(617e) Understanding the Chemistry of Thiol-Amine Solutions: Versatile Solvents for Solution-Processed Thin Film Photovoltaics

Solution processing of inorganic electronic materials is an attractive alternative to vacuum based methods due to its promise of high-throughput, roll-to-roll fabrication at low cost. Recently, significant work has been done to advance the so-called molecular precursor route to inorganic thin films. This route involves the dissolution of appropriate precursors to form a homogenous solution, followed by an appropriate coating method, and subsequent heat treatments to promote the growth of a polycrystalline film of the desired final phase. This method has been applied with great success for thin film PV such as Cu(In,Ga)Se­₂, Cu₂ZnSn(S,Se)­₄, CdTe, and others--though the system is generally applicable to inorganic thin films of diverse applications.

An especially promising solvent system, a mixture of an amine and a thiol, has been identified. Recent work has demonstrated the incredible flexibility of the amine-thiol solvent system in dissolving a host of salts and precursors that are insoluble in either solvent by itself.^1,2 The system has found significant use in processing photovoltaic absorber layers, luminescent quantum dot films, and a variety of other thin-film materials, making it a very general solvent system for processing of inorganic thin films.^3–7

Currently, little is known about the solution chemistry enabling this powerful solvent. In this study we present electrospray ionization mass spectrometry, X-ray absorption spectroscopy, and Raman spectroscopy results obtained for copper chloride precursors dissolved in thiol-amines to reveal the species formed in solution. In short, they were found to be copper thiolates, copper chlorides, copper thiolate-chlorides, and alkylammonium chlorides. We also show that the final oxidation state of the complexed Cu is +1, even when the initial state of the Cu is +2. We also present similar analysis on Se and Te solutions and elucidate the differences when chalcogens are dissolved in contrast to metal salts—as ultimately chalcogens and metal precursors are typically co-dissolved or combined in a single solution prior to deposition.

We also present SEM-EDS, XRD, and Raman spectroscopy studies of annealed films prepared from these solutions to understand the conditions under which precursor impurities (such as chlorides) are removed from the films making them suitable for use in high efficiency devices. We find that chlorides are difficult remove from the final films due to the species formed in solution and suggest future straegies to avoid this contamination. We also present results obtained by applying this film preparation method to CdTe,⁷ CIGS,⁶ and other metal chalcogenide photovoltaic devices.^4,5

References

1 D. H. Webber and R. L. Brutchey, J. Am. Chem. Soc., 2013, 135, 15722–15725.

2 B. C. Walker and R. Agrawal, Chem. Commun., 2014, 50, 8331–8334.

3 Q. Tian, G. Wang, W. Zhao, Y. Chen, Y. Yang, L. Huang and D. Pan, Chem. Mater., 2014, 26, 3098–3103.

4 R. Zhang, S. Cho, D. G. Lim, X. Hu, E. A. Stach, C. A. Handwerker and R. Agrawal, Chem. Commun., 2016, 52, 5007–5010.

5 R. Zhang, S. M. Szczepaniak, N. J. Carter, C. A. Handwerker and R. Agrawal, Chem. Mater., 2015, 27, 2114–2120.

6 X. Zhao, M. Lu, M. J. Koeper and R. Agrawal, J. Mater. Chem. A, 2016, 4, 7390–7397.

7 C. K. Miskin, A. Dubois-Camacho, M. O. Reese and R. Agrawal, J. Mater. Chem. C, 2016, 4, 9167-9171.