I have seen in some papers and theses (link 1 is an example) that a constant generation rate is assumed for solving drift-diffusion equations. My questions are, why this assumption is valid?
Is there any case in which we can't use this assumption? Thank you.
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.85.155201
Hello,
In organic solar cells, the main exciton dissociation occurs at the donor-acceptor interfaces and depends on the electric field at the interface (greater field means electron-hole pair is more likely to dissociate). In a bulk heterojunction device (like in your attached paper), there is a nanoscale blend of donor-acceptor materials. Since the interfaces are oriented in all random directions, there might not be a significant effect of the applied voltage on these interface fields, so using a constant generation rate could give reasonable results.
However, if we want to model a bilayer solar cell, where there is just 1 large donor-acceptor interface parallel to the electrodes we would need to consider the generation rate as being dependent on the field at the donor-acceptor interface. This is because the field generated from the applied voltage and the built-in interface field are parallel to each other, so the interface field is affected by the applied voltage. See the Koster et. al. paper for an example of using a non-constant generation rate:
Article Device model for the operation of polymer/fullerene bulk het...
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