A doping mechanism for organic semiconductors derived from SXPS measurements on co-evaporated films of CuPc and TCNQ and on a TCNQ/CuPc interface

Synchrotron induced photoelectron spectroscopy on in situ co-evaporated blends of CuPc and TCNQ and on TCNQ/CuPc interfaces is applied for monitoring the electronic interaction indicated in systematic shifts of the CuPc and TCNQ HOMO and core orbitals. These shifts correspond to a movement of the Fermi level within the HOMO LUMO energy gap. The shifts in CuPc induced by the interaction with TCNQ are similar in the composites and in the interface model experiment. At the interface an additional induced dipole potential can be measured. The interface-dipole plus the Fermile level shift add up to the work function difference of pure CuPc and TCNQ. We conclude that TCNQ forms a separate phase in CuPc rather than single isolated acceptor molecules. Charge transfer at the bulk hetero-junction induces the Fermi level variations, which may be called doping.