(574g) Understanding the Formation and Pyrolysis of Metal Thiolate Complexes for Solution-Processed Thin Film Photovoltaics

Significant improvements in photovoltaic manufacturing capacity are necessary if society is to sustainably satisfy an ever-growing global energy demand.[1] One prominent opportunity for realizing this increase in production is through the roll-to-roll printing of thin-film photovoltaics, a manufacturing approach which has the potential to easily meet growing energy needs if properly implemented.[1] One particular thin-film material, copper indium gallium diselenide (CIGSe), is notable for having no toxic constituent elements, long-term stability, and a certified power conversion efficiency of 22.9% for a vacuum-deposited device.[2] Many researchers have endeavored to create high-efficiency CIGSe devices with various printing techniques. The highest efficiency from a lab-scale device thus far is 17.3% and was processed using a hydrazine ink.[3] However, hydrazine-based printing technologies would likely be prohibitively expensive for industrial scale applications due to the significant hazards presented by the hydrazine solvent.[3] Instead, we propose the judicious use of a mixture of a monoamine and 1,2-ethanedithiol in a 2:1 mole ratio for the dissolution of pure metals Cu, In, and Ga for the fabrication of CIGSe devices in a first-ever demonstration of the dissolution of metallic gallium without the use of more hazardous diamine species.

Recent work conducted on the fabrication of CIGSe photovoltaic devices using amine-thiol mixtures yielded a champion efficiency of 13.12%, achieved through the implementation of a gallium gradient within the device.[4] Despite these successes, there has been no work that the authors are aware of that has investigated the underlying chemistry of amine-thiol reactions with pure metals and the way in which they form the desired molecular precursors.[5] This represents a significant gap in the understanding of these promising systems.

We will present our recent work determining the structure of the compounds formed when metallic copper, indium, and gallium are dissolved in a mixture consisting of a primary alkylamine (1-hexylamine or 1-propylamine) and 1,2-ethanedithiol. Data collected from XANES, EXAFS, Mass Spectrometry and Nuclear Magnetic Resonance spectroscopy shows that indium and gallium each form a complex in which they are chelated by two 1,2-ethanedithiolate ligands and suggests that copper may be forming a polymer consisting of Cu(I) cations and 1,2-ethylenedithiolate anions. Based on the identification of these reaction products, we have proposed a potential reaction pathway governing the dissolution of these metals. The resulting insights in to the underlying mechanisms of these reactions has enabled greater control over the use of the amine-thiol system, allowing us to reduce amine-thiol usage down to the stoichiometric quantities necessary for the reaction, minimizing residual carbon contamination in the final films and preventing the formation of undesirable by-products.

Following dissolution, these compounds are cast on molybdenum-coated glass using a blade-coating approach and annealed at 300ºC to drive off the organic ligands and form a copper indium gallium disulfide film, which can then be further processed into a complete device. We have done preliminary work to investigate the products released in the pyrolysis of these metal thiolate complexes, and initial results from pyrolysis-mass spectrometry suggests that ethylene sulfide is released upon heating. Further work is underway to confirm the identity of species evolved during decomposition, allowing for greater control over the processing of films formed with these complexes. Such an understanding will lead to greater control over the final device absorber layer, resulting in high-efficiency photovoltaic devices. Thus far, in our lab, champion device efficiencies of 11.8% for the CIGSe material system and 11.5% for the copper indium diselenide system (an amine-thiol record[5]) have been achieved with this approach and both are expected to rise as further insights in to this system are uncovered and processing conditions are tailored accordingly.

Our work can be easily extended to other semiconductor materials. Other photovoltaic materials such as copper zinc tin sulfoselenide (CZTSSe)[6] and CdTe[7] have been processed with amine-thiol chemistry in our group, and lead chalcogenide thermoelectric materials are also under development. Future work extending this newly understood chemistry to these systems will help clear the way for high quality semiconductor films and particles. The chemistry described during this presentation has the potential to significantly improve the understanding of the amine-thiol system and represents a substantial contribution to the field.

References

[1] J. J. Berry, J. van de Lagemaat, M. M. Al-Jassim, S. Kurtz, Y. Yan, and K. Zhu, “Perovskite Photovoltaics: The Path to a Printable Terawatt-Scale Technology,” ACS Energy Lett., vol. 2, no. 11, pp. 2540–2544, Nov. 2017.

[2] Solar Frontier K.K. "Solar Frontier Achieves World Record Thin-Film Solar Cell Efficiency of 22.9%" 2017. [Online] Avaiable: http://www.solar-frontier.com/eng/news/2017/1220_press.html [Accessed: 15-April-2018]

[3] T. Zhang et al., “High efficiency solution-processed thin-film Cu(In,Ga)(Se,S)₂ solar cells,” Energy Environ. Sci., vol. 9, no. 12, pp. 3674–3681, 2016.

[4] Q. Fan, Q. Tian, H. Wang, F. Zhao, J. Kong, and S.-X. Wu, “Regulating the Starting Location of Front-Gradient Enabled Highly Efficient Cu(In,Ga)Se₂ Solar Cells Via a Facile Thiol–Amine Solution Approach,” J. Mater. Chem. A, vol. 6, no. 9, pp. 4095-4101, 2018.

[5] C. L. McCarthy and R. L. Brutchey, “Solution processing of chalcogenide materials using thiol–amine ‘alkahest’ solvent systems,” Chem. Commun., vol. 53, no. 36, pp. 4888–4902, 2017.

[6] R. Zhang, S. M. Szczepaniak, N. J. Carter, C. A. Handwerker, and R. Agrawal, “A Versatile Solution Route to Efficient Cu 2 ZnSn(S,Se)₄ Thin-Film Solar Cells,” Chem. Mater., vol. 27, no. 6, pp. 2114–2120, Mar. 2015.

[7] C. K. Miskin, A. Dubois-Camacho, M. O. Reese, and R. Agrawal, “A direct solution deposition approach to CdTe thin films,” J. Mater. Chem. C, vol. 4, no. 39, pp. 9167-9171, 2016.