How is Gas Analysis Employed in Solar Cell Optimization?

By Bhavna KavetiJun 10 2022

Perovskite solar cells (PSC’s) have promising energy conversion efficiencies. However, its underperforming durability hampers its commercialization.

Image Credit: Alberto Masnovo/Shutterstock.com

The present article discusses the application of gas chromatography-mass spectrometry (GC-MS) to detect volatile products that evolved during the decomposition of perovskites and polymeric encapsulants. In addition, the use of time-of-flight secondary ion mass spectrometry (TOF-SIMS) to detect phosphorus (P) dopant in nanocrystalline silicon oxide (nc-SiO_x) is also highlighted.

Analysis of Gases Evolved PSC’s Decomposition

Organic-inorganic halide perovskites have unique properties such as high absorption coefficients and long carrier lifetimes, which render robustness in solar cells. Metal halide PSCs have progressed with their power conversion efficiencies (PCEs) increasing from 3.8% to 25.2% in the past 10 years. For the commercialization of PSCs, they must withstand long-term environmental stresses imposed by moisture, heat, and light.

To this end, industrial and academic researchers developed technologies for constructing moisture barriers, such as epoxy, butyl rubber, ceramic thin-films, and dyads. Other strategies of varying cell design and material choice to improve moisture and thermal stability include the application of two-dimensional (3D) and two-dimensional (2D) perovskite material and using alternative electron/hole transport materials.

Thermal stability in PSCs is improved by preventing stress-induced gaseous decomposition of the perovskite materials.

Hence, detecting these gases is critical in optimizing PSCs. To this end, previous studies mentioned the stabilization of methylammonium lead iodide (MAPbI₃)-based PSC using tin oxide (SnO₂)/indium tin oxide (ITO) barrier around the cell to prevent the escape of volatile gases. The heat-induced mass loss was confirmed by employing thermal gravimetric analysis (TGA), which revealed that MAPbI₃ reversibly decomposes to methylamine (CH₃NH₂) and hydrogen iodide (HI) gases.

Further, coupling TGA with Fourier transform infrared spectroscopy (FTIR) confirmed that the methylammonium iodide (MAI) barrier on PSCs did not release the gaseous products until 240 °C. Employing the combination of TGA-MS helped investigate the mass loss in solvent-free MAPbI₃ single crystals and the determination of gaseous products released under heat-induced stress.

GC-MS for Identifying Decomposition Products

In an article published in the journal Science, the researchers used GC-MS to identify the decomposition products of perovskite precursors with high specificity. They used unencapsulated and encapsulated perovskite test structures and full cells at elevated temperatures.

The research findings confirmed the outgassing behavior of multi-cation mixed halide perovskites and identified thermal degradation pathways. Additionally, the team used GC-MS to quantitatively determine the effectiveness of the glass/polymer-stack blanket-encapsulation scheme in suppressing such outgassing.

The results revealed that annealing organic perovskite precursor powders, including MAI, formamidinium iodide (FAI), and methylammonium bromide (MABr) resulted in the evolution of CH₃I, hydrogen cyanide (HCN), and methyl bromide (CH₃Br) as decomposition products.

Similarly annealing the un-encapsulated test structures of formamidinium / methylammonium (FAMA), poly[bis(4-phenyl) (2,4,6-trimethylphenyl) amine] (PTAA) revealed evolution of CH₃I, CH₃Br, ammonia (NH₃), HCN, triazine (H₃C₃N₃). The encapsulated cells with polyolefin (PO) and polyisobutylene (PIB)-based polymeric blanket-cover encapsulation reduced intensities of CH₃I, CH₃Br, and NH₃ as observed in GC-MS. The authors reported that PO is more effective than PIB in suppressing CH₃I and CH₃Br outgassing.

Related Studies

In another article published in the journal Polymers and Polymer Composites, researchers used GC-MS to identify and control volatile organic compounds (VOCs) released by the polymeric encapsulant during photovoltaic (PV) module encapsulation. They demonstrated how GC-MS help understand the curing process, mainly by identifying the VOCs emanating from poly(ethylene-co-vinyl acetate) (EVA) under the effect of temperature and pressure.

The results provide chemical insights into the EVA encapsulation process, which are valuable for further optimization of the PV module manufacturing process and evaluation of its environmental impact.

In the other article published in the journal ACS Applied Energy Materials, the researchers used phosphine gas fraction (f_PH3) to investigate the optoelectronic properties of n-type hydrogenated nanocrystalline silicon oxide (nc-SiOx:H) films. In addition, they confirmed the transformation of the nc-SiOx:H microstructure during the variation of f_PH3.

They observed that when f_PH3 was low, the growth of the crystalline phase was accelerated. However, further increasing f_PH3 led to excessive P inactive dopants, causing a phase transition from nanocrystalline silicon to amorphous silicon, that is, a transition from nc-SiOx:H to a-SiOx:H. While ultraviolet (UV)-Raman scattering spectroscopy and FTIR spectroscopy quantified the fraction of crystallites, the TOF-SIMS was applied to evaluate the phosphorus dopant intensity profiles.

Conclusion

In conclusion, GC-MS can be effectively applied in identifying the volatile decomposition products of organic-inorganic halide perovskite precursor materials as well as completed perovskite solar cells with high specificity. Additionally, GC-MS helped in gaining knowledge on the type and the rough amounts of VOCs produced from EVA during heating at encapsulation temperatures.

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References and Further Reading 

Shi, L., Bucknall, M. P., Young, T. L., Zhang, M., Hu, L., Bing, J., and Ho-Baillie, A. W. (2020). Gas chromatography–mass spectrometry analyses of encapsulated stable perovskite solar cells. Science, 368 (6497), eaba2412.https://www.science.org/doi/10.1126/science.aba2412?cookieSet=1

Qiu, D., Duan, W., Lambertz, A., Wu, Z., Bittkau, K., Qiu, K., and Ding, K. (2021). Function Analysis of the Phosphine Gas Flow for n-Type Nanocrystalline Silicon Oxide Layer in Silicon Heterojunction Solar Cells. ACS Applied Energy Materials, 4(8), 7544-7551. https://pubs.acs.org/doi/full/10.1021/acsaem.1c00654

Li, H. Y., Théron, R., Röder, G., Turlings, T., Luo, Y., Lange, R. F., and  Perret-Aebi, L. E. (2012). Insights into the Encapsulation Process of Photovoltaic Modules: GC-MS Analysis on the Curing Step of Poly (ethylene-co-vinyl acetate) (EVA) Encapsulant. Polymers and Polymer Composites, 20(8), 665-672. https://journals.sagepub.com/doi/abs/10.1177/096739111202000801

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