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Slashing the cost of solar cells with antifreeze

The combination of using a continuous flow reactor, which is much faster than batch mode synthesis, commonly used for CIGS, and the use of cheap environmentally friendly materials promises to cut costs
A process combining some comparatively cheap materials and the antifreeze used in vehicles could make cheaper solar cells that avoid toxic compounds, while further expanding the use of solar energy.

And when perfected, this approach might also cook up the solar cells in a microwave oven similar to the one in most kitchens.

Engineers at Oregon State University have determined that ethylene glycol, commonly used in antifreeze products, can be a low-cost solvent that functions well in a “continuous flow” reactor, an approach to making thin-film solar cells that is easily scaled up for mass production at industrial levels.

The research, just published in Material Letters, also concluded this approach will work with CZTS (copper zinc tin sulphide). This a compound is of significant interest for solar cells due to its excellent optical properties and the fact the materials in the compound are cheap and environmentally friendly.

“The global use of solar energy may be held back if the materials we use to produce solar cells are too expensive or require the use of toxic chemicals in production,” says Greg Herman, an associate professor in the OSU School of Chemical, Biological and Environmental Engineering. “We need technologies that use abundant, inexpensive materials, preferably ones that can be mined in the U.S. This process offers that.”

By contrast, many solar cells today are made with CIGS (copper indium gallium diselenide). Indium is comparatively rare and costly, and mostly produced in China. Last year, the prices of indium and gallium used in CIGS solar cells were about 275 times higher than the zinc used in CZTS cells.

The technology being developed at OSU uses ethylene glycol in meso-fluidic reactors that can offer precise control of temperature, reaction time, and mass transport to yield better crystalline quality and high uniformity of the nanoparticles that comprise the solar cell - all factors which improve quality control and performance.

SEM micrograph of solar CZTS nanoparticle (Credit:OSU)

This approach is also faster - many companies still use “batch mode” synthesis to produce CIGS nanoparticles, a process that can ultimately take up to a full day, compared to about half an hour with a continuous flow reactor. The additional speed of such reactors will further reduce final costs.

“For large-scale industrial production, all of these factors - cost of materials, speed, quality control - can translate into money,” Herman points out. “The approach we’re using should provide high-quality solar cells at a lower cost.”

The performance of CZTS and CZTSe (copper zinc tin selenide) cells right now is lower than that of CIGS cells, which have a typical efficiency of 20 percent. For example, research institute imec and Solliance reported this week that they had achieved a CZTSe solar cell with an efficiency of 9.7 percent. But with further research on the use of dopants and additional optimisation it should be possible to create solar cell efficiencies that are comparable to that of CIGS cells

This research is described in more detail in the paper, "Continuous flow mesofluidic synthesis of Cu2ZnSnS4 nanoparticle inks," by B. Flynn et al in Materials Letters, Volume 107, 15th September 2013, Pages 214–217.

This project is one result of work through the Centre for Sustainable Materials Chemistry, a collaborative effort of OSU and five other academic institutions, supported by the National Science Foundation. Funding was provided by Sharp Laboratories of America. The goal is to develop materials and products that are safe, affordable and avoid the use of toxic chemicals or expensive compounds.

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