Dependence of organic interlayer diffusion on glass-transition temperature in OLEDs

McEwan, Jake A., Clulow, Andrew J., Nelson, Andrew, Yepuri, Nageshwar Rao, Burn, Paul L. and Gentle, Ian R. (2017) Dependence of organic interlayer diffusion on glass-transition temperature in OLEDs. ACS Applied Materials and Interfaces, 9 16: 14153-14161. doi:10.1021/acsami.7b01450


Author McEwan, Jake A.
Clulow, Andrew J.
Nelson, Andrew
Yepuri, Nageshwar Rao
Burn, Paul L.
Gentle, Ian R.
Title Dependence of organic interlayer diffusion on glass-transition temperature in OLEDs
Journal name ACS Applied Materials and Interfaces   Check publisher's open access policy
ISSN 1944-8252
1944-8244
Publication date 2017-04-13
Year available 2017
Sub-type Article (original research)
DOI 10.1021/acsami.7b01450
Open Access Status Not yet assessed
Volume 9
Issue 16
Start page 14153
End page 14161
Total pages 9
Place of publication Washington, DC, United States
Publisher American Chemical Society
Language eng
Subject 2500 Materials Science
Abstract Organic light-emitting diodes (OLEDs) are subject to thermal stress from Joule heating and the external environment. In this work, neutron reflectometry (NR) was used to probe the effect of heat on the morphology of thin three-layer organic films comprising materials typically found in OLEDs. It was found that layers within the films began to mix when heated to approximately 20 °C above the glass-transition temperature (T) of the material with the lowest T. Diffusion occurred when the material with the lowest T formed a supercooled liquid, with the rates of interdiffusion of the materials depending on the relative T's. If the supercooled liquid formed at a temperature significantly lower than the T of the higher-T material in the adjacent layer, then pseudo-Fickian diffusion occurred. If the two T's were similar, then the two materials can interdiffuse at similar rates. The type and extent of diffusion observed can provide insight into and a partial explanation for the "burn in" often observed for OLEDs. Photoluminescence measurements performed simultaneously with the NR measurements showed that interdiffusion of the materials from the different layers had a strong effect on the emission of the film, with quenching generally observed. These results emphasize the importance of using thermally stable materials in OLED devices to avoid film morphology changes.
Formatted abstract
Organic light-emitting diodes (OLEDs) are subject to thermal stress from Joule heating and the external environment. In this work, neutron reflectometry (NR) was used to probe the effect of heat on the morphology of thin three-layer organic films comprising materials typically found in OLEDs. It was found that layers within the films began to mix when heated to approximately 20 °C above the glass-transition temperature (Tg) of the material with the lowest Tg. Diffusion occurred when the material with the lowest Tg formed a supercooled liquid, with the rates of interdiffusion of the materials depending on the relative Tg’s. If the supercooled liquid formed at a temperature significantly lower than the Tg of the higher-Tg material in the adjacent layer, then pseudo-Fickian diffusion occurred. If the two Tg’s were similar, then the two materials can interdiffuse at similar rates. The type and extent of diffusion observed can provide insight into and a partial explanation for the “burn in” often observed for OLEDs. Photoluminescence measurements performed simultaneously with the NR measurements showed that interdiffusion of the materials from the different layers had a strong effect on the emission of the film, with quenching generally observed. These results emphasize the importance of using thermally stable materials in OLED devices to avoid film morphology changes.
Keyword Diffusion
Intermixing
Neutron reflectometry
Organic light-emitting diodes
Photoluminescence
Q-Index Code C1
Q-Index Status Provisional Code
Grant ID DP120101372
Institutional Status UQ

Document type: Journal Article
Sub-type: Article (original research)
Collections: HERDC Pre-Audit
School of Chemistry and Molecular Biosciences
 
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Created: Fri, 21 Apr 2017, 11:38:32 EST by Mrs Louise Nimwegen on behalf of School of Chemistry & Molecular Biosciences