Understanding of Electronic Processes
To understand and improve electronic processes in organic devices it is important to obtain fundamental knowledge about the electronic structure of the used materials and at the interface of two materials. One possibility is using a combination of ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). While UPS can be used to examine the occupied states, the complementary method IPES gives information about the unoccupied states above the Fermi energy EF. With this knowledge it is for example possible to shift injection barriers with the help of specific surface modifications to enable an improved transport of conduction carriers.
On top of that the combination of UPS and IPES allows the identification of the band gap and position of doping states which are important for transport. Also the shift of states or the creation of additional states, for example by chemical interaction of two materials at an interface, can be observed and analyzed in combination with DFT-calculations.
In the figure the basic principle of the two methods is shown. Likewise the highest occupied orbital (HOMO) and the lowest unoccupied orbital (LUMO) of a molecule, as well as the electron affinity (EA), the ionization energy (IE) and the work function Φ with regards to the Fermi energy (EF) and the vacuum level (EVAC) is depicted. The secondary electron cutoff Ecutoff marks the photoionization threshold and can be employed to evaluate the work function Φ via Φ = hν - Ecutoff.