Paper TF-ThP7
Characteristics of the Top-Emitting Organic Light-Emitting Diodes Based-on the Electronic Structure of the BaO-on-Alq3 Interface

Thursday, October 23, 2008, 6:00 pm, Room Hall D

Top-emitting (TE) type of organic light-emitting diode (OLED) have been actively developed in recent years, to improve aperture ratio to the top cathode direction in active matrix (AM) displays. Therefore, developing thin semitransparent conducting cathodes (STCCs) have become one of the essential key-element technologies to obtain a high light out-coupling of the devices. In this study, STCC of the BaO/Ag (20 nm)/ITO (100 nm) structure were used to fabricate the TEOLED. The TEOLED structure is consisted of glass/Ag (150 nm)/tin-doped indium oxide (125 nm)/4,4’,4”-tris[2-naphthylphenyl-1-phenylamino]triphenylamine (2-TNATA, 30 nm)/4,4’-bis[N-(1-napthyl)-N-phenyl- amino]-biphenyl (NPB, 18 nm)/tris(8-quinolinolato)aluminum (III) (Alq3, 62 nm)/BaO (x nm)/Ag (20 nm)(x: 0, 0.5, 1, and 1.5 nm)/ITO (100 nm). The device with 0.5-nm-thick BaO showed the highest driving performance as the luminous efficiency of 2.7 % and the maximum luminance of 47000 cd/m2, compared with other devices. The driving performance of these devices can be explained by analyzing as electronic structures of the BaO-on-Alq3 interface. In the ultraviolet photoemission spectroscopy (UPS) study, the barrier height of an electron injection (ΦnB) from the BaO/Alq3 interface to the Alq3 layer is increased with increasing the coverage of BaO on Alq3. In the X-ray photoemission spectroscopy (XPS) studies of the O, N, and Ba core levels, the staged interface reaction of two steps was observed, which shows the formation of stable radical anion at low BaO coverage of below 0.5 nm.

Alq₃. cd/m². φ_n^B.