5. Measuring the energetics of organics and their contacts is key to understanding transport. The most common means for measuring ionization potentials and electron affinities is ultraviolet photoelectron spectroscopy (UPS) and inverse photoelectron spectroscopy (IPES, see Appendix C). The spectra of an organic semiconductor with progressively thicker layers of Au on its surface are shown in Fig. P4.5. To gather the UPS spectra, the He I emission line (21.22 eV) was used as the photon source. (a) On the spectra, label all the relevant transitions needed to determine the energetic properties of a semiconductor (the vacuum level, VL or EVAC, the LUMO (ELUMO) and HOMO (HOMO) energies, the deeper energy levels away from the frontier orbitals that can be observed, the energy gap, EG, and the Fermi energy, EF), and discuss how these energies are determined. Intensity (a.u.) (a) Cut-off Region (b) HOMO region 5.2 eV 64 64 32 32 16 16 8 8 4 4 2 2 1 1 0.5 Au 0.5 Au 4.4 eV |1.0 eV 17.0 16.5 16.0 15.5 5 4 3 2 1 0-1 Binding Energy (eV) L (c) Expanded view 64 32 16 8 4 2 1 0.5 Au 0.6 0.4 0.2 0.0-0.2-0.4-0.6 (d) IPES 10-1-2-3-4-5-6-7 Figure P4.5 (a, b) UPS spectra of an organic in different energy regions; (c) a detail of (b) near the Fermi energy. (d) IPES data for the same material. (Liu et al., 2015)