Influence of the direction of spontaneous orientation polarization on the charge injection properties of organic light-emitting diodes

Spontaneous orientation polarization (SOP) is inherent in evaporated films of many organic semiconducting molecules with a permanent dipole moment. A significant electric field is formed in the film due to SOP. Consequently, the properties of organic light-emitting diodes (OLEDs) incorporating such films are influenced. The polarization charge appearing at heterointerfaces dominates the charge injection and accumulation properties.Moreover, SOP correlates to device degradation. In this article, we review SOP of organic semiconductor films and its influences on the device properties of OLEDs.

A tris(7-propyl-8-hydroxyquinolinato) aluminum [Al(7-Prq) 3 ] film shows negative giant surface potential (GSP) because of spontaneous orientation polarization. The polarity of this film is opposite to those of tris-(8-hydroxyquinolate) aluminum films. In Al(7-Prq) 3-based organic light-emitting diodes, negative GSP leads to the positive interface charge and governs the electron injection and accumulation properties. In addition, a high resistance to the electron injection at the Al(7-Prq) 3 /Ca interface is suggested possibly because of the negative polarization charge at the interface. These results show the importance of orientation polarization in controlling the charge injection and accumulation properties and potential profile of the resultant devices. V

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