Photovoltaic materials generate electricity from solar illumination. The widespread use of PVs for solar energy generation is limited by the efficiency, stability and cost of the systems, a problem being taken on by Energy Frontier Research Centers. The maximum theoretical efficiency of a basic semiconductor solar cell (p-n junction) is defined by the Shockley-Queisser limit. One of the most promising new approaches can potentially produce solar cells with efficiencies exceeding the Shockley-Queisser limit. These new cells take advantage of the unique physical and modular electronic properties of quantum-confined semiconductors. For a classic semiconductor that has an adsorption spectrum, or band gap, optimized for the solar spectrum, the limit is around 33.7%.
The new approach, using nanoscale materials, confers several advantages. One is the ability to vary the electronic properties by nanocrystal size. This allows scientists to turn more economical and stable materials into excellent solar collectors.
Another advantage is the greater potential for the materials to collect more energy from sunlight that would otherwise be radiatively or non-radiatively lost as heat. Gathering this otherwise lost energy could result in solar cell efficiencies that exceed the Schockley-Queisser limit and is a major focus of the Center for Advanced Solar Photophysics, an Energy Frontier Research Center.
Advancements into the development of solar cells composed of nanostructured materials are being conducted at the Center, and recent progress was made in the fabrication of a lead sulfide and zinc oxide solar cell displaying high stability upon solar radiation in air. Lead sulfide is an inexpensive and abundant semiconductor but cannot collect enough solar energy to be efficient. Nanocrystals of lead sulfide, however, can be optimized to be more efficient than in bulk lead sulfide.
Lead sulfide crystals were incorporated into a solar cell device with zinc oxide nanocrystals. When tested in air with a solar simulator, the device maintained 95% of its initial efficiency even after 1000 hours with little degradation to the materials. This is a significant improvement in durability over previously reported solar cells of this type. The 3% efficiency and 1000-hr stability benchmarks reported are instrumental in establishing standards for the evaluation of future nanostructured solar cells.
This work has led to solar cells being included in standardized PV progress tables and published PV technology efficiency vs. year charts, allowing scientists to directly compare the results and measure the success of this technology. Further development along this line of this work led to a certified 4.4% lead sulfide device in March 2011.
