Silicon Nanoparticles Enhance Performance of Solar Cells
4 Sep, 2007 02:15 pm
Semiconductor fluorescent nanomaterial may find applications in photovoltaic (PV) solar cell technology through the development of cells based entirely on the nanomaterial or in combination with existing technology. Efficiency improvement or expansions of operating spectral range toward the ultra-violet/blue is an example of the targeted improvements. Researchers have suggested the use of several semiconductor nanomaterials that have the potential of enhancing solar cells.
Recently the research group of Munir Nayfeh at the University of Illinois synthesized Si nanoparticles with characteristics that alleviate these difficulties. The procedure provides macroscopic amounts of highly-luminescent, mono-dispersed nanoparticles, which also allow formation of homogeneous films, integrable directly on Si cells without glass or polymer matrices.
The researchers improved the efficiency of solar cells by dispensing silicon nanoparticles of the desired size dissolved in alcohol onto the face of the solar cell. As the alcohol evaporated, a film of closely packed nanoparticles was left firmly fastened to the solar cell. The finding shows that “Placing a film of silicon nanoparticles onto a solar cell can boost power, reduce heat and prolong the cell’s life, while the process of coating the solar cells with silicon nanoparticles could be easily incorporated into the manufacturing process with little additional cost.” In key experiments, researchers integrated ultra thin films of silicon nanoparticles of approximately 1nm and 3nm in diameter directly onto conventional polycrystalline solar cells (BP Solarex Si cells), forming a “transparent” layer of silicon nanoparticles.
The particles were first made by converting bulk silicon into discrete, nano-sized particles using a patented process they developed. Depending on their size, the nanoparticles will fluoresce in distinct colors. The particles are manufactured utilizing common raw materials and chemicals. It uses silicon, the most abundant, cheapest and least toxic semiconductor material as well as common chemicals of acids and hydrogen peroxide and low power electricity in table-top setups. They start with same silicon that is used in silicon solar cells or computer chips. They ground the silicon to a consistency of flour through a special room temperature electrochemical dispersion process that has been protected by 24 US patents (10 issued) as well a number of foreign patents. The process is efficient, scalable and can be automated, taking 30-45 minutes to completion
The silicon nanoparticles come in discrete uniform sizes or identical sizes, which allows reconstitutions in high-quality solids and films. Moreover they are long-lasting, being photostable with their fluorescence lasting several years. Most importantly, they are made of the least toxic material and are expected to be more compatible with the environment, requiring no further processing or shielding to make them safe thus saving material and energy.
In experiments where solar cells were coated with a film of 1 nanometer, blue luminescent particles, the cells showed a power enhancement of about 60 percent in the ultraviolet/blue range of the spectrum, but less than 3 percent in the visible range, the researchers report. Solar cells coated with 2.85 nanometer red particles showed an enhancement of about 67 percent in the ultraviolet range, and about 10 percent in the visible. In conventional solar cells, ultraviolet light is either filtered out at extra cost or absorbed by the silicon and converted into potentially damaging heat that limits the lifespan of the cell.
The improved performance is a result of enhanced voltage rather than current. Our results point to a significant role for charge transport across the film and rectification at the nanoparticle interface.
The research was funded by the National Science Foundation, the State of
Stupca M., Alsalhi, M., Saud, T. Al, Almuhanna, A. and Nayfeh, M. H., Enhancement of polycrystalline silicon solar cells using ultrathin films of silicon nanoparticle, Applied Physics Letters, 6 August 2007