One of the biggest factors in solar power remaining an expensive power source, despite constantly improving efficiencies, is the inherent cost of materials and processing for solar cells made of polysilicon. Cutting these costs could make the solar power the preferred energy source for mankind, but thus-far there has been little high-performance designs made with cheaper processes or materials.
Now University of California, Berkeley researchers have created a new type of solar cells that may offer exactly that -- lots of solar energy with low processing and materials costs. The new type of solar cells are composed of tiny nanopillars in a thin film layer atop aluminum foil. The foil is enclosed in a protective layer of transparent, rubbery polymer.
The total materials costs are quite low, and the production costs, while not fully determined also look promising. Ali Jarvey, an electrical-engineering and computer-sciences professor who led the work, cheers, "You won't know the cost until you do this using a roll-to-roll process, but if you can do it, the cost could be 10 times less than what's used to make [crystalline] silicon panels."
The cells use a nanofilm of cadmium telluride with uniform 500-nanometer-high pillars of cadmium sulfide laid on top of it. Other thin-film solar cells with pillars have been made before, says Professor Jarvey, but they have relied on more expensive deposition techniques. Further, the new cells have an efficiency of 6 percent in transforming sunlight into electricity, where past designs had efficiencies of less than 2 percent.
Silicon-based photovoltaics still have the cell beat in efficiency with 20 percent or more in commercially available designs; however, they are extremely pure, expensive crystalline silicon. Impurities can cause electrons to get trapped in the semiconductor, so the expensive process of making this high quality crystal material is unavoidable for that design. Purity is much less of a cost concern in the new design.
Creating an equivalent amount of power would require three times the area (panels) of photovoltaic cells, given their respective efficiencies. This means that given the cost estimates, solar power costs could be cut to a third of the current levels.
Another key advantage of the new design over traditional photovoltaic panels is flexibility. Traditional crystalline panels would break if flexed. The thin film nanopillar cells, though, can be rolled and unrolled with ease.
The new design essentially divides silicon's responsibilities. The thin film material absorbs light and generates electrons, while the pillars conduct the electrons to the circuit and help to trap light. As electrons have a shorter distance to travel to reach the pillars they're less likely to get trapped by defects, and thus crystal quality is less of a concern.
Currently the cells are produced using a relatively cost-effective anodizing design to grow the pillars on a thin aluminum foil film, the bottom electrode. The thin semiconductor film is then layered over the pillars and a top electrode of copper and gold is layered thinly to complete the circuit.
Two key areas of improvement are the top film and the production process. Adopting a roll-to-roll production system could speed up the assembly and make it cheaper. Also, currently the gold only allows half the sunlight to enter the cell as its semi-opaque. Replacing the gold with a transparent material like indium oxide could double the efficiency to 12 percent or more, while not significantly impacting the cost of materials.
States Professor Yang, "(The) architecture is most important--materials we can continue working on. The beauty of this paper is the demonstration of how well the architecture works."
The research appears in this month's edition of the journal Nature Materials.
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