University of Utah metallurgists used an old microwave oven to produce a nanocrystal semiconductor using cheap, abundant and less-toxic metals than other
semiconductors. They hope it will serve as a launching point for more efficient photovoltaic solar cells and LED lights, 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,” says Michael Free, a professor of metallurgical engineering.
Free and the study’s lead author, Prashant Sarswat, a research associate in metallurgical engineering, published their study of the microwaved photovoltaic semiconductor — known as CZTS for copper, zinc, tin and sulfur — in the June 1 issue of Journal of Crystal Growth.
In the study, researchers determined the optimum time required to produce the most uniform crystals of the CZTS semiconductor — 18 minutes in the microwave oven — and confirmed the material actually was CZTS using a range of tests. They 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 suitable for different uses.
Sarswat says that compared with photovoltaic semiconductors that use highly toxic cadmium and arsenic, ingredients for CZTS photovoltaic material “are more environmentally friendly.”
The method developed by Sarswat and Free has some unique characteristics, including different precursor chemicals (acetate salts instead of chloride salts) used to start the process of making CZTS, and a different solvent (oleylamine instead of ethylene glycol).
Sarswat says many organic compounds are synthesized with microwaves, and Free notes microwaves sometimes are used in metallurgy to extract metal from ore for analysis. They say using microwaves to process materials is fast and often suppresses unwanted chemical-side reactions, resulting in higher yields of the desired materials.
CZTS previously was made using various methods, but many took multiple steps and four to five hours to make a thin film of the material, known technically 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 min.
By controlling how long they microwave the ingredients, the metallurgists could control the size of the resulting nanocrystals and thus their possible uses. Formation of CZTS began after eight minutes in the microwave, but the researchers found they came out most uniform in size after 18 minutes.
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,” says Free. “It is the absorber layer — the active layer — of the solar cell.”
The microwave method produced crystals ranging from 3 to 20 nanometers in size, and the optimum sought by researchers was between 7 and 12 nanometers, depending on the intended use for the crystals.
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