Proceedings of nanoGe Fall Meeting19 (NFM19)
Publication date: 18th July 2019
Halide perovskite photo-active materials require careful processing protocols to control the crystallization process. This may infer some extremely strict processing requirements (timing, temperature, etc.) and as such is doomed to irreproducibility.[1] For the frequently used solvent engineering approach, establishing or changing a perovskite process or equipment typically requires tedious fine-tuning and skilled personnel.
A number of these issues can be mitigated by avoiding solvent-solvent interaction and the use of the so called “complex assisted gas quenching” (CAGQ) approach instead of an “anti-solvent” step.[2]
On the other hand, we will show that the use of the CAGQ approach with the most commonly used solvent system dimethylformamide (DMF) : dimehylsulfoxide (DMSO) may bring severe issues in terms of precursor de-wetting and pinhole formation - especially if used on top of hydrophobic hole transport layers like polytriarylamine (PTAA). We identify the high boiling point and highly polar DMSO as main driving force for precursor de-wetting on the hydrophobic PTAA. The higher vapor pressure of DMF compared to DMSO leads to an increasing concentration of DMSO and a consequently increasing contact angle of the precursor on the PTAA.
In striking contrast, we demonstrate that the introduction n-methyl-2-pyrolidon (NMP) as a replacement of DMSO in the CAGQ effectively mitigates these wetting issues. This can mainly be attributed to the lower hydrophilic-lipophilic-balance of NMP and a resulting more favorable contact angle on PTAA. The deposition of high-quality perovskite layers is largely independent from solvent mixing ratios, process handling and timing. Finally, our findings afford an outstandingly robust, easy to use and failsafe deposition technique yielding single (MAPbI3) and double (FA0.94Cs0.06PbI3) cation perovskites as well as mixed halide, wide bandgap (FA0.6Cs0.4PbI2Br) perovskites. The resulting solar cells show high efficiencies and low process deviations without the need to adjust process parameters when switching between material systems.
We expect our results to be an important step for more robust perovskite processing and for reduced sample-to-sample variations. Ultimately, our work will help to pave the way for cost effective upscaling of perovskite solar technology.
We acknowledge the Deutsche Forschungsgemeinschaft (DFG) (Grants: RI1551/4-2 and RI1551/12-1) for financial support. The research leading to these results has received partial funding from the European Unions's 7th Framework Programme under Grant Agreement no. 604148 (MUJULIMA).
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