If we can make an energy level below the LUMO of active layer with cathode electrode, why we require ELT in between them? Why do we need an extra energy level between LUMO of the active layer and cathode work-function? How can we minimize the potential barrier at active layer/electrode junction by making an ohmic contacts?
Dear Shamjid,
The purpose of an electron transport layer (or of a hole transport layer ) between the active layer and the cathode (or the anode) is to reduce the recombination of the free charge carriers (electrons and holes) with their counterparts on defects which exist on the interfaces. So, regarding the active layer and the cathode interface, it is not an extra energy level between LUMO of the active layer (namely the acceptor material) and cathode work-function that is needed, but an interface without, ideally, any recombination sites for the electrons. Of course the LUMO level of the material that will be used as an inter-layer between the active layer and the cathode must be aligned to the LUMO level of the cathode material so as not to render the electron transport towards the cathode. Also, the inter-layer materials usually have the advantage that they can be further processed so that their recombination sites get further reduced. So, the inter-layer material may not contribute to the enhancement of the Coulombic attraction of the electron towards the cathode, but it's surface has better compatibility with both the active layer and the cathode, leading to less surface defects and more efficient electron transport towards the cathode.
Dear Shamjid
Addition to the explained role of ETL by NIkolaos, the stability of the device can be improved by inserting HTL. It can work as an injection layer and also a barrier for water and oxygen molecules.
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