If a semiconductor is measured by the Kelvin probe, is it possible to obtain some information about the valence band edge (HOMO level)?
Clearly you are dealing with organic semiconductors. At a conference in Riga in 2006 there was a discussion of the electrical behaviour of traditional and organic semiconductors and in many ways they appear to be the same. For a number of years here at KP Technology we have been consulting for several large OLED manufacturers many of whom purchased our equipment. The principle research was into energy barrier height minimisation at the cathode, anode and HIL. However things are moving on and as solar cells/detector devices are being considered a better understanding of interface defects is also required (as has been the case with traditional materials).
Starting from the beginning: the Kelvin probe is the only method that gives the position of the semiconductor Fermi-level. Photoemission is of course sensitive to the topmost populated states of the valence band, inverse PE respectively for conduction. However with hole based conduction in organics PE data is a little unclear. In the best case (with electronic states) one cannot distinguish a PE threshold better than 30 - 50 meV. In fact photoemission data can have several turn-on points indicating several photoelectric thresholds and thus state distribution in the energies near the valence band maxima, so the PE data (and corresponding Work Functions) reported are sometimes averages and recent data suggests 0.25 - 0.3 eV lower than that predicted by the Kelvin (capacitance) method.
In this picture HOMO is not perhaps an ‘edge’, but a series of energy states.
Typically the way to get information with the Kelvin method on semiconductors is to make a measurement in darkness, then illuminate the semiconductor. If the illumination is sufficiently intense the surface energy bands will flatten as the photon flux is sufficient to induce intrinsic characteristics with ni=pi. The surface band-bending is thus obtained from the shift in Fermi level upon illumination. As the Fermi level is known from the dark measurement and the band-bending, the doping gives the bulk Fermi level, the band-gap is known (or can be determined from another measurement) so in theory everything that is necessary to draw the energy band diagram including HOMO level.