期刊
ORGANIC ELECTRONICS
卷 88, 期 -, 页码 -出版社
ELSEVIER
DOI: 10.1016/j.orgel.2020.106014
关键词
Organic semiconductor thin-films; Organic light-emitting diode; Molecular orientation; Thermal stability; Ellipsometer
资金
- National Natural Science Foundation of China [U1402273, 11774304]
- Natural Science and Engineering Research Council of Canada
- Yunnan University [2017YDQN07]
The orientation of organic molecules in physical-vapor deposited films varies drastically depending on thickness, as does the thermal stability of the molecular network. A simple function to quantify the molecular dipole orientation as a function of film thickness has been proposed, which is found to describe experimental data well. Additionally, the molecular orientation parameter is found to impact material density changes after thermal anneal and glass transition.
Molecular orientation in organic semiconductors plays a critical role in maximizing external quantum efficiencies of organic light-emitting diodes. It was generally believed that the molecular packing of organic semiconductors is either amorphous or liquid-crystal-like with a preferred molecular orientation distributed uniformly throughout the film. In this paper, however, we report that the orientation of organic molecules in physical-vapor deposited films varies drastically depending on thickness. The thermal stability of the molecular network, measured by its characteristic glass transition temperature, also varies as a function of the film thickness. Based on a two-layered film-structure model, we propose a simple function to quantify the molecular dipole orientation S parameter as a function of film thickness. This function describes well experimental data. In addition to contributing to external quantum efficiency, the molecular orientation parameter S is found to have a strong impact on disruptive change in material density after thermal anneal and glass transition.
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