Richard Loomis is trying to make a better solar cell. And he’s taken a road off the beaten path to achieve that goal.
Loomis, a Ph.D., is a Professor of Chemistry and the Director of Graduate Studies for the Chemistry Ph.D. program at Washington University in St. Louis, and is also a member of the Institute of Materials Science and Engineering. Loomis experiments with the composition and shape of the semiconductor quantum nanoparticles. His goal is to produce the most efficient conversion of light, which is comprised of packets of energy called photons, into charged particles, and thus voltage.
But he’s really an economist. His job is to get the greatest voltage output from a fixed amount of light and energy from the sun. The challenge here is the sun generates light with different colors spanning from the ultraviolet to the infrared. Each semiconductor material prefers to absorb specific colors. If a highly energetic blue photon from the sun is absorbed by a solar cell made of material that preferentially absorbs low-energy red photons, the excess energy of the photon is wasted as heat.
Some solar cell designs are now incorporating semiconductor quantum nanoparticles to absorb sunlight and make charged particles. A semiconductor quantum nanoparticle is a particle on the scale of nanometers that can be used to efficiently convert light into charges and voltage. These particles have diameters of nanometers; one billion nanometers equals a meter. To give a sense of proportion, a human hair is about 100,000 nanometers in diameter.
Why use nanomaterials in the construction of solar cells? A little background in order.
Every semiconductor material has an energy, termed the band gap energy, that must be overcome to generate mobile electrons. When light strikes a semiconductor photovoltaic material like silicon, if the photons are energetic enough, they can stimulate the electrons, and these electrons can be captured. By doing so, the light has been converted from solar energy into electrical energy, which is stored as voltage. Following the principles of Albert Einstein and his photoelectric effect, for every photon (or packet of light energy) absorbed, there is a preference for only one electron to be generated. If the energy of the photon is more than the band gap energy, then this excess energy is not converted into electrical energy, but is instead wasted as heat.
A solar cell has traditionally been made of silicon wafers. The property that’s so important in silicon is the conductivity of the material. Silicon, besides being able to convert photons to energy, is highly conductive. That means less energy is lost during the transmission of the energy, or voltage, across the silicon to the conductors that transmit the voltage beyond the wafer. That means better efficiency of the solar cell.
Other semiconductor materials are now being used because their band gap energies overlap better with the optical spectrum from the sun. Even more recently, researchers have found that by shrinking the size of a semiconductor down to nanometer size scales, the band gap energy of a semiconductor can be tuned to higher energies. It is thus possible to optimize the conversion of a light made up of specific colors into electrical energy by controlling the size of a semiconductor nanomaterial. It also means that by using appropriately sized semiconductor nanomaterials, less energy is lost in the transmission of the charges along the photovoltaic material when being collected. Scientists have traditionally used quantum dots, spherical molecules about 3nm to 20nm in size, as these advanced photovoltaic materials.
In addition to quantum dots, researchers have also incorporated semiconductor nanoparticles with varying chemical compositions that have shapes of quantum rods (like small grains of rice) and quantum platelets (like little pancakes) in solar cells. Many solar cell designs have gone away from using silicon as the semiconductor, and now use cadmium telluride (CdTe) since it can better absorb more of the solar spectrum than silicon.
Loomis and his associate William Buhro, Chairman of the Department of Chemistry at Washington University in St. Louis, have taken a different approach. They also use CdTe, or cadmium selenide (CdSe), as the absorbing semiconductor material, but they make and use nanomaterials with different dimensionality.
“We make semiconductor quantum wires very, very small...CdTe or CdSe wires that have dimensions of a few nanometers, but have long lengths, 2 microns to 100 microns (up to a tenth of a millimeter),” Loomis said. “Not a lot of people can make semiconductor quantum wires with both the high physical and optical properties necessary for efficiently converting light into charges,” Loomis said.
The shape of quantum wires offers the ability to tune the band gap energy of the semiconductor by adjusting the diameter, just like quantum dots. But, these one-dimensional quantum wires also offer a conduit or pathway for the charges to efficiently move within a solar cell device until they can be collected and stored. This is in contrast to the charge movement in films of quantum dots or other shapes, where the charges move in a more random path, jumping from particle to particle until the charges can be collected at an electrode.
“Quantum wires offer a dimension where the charges can move,” Loomis said. “With semiconductor nanoparticles, the limit is not just forming the electrons, it is also getting the electrons to an electrode and out of the medium so that they can be stored. In other nanoparticle solar cell designs, the electrons go from the semiconductor nanoparticle into a solution or a polymer, or they hop from nanoparticle to nanoparticle, until they are collected.”
“This can be an inefficient process,” he said. “We want to minimize inefficient processes, and we want the charges to transport along the semiconductor quantum wire to the electrode. That’s our idea.”
The group wants to make high quality semiconductor quantum wires and put them in a solar cell design. As long as they can make quantum wires of similar physical and optical properties as quantum dots, their quantum wire solar cells should be more efficient than those using quantum dots, they theorize. At this point, the Loomis and Buhro team is optimizing the optical properties of CdTe and CdSe quantum wires.
“On one side we have a spectrofluorometer, so we can quickly capture photoluminescence spectra of the quantum wires suspended in solution,” he said. “And on the other side, we have a scanning monochromator with a photomultiplier tube. This enables us to collect spectra with more quantitative intensities over spectral ranges from 300nm to 2 microns. There aren’t a lot of instruments that can span that broad a wavelength with quantitative intensities.”
Loomis uses the photoluminescence quantum yield of a quantum wire sample to judge their optical qualities. The quantum yield is a measure of the efficiency of photon emission as defined by the ratio of the number of photons emitted to the number of photons absorbed. When semiconductor nanoparticles are not embedded in a solar cell, but are suspended in solution, the charge carriers tend to remain in the nanoparticles, and they ultimately relax energetically by giving off light, or photoluminescence. The photoluminescence quantum yield of a sample is a key measure of the quality of semiconductor quantum nanoparticles and an indicator of how efficient they will be in a solar cell. In principle, the higher the photoluminescence quantum yield of a sample, the better it will be in a solar cell since loss pathways for the charge carriers that would decrease the solar to electrical conversion have been minimized.
Loomis and Buhro are trying to make materials that will improve solar cell efficiencies by using advanced synthetic schemes coupled with lasers and optical spectrometers to characterize the physical and optical properties of these materials. They use photoluminescence measures to both gauge the absorption and emission of light, and the quantum yield.
There are two major applications of semiconductor quantum nanoparticles: solar cell design and circuits in LED displays. Loomis says the next step for quantum wires could be in quantum computers or ni nanoelectronics, which fuel Moore’s Law. (That’s the observation that the number of transistors in a dense integrated circuit doubles about every two years.) Miniaturization in cell phone design and function, essentially building a computer that fits in your shirt pocket, is an example of the ongoing, although slowing, process of this “law.”
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