06 September 2012
The technology behind microwave ovens people use to heat food could soon be used to synthesise nanoparticle materials in the production of thin-film photovoltaics. Engineers at Oregon State University (OSU) in the US for the first time ever have succeeded in using microwave heating in the synthesis of a new solar cell compound, copper zinc tin sulfide (CZTS), which is benign and inexpensive. Cu2ZnSnS4 could be a promising alternative to CIGS absorbers in thin-film solar cells. The zinc and tin would replace the expensive indium and gallium, leading to much lower materials costs.
The synthesis of high-quality, phase-pure nanoparticle materials typically takes a considerable amount of time and requires the use of expensive chemicals. So combining low-cost synthetic methods for the thin-film precursor inks and using high-yield printing methods could lead to significant cost savings in fabricating thin-film solar cells.
The inspiration for nuking solar cell materials in the microwave did not, as one may first think, arise over lunch in some breakroom at an OSU lab. Rather, “The idea to use microwaves for the synthesis of CZTS nanoparticles came partly from a joint research project that OSU and HP [Hewlett-Packard] had, where we observed the formation of unique structures when we used microwave radiation to heat our solutions,” says Greg Herman, PhD, an associate professor in OSU's School of Chemical, Biological and Environmental Engineering.
“The primary goal of this research is to develop methods that can be used to scale up the synthesis of high quality, highly uniform nanoparticle materials,” says Herman, who is also the senior author of the paper detailing the production method for CZTS cells, “Microwave assisted synthesis of Cu2ZnSnS4 colloidal nanoparticle inks,” published in the peer-reviewed journal Physica Status Solidi A. “Microwave heating provides uniform heating of the precursor solutions, which allows uniform nucleation of the nanoparticles to occur. This nucleation event, and the subsequent growth mode both require extremely high temperature uniformity so that the synthesised nanoparticles are highly homogeneous.” Such materials could be used as inks for printed solar cell manufacturing, which is the focus The Oregon Process Innovation Center for Sustainable Solar Cell Manufacturing, OSU’s signature research facility of the Oregon Built Environment & Sustainable Technologies Center, or Oregon BEST for short. “Not only have we used microwave radiation, but also ethylene glycol, which is a low-cost chemical (automotive antifreeze) as opposed to expensive coordinating solvents like trioctylphosphene,” adds the researcher.
The Oregon researchers’ focus is to fabricate printed and flexible solar cells. Their devices have fairly low performance, due to the low annealing temperatures in the experiments. “We have achieved 0.25% efficiency for the devices, but are improving our cell fabrication methods to improve performance,” Herman says.
This much simpler one-pot microwave heating method, which potentially reduces reaction times to seconds and allows for precise control over heat and energy to achieve the desired reactions, could work for other PV compounds, too. “We are exploring the synthesis of a variety of semiconducting nanoparticles using the basic microwave reactor and methods described in our paper,” Herman reports. “These include, CuInSe2, PbS, PbSe, ZnS, SnS2, Cu2S, and others.”
Next, the OSU engineers are gearing up their method for application on an industrial scale. “We are currently developing a continuous flow microwave system that will allow us to scale these synthetic approaches up to tens of kilogram per day quantities, which will be necessary for the expanded application of nanoparticles to solar cell manufacturing and other industries,” Herman explains.
Written by Sandra Henderson, Research Editor, Solar Novus Today
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