: Horizontal Molecular Orientation of Light-Emitting Oligofluorenes in Solution-Processed Glassy Organic Thin Films and its Application in Organic Light-Emitting Diodes

九州大学学術情報リポジトリ

Kyushu University Institutional Repository

溶液プロセスを用いたオリゴフルオレン薄膜の水平 配向性評価と有機ELへの応用

趙, 理

https://doi.org/10.15017/1806984

出版情報：Kyushu University, 2016, 博士（工学）, 課程博士 バージョン：

権利関係：Fulltext available.

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: Horizontal Molecular Orientation of Light-Emitting Oligofluorenes in Solution-Processed Glassy Organic Thin Films and its Application in Organic Light-Emitting Diodes

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This thesis mainly focuses on the first demonstration of horizontally oriented oligofluorene emitters in a host matrix fabricated by spin-coating method. Also, the influence of the mainly horizontal oriented emitters on device properties of the solution processed OLEDs was examined.

In Chapter 2, the combined study of VASE and angle dependent PL demonstrates that such a horizontal orientation can be achieved in glassy spin-coated organic films based on a composite blend of a heptafluorene derivative as a dopant and a 4,4'bis(N-carbazolyl)-1,l'biphenyl (CBP) as a host. Solution-processed fluorescent OLEDs with horizontally oriented heptafluorene emitters were then fabricated and emitted deep blue electroluminescence with an EQE as high as 5.3%, which is higher than the theoretical limit of the EQE predicted from random oriented emitters.

In fact, the original purpose that I studied oligofluorene materials was not that I expected

molecular orientation in these materials, but that I realized that oligofluorenes were excellent

lasing materials, and I fabricated organic laser devices based on the oligofluorenes. Thus , in

Chapter 3, I report on the use of oxygen as triplet quencher to reduce singlet-triplet

annihilation in light-emitting organic solid films and solvent-free molecular semiconducting

liquids . For this purpose, a fluorescent heptafluorene derivative is dispersed either in a wide

bandgap CBP host or in a solvent-free liquid matrix based on 9-(2-ethylhexyl)carbazole (EHCz) .

To introduce oxygen in the samples, a modified cold isostatic pressure technique is used in the

case of spin-coated CBP blends while oxygen is bubbled in the liquids. The influence of the

oxygenation on their photophysical and amplified spontaneous emission (ASE) properties is

examined. Both the solid blend films and the liquids showed a PL quantum yield higher than

80% and an ASE threshold lower than 0.4

While oxygen does not quench significantly

the singlet excitons in these systems, singlet-triplet annihilation is strongly reduced in solid

thin films and nearly suppressed in the liquid environment. This study demonstrates that

liquid organic semiconductors are promising candidates for optically pumped continuous wave

lasers and provides important insights for a more effective triplet management in organic

semiconductor lasers.

Following the study m Chapter 2, I demonstrate that a horizontal orientation of light-emitting molecules can be achieved in solution-processed blend films containing a series of highly fluorescent oligofluorenes dispersed into various small molecule host materials in Chapter 4. Influences of the host material, the solvent used for the spin-coating, the doping concentration of the oligofluorenes and the oligofluorene chemical structure on these orientational processes are investigated using VASE and angle dependent PL measurements.

By clarifying the roles played by these parameters on the horizontal orientation of the emitters,

the results provide crucial insights into these molecular orientational processes in spin-coated

glassy organic thin films.