(775f) Solution Processed Thin Film Photovoltaics Using Amine-Thiol Chemistry

Future development of the thin film electronics industry relies on cost effective solution processing techniques. Solution processing enables us to print electronic materials on flexible substrates using roll-to-roll techniques, which is a challenge with high temperature vacuum processes. Along with ease of scalability, solution processing also reduces the cost of production leading to cheap, flexible, wearable electronic devices like photovoltaics, batteries, sensors and thermo-electrics. Solution processing involves either making nanoparticles of the desired material and then casting films with a nanoparticle ink or using a molecular precursor approach where the precursor solution is first cast as a film and then reacted to make the desired material. The molecular precursor approach is easier to scale up, reduces the steps in making films and hence the cost.

Many researchers working in this field are developing different solvent systems for dissolution of precursors and had most success with hydrazine based solution system.¹ Scaling up a process with such a highly toxic chemical is both costly and challenging. Hence, we have developed a mixture of commonly used amines and thiols to dissolve a host of materials including various elemental metals (Cu, Zn, In, Ga), elemental chalcogens (S, Se, Te), metal chalcogenides (Cu₂S, SnS, Cu₂Se, SnSe, In₂Se₃), metal oxides (ZnO, Cu₂O) and metal salts (CuCl, CuCl₂, InCl₃, GaI₃), which were otherwise insoluble in either amines or thiols alone.^2–6 This system allows us to adjust the cation to anion ratio in the solution during precursor preparation by choosing the appropriate ratio of amine or diamine with mono or dithiol. Taking advantage of this mixture’s dissolution ability, we synthesized and cast thin films of electronic materials specifically for applications in photovoltaic devices. Our typical fabrication of a solar cell starts with the dissolution of appropriate metal salts or metal chalcogenides in a selected amine-thiol mixture and then casting a thin film (around 1 micron thick) from this precursor solution on to a Mo coated glass substrate with an optimized coating and drying procedure. This film is then selenized in a tube furnace and the device is finished with subsequent deposition of CdS, ZnO, and ITO layers followed by Ni/Al metal grids. For the CZTSSe material system, we used a mixture of hexylamine and propanethiol to dissolve metal chlorides (ZnCl₂, CuCl₂, SnCl₂) and chalcogens (S, Se) while for the CIGSe material system ethanedithiol is instead used along with hexylamine to dissolve a metal chalcogenide (Cu₂Se) and metal salts (Ga-acetylacetonate, In-acetate). Films obtained by using these inks yielded a power conversion efficiency of 7.86% for CZTSSe and 12.2% for CIGSe.^4,7 SEM cross section imaging of both devices showed a fine grain layer in the final architecture, which is thinner, compared to that seen in other solution processing routes for these materials. Using similar amine thiol chemistry, we also fabricated the first solution processed CdTe solar cell, with an initial power conversion efficiency of 0.5%.⁸

We have found that the use of metal salts in precursor solutions introduces unnecessary species in the film, which act as impurities affecting the performance of the cell. Here we will present our solution to this issue by using different combinations of amine and thiol to dissolve pure metals instead of metal salts for making inks. A challenge that emerges with the use of dissolved metals in as synthesized amine-thiol inks is the appearance of thick carbon residue in the film, deteriorating the cell performance. To address this challenge, we have developed alternative routes to utilize these inks while maintaining little carbon residue in the final film. In addition to the traditional photovoltaic material systems, we will also present use of amine thiol chemistry to synthesize lead chalcogenide nano/microstructures at room temperature, which can be used in photovoltaic and thermoelectric devices. Indeed our results are very promising and exciting.

References

(1) Zhang, T.; Yang, Y.; Liu, D.; Tse, S.; Cao, W.; Feng, Z.; Chen, S.; Qian, L. High Efficiency Solution-Processed Thin-Film Cu(In,Ga)(Se,S)₂ Solar Cells. Energy Environ. Sci. 2016, 9 (12), 3674–3681.

(2) Webber, D. H.; Buckley, J. J.; Antunez, P. D.; Brutchey, R. L. Facile Dissolution of Selenium and Tellurium in a Thiol–amine Solvent Mixture under Ambient Conditions. Chemical Science 2014, 5 (6), 2498.

(3) Zhang, R.; Cho, S.; Lim, D. G.; Hu, X.; Stach, E. A.; Handwerker, C. A.; Agrawal, R. Metal–metal Chalcogenide Molecular Precursors to Binary, Ternary, and Quaternary Metal Chalcogenide Thin Films for Electronic Devices. Chem. Commun. 2016, 52 (28), 5007–5010.

(4) Zhang, R.; Szczepaniak, S. M.; Carter, N. J.; Handwerker, C. A.; Agrawal, R. A Versatile Solution Route to Efficient Cu₂ZnSn(S,Se)₄ Thin-Film Solar Cells. Chemistry of Materials 2015, 27 (6), 2114–2120.

(5) Webber, D. H.; Brutchey, R. L. Alkahest for V2VI3 Chalcogenides: Dissolution of Nine Bulk Semiconductors in a Diamine-Dithiol Solvent Mixture. Journal of the American Chemical Society 2013, 135 (42), 15722–15725.

(6) Agrawal, R.; Zhang, R.; Walker, B. C.; Handwerker, C. Homogeneous Precursor Formation Method and Device Thereof. US20160333200 A1, 2016.

(7) Zhao, X.; Lu, M.; Koeper, M. J.; Agrawal, R. Solution-Processed Sulfur Depleted Cu(In,Ga)Se₂ Solar Cells Synthesized from a Monoamine–dithiol Solvent Mixture. J. Mater. Chem. A 2016, 4 (19), 7390–7397.

(8) Miskin, C. K.; Dubois-Camacho, A.; Reese, M. O.; Agrawal, R. A Direct Solution Deposition Approach to CdTe Thin Films. J. Mater. Chem. C 2016, 4 (39), 9167–9171.