A plasmonic-enhanced solar cell, commonly referred to simply as plasmonic solar cell, is a type of solar cell (including thin-film or wafer-based cells) that converts light into electricity with the assistance of plasmons, but where the photovoltaic effect occurs in another material.
A direct plasmonic solar cell is a solar cell that converts light into electricity using plasmons as the active, photovoltaic material. The active material thickness varies from that of traditional silicon PV (~100-200 μm wafers) , to less than 2 μm thick, and theoretically could be as thin as 100 nm. The devices can be supported on substrates cheaper than silicon, such as glass, steel, plastic or other polymeric materials (e.g. paper). One of the challenges for thin film solar cells is that they do not absorb as much light as thicker solar cells made with materials with the same absorption coefficient. Methods for light trapping are important for thin film solar cells. Plasmonic-enhanced cells improve absorption by scattering light using metal nano-particles excited at their localized surface plasmon resonance. Plasmonic core-shell nanoparticles located in the front of the thin film solar cells can aid weak absorption of Si solar cells in the near-infrared region—the fraction of light scattered into the substrate and the maximum optical path length enhancement can be as high as 3133. On the other hand, direct plasmonic solar cells exploit the fact that incoming light at the plasmon resonance frequency induces electron oscillations at the surface of the nanoparticles. The oscillation electrons can then be captured by a conductive layer producing an electrical current. The voltage produced is dependent on the bandgap of the conductive layer and the potential of the electrolyte in contact with the nanoparticles. There is still considerable research necessary to enable these technologies to reach their full potential and enable the commercialization of plasmonic solar cells.
History
Devices There are currently three different generations of solar cells. The first generation (those in the market today) are made with crystalline semiconductor wafers, with crystalline silicon making "up to 93% market share and about 75 GW installed in 2016". Current solar cells trap light by creating pyramids on the surface which have dimensions bigger than most thin film solar cells. Making the surface of the substrate rough (typically by growing SnO2 or ZnO on surface) with dimensions on the order of the incoming wavelengths and depositing the SC on top has been explored. This method increases the photocurrent, but the thin film solar cells would then have poor material quality.
The second generation solar cells are based on thin film technologies such as those presented here. These solar cells focus on lowering the amount of material used as well as increasing the energy production. Third generation solar cells are currently being researched. They focus on reducing the cost of the second generation solar cells. The third generation SCs are discussed in more detail under the "Recent advancements" section.
Design The design for plasmonic-enhanced solar cells varies depending on the method being used to trap and scatter light across the surface and through the material.
Nanoparticle cells
A common design is to deposit metal nano-particles on the top surface of the solar cell. When light hits these metal nano-particles at their surface plasmon resonance, the light is scattered in many different directions. This allows light to travel along the solar cell and bounce between the substrate and the nano-particles enabling the solar cell to absorb more light. The concentrated near field intensity induced by localized surface plasmon of the metal nanoparticles will promote the optical absorption of semiconductors. Recently, the plasmonic asymmetric modes of nanoparticles have found to favor the broadband optical absorption and promote the electrical properties of solar cells. The simultaneously plasmon-optical and plasmon-electrical effects of nanoparticles reveal a promising feature of nanoparticle plasmon. Recently, the core (metal)-shell (dielectric) nanoparticle has demonstrated a zero backward scattering with enhanced forward scattering on Si substrate when surface plasmon is located in front of a solar cell. The core-shell nanoparticles can support simultaneously both electric and magnetic resonances, demonstrating entirely new properties when compared with bare metallic nanoparticles if the resonances are properly engineered. Despite these effects, the application of metal nanoparticles at the solar cells' front can bring considerable optical losses, chiefly due to partial shading and reflection of the impinging light. Instead, their integration at the rear side of thin-film devices, particularly in between the absorber layer and the rear metallic contact (acting as reflective mirror), can circumvent such issues since the particles interact only with the longer-wavelength light that is weakly-absorbed by the cell, for which the plasmonic scattering effects can allow pronounced photocurrent gains. Such so-called plasmonic back reflector configuration has allowed the highest PV efficiency enhancements, for instance as demonstrated in thin-film silicon solar cells.
Metal film cells Other methods utilizing surface plasmons for harvesting solar energy are available. One other type of structure is to have a thin film of silicon and a thin layer of metal deposited on the lower surface. The light will travel through the silicon and generate surface plasmons on the interface of the silicon and metal. This generates electric fields inside of the silicon since electric fields do not travel very far into metals. If the electric field is strong enough, electrons can be moved and collected to produce a photocurrent. The thin film of metal in this design must have nanometer sized grooves which act as waveguides for the incoming light in order to excite as many photons in the silicon thin film as possible.
Principles
General
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