n-Type doping effect on the electronic structure of organic semiconductor: doping of tetrathianaphthacene (TTN) into hexadecafluorophthalocyaninatozinc (F16ZnPc) by co-evaporation

The effect of n-type doping of hexadecafluorophthalocyaninatozinc (F 16 ZnPc) with an electron-donating organic molecule tetrathianaphthacene (TTN) was investigated by ultraviolet photoemission spectroscopy (UPS). We measured the UPS spectra over a wide range of the film thickness for pristine F 16 ZnPc, pristine TTN, and TTN-doped F 16 ZnPc. From the change of the energy of the vacuum level (VL), we could deduce the energy level alignment at organic/metal interfaces. The results for pristine F 16 ZnPc showed (1) abrupt shift of VL at small thickness corresponding to dipole layer formation, and (2) more gradual shift of VL, which might correspond to the band bending to align the Fermi level by residual impurity acting as unintentional dopant. For F 16 ZnPc doped with TTN, the region (2) is merged into the region (1), indicating that the thickness of the band-bending region of the doped F 16 ZnPc was much reduced from that of pristine F 16 ZnPc. This demonstrates that F 16 ZnPc was effectively doped with carriers from the TTN molecules. It was also found that the energy of the highest occupied molecular orbital of F 16 ZnPc was shifted to higher binding energy by doping TTN molecules. These results suggest that TTN acts as a good donor in F 16 ZnPc.