Highest occupied molecular orbital electron injection

Fig. 1.20. Schematic energy band diagram of a two-layer organic light emitting diode (OLED), in which tin-doped indium oxide (ITO) is used to inject holes into the highest occupied molecular orbital (HOMO) and a low work function metal to inject electrons into the lowest unoccupied molecular orbital (LUMO)...

An important issue for the performance of an organic electronic device like an OFET is the injection of charge carriers, electrons or holes, from the electrode into the organic material. In case of the commonly used metal electrodes an efficient electron injection is possible only if the Fermi level of the metal and the energy of the lowest unoccupied molecular orbital (LUMO) of the organic material differs by a small amount only. A similar statement applies for hole injection, in this case the position of the highest occupied molecular orbital (HOMO) has to match with the position of the Fermi level. When noble metals, in particular Au, are being used for an electrode one may naively assume... 

The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of fluorenone are shown in Fig. 4.28. Consistent with charge trapping, the fluorenone defects function as both a hole trap and an electron trap the HOMO and LUMO of fluorenone fall within the Jt-it gap of PFO [47]. In addition, the hole (electron) can be injected from the PEDOT PSS (Ca) electrode directly into the HOMO (LUMO) of fluorenone because of the small energy barrier between PEDOT PSS and the HOMO (or between Ca and the LUMO) of fluorenone. 

Depending on the positions of the highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO) of the dye, the photoexcited state may inject either electrons or holes into the conduction or valence band, respectively, of the solid [7] as shown in Fig. 1. (Note the broken arrows in Fig. 1 which show the direction in which electrons move). 

Fig. 179. Schematic energy-level diagram for an ITO/PPV/Al LED under forward bias, showing the ionization potential (Ip) and electron affinity (EyO of PPV, the work functions of ITO and Al (4>ito nd and the barriers to injection of electrons and holes (ISEe and A ,). There is a small barrier for hole injection from the ITO electrode into the valence band (of highest occupied molecular orbital, HOMO), and with aluminum as cathode, a considerably larger barrier for electron injection into the PPV conduction band states (of lowest unoccupied molecular orbital, LUMO). Reproduced by permission of Nature from R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Bredas, M. Logdlund, and W. R. Salaneck, Nature 397,121 (1999).

---