Importance of HOMO AND LOMO Level
As seen in an OLED TV technology video, when we connect the organic semiconductor to an external power supply using anode and cathode, electrons move from the HOMO (highest occupied molecular orbital) to the LUMO (lowest unoccupied molecular orbital) levels via a power supply, creating holes. As soon as these electrons enter the LUMO layer, they recombine with the holes and emit light due to the natural tendency.
However, this process is not simple. Let’s look at the anode side first. When we connect the battery’s positive terminal to the anode, it tries to extract electrons from the organic layer. It must be noted that there is an energy difference between the HOMO level of the organic layer and the anode, which will act as a barrier for electrons. The same is the case with the cathode side. Thus, the cathode will be unable to inject electrons easily and consumes more energy.
Energy barriers at contact
Let’s study the energy barriers in greater detail. An energy barrier is defined as a potential field that opposes the localization or transfer of charged particles. It arises due to differences in the Fermi energy levels of materials. The highest energy level that an electron can occupy at the absolute zero temperature is known as the Fermi energy level. The Fermi level lies between the valence band and conduction band because at absolute zero temperature, the electrons are all in the lowest energy state.
When we connect two dissimilar materials, the free electrons travel from one material to another for maintain equilibrium near contact. In this process, the Fermi levels equalize and the conduction & valence band bending process happens. This movement of electrons also creates contact potential differences, which act as a barrier for new electrons.
Work function and its significance
The work function of material plays an important role. It describes the minimum amount of energy required to remove an electron from a solid to a certain point immediately outside the solid surface.
The greater the work function, the more easily electrons can be extracted from the organic layer. If the difference in work function or Fermi level of materials is small, then the bands will experience less bending, and the contact potential difference at a junction will be decreased because it depends on the difference between the work functions. In this condition, the electron can easily travel from one material to another.
That’s why OLED uses many intermediate work function layers to reduce energy barriers and increase overall efficiency of the device.