Type of solar cell
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.
When a photon is excited in the substrate of a solar cell, an electron and hole are separated. Once the electrons and holes are separated, they will want to recombine since they are of opposite charge. If the electrons can be collected prior to this happening they can be used as a current for an external circuit. Designing the thickness of a solar cell is always a trade-off between minimizing this recombination (thinner layers) and absorbing more photons (thicker layer).
Nano-particles.
Scattering and Absorption.
The basic principles for the functioning of plasmonic-enhanced solar cells include scattering and absorption of light due to the deposition of metal nano-particles. Silicon does not absorb light very well. For this reason, more light needs to be scattered across the surface in order to increase the absorption. It has been found that metal nano-particles help to scatter the incoming light across the surface of the silicon substrate. The equations that govern the scattering and absorption of light can be shown as:
This shows the scattering of light for particles which have diameters below the wavelength of light.
This shows the absorption for a point dipole model.
This is the polarizability of the particle. V is the particle volume. formula_4 is the dielectric function of the particle. formula_5 is the dielectric function of the embedding medium. When formula_6 the polarizability of the particle becomes large. This polarizability value is known as the surface plasmon resonance. The dielectric function for metals with low absorption can be defined as:
In the previous equation, formula_8 is the bulk plasma frequency. This is defined as:
N is the density of free electrons, e is the electronic charge and m is the effective mass of an electron. formula_10 is the dielectric constant of free space. The equation for the surface plasmon resonance in free space can therefore be represented by:
Many of the plasmonic solar cells use nano-particles to enhance the scattering of light. These nano-particles take the shape of spheres, and therefore the surface plasmon resonance frequency for spheres is desirable. By solving the previous equations, the surface plasmon resonance frequency for a sphere in free space can be shown as:
As an example, at the surface plasmon resonance for a silver nanoparticle, the scattering cross-section is about 10x the cross-section of the nanoparticle. The goal of the nano-particles is to trap light on the surface of the SC. The absorption of light is not important for the nanoparticle, rather, it is important for the SC. One would think that if the nanoparticle is increased in size, then the scattering cross-section becomes larger. This is true, however, when compared with the size of the nanoparticle, the ratio (formula_13) is reduced. Particles with a large scattering cross section tend to have a broader plasmon resonance range.
Wavelength dependence.
Surface plasmon resonance mainly depends on the density of free electrons in the particle. The order of densities of electrons for different metals is shown below along with the type of light which corresponds to the resonance.
If the dielectric constant for the embedding medium is varied, the resonant frequency can be shifted. Higher indexes of refraction will lead to a longer resonant wavelength.
Light trapping.
The metal nano-particles are deposited at a distance from the substrate in order to trap the light between the substrate and the particles. The particles are embedded in a material on top of the substrate. The material is typically a dielectric, such as silicon or silicon nitride. When performing experiment and simulations on the amount of light scattered into the substrate due to the distance between the particle and substrate, air is used as the embedding material as a reference. It has been found that the amount of light radiated into the substrate decreases with distance from the substrate. This means that nano-particles on the surface are desirable for radiating light into the substrate, but if there is no distance between the particle and substrate, then the light is not trapped and more light escapes.
The surface plasmons are the excitations of the conduction electrons at the interface of metal and the dielectric. Metallic nano-particles can be used to couple and trap freely propagating plane waves into the semiconductor thin film layer. Light can be folded into the absorbing layer to increase the absorption. The localized surface plasmons in metal nano-particles and the surface plasmon polaritons at the interface of metal and semiconductor are of interest in the current research. In recent reported papers, the shape and size of the metal nano-particles are key factors to determine the incoupling efficiency. The smaller particles have larger incoupling efficiency due to the enhanced near-field coupling. However, very small particles suffer from large ohmic losses.
Nevertheless, in certain types of nanostructured solar cells, such as the emerging quantum-dot intermediate band solar cells, the highly intense scattered near-field produced in the vicinity of plasmonic nanoparticles may be exploited for local absorption amplification in the quantum dots that are embedded in a host semiconductor. 
Recently, the plasmonic asymmetric modes of nano particles 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.
Metal film.
As light is incident upon the surface of the metal film, it excites surface plasmons. The surface plasmon frequency is specific for the material, but through the use of gratings on the surface of the film, different frequencies can be obtained. The surface plasmons are also preserved through the use of waveguides as they make the surface plasmons easier to travel on the surface and the losses due to resistance and radiation are minimized. The electric field generated by the surface plasmons influences the electrons to travel toward the collecting substrate.
Applications.
There are many applications for plasmonic-enhanced solar cells. The need for cheaper and more efficient solar cells is considerable. In order for solar cells to be considered cost-effective, they need to provide energy for a smaller price than that of traditional power sources such as coal and gasoline. The movement toward a more green world has helped to spark research in the area of plasmonic-enhanced solar cells. Currently, solar cells cannot exceed efficiencies of about 30% (first generation). With new technologies (third generation), efficiencies of up to 40-60% can be expected. With a reduction of materials through the use of thin film technology (second Generation), prices can be driven lower.
Certain applications for plasmonic-enhanced solar cells would be for space exploration vehicles. A main contribution for this would be the reduced weight of the solar cells. An external fuel source would also not be needed if enough power could be generated from the solar cells. This would drastically help to reduce the weight as well.
Solar cells have a great potential to help rural electrification. An estimated two million villages near the equator have limited access to electricity and fossil fuels, and approximately 25% of people