I produce P3HT PCBM solar cells on ITO-coated glass substrates and evaporate six electrodes on one substrate. The problem is that I get different power conversion efficiencies on different electrodes. The difference between efficiencies is sometimes larger than 1%. What could be the problem?
Do you mean 1% absolute or relative? 1% relative is very little, and in my view well within random process variations. If it's 1% absolute it's a genuine problem. For inorganic semiconductors contacts are normally annealed - is this an option?
I mean absolute difference. The maximum relative difference is about 30%.
I always anneal samples after evaporation of electrodes for 10 or 15 min at 150oC but still have a problem.
Well that's the limit of my ideas but I think other people might find it helpful if you tell them what electrode material you used.
Dear Nevena,
I think that there are 3 possible reasons.
a) Your solar simulator is not well focused across the surface of your solar cell.
b) Your contacts are very thin and sensitive. Therefore, it is possible to skin them each time, with your solar probes.
c) Your hotplate you use for the annealing process doesn't heat uniformly.
I hope to give you some options.
Do you mean efficiences of samples coated at the same time are different from each other or ones coated at different times are different?
If you have the first case, I agree with reason C written by Minas.
If you have the second case, I think you should repeat coating several times,
I have nothing more than this to say,
I hope it will be useful.
Good luck!
In addition to what Mr. Stylianakis mentioned above-
1) It's possible that different devices on same substrate have different area because of your shadow mask and/or evaporation technique, which leads to variation in short circuit current density.
2) If your film doesn't have uniform thickness, it's possible to have variation in performance, i. e. efficiency.
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