Although the solar panel market (18.2 GWP produced in 2010) is currently dominated by crystalline silicon solar cells, thin-film solar cell technologies based on chalcogenides (S, SE and TE) will significantly increase their market penetration. For example, First Solar produced 1.5 billion watts of cadmium telluride (CdTe) modules in 2010 and estimates it will reach 2.3 billion watts by the end of 2011. Other cadmium telluride companies, such as Primestar, owned by energy giant General Electric, are accelerating cadmium telluride production, while many others, such as Solar Frontier, Maia Solar, Miasole, Ascent, Nanosolar, etc. are all focusing on copper indium gallium (DI) selenide (CIGS: copper indium gallium (di)selenide) solar cells. Copper indium gallium selenide has the potential to achieve higher efficiencies, with laboratory-scale equipment efficiency reaching 20.3% ("Copper Indium Gallium Selenide Thin Film Solar Cell Efficiency Exceeds 20% to Create New World Record Efficiency").

　　In addition to device performance, price fluctuation issues (particularly for indium), rare earth scarcity issues (such as tellurium), and potential environmental issues (such as the toxicity of cadmium) have caused some concerns about cadmium telluride and copper indium gallium selenium. The first next-generation thin-film photovoltaic materials found are low-cost quaternary copper-zinc-tin-sulfide (CZTS: quaternary copper-zinc-tin-sulfide) and copper-zinc-tin-chalcogenide (CZTSSe: copper-zinc-tin-chalcogenide). It is worth noting that these materials contain natural elements abundant in the earth's crust and have very low toxicity (see "Abundant inorganic materials can replace platinum for dye-sensitized solar cells").

　　“Historically, most quaternary copper-zinc-tin-sulfur thin-film solar cells have been synthesized using vacuum deposition of metal precursors followed by sulfidation,” said Hugh W. Hillhouse, co-author of the paper. Rehnberg Chair Professor of Chemical Engineering at the University of Washington. "However, this approach is challenging due to cost, spatially heterogeneous compositions and the binary compounds formed, such as zinc sulfide (ZnS). Therefore, they are developing some solution-based inorganic solar cell chemistries that to significantly reduce production costs and improve performance.”

　　The current record for photoelectric conversion efficiency of copper-zinc-tin-chalcogenide solar cells exceeds 10.1%, achieved by Mitzi and colleagues at IBM ("High-efficiency solar cells employ Earth's abundant liquid process absorbers" ), the precursor used needs to be stabilized with hydrazine, which is a hepatotoxic, explosive and carcinogenic solvent.

　　There is a paper published in the latest issue of "Advanced Energy Materials", which is "Earth-Abundant Element Photovoltaics Directly from Soluble Precursors with High Yield Using a Non-toxic Solvent" Non-Toxic Solvent), Hillhouse and his group showed that there are other chemical approaches that use more benign solvents, which required the demonstration of a simple and lightweight solution phase method to make quaternary solvents. Copper-zinc-tin-sulfur thin-film solar cells can also be produced at high yields using commercially available precursors and non-toxic solvents.

　　A key finding in this work is the availability of alternatives so that highly efficient solution-processed solar cells can be produced without the use of nanocrystal inks or toxic solvents such as hydrazine.

　　"In fact, we've probably only scratched the surface of what's possible with solution-processed inorganic solar cells," Hillhouse said.

　　His simple new chemical method for preparing copper-zinc-tin-chalcogenide thin-film solar cells requires the use of a spin-coating solution, a highly soluble and cheap commercial precursor, and an environmentally friendly, non-toxic solvent, dimethyl sulfoxide. (dimethyl sulfoxide), in this way, a high-quality copper-zinc-tin chalcogenide film can be formed, which is subsequently selenized on molybdenum-coated lime (sodalime) glass. The manufactured solar cell device is air-stable and exhibits an efficiency of 4.1%.

　　Hillhouse pointed out that the amount of metal incorporated from precursor to film can be close to 100% with this method. This is not the case with the nanocrystal ink method, but a device with an efficiency of 7.2% can be made.

　　This novel processing method opens the door to low-cost, solution-processed thin-film solar cells using elements that are abundant on Earth. The absorption layer can be deposited using slot die coating, partition coating, or other easily scalable processes.

　　"Until now, little is known about the fundamental electronic properties, defect physics, thermodynamics, or formation kinetics of quaternary copper zinc tin sulfur, and even less about heterostructured optoelectronic devices based on quaternary copper zinc tin sulfur. " Hillhouse said. “The theoretical efficiency limit of single-junction solar cells using quaternary copper-zinc-tin-sulfur or copper-zinc-tin-sulfur group exceeds 30%. It is likely that once these basic conditions are in place, quaternary copper-zinc-tin-sulfur solar cells can one day replace copper indium gallium. Selenium, cadmium telluride, and even silicon-based photovoltaics.”

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