Hole transport layer:
This layer supports to transport of the holes across it so they can reach the emissive layer.
It is the main point of this component. The emissive layer is also known as the heart of the OLED and this layer consists of color-defining impurities doped into the base material or host. This is the layer where the electrical energy converts into optical energy.
It is commonly used to improve OLED technology by confining electrons to the emissive layer.
Electron transport layer:
The electron transport layer (ETL) supports the transportation of electrons across it so they can reach the emissive layer. This layer is deposited over the cathode terminal of OLED.
Cathode electrodes may or may not be transparent depending upon the type of OLED. It is negatively charged to inject the electrons into the organic layers of the OLED.