Physicists from the University of Illinois at Urbana-Champaign (UIUC) have improved the performance of solar cells by 60 percent. And they obtained this spectacular result by using a very simple trick. They've coated the solar cells with a film of 1-nanometer thick silicon fluorescing nanoparticles. The researchers also said that this process could be easily incorporated into the manufacturing process of solar cells with very little additional cost.
This research has been led by Munir Nayfeh, Professor of Physics at UIUC. Here is a photo of Nayfeh holding a silicon solar cell coated with a film of silicon nanoparticles (Credit: L. Brian Stauffer, for UIUC). Here is a link to a larger version of this picture. And Nayfeh said that "integrating a high-quality film of silicon nanoparticles 1 nanometer in size directly onto silicon solar cells improves power performance by 60 percent in the ultraviolet range of the spectrum."
According to the physicist, "in conventional solar cells, ultraviolet light is either filtered out or absorbed by the silicon and converted into potentially damaging heat, not electricity. In previous work, however, Nayfeh showed that ultraviolet light could efficiently couple to correctly sized nanoparticles and produce electricity."
This previous work was published in the IEEE Photonics Technology Letters under the name "Thin film silicon nanoparticle UV photodetector" (Volume 16, Issue 8, Pages 1927-1929, August 2004). Here is the abstract. "We constructed ultraviolet (UV) photodetectors by room-temperature deposition of Si nanoparticle films on Si p-type substrates. Silicon nanoparticles of 1-nm diameter are dispersed from Si wafers using electrochemical etching. The current-voltage characteristics indicate a photoconductor in series with a diode-like junction with a large enhancement in the forward current under UV illumination. With increasing wavelength, the response drops rapidly, dropping to a few percent at 560 nm. These results point to a sensitive UV detector with good visible blindness where the particle films effectively constitutes a wide-bandgap material."
So how did Nayfeh improved this process in the last 3 years? "To make their improved solar cells, the researchers began by first 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. Nanoparticles of the desired size were then dispersed in isopropyl alcohol and dispensed 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."
And it really worked. "Solar cells coated with a film of 1 nanometer, blue luminescent particles showed a power enhancement of about 60 percent in the ultraviolet 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."
This latest research work was published in Applied Physics Letters under the name "Enhancement of polycrystalline silicon solar cells using ultrathin films of silicon nanoparticle" (Volume 91, Issue 6, Article 063107, August 6, 2007). Here is the abstract. "Ultrathin films of highly monodispersed luminescent Si nanoparticles are directly integrated on polycrystalline Si solar cells. The authors monitor the open-circuit voltage and the short circuit current. The results demonstrate that films of 1 nm blue luminescent or 2.85 nm red luminescent Si nanoparticles produce large voltage enhancements with improved power performance of 60% in the UV/blue range. In the visible, the enhancements are ~10% for the red and ~3% for the blue particles. The results point to a significant role for charge resonant transport across the nanofilm and Schottky-like rectification at nanoparticle-metal interface."
One thing puzzles me. Why did the researchers tried films with a thickness of 1 and 2.85 nanometers only? Why not 2, 4 or 6? Did they lack time? Or did they try with poor results? Please send me a note if you have the answers?
Sources: University of Illinois at Urbana-Champaign news release, via EurekAlert!, August 20, 2007, 2007; and various websites
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