
Issue Date:2019-10-29
Rights Information:Copyright 2019 Steven Rogers
Date Available in IDEALS:2020-03-02
Date Deposited:2019-12

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