Researchers Discuss How Impurities Impact Halide Perovskite Inks

By Chinmay ChariReviewed by Susha Cheriyedath, M.Sc.Nov 14 2022

In a paper recently published in the journal ACS Energy Letters, researchers characterized the specific impact of nitrate (NO₃^-), acetate (OAc^-), and significant amounts of potassium (K) in commercial PbI₂ reagents based on the comparative condensation rate of methylammonium (MA) with formamidinium (FA) to generate N-methylformamidinium (MFA).  

Background

Impurities in reagents and solvents will unavoidably have a negative impact on the resultant perovskite as the stability and performance of solar halide perovskite cells improve over time. Furthermore, it is well known that subtle ink reactions, reagent batch-over-batch variance, and ambient parameters like humidity reduce the repeatability of device metrics.

It is vital to regulate these characteristics for production on a larger scale since researchers' ability to replicate their findings, as well as those of others, is hampered by a lack of understanding of the details of these parameters.

A recent study determined the conflicting effects of KI, OAc^-, and NO₃^- impurities in various commercially available PbI₂ reagent sources and their effect on the performance of the devices used to produce thin films of halide perovskites in optoelectronic devices.

Moreover, it is known that common A-site alloyed MA–FA–containing perovskite inks deteriorate through a condensation process between FA and methylamine (deprotonated MA), producing MFA or N,N′-dimethylformamidinium (DMFA), which releases NH₃.

In another study, two small levels of PbO have been shown to significantly speed up this process in PbI₂ of lower grades, while being a plausible contaminant. As a result, it is essential to understand the impact of the whole assay of contaminants in PbI₂ to better realize the interconnection between the reagent purity and the degradation processes of the halide perovskite inks.

The present study described the reaction rate during the presence of NO3- and OAc- impurities. The findings highlighted the crucial roles that perovskite reagent quality and enhanced purification techniques would play as these semiconductor manufacturing processes move closer to commercialization in ensuring process reproducibility.

About the Study

The team compared the spectra of hydrogen-1 nuclear magnetic resonance (¹H NMR) for a stock solution of FA:MA in deuterated dimethyl sulfoxide (DMSO) following 24 hours under room temperature to those of samples made from the stock solution of FA:MA with further low- as well as high-purity PbI₂ (TCI perovskite grade). All glassware was oven dried with the samples completely readied in N₂ glove boxes.

Furthermore, using a standard FA:MA solution, the team was able to separate the influence on the degradation rate for different impurities. Spectra of samples spiked with 2 μL of 0.2 M water, 1 mg of Pb(OAc)₂^.3H₂O, 1 mg of PbO or 2 mg of Pb(NO₃^-)₂ after 1.5–3-hour reaction times at room temperature were compared to the ¹H NMR spectrum of a control aliquot.

Observations

Peak integration calculations showed that 0.04% MA was used up in the control sample, while the rate remained unaffected by PbI₂ with higher purity. The rates of degradation of the PbI₂ samples of high purity and the control were slower than those previously reported. This could be attributed to using better FA and MA solvents or reagents in the present study.

Moreover, a higher degree of FA-NH hydrogen peak splitting was detected, which was indicative of a reduced rate of proton exchange and, consequently, fewer methylamine or NH3 impurities in the organic halide reagents. Conversely, PbI₂ with low purity, which included visible 1-2% OAc^- at a concentration of 1.7 ppm, produced quicker degradation. About 2.1% of MA was used up after two hours, with 22% following 24 hours. As anticipated, NO₃^- was a weaker base for proton removal from MA to liberate methylamine, even at catalytic levels, and the reaction rate of MFA was unaffected compared to the control.

Furthermore, MA's deprotonation by OAc^- was comparable with alkylammonium and acetic acid’s pKa values in DMSO of 11.1 and 12.7, respectively. In contrast, the addition of OAc^- resulted in 80% degradation after two hours, whereas almost total degradation occurred due to the presence of PbO.

Conclusions

To summarize, the researchers demonstrated the sensitivity of the stability of the MA:FA mixed perovskite inks to certain contaminants within PbI₂ reagents, including PbO, OAc^-, and possibly PbIOH, whereas other contaminants, such as NO₃^- and H₂O, were generally neutral to such a degradation pathway.

Compositions utilizing only FA or MA as an organic cation would be significantly less susceptible to these contaminants and may be more amenable to scalability but would still benefit from thorough decontamination of PbX₂ reagents.

Moreover, the observations implied that FA and MA impurities also affected the reaction rates. According to the authors, this establishes a larger significance of purity and emphasizes the need to maintain the additive, solvent, and reagent qualities used to produce halide perovskite semiconductors.

Reference

Ross A. Kerner, Kelly Schutt, Kai Zhu, and Joseph J. Berry, (2022) PbI₂ Reagent Impurities Catalyze Mixed-Cation Halide Perovskite Ink Degradation, ACS Energy Letter, ​​​​​​​7, pp. 4333-4335, DOI: 10.1021/acsenergylett.2c01953

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