(390c) Direct Solution Deposition of Metal Selenide Semiconductors Using Novel Metal-Selenium Complexes

In the field of semiconductor materials, considerable research effort has been dedicated to the development of solution-deposition techniques due to the potential of these methods to reduce costs while increasing manufacturing throughput compared to analogous vacuum deposition techniques. In the vein of solution-processed thin-film solar cells, Cu(In,Ga)(S,Se)₂ has received significant attention due its combination of high efficiencies and stability. As with many other solution-deposited metal chalcogenide semiconductors, hydrazine-based molecular precursors have set the bar by producing solar cells with efficiencies exceeding 17%.¹ The success of hydrazine-based methods in solution-deposited Cu(In,Ga)(S,Se)₂ and other metal chalcogenidescan be explained by understanding its reactive-dissolution chemistry. Hydrazine, with the addition of excess chalcogen, is capable of dissolving a range of metal chalcogenides, forming a number of hydrazinium thiometallates and hydrazinium selenometallates.² These soluble species will then decompose upon heating into the desired metal chalcogenides and other volatile products that are easily removed from the film. However, hydrazine is both explosive and highly toxic, potentially hindering the scale-up of this method.

While other chemistries have been developed to replace hydrazine in the solution deposition of metal sulfides, limited success has been seen in developing a molecular precursor approach to metal selenides.³ This is in large part due to the difficulty in producing soluble and stable selenium containing species. One promising route to overcome this limitation is through the use of the amine-thiol reactive solvent system. The amine-thiol system has emerged as one of the most promising alternatives to hydrazine, producing Cu(In,Ga)(S,Se)₂ devices with efficiencies above 15%.⁴ Not only can this reactive solvent system dissolve metals and metal sulfides, but the ability of this system to dissolve selenium and metal selenides provides an opportunity to match hydrazine in producing metal selenide films.⁵ However, one significant challenge to this prospect is that the metal-thiolates produced in this dissolution can easily decompose into metal sulfides upon heating, often leading to significant sulfur content in the precursor film.⁶

In this research, we will discuss our investigations into the metal-chalcogenide complexes that are formed with the amine-thiol solvent system through the use of ¹H-NMR, Raman, and ESI-MS. We will then present how an understanding of this chemistry allows for the synthesis of novel metal-selenium complexes that can be used for the direct deposition of Cu(In,Ga)Se₂ films. We will further discuss the role of sulfur and selenium in the precursors film and how the chalcogen species alters the grain growth mechanism, contributing to the formation of the notorious “fine-grain” layer that is often observed during selenization of sulfide precursor films. Finally, we will show the broader applicability of these techniques by extending these methods to the direct deposition of pure selenide Cu₂ZnSnSe₄.

In conclusion, this research focuses on furthering our understanding of soluble metal complexes, using that knowledge to alter their molecular structure and produce novel metal-selenium complexes, and applying these complexes to the solution-processed fabrication of Cu(In,Ga)Se₂ thin films. While the primary focus of this work was directed towards Cu(In,Ga)Se₂, we have also shown that these methods have broader applicability to other solution processed metal selenides.

(1) Zhang, T.; Yang, Y.; Liu, D.; Tse, S. C.; 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, 3674. https://doi.org/10.1039/c6ee02352e.

(2) Chung, C.-H.; Li, S.-H.; Lei, B.; Yang, W.; Hou, W. W.; Bob, B.; Yang, Y. Identification of the Molecular Precursors for Hydrazine Solution Processed CuIn(Se,S)₂ Films and Their Interactions. Chem. Mater 2011, 23, 964–969. https://doi.org/10.1021/cm103258u.

(3) Clark, J. A.; Murray, A.; Lee, J.-M.; Autrey, T. S.; Collord, A. D.; Hillhouse, H. W. Complexation Chemistry in N,N-Dimethylformamide-Based Molecular Inks for Chalcogenide Semiconductors and Photovoltaic Devices. J. Am. Chem. Soc. 2018, 141, 298–308. https://doi.org/10.1021/jacs.8b09966.

(4) Yuan, S.; Wang, X.; Zhao, Y.; Chang, Q.; Xu, Z.; Kong, J.; Wu, S. Solution Processed Cu(In,Ga)(S,Se)₂ Solar Cells with 15.25% Efficiency by Surface Sulfurization. Appl. Energy Mater. 2020, 3, 6785–6792. https://doi.org/10.1021/acsaem.0c00917.

(5) 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. https://doi.org/10.1039/C5CC09915C.

(6) Deshmukh, S. D.; Ellis, R. G.; Sutandar, D. S.; Rokke, D. J.; Agrawal, R. Versatile Colloidal Syntheses of Metal Chalcogenide Nanoparticles from Elemental Precursors Using Amine-Thiol Chemistry. Chem. Mater. 2019, 31 (21), 9087–9097. https://doi.org/10.1021/acs.chemmater.9b03401.