Monday, May 9, 2011 - 16:00
Wngr 153
Event Speaker: 
Hiroshi Yanagi, Yamanashi University
Local Contact: 


Carrier injection barriers at electrode/organic semiconductor interfaces are in general large (e.g. > 1 eV) in organic light emitting diodes (OLEDs) and a serious obstruction for stable and low power consumption device operation. To reduce an electron injection barrier (EEIB), a low work function material is suitable for a cathode because electron affinities of electron transport organic layers are smaller than the work functions of typical cathode materials. On the other hand, indium tin oxide (ITO) is widely used as a transparent anode in OLEDs with a relatively small hole injection barriers (EHIB) of 0.6–1 eV although ITO is an n-type conductor. The low EHIB is attained by oxidizing the ITO surface to reduce the Fermi level, but it increases the contact resistance. In this sense, using p-type materials would be more adequate for lowering EHIB, because a higher work function is expected.
12CaO·7Al2O3 electride (C12A7:e-) is a good candidate for a high-efficiency cathode because C12A7:e- has a small work function of ~2.4 eV and chemical stability at the same time [1]. On the other hand LaCuOSe and CuxSe are possible candidate of anodes: LaCuOSe is a p-type transparent degenerate semiconductor [2]. CuxSe film deposited at room temperature shows high p-type conductivity [3]. Interfacial electronic structure between these inorganic materials and typical organic semiconductors were studied by photoelectron spectroscopy. Small EEIB of ~0.8 eV (at C12A7:e-/Alq3 interface) and EHIBs of ~0.3 eV (at LaCuOSe/NPB) and ~0.4 eV (Cu2Se/NPB) were obtained by optimizing surface treatment conditions. Finally, prototype OLEDs were fabricated by using these electrodes. The combination of C12A7:e- and CuxSe was effective to fabricate a highly efficient inverted top-emitting OLED.
(This work was carried out in Tokyo Tech, Hosono Group.)

[1] Matsuishi et al. Science, 301, 626 (2003); S.W. Kim et al. Nano Letters, 7, 1138 (2007); Toda et al. Adv. Mater., 19, 3564 (2007).
[2] Hiramatsu et al. Phys. Status Solidi (a) 203 2800 (2006).
[3] Hiramatsu et al. Phys. Stat. Solidi (a), 205, 2007 (2008).

Related publications
[1] K.B. Kim et al. J. Phys. Chem., C, 111, 8403 (2007).
[2] H. Yanagi et al. Organic Electronics, 9, 890 (2008).
[3] H. Hiramatsu et al. J. Appl. Phys., 104, 113723 (2008).
[4] H. Yanagi et al. J. Phys. Chem. C, 113, 18379 (2009).