Energy Level Alignment at Bebq 2 /PEI/ITO Interfaces Studied by UV Photoemission Spectroscopy
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Energy Level Alignment at Bebq2/PEI/ITO Interfaces Studied by UV Photoemission Spectroscopy Kohei Shimizu1, Hirohiko Fukagawa2, Katsuyuki Morii3, Hiroumi Kinjo1, Tomoya Sato1, and Hisao Ishii1,4,5 1 Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba-shi, Chiba 263-8522, Japan. 2 NHK Science & Technology Research Laboratories, 1-10-11 Kinuta, Setagaya-ku, Tokyo 1578510, Japan. 3 Suita Research Center, Nippon Shokubai Co., Ltd., 5-8 Nishiotabi-cho, Suita-shi, Osaka 5640034 Japan. 4 Center for Frontier Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba-shi, Chiba 263-8522, Japan. 5 Molecular Chirality Research Center, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba-shi, Chiba 263-8522, Japan. ABSTRACT A polyethyleneimine (PEI) interlayer has been applied on indium tin oxide (ITO) to improve electron injection in organic devices including inverted organic light-emitting diodes (OLEDs). To understand the improvement effect by PEI insertion, the energy level alignment at bis(10hydroxybenzo[h]quinolinato)beryllium (Bebq2)/PEI/ITO interfaces was investigated by UV photoemission spectroscopy (UPS). The deposition of a PEI layer was found to reduce the absolute work function of ITO by 1.4 eV. The vacuum level shifts at Bebq2/ITO and Bebq2/PEI interfaces were also determined as 0.3 eV and 0.1 eV in the direction to reduce the electron injection barrier, respectively. Thus the work function reduction by PEI and downward vacuum level shift at the Bebq2/PEI interface can contribute to the improvement effect. Kelvin probe measurement revealed the weak orientation polarization in Bebq2 film with the bottom side positively polarized. This polarization polarity is also advantageous for electron injection in inverted devices. INTRODUCTION Organic light-emitting diodes (OLEDs) have attracted much attention for applications in display and lighting technology because they have a great potential for low-cost fabrication of large-area, lightweight and flexible devices. One of the major problems that limit the practical application of OLEDs is their intrinsic low environmental stability [1]. Conventional OLEDs commonly use air-sensitive materials such as aluminum for the top cathode, and LiF, cesium, and calcium for the electron injection layer (EIL). Because of those reactive layers, the devices require strict encapsulation to extend their lifetime. This factor pushes up the production cost of the device, resulting in the reduced competitiveness of OLEDs. Furthermore, the need for strict encapsulation can be problematic especially in the development of flexible OLED devices. Inverted OLEDs with a bottom cathode have been proposed as an ideal structure to realize an air-stable OLED. One advantage in employing an inverted structure is that the EIL is formed before the organic layer, allowing the use of other deposition methods, such as sputtering, that may otherwise damage the organic layer. This development has opened up a range of options for
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