Two step methods consist of,
1) spin coating PbI2-DMF solution (462 mg/ml) on substrate and then annealing to form PbI2 layer
2) dipping the PbI2 coated substrate in CH3NH3I- Isopropanol(IPA) Solution (10 mg/ml) to form brownish CH3NH3PbI3 coating
The problem occurs after removing substrate from 2nd step. The IPA evaporates off immediately leaving behind whitish patches, thus damaging the base layer too.
Perovskites, especially ones derived from methyl ammonium iodide salt, are prone to degradation when exposed to moisture. I encountered the exact same problem.
You have to be very carefull when working with this material, specially because degradation in very influenced by humidity. The water in the air can cause problems in just a couple of minuts. The DMF can also produce degradation.
Do you conduct the processing in air or inert ambient? If you do in inert ambient, how long do you leave the substrate after dipping into IPA solution? I never noticed this issue.
One alternative to your problem is to spin-coat MAI solution (in IPA) instead of dip-coating.
Or you can also do dip-coating immediately followed by spin-coating.
Sanjib Das, would you please suggest annealing parameter for PbI2 (spin coated from DMF solvent). I would like to try two step perovskite with MAI (34 or 36 mg/mL from IPA).
Also, please comment on whether two step coating will work on ITO/PEDOT:PSS substrates?
Last time, I tried both by spin coating MAI (34 or 36 mg/mL from IPA) and dipping into 10 mg/mL in IPA for 1 min. The film is turning brown color. However, when we expose it to the air, i am seeing whitish color coming on the top. Please suggest any suitable alternatives.
I found this article (http://pubs.rsc.org/en/content/articlepdf/2014/ta/c4ta03674c) dealing with PEDOT:PSS/two step coating. I tried to repeat the procedure, however perovskite film could not be formed.
Hi Sai-Anand:
1) Yes, you can use sequential method with both conventional (e.g. ITO/TiO2) and inverted (ITO/PEDOT:PSS) architectures. Please keep in mind the following - people use various concentrations of PbI2-in-DMF and MAI-in-IPA; it varies from lab to lab. So, you need to optimize it.
2) You noticed whitish color on top while exposing the film to air. It could be either because diffusion between MAI and PbI2 is not complete, or perovskite degrades. By the way, did you check the humidity level when you exposed the film to air?
Please check the following papers-
Angew. Chem. Int. Ed. 2015 (DOI: 10.1002/anie.201505882), J. Am. Chem. Soc. 2015, 137, 9210.
Please let me know if you have further questions. Good luck!
Thank you for the answer. Humidity was over 40 % that time. I tried few months back in open-air atmosphere. I will check those papers. :)
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