What is the most significant recombination happening in organic solar cells. In bulk heterojunction configuration, how about the recombinations taking place at the donor-acceptor interface Vs the whole bulk of the active layer.
That is probably one of the most important and not-yet resolved questions in OPV. What is sure is that most of the recombination is non-radiative. That is why Voc is relatively low.
To further discuss recombination, you have to specify what you mean. Is it recombination between (more or less separate) positive and negative charge or do you consider decay of molecular excited states as recombination as well.
For optimized dev ices (high internal quantum efficiency at short circuit), only the first is important. There, recombination is expected to happen at the D/A interface. In bulk heterojunction configuration you have donor-acceptor interfaces spread everywhere and due to the energetics, you have electrons and holes rather located on acceptor and donor, respectively. Many results also show that charge carrier trapping in the BHJ plays an important role for recombination through defects, which are not yet completely identified.
Relaxation of excited states in particular when donor and acceptor are not well intermixed (compared to the exciton diffusion length) can happen in the whole bulk.
I'm considering both recombination of separate charges and decay of excited states, as I'm working on relating optical performance to JV characteristics. I would like to know which mechanism dominates under various conditions, and also how much is the net effect of the two. Does the recombination process depend on the particular polymers involved or is it same for all organic solar cells. There are lot of literature that model the recombination of separated charges through direct/Langevin type recombination, but there aren't many papers I could find which model the recombination as a trap assisted process and consider charge carrier lifetime. Are there any indicators (experimental results, particular trends etc) that indicate direct/Langevin recombination is more dominant?
Dear Waranatha,
You question is very interesting. In both organic an metallic semiconductor solar cells the recombination of the generated electron-hole pairs generated by the incident photons of sufficient quantum energy plays the dominant role in the photcurrent resulting from the incident solar radiation. The higher the recombination rate, the lower will be the the photocurrent. The higher recombination rate increases also the dark current, and lowers the open circuit voltage.
The most important difference between the two materials is that the dielectric constant of the organic material is much less than that of the metallic semiconductor. Consequently the excitons in the organic semiconductor is bound by the self electrostatic force. This state has its lifetime and or diffusion length which is of the order of tens of nanometer. In order to prevent the decay of the excitons they have to be made to dissociate at the donor-acceptor interface.Then by making the active material thickness smaller than diffusion length of the excitons. The remaining is the recombination of electrons and holes after dissociation of excitons. In this state the electron hole pairs on the different sides of the donor- acceptor interface are termed polaron. Polaron recombination has specific recombination
mechanisms:
-The direct recombination
- trap assisted recombination
each recombination mechanism has its rates and statistics.
The transport of polarons is subjected to current and continuity equations for the semiconductors.
You can find very good work in the phd thesis in the Link:
opus.bibliothek.uni-wuerzburg.de/files/6870/foertig_diss.pdf
The title of the thesis is:Recombination Dynamics
in Organic Solar Cells.
Wish you success
Waranatha, to the best of my knowledge, the recombination is highly dependant on the polymer (or small molecule) system used. For example, P3HT, PCDTBT and PTB7 show different stacking behaviours and therefore lead to different recombination mechanisms. To make matters worse, under realistic conditions, different suppliers provide different molecular weights, purities, regioregularities for the same polymer which makes a direct comparison on the work reported by different labs difficult. If you are working on theoretical aspects, i suggest that you focus on the results published by a single group as this cuts down on the variation of the above factors (though not all the time). Hope this helps.
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