Effect of electrode on charge transfer phenomena of charge transport material

It is shown that the interface charge transfer between the charge transport layer (CTL) and anode is affected by the work function of the anode metal. The charge transfer of the CTL/anode interface under low electric field is greatly influenced by a difference of injection barrier between the localized energy state of the CTL and the work function of the anode. From the results of dark current measurements under high electric field, we conclude the cause to be the transfer of electrons between the HOMO level of the CTL and the valence band of the anode metal. On the other hand, the photothreshold energies for the specimens with anodes of different work function are not affected by electric field. These experimental findings suggest that surface states do not exist on the CTL-3. Moreover, as the LUMO level of the CTL-3 is about 2.7 eV, the photothreshold energy should correspond to the LUMO level of the CTL-3. We conclude that the CTL has two electronic energy levels and that surface states do not exist