What is the potential barrier between metal-semiconductor contact? Why does it reduce the overall efficiency of an OLED device?

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.

Fig:1 - HOMO AND LOMO Level of alq3 molecules
Fig:1 - HOMO AND LOMO Level of alq3 molecules

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.

Fig:2 - Energy barrier between the metal and semiconductor
Fig:2 - Energy barrier between the metal and semiconductor

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.

Fig:3- Energy level diagram of conduction and valence band.
Fig:3- Energy level diagram of conduction and valence band.

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.

Fig:4 - Animated explanation of work function.
Fig:4 - Animated explanation of work function.

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.

Fig:5a - Band bending diagram for metal and semiconductor contact
Fig:5a - Band bending diagram for metal and semiconductor contact
Fig:5b - small band bending at Equilibrium.
Fig:5b - small band bending at Equilibrium.

That’s why OLED uses many intermediate work function layers to reduce energy barriers and increase overall efficiency of the device.

Fig:6- Intermediate layers of OLED for improving efficiency
Fig:6- Intermediate layers of OLED for improving efficiency

 152