SOLAR NANOTECHNOLOGY

e live in an exciting time with the Technology rapid advancement of and emerging solar technology. One of the exciting areas that is Breakthrough increasing collection efficiencyW and practicality is processing photovoltaic solar panels. The use of semiconductor materials has boosted the efficiency of solar panels. for Boosting The propensity for these materials to 'give up' an electron makes them a great choice, when combined with complementary elements, to collect energy from Power the sun. The photons from sunshine can excite the material and cause the flow of electricity that can be SHIKHA NAGPAL directed through wires to batteries or directly to the applications for which they were intended. Solar energy is the most Unfortunately, the use of semiconductor materials in abundant form of renewable the solar cells causes production issues that affect the price and output of manufacturers. energy today. But the production Solar cells are made by placing material used as a substrate into a vacuum chamber. Once the vacuum of efficient and cost sensitive is fully engaged; a semiconductor substance, such as solar cell for large scale energy , is placed on the substrate to create the solar cells. Because this process is time consuming and generation is the challenge into only a small amount can be done at a time, solar panels remain expensive. With advancement in the consideration. The field of nanotechnology, new methods have become nanotechnology finds a way. available that eliminate the need to use a vacuum to work with semiconductor material. For example, a new ink made of nano-particles has been formulated that can be sprayed directly onto a substrate in order to lay down the necessary layers for a solar panel. This could greatly increase production capacity and reduce the price of solar panels dramatically. As the nanoparticle processing solar panel emerges and is incorporated into practical solar products, it could change the landscape of energy consumption. The implications are far reaching on an economic, environmental, and political level. Background Thin-film photovoltaic solutions are gaining ground quickly and are expected to capture up to 30% of solar panel market share by 2013. Production costs are significantly less than with crystalline products for a variety of reasons: deposition processes use a fraction of the raw material as wafers, the raw material need not be grown or cast, and there is no sawing or waste from unused portions. Thin-film solar cells are presently considered the route to low cost . This is achieved by reducing the photoactive layer, which is mostly silicon. However low cost is achieved by compromising the efficiency of these cells. This is Australian innovates world's most efficient because as the active layer is thinned down the broadband nanoplasmonic solar cells charge transport properties deteriorate thereby SOLAR NANOTECHNOLOGY

Light trapping technology is of paramount reported photocurrent enhancement results for inorganic devices are explained by the first importance to increase the performance mechanism of scattering and for organic devices by of solar cells and make them near field enhancement. Silver Silver competitive with silicon cells nanoparticles are nanoparticles of silver, i.e. silver particles of between 1 nm and 100 nm in size. Silver nanoparticles have unique optical properties affecting cell performance. For the same reason because they support surface plasmons. At specific light trapping becomes an important factor. wavelengths of light the surface plasmons are Conventional methods of light trapping use driven into resonance and strongly absorb or textures that have features that are comparable or scatter incident light. This effect is so strong that it much larger than the thickness of the actual thin- allows for individual nanoparticles as small as 20 film solar cells. This would not be feasible for thin- nm in diameter to be imaged using a conventional film cells. Textured surface can also result in dark field microscope. This strong coupling of considerable surface recombination losses due to metal nanostructures with light is the basis for the increased surface area, thereby degrading the cell new field of plasmonics. There is increasing interest performance. Hence one of the most crucial factors in utilizing the large scattering and absorption cross determining the efficiency of thin-film cells is the sections of plasmonic silver nanoparticles for solar light trapping effect or the amount of light scattered applications. Since the nanoparticles act as into the substrate. When excited, surface plasmon efficient optical antennas, very high efficiencies can excitation can result in scattering and giant be obtained when the nanoparticles are enhancement of the electric field. incorporated into collectors. Plasmonic Solar Cells Aluminum Particles Plasmonics is an emerging field that makes use of Metallic nanoparticles can direct light better into the nanoscale properties of metals. Though the solar cell and prevent light from escaping. In plasmonics is a wide area of study, its application conventional ‘thickfilm’ solar cells, the for solar cells has seen a recent surge of interest as nanoparticles would have little effect because all is evident from the increasing number of the light is absorbed by the film due to its thickness. publications over the last couple of years. Metals For thin films, however, the nanoparticles can make support surface plasmons that are the collective a big difference. Their scattering increases the oscillation of excited free electrons and duration the light stays in the film, bringing the total characterized by a resonant frequency. They can be absorption of light up to a level comparable with either localized as for metal nanoparticles or that for conventional solar cells. The strategy allows propagating as in the case of planar metal us to reduce the production costs of solar cells by surfaces. By manipulating the geometry of the several times and makes photovoltaics more metallic structures, the surface plasmon resonance competitive with respect to other forms of power or plasmon propagating properties can be tuned generation. depending on the applications. The resonances of The researchers modeled the light absorption noble metals are mostly in the visible or infrared efficiency of solar cells for various nanoparticle region of the electromagnetic spectrum, which is materials and sizes, specially they compared the the range of interest for photovoltaic applications. properties of silver versus aluminum nanoparticles. The surface plasmon resonance is affected by the In most studies on the subject, silver particles have size, shape and the dielectric properties of the been preferred. These have optical resonances in surrounding medium. Silver and gold have the visible part of the spectrum that are even better dominated experimental research in this area at focusing the light into the solar cell. although other metals also support surface Unfortunately, there is a trade off: the optical plasmons. Three different mechanisms that could resonances also cause the absorption of light by the be utilized for photovoltaic applications are (a) the nanoparticles, which means the solar cell is less scattering from the metal particles that also act as efficient. dipoles (far-field effect), (b) the near field In the case of silver, this resonance is right in the key enhancement and (c) direct generation of charge part of the solar spectrum, so that light absorption is carriers in the semiconductor substrate. Most considerable. But not so for aluminum SOLAR NANOTECHNOLOGY

nanoparticles, where these resonances are outside Laboratory (NREL), showed that silicon the important part of the solar spectrum. nanocrystals can produce two or three electrons Furthermore, the aluminum particles handle per photon of high-energy sunlight. The effect, oxidation well and their properties change little with could lead to a new type of solar cell that is both variations in shape and size. And more importantly, cheap and more than twice as efficient as today's their scattering properties are robust in comparison typical photovoltaics. with silver nanoparticle. Researchers found that As in earlier work with other materials, the extra nanoparticles made of aluminum perform better electrons come from photons of blue and than those made of other metals in enhancing light ultraviolet light, which have much more energy trapping in thin-film solar cells. than those from the rest of the solar spectrum, especially red and infrared light. In most solar cells, Breakthrough Technology the extra energy in blue and ultraviolet light is The UNSW researchers have devised a way to wasted as heat. But the small size of nanoscale deposit a thin film of silver (about 10 nanometres crystals, also called quantum dots, leads to novel thick) onto a solar cell surface and then heat it to quantum-mechanical effects that convert this 200° Celsius. This breaks the film into tiny 100- energy into electrons instead. nanometre “islands” of silver that boost the cell’s By generating multiple electrons from high-energy light trapping ability, thereby boosting its efficiency. photons, solar cells made of silicon nanocrystals A typical solar cell generates only one electron per could theoretically convert more than 40 percent of photon of incoming sunlight. Some exotic materials the energy in light into electrical power, are thought to produce multiple electrons per Concentrating sunlight with mirrors or lenses could photon, but for the first time, the same effect has raise that figure to about 40 %, but the same been seen in silicon. approach could boost the efficiency of a silicon Researchers at the National nanocrystal solar cell to well over 60 %.