Solar Cell Material Made in the Microwave

Most people use microwaves to reheat leftovers, and some even experiment with marshmallows, inflating them like a balloon; but what if you could use the same home oven to make solar cells?

University of Utah metallurgists have done just that, cooking up a nanocrystal semiconductor — known as CZTS for copper, zinc, tin and sulfur — in just 18 minutes in an old microwave being discarded by the Department of Metallurgical Engineering. They believe the semiconductor, which uses cheap, abundant and less toxic metals than other semiconductors, could lead to more efficient photovoltaics, LEDs, biological sensors and systems to convert waste heat to electricity.

Using microwaves “is a fast way to make these particles that have a broad range of applications,” said Michael Free, a professor of metallurgical engineering. “You can use just a simple microwave oven to make the CZTS semiconductor,” he said, adding: “Don’t do it at home. You have to be cautious when using these kinds of materials in a microwave.”

A small prototype solar cell that uses CZTS, a photovoltaic semiconductor that University of Utah metallurgists produced in an old microwave oven that once heated student lunches. Courtesy of Lee J. Siegel, University of Utah.

Swiss researchers first invented CZTS in 1967 using another method, and in 1998 other researchers discovered that it could serve as a photovoltaic material. But until recently, “people haven’t explored this material very much,” said research associate Prashant Sarswat.

The material was previously made using various methods, but many took multiple steps and four to five hours to make a thin film, known as a p-type photovoltaic absorber, which is the active layer in a solar cell to convert sunlight to electricity.

A more recent method known as colloidal synthesis — preparing the crystals as a suspension or “colloid” in a liquid by heating the ingredients in a large flask — reduced preparation time to 45 to 90 minutes.

By controlling how long they microwave the ingredients, the Utah researchers could control the size of the resulting nanocrystals and their possible uses. Formation of CZTS began after 8 minutes in the microwave, but researchers discovered the semiconductors came out most uniform in size after 18 minutes.

This transmission electron microscope image shows a single nanocrystal of the semiconductor CZTS dissolved in an organic solvent. The nanocrystal is faintly visible in the center of the photo, shaped like a table-tennis paddle. The CZTS was produced by University of Utah researchers using an old microwave oven. University of Oregon researchers made the image for their Utah colleagues. Courtesy of the Center for Advanced Materials and Characterization of Oregon.

They confirmed the material using a variety of tests such as x-ray crystallography, UV spectroscopy, and electron and atomic force microscopy. The researchers also built a small photovoltaic solar cell to confirm that the material works and to demonstrate that smaller nanocrystals display quantum confinement, a property that makes them versatile for different uses.

“It’s not an easy material to make,” Sarswat said. “There are a lot of unwanted compounds that can form if it is not made properly.”

To make CZTS, salts of the metals are dissolved in a solvent and then heated in a microwave, forming an “ink” containing suspended CZTS nanocrystals. The ink then can be painted onto a surface and combined with other coatings to form a solar cell.

“This [CZTS] is the filling that is the heart of solar cells,” Free said. “It is the absorber layer — the active layer — of the solar cell.”

He says the easy-to-make photovoltaic semiconductor can be used in more efficient, multilayer solar cell designs.

University of Utah metallurgical engineers Prashant Sarswat and Michael Free used an old office microwave oven to produce a nanocrystal semiconductor known as CZTS that is made from cheaper, less toxic materials than other semiconductors and holds promise for more efficient solar power cells and lighting by LEDs as well as sensors for medical tests and systems to convert waste heat to electricity. Courtesy of Lee J. Siegel, University of Utah.

The method produced crystals ranging from 3 to 20 nm in size, and the optimum sought by researchers was between 7 and 12 nm, depending on the intended use for the crystals.

Larger CZTS crystals make a good photovoltaic material; crystals smaller than 5 nm are best for quantum confinement, meaning that the nanocrystals can be tuned to emit specific light, making them useful for LEDs.

“We hope in the next five years there will be some commercial products from this, and we are continuing to pursue applications and improvements,” Free said. “It’s a good market, but we don’t know exactly where the market will go.”

The study will appear June 1 in the Journal of Crystal Growth (doi: 10.1016/j.jcrysgro.2013.03.022).

For more information, visit: www.utah.edu