The difference in electron and hole mobility in inorganic semiconductor can be understand easily but it is very difficult to understand the electron and hole mobility difference in organic semiconductors.
Dear Mohammad,
The main difference between holes and electrons with respect to the mobilities is related to the level spectrum of transport states. P-type doping requires the transfer of an electron from the filled HOMO of the host to the LUMO of the dopant at no expenses of energy or only a little bit. While the n-type needs more energy because the HOMO of the dopant has to be close to the LUMO of the host in order to promote such doping.
This puts serious constraints on the mutual energy levels and makes the mobility of the holes much more low cost in energy than for the electrons ( or better polarons due to the importance of the phonons in these materials).
I hope this helps.
I totally agree with the previous answer. Basically the mobility of holes in such organic semiconductors depends upon the energy difference of the valance bands of two adjacent (different) layers. More the slope or difference in energy band diagram (uphill transportation for holes) easier for the holes to migrate as a result mobility also increases.
Sarthak, I think you are referring to the likelihood of hole transfer at a (bulk)heterojunction interface, which depends on the HOMO level offset. This is not the same thing as hole mobility, which depends on the properties of the individual layer, i.e. a single material, and not the interface with a second material. I think Mohammad is instead asking why most organics have a higher hole mobility than their electron mobility.
Dear Paul,
Speaking more specifically on the mobilities, they are intrinsically similar in organic semiconductor materials, but due to the energy structure that I have told above (for HOMO-LUMO levels), the electrons can be easily trapped while the holes have more difficulties. It is important to remark that usually a p-organic material is when we can inject holes in the material while the n-semicondutor if we can do it with electrons from the electrodes.
In inorganic semiconductors, charge transport occurs via band transport mechanism. Organic semiconductors being amorphous, lack long range order and hence charge transport occurs via a hopping mechanism.
There are two theories to understand charge transport in organic semiconductors. They being Gaussian disorder model and Polaron theory. In Gaussian disorder model, the rate of charge transfer between molecules depends on the distance between levels and the overlap of wave functions corresponding to the levels of the molecules. For hole transport, overlap of HOMO levels between molecules is required and for electrons it is the overlap of LUMO levels between molecules which determines charge hopping between molecules. In Polaron theory the rate of charge carrier transport is explained by Marcus theory of electron transfer. There are molecules which belong to either type.
The current in a device can be explained by above mechanisms only if it is space charge limited and not injection limited from the contact.
Organic semiconductors can be also molecular solids (besides amorphous) binding by much weak interactions that the inorganic semiconductors. This makes so important the phonons in such materials and also the polaron (hole or electron) quasiparticles instead of pure holes or electrons mobilities.
Most of the organic semiconductors have small polarons which therefore differ quite a lot of band semiconductors for the electronic transport. In particular there is a thermally activated hopping following a Arrhenius-like expression which depends exponencially of the binding energy (HOMO-LUMO) that in the case of electrons have a higher energy cost. Thus this is another form to answer the question of this post.
High mobilties typically characteristic of band transport have also been seen in some organic polymers apart from molecular solids.
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