To produce efficient charge extraction in solar cell devices, the potential barrier at the active layer/electrode junction should be minimized by creating an ohmic contacts at the junction. This ohmic contact is achieved by matching work function of metal electrode (cathode) with active material (acceptor LUMO).
Could you please tell me the physics behind the work function matching?
If work function of cathode electrode is higher than the acceptor LUMO, electron in the acceptor LUMO can see a lower energy level at the cathode. So it can easily move to that lower energy state without overcoming any barrier. Then what is the need of work function matching at the active layer/electrode junction?
If two energy levels of two systems are same or almost same then electrons can move from one to the other by a non-resonant process such as thermal fluctuation while a gap in the energy levels require a resonant perturbation such as a matched photon, i.e. a photon with a fixed frequency. The second process is much less probable than the first.
@ Datta. Thanks for your answer....
I hope you can give more detailed explanation ..... Such as non resonant transfer of electrons in matched energy levels, etc..
Shamjid,
The word matching and or alignment has the same meaning. It means there must exist some relationship between the corresponding energy levels at both sides of a junction between to materials. To be specific, it is required to extract the electrons arriving the interface by diffusion from the active material to dispatch it to the cathode.
One of the solutions is to use an electron transport layer ETL to perform this function.
For properly functioning its conduction energy level must be lower than that of the LUMO level of the active material. In this way the arriving electrons from the active layer to the interface will slide down the potential gradient to the ETL.
The extraction process dictates also that the EHL must have high conductivity to electrons such that absorbs small voltage form the cell.
The third condition is that the valence level of the ETL must behigher than that of the active layer to reflect the holes in the direction of the hole transport layer HTL.
So the alignment concerns not only the THE CONDUCTION LEVELS BUT ALSO THE VALENCE LEVEL in the manner discussed above.
ON the other side if the there is a potential barrier then a potential fall, the transport of electrons will be impeded and one has to drop an appreciable voltage at the interface to dispatch the electrons from the active region. In this case, the interface will be rectifying rather than ohmic. This detracts the operation of the solar cell by heavily reducing its fill factor.
@ zekry
If we can make an energy level below the LUMO of active layer with cathode electrode, what is the role of ETL there? Why do we need an extra energy level between LUMO of the active layer and cathode work-function?
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