I am trying to evaporate Au top contacts on TiOx/perovskite/spiro-OMeTAD solar cells. However, I often observe that after deposition the Au appears dark, as if the device had been “burned”, and the solar cell shows no efficiency. This issue occurs randomly, not every sample is affected, and the “burned” area is not always in the same position. In the attached photo, for example, all cells came from the same evaporation batch: the first and the second one appear partially burned, while the fourth one is completely affected
My samples are placed above the source and do not rotate. I’ve also noticed significant variability in efficiency between different evaporations of the same batch, where the only difference is the Au deposition step.
I’m wondering if this is due to the substrate reaching too high temperatures during Au evaporation. If so, what is the maximum substrate temperature I should aim for to prevent the perovskite from degrading? Are there other precautions I can take?
Thank you very much for any insights or suggestions!
Maybe a lowering deposition rate (1 ångström per second) could help with this. Another aspect is the pressure, try to reach close to 10E-7 bar, this would help in reducing the heat transfer too. The distance from the source to the substrate could affect the uniformity of PVD deposition. Hope this helps.
As Sharun Parayil Shaji mentionned, it is important to lower the deposition rate. In my case for my perovskite devices, I deposit around 0.2 Å/s the first few nm, then I can increase, but up to 1.5 Å/s maximum.
Secondly, having a high vacuum pressure below 8.0 E-7 Torr (i.e. below 1.0 E-4 Pa) is very important for a nice metal deposition. Above that you might have undesired "burned" metal electrodes (it happened to me with Al in the past).
Thirdly, make sure the tungsten boats are not too much deformed (they may be used several times, but they can break with the high current used for so long), well centered when placed and tightly fixed (usually with screws).
Finally, if your devices could rotate even slowly it would help for the homogeneity of the deposition. Good luck.
gold evaporation needs 60Amps, and the temperature of the substrate can be over 130degC. Maybe the intrinsic properties of perovskites are affected by the temperature.
I recommend to replace evaporation by sputtering (because it is a "cold" process).
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