Columbus (OH) - Researchers at Ohio State University have accidentally discovered a new solar cell material capable of absorbing all of the sun's visible light energy. The material is comprised of a hybrid of plastics, molybdenum and titanium. The team discovered it not only fluoresces (as most solar cells do), but also phosphoresces. Electrons in a phosphorescent state remain at a place where they can be "siphoned off" as electricity over 7 million times longer than those generated in a fluorescent state. This combination of materials also utilizes the entire visible spectrum of light energy, translating into a theoretical potential of almost 100% efficiency. Commercial products are still years away, but this foundational work may well pave the way for a truly renewable form of clean, global energy.
A complete study of the team's work appears in the current issue of "Proceedings of the National Academy of Sciences" (PNAS).
Fluorescence and phosphorescence
Traditional solar cell materials use a property called fluorescence to gather electricity. Energy from the sun strikes whatever material they are made of resulting in a momentary "dislodging" of electrons into an excited state. The excited electrons exist due to a property called fluorescence. They last only a dozen or so picoseconds (trillionths of a second) in this state, which is also called a "singlet state." The many picosecond dwell there is fairly typical among traditional solar cell material in use today.
The new material, which was accidentally discovered using supercomputers to determine possible theoretical molecular configurations, causes not only fluorescing electrons in the singlet state to be created, but also phosphorescing electrons in what's called a "triplet state."
These triplet state electrons remain in their excited state of phosphorescence for scores of microseconds (up to about 200 microseconds, or 0.0002 seconds). With such a long lasting state of free electron flow, their ability to be captured is theoretically significantly greater than existing technologies.
And if the research team's current efforts (of using only a few molecules of the hybrid materials suspended in a liquid solution) can be extended into practical real-world scales, then products yielding nearly 100% solar efficiency may soon be achievable.
Solar cell technology
Today's best solar cell technologies utilize several material layers to convert the infrared, ultraviolet and visible portions of the spectrum into electrical energy. This equates to about 61% efficiency in the furthest extremes of the technology, though something around mid-40% is far more typical. Solar cells like these are also incredibly expensive, fragile and impractical for mass production, making them useful for projects like satellites. They have no real potential to become real alternatives for the base consumer's energy needs.
Quite recently, plastic solar cells have been created which achieve between 7% and 11% efficiency. While this may not sound like a lot, such products and materials are extremely inexpensive to produce in bulk quantities, costing about $3 per square meter. The idea of having a rooftop covered by plastic solar cells in place of tar-based shingles has drawn many a consumer's thought since being first reported in 2007. Commercial consumer products based on the technology, which could offer up to 14% efficiency if theories are to be believed, are promised within the next five years.
Alternate forms of using solar power
One of the biggest downfalls of using solar energy on the Earth's surface is that it only works when there is strong sunlight. If it is overcast or if there are clouds, then the resulting efficiency drops sharply and much less power is generated. Also, on most places during most of the year it is dark about 50% of the time. This means some kind of battery storage system must be used to gather the energy during the sun's brilliance in daylight hours, only to then rely on batteries during the night. This adds expense and complexity to solar cell solutions and produces a solution which has peaks and valleys of available power.
Another form of solar power, however, has bypassed some of those limitations. A phenomenal heat absorbing material (made primarily of sodium) uses a relatively simple technology to power itself. By directing the sun's rays through a large array of mirrors which focus the sun's heat and light onto a single spot of the material, it quickly heats up to a few thousand degrees. The material's properties allow it to absorb and store much heat, and then release it slowly over time.
Building technologies around this solution have allowed the sun's direct energy to continue to give off power during darkened times, much like a battery solution but without the need of a battery. The heat is stored in an insulating container, only to be tapped to power steam turbines or some other form of heat-sensitive motor technology.
Still not enough, more to come
The materials these researchers have created is not ready for prime time. Only a few molecules were created through a joint effort of the Ohio State University team and a team of chemists from the National Taiwan University. They synthesized enough of the material to carry out preliminary tests. And while these early findings are truly remarkable, there are still more on the horizon.
Supercomputers are enabling an entire new area of materials. No longer do scientists have to physically create samples of every possible material in the lab, only to test and document everything they find about it. Today they can set up a series of parameters and instruct a supercomputing machine to find the one that best aligns with their desires, wants and wishes. And while such computations often takes many days or even weeks for each trial material, it's more economical and feasible than the old route. Plus, it enables materials like these which were, in this context, accidentally discovered using computers.
The materials analysis these supercomputers carry out is only as good as they are properly designed, and the machine is powerful. Technology sciences like semiconductors and machine manufacturing are quickly overcoming every aspect of limitations regarding the machine's power. And ironically, faster computers are allowing research teams to develop better and more comprehensive models for materials research.
It won't be too long before supercomputers light the way for the truly revolutionary form of renewable energy generation. Who knows, it may come from a bacteria inside the digestive tract of a beetle. But, if you believe anything in science then you must believe it's out there. We just have to find it. And tools like supercomputers, and efforts like these at Ohio State University, are proving time and time again how valuable they are in increasing man's knowledge.
