In bilayer organic solar cell there has large contact between thin films of two organic material, then why the interface region is less than that of Bulk Heterojunction solar cell and other Nanostructured solar cells
In a planar bilayer structure interface area is equal to area of cross section of the device while in bulk heterojunction interface area is much larger than cross section. Later case has interface spread over in 3D. e.g. If you take cream and spread over a piece of bread than maximum interface you can have is the surface area of bread while if you dip the same piece of bread in milk then interface between milk and bread is very large than surface area of bread.
I hope this makes your doubt clear.
Thanks
Sumanshu
Provided the dimensions are the same. If I take a smaller bread piece and dip it in milk, then the interface contact may be* less than that of a larger bread slice with cream spread over it. Just wondering if my assumptions are correct.
It may be but when we talk about the solar cell then we are interested in carrier generation, carrier separation and carrier collection. To have high generation you need to have thicker material. In bilayer architecture thicker active layer result in low separation efficiency, as the carriers generated away from junction may not reach the junction to get separated (generated beyond diffusion length). While in bulk heterojunction absorber layer is within full thickness of the cell (leaving contact and ETL/HTL aside). In this architecture whenever a carrier is generated, it finds a junction in the vicinity with higher probability. Carriers can get separated there. Once separation occurs then it is only the collection of carrier is the task to be performed. Now large interface cause high recombination also which lowers the open circuit potential of the cell. So one with bilayer structure can have high VOC while other with bulk heterojunction can have high JSC. An optimized geometry is still not understood for these solar cells. If you look at the research in this field you will find numerous geometries have been proposed but none of them says it is optimized.
I think it lighten your insight.
Thanks
Sumanshu
@Sumanshu.Thank you for the insightful answer. The idea of using a large interface to increase the chances of exciton dissociation at a junction also increases recombination rate due to interface trap states across a longer interface.
@Ananth Sardhi. Yes, This is what observed in these solar cells (at least reports say so)
Then, how can we increase interface without increasing recombination rate...
If it is possible to create a large interface with very small interface trap density, then recombination would decrease. I read a few papers that consider annealing as a means to reduce interface traps in BHJ OSCs. (I don't have the link at the moment, though).
Here optimization comes into picture :). With increasing interface you are increasing current density as well recombination. So an optimized geometry is sought.
how we can know interface trap for large interface region. Any specific method is available or it is a assumption
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