This is regarding P3HT:PCBM bulk heterojunction organic solar cells. I have noticed that open circuit voltage is a little less for the annealed devices when compared to un-annealed devices. (of course the performance improves with annealing, with significant enhancements in Jsc and fill factor). I want to know if this is a well known scenario, and if so what can be the reason for this. Is this common to other polymer blends as well?
Thank you.
How do you anneal it? And what is the chemical/structural purpose of annealing?
this is thermal annealing, to improve the active layer morphology for better charge extraction.
There are numerous examples in the literature for P3HT:PCBM blends, where Voc decreases due to annealing. I don't remember reading a detailed discussion of this issue, but the most likely explanation to me is that annealing causes a reduction in the effective band gap of the blend. I have linked a paper which may help you understand these issues in more detail.
Article On the origin of the open-circuit voltage of polymer-fullere...
Can you indicate the steps followed in your device fabrication process? We work with a lot of polymer:fullerene samples including P3HT:PCBM so might be able to help. Also, what sort of Vocs are you getting and is it PC70BM or PC60BM you are using?
When you anneal the film, the crystallanilty of P3HT increase and reduce the effective bandgap by changing the HOMO level. Voc is directly propotional to the difference of the HOMO(donor) and LUMO (acceptor). Hence Voc changes if any one of the level is affected by processing.
@Imalka,
The basic device structure is ITO/pedot:pss/(P3HT/PCBM)/Al. Polymers dissolved in DCB. spin coating carried out in glove box, Al back electrode deposited by thermal evaporation. then it is annealed at 150 C for 5 mins. I'm using PC60BM, and Voc, after annealing is around 0.55V
Hi Waranatha,
Based on the details given, a Voc of 0.55 V is quite good. The reduction in Voc can be due to a number of possible reasons. For example, it has been reported in the past that P3HT tends to segregate near the surface which can result in a decrease in the Voc. another possibility is the crystallinity as explained by Ankur.
A few inetersting points that I've noted with your process:
1) You haven't mentioned the use of a slow drying process. I take it that you do this before you deposit the Al?
2) In our labs, we normally thermally anneal the P3HT:PCBM layer before deposition of the contact. Not really sure if both procedures lead to same result, but might be worthwhile trying out and see how that affects the device parameters. The annealing temperature and times are different for different labs. Hopefully you have done a series of tests under different temperatures and times which has led to the number you are using.
3)One point I've noted is that you don't seem to have a buffer layer between Al and the active layer. Usually a thin film of BCP, Ca, LiF etc might help you to push that slightly higher.
Also any particular reason why you are using PC60BM instead of PC70BM (or even ICBA)?
While in principle, the use of Al should allow you to form the required contact (at least on paper), a buffer layer usually helps to mitigate any exciton quenchign effects in the presence of a metal contact and also prevents hot metal diffusing into the device during the Al deposition.
Although eh above are not directly related to your original question, hope it helps
Thank you very much for your detailed response. These are devices I have made sometime back. Looking back at the data, (now I'm working on modelling organic solar cells) I have noticed that Voc is lower in annealed devices. I was wondering if this is due to error in my fabrication process, or if this is widely known.
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