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

Abstract

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. (C) 2013 AIP Publishing LLC.Y.N. and H.L. would like to thank members of Brutting's group (Universitat Augsburg) for their kind cooperation in our experiments. We would also like to thank Nippon Steel & Sumikin Chemical Co., Ltd. for providing alpha-NPD and Alq<INF>3</INF> samples. This research was supported by JSPS through the "Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program)" initiated by the Council for Science and Technology Policy, the Global-COE Project of Chiba University (Advanced School for Organic Electronics), and KAKENHI (Grant Nos. 21245042 and 22750167)