Influence of Polarity of Polarization Charge Induced by Spontaneous Orientation of Polar Molecules on Electron Injection in Organic Semiconductor Devices

Yuya TANAKA  Takahiro MAKINO  Hisao ISHII  

Publication
IEICE TRANSACTIONS on Electronics   Vol.E102-C   No.2   pp.172-175
Publication Date: 2019/02/01
Online ISSN: 1745-1353
DOI: 10.1587/transele.2018OMS0014
Type of Manuscript: BRIEF PAPER
Category: 
Keyword: 
giant surface potential,  displacement current measurement,  organic light-emitting diode,  charge injection,  polarization charge,  

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Summary: 
On surfaces of tris-(8-hydroxyquinolate) aluminum (Alq) and tris(7-propyl-8-hydroxyquinolinato) aluminum (Al7p) thin-films, positive and negative polarization charges appear, respectively, owing to spontaneous orientation of these polar molecules. Alq is a typical electron transport material where electrons are injected from cathode. Because the polarization charge exists at the Alq/cathode interface, it is likely that it affects the electron injection process because of Coulomb interaction. In order to evaluate an impact of polarization charge on electron injection from cathode, electron only devices (EODs) composed of Alq or Al7p were prepared and evaluated by displacement current measurement. We found that Alq-EOD has lower resistance than Al7p-EOD, indicating that the positive polarization charge at Alq/cathode interface enhances the electron injection due to Coulomb attraction, while the electron injection is suppressed by the negative polarization charge at the Al7p/Al interface. These results clearly suggest that it is necessary to design organic semiconductor devices by taking polarization charge into account.