Bathocuproine (BCP) is used as a buffer layer material at organic/metal interface in organic devices and has an effect of improving electrical characteristics.1,2) The improvement in electrical characteristics is considered to be due to a formation of interface states which was observed in the combination of BCP with metals such as Ca, Mg, Al, and Ag.3) In this work, the interaction of BCP with Ca and Ag was studied by ultraviolet photoemission spectroscopy (UPS) to clarify the electronic properties of the interface states.
Two kinds of specimens were prepared. The one is the BCP thin film deposited on metal (Ca or Ag) surface and another one is the BCP and metal (Ca or Ag) co-deposited film. The BCP, Ca, and Ag were deposited at room temperature by thermal deposition method under the vacuum of less than 4x10-6 Pa. The thickness of the BCP deposited on metal surface was varied from 0.4 nm to 10 nm. The composition ratio was varied by almost two orders of magnitude for the composite films. The deposited films were transferred to the UPS measurement chamber with keeping a vacuum. The UPS measurements were carried out using fixed photon energy of 21.2 eV at the beam line 11C in KEK Photon Factory.
In the BCP film on Ca and Ag, new UPS peaks which are not detected in pure BCP, Ca and Ag were observed near the Fermi level. Such new peaks may be generated by an interaction between BCP and metal suggesting a formation of interface states. In the composite film, new UPS peaks were also observed near the Fermi level. The formation mechanisms for the interface states are similar between the two specimens. It was found that the energy difference between Fermi level of metal and the vacuum level of BCP are almost constant for the whole composition ratio. These results may suggest that the LUMO of BCP plays an important role in forming the interface states.
1) P. Peumans et al, Appl. Phys. Lett. 2000, 76, 2650.
2) T. Taima et al, Appl. Phys. Lett. 2004, 85, 6412.
3) S.Toyoshima et al, International Conference on Solid State Device and Materials, 304-305, 2006 : Jpn. J. Appl. Phys., in press.
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