p-Type Doping of Spiro-MeOTAD with WO3 and the Spiro-MeOTAD/WO3 Interface Investigated by Synchrotron-Induced Photoelectron Spectroscopy

p-Doping of the organic hole conductor Spiro-MeOTAD with tungsten oxide (WO3) is investigated by synchrotron-induced photoelectron spectroscopy (SXPS). Similar valence band spectra and electronic-state energies are shown for Spiro-MeOTAD films evaporated in UHV and prepared by drop-casting from cyclohexanone solution. In coevaporated Spiro-MeOTAD:WO3 films with varying amounts of WO3, a maximum slift of the HOMO binding energy by 0.98 eV toward the Fermi level is found. Similar shifts are induced in Spiro-MeOTAD at interfaces of Spiro-MeOTAD on WO3 and WO3 on Spiro-MeOTAD. In addition, interface dipole potentials of 0.87 and 1.36 eV, respectively, are induced in the two deposition sequences. The exchanged charge appears as a reduced W5+ component in W4f core orbital and additional W5d gap-state emissions. A correlation of the interface charge transfer to the doping mechanism is discussed.