The rows of blue solar panels that dot landscapes and rooftops are typically made out of crystalline silicon, the workhorse semiconductor found in virtually every electronic device. Last week, researchers at Colorado State University published a paper in the monthly journal Nature Energy on fabricating solar cells using a material that shows promise for replacing silicon, called cadmium telluride.
“Our paper goes right to the fundamental understanding of what happens when we alloy selenium to cadmium telluride,” said Kurt Barth, a director of the Next Generation Photovoltaics Center and an associate research professor in the Department of Mechanical Engineering.
Although getting quite common nowadays, the idea of getting free electricity from sunlight goes back well over 150 years. The research team at Colorado State have increased the conversion efficiency to one of the highest levels recorded for a single junction solar cell. Before we go into their technique, a little review is in order.
First demonstrated by French physicist Edmond Becquerel (father of Antoine Henri Becquerel of radioactivity fame) who at age 19, in 1839, built the world’s first photovoltaic cell using silver chloride in his father’s laboratory. Forty years later, Willoughby Smith first described the “Effect of Light on Selenium during the passage of an Electric Current” in a February 1873 issue of Nature. It wasn’t until 1946 when Russell Ohl, working at Bell laboratories, patented the modern junction semiconductor solar cell, this one being composed of silicon.
The practicality of using solar cells to produce electrical power gained much prominence with their incorporation onto the many satellites launched in the late 1950s. The solar cells you see at roadside call boxes or flashing traffic signals, or even on some calculators are based on the semiconductor silicon. But, new materials are making great progress.
Cadmium telluride is a compound semiconductor made of the metal cadmium and the semi-metal tellurium. Besides finding use in photovoltaic systems as you will soon see, it is also used as an infrared optical window because, unlike glass, CdTe is very transparent in the infrared, from 830 nm to greater than 20 µm – an enormous range. This allows it to be used in lens fabrication for high end thermal imaging systems such as those used by Homeland Security.
Besides image processing, CdTe is now emerging as a strong competitor to silicon based thin film solar cells because of its relatively high conversion efficiency. Since everyone who lives off the grid wants to squeeze as much energy from their solar arrays as possible, efficiency is an important parameter.
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To make solar energy feasible in the long run a system should have a reasonable energy payback time. Ten years ago it was estimated that you could recoup your money spent in setting up such a system within 10 years time, this based on a crystalline silicon system. Because such systems have a typical lifetime of 20-30 years, you would begin to save money after a while – hopefully years before you had to replace the entire array.
Thin film silicon systems, as opposed to crystalline, show a shorter break-even time because of their lower initial cost, something manufacturers say is in the area of three years even though the efficiency of thin films silicon is roughly half that of crystalline material. But that can change when higher efficiency thin film CdTe cells are used, reducing the break even time even further.
How did the Colorado team push the efficiency number higher? They sprinkled a tiny amount of selenium into each cell. Their experiments revealed that selenium overcomes the effects of atomic-scale defects in the cadmium telluride crystals that limited the conversion process.
According to their published article, electrons generated when sunlight hits the selenium-treated solar panel are less likely to be trapped and lost at the material’s defects, located at the boundaries between crystal grains as they are grown. This increases the amount of power extracted from each solar cell with a record breaking 22.1%.
Working with materials fabricated at CSU via advanced deposition methods, the team discovered this unexpected behavior by measuring how much light is emitted back from selenium-containing panels. As an extra bonus, the cadmium telluride thin films that the CSU team makes in the lab use 100 times less material than conventional silicon solar panels. They are thus easier to manufacture, and they absorb sunlight at nearly the ideal wavelength.
Electricity produced by cadmium telluride photovoltaic cells is the lowest-cost available in the solar industry, undercutting fossil fuel-based sources in many regions of the world.
According to Wikipedia, the prominent CdTe manufacturer is the United States is First Solar based in Tempe, Arizona. They are producing CdTe-panels with an efficiency of about 14 percent at a reported cost of $0.59 per watt.