Dissertation
Magnetic field effects in exciplex- and exciton-based organic light emitting diodes and radical-doped devices
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Autumn 2017
DOI: 10.17077/etd.p31o1slq
Abstract
<p>Organic semiconductors (OSCs) have already been shown to have great potential to play an important role in the future of clean energy generation (organic solar cells) and provide energy efficient lighting (organic light-emitting diodes, OLED). Prior research has found that the light-emission efficiency of OLED is severely limited by the magnetic state (technically the spin-configuration) of the light-emission process. In this thesis, we work on the processes using external magnetic fields that can overcome these magnetic limitations. A major focus of this research is to enhance the performance of OLED, while at the same time to unravel the scientific mechanisms by which magnetic fields act on OSCs devices.</p>
<p>Thermally activated delayed fluorescence (TADF) is a next-generation OLED emission technology which enables nearly 100% light-emission efficiency without using heavy precious metals. TADF characteristics depend on the probability of reverse intersystem crossing (RISC) from the triplet excited states (T1) to singlet excited states (S1). The conversion (T1 to S1) process depends strongly on spin dynamics, thus we predict a dramatic magnetic field effects (MFEs) in such TADF OLED devices. In subsequent experiments we observed that changes in TADF devices due to various forms of electrical stress can lead to enormous increases in magnetic field effects (MFEs) on the current (> 1400%) and electroluminescence (> 4000%). Our work provides a flexible and inexpensive pathway towards magnetic functionality and field sensitivity in current organic devices. Such OLED pave the way for novel magnetic sensitive OSCs devices with integrated optical, electronic and magnetic characteristics.</p>
<p>Organic magnetoresistance (OMAR) has been observed to alter the current and efficiency of OLED without any ferromagnetic components. Here we utilizes slight alterations to the device properties, the addition of a radical-doped functional layer, in which the spin-relaxing effects of localized nuclear spins and electronic spins interfere, to address the assumption about the importance of the hyperfine interaction and to attempt to differentiate between the different models for OMAR. A feature where the magnitude of OMAR exhibits a plateau over a wide range of doping fraction was observed at all temperatures investigated. This phenomenon is well explained by a theory in which a single dopant spin strongly interacts, by exchange, with one of the bottleneck sites. A similar can be used to explain the efficiency increases observed in organic solar cells for certain doping fractions.</p>
Details
- Title: Subtitle
- Magnetic field effects in exciplex- and exciton-based organic light emitting diodes and radical-doped devices
- Creators
- Yifei Wang - University of Iowa
- Contributors
- Markus Wohlgenannt (Advisor)Michael E. Flatté (Committee Member)John P. Prineas (Committee Member)David R. Andersen (Committee Member)Leonard R. MacGillivray (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Physics
- Date degree season
- Autumn 2017
- DOI
- 10.17077/etd.p31o1slq
- Publisher
- University of Iowa
- Number of pages
- xiv, 105 pages
- Copyright
- Copyright © 2017 Yifei Wang
- Language
- English
- Date submitted
- 05/04/2018
- Description illustrations
- color illustrations
- Description bibliographic
- Includes bibliographical references (pages 98-105).
- Public Abstract (ETD)
Whereas incandescent light-bulbs were used in the past, the lighting industry is now dominated by much more energy efficient solid-state devices called light emitting diodes (LED). In addition, most portable displays nowadays use flexible organic LED (OLED) to convert electric energy into light, but the magnetic character of the electron prevents a majority of electrons from producing light efficiently. A major focus of this research is to scramble the magnetic character of the electron, generating light more efficiently.
In this work, we explore the mechanism of limitation of magnetic character in OLED and successfully achieve more than 10-fold current and 40-fold light efficiency enhancement by a small external magnetic field. This discovery is the largest one measured for thin film OLED and opens a new and intriguing physical scenario that paves the way for new applications in novel, ultra-sensitive, multi-functional organic devices.
- Academic Unit
- Physics and Astronomy
- Record Identifier
- 9983776767202771
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