|Fig. 1: Three plasmonic approaches to enhance light absorption in the active layer of a solar cell. The blue layer is a dielectric spacer and the red layer is the absorber layer. a) Metal nanoparticles at the front surface of a solar cell. b) Metal nanoparticles embedded inside the cell. c) Metal corrugations at the back surface of a cell.  (Adapted by permission © 2010 Macmillan Publishers Ltd.)|
Plasmonic light trapping via metallic elements is of particular interest for enhancing the efficiency of thin film solar cells. Interface between a metal and a dielectric/semiconductor can support surface plasmons, which are collective oscillations of conduction electrons in metals bound to light oscillations in both metal and dielectric. Surface plasmons can be excited in metallic nanoparticles, where it is localized and has limited spatial extent, or along a metal-dielectric interface called surface plasmon polaritons (SPP), where it can propagate and confine light at the interface. Using plasmonic elements not only can improve the solar cell efficiency by trapping or concentrating light at the absorber layer but it can also serve as a back contact or a cheap anti-reflective electrode in some configurations. [1,2]
Plasmonic structures can be integrated with a solar cell in three ways.  First, a plasmonic structure can be placed on the top surface of a solar cell. If it is in the form of metallic nanoparticles, then it can enhance absorption in the absorber layer by scattering the sunlight into it and reducing the reflection (Fig. 1a). If it is in the form of a metallic mesh directly on the absorber layer, then it can serve both as a scatterer and an electrode. Second, plasmonic nanoparticles can be embedded inside the absorber (Fig. 1b). They behave as subwavelength lenses and enhance light absorption by concentrating the light locally. Third, metallic nanoparticles or corrugated metal films can be placed at the back surface of a cell (Fig. 1c). In the case of a metallic film, SPPs are excited at the interface between the metal and the absorber, which enhance optical field concentration and also couple the incident light into the guided modes of the thin absorber. It can also serve as the back contact of the cell where charge carries are efficiently collected due to a very short distance they travel to reach the contact. These three configurations are discussed in more details in the next sections.
The nanoparticle geometry in Fig. 1a can achieve light trapping by scattering the incident light into the absorber layer. Although the number of optical modes available in a thin absorber is much less than a thick absorber, metallic nanoparticles are able to excite them very efficiently because they scatter light into many different possible directions. If any light escapes the absorber layer, it can partially be scattered back by the nanoparticles. Although dielectric particles can also provide scattering, metallic counterparts are preferable because they have a much larger scattering cross section.  This means they can scatter light over an area much larger than their physical geometry. For example, the scattering cross section of a 20 nm Ag nanoparticle embedded in Si is 30 times larger than its geometric cross section, therefore small surface coverage with these particles can highly scatter the incident light.  Studies have shown that type of metal, shape, and size and density of particles are important factors determining the scattering efficiency.  However, very small particles suffer from significant Joule heating losses and careful engineering is required to optimize metal loss versus scattering efficiency. Inexpensive and scalable fabrication techniques are required to make metal nanoparticles useful for solar cells. A simple way of forming a random array of metal nanoparticles is to evaporate a thin metal film and then heat it at a moderate temperature. The surface tension of the metal film causes the film to convert into a random array of nanoparticles.  A periodic array can also be made by employing a recently developed method of substrate conformal imprint lithography. 
The plasmonic in-coupling geometries are not limited to nanoparticles. Arrays of periodic or aperiodic metal stripes or grids can also serve as very efficient couplers. An example of such a cell decorated with a periodic array of thin silver stripes is given in Pala et al.  In this case, both localized surface plasmons and coupling into waveguide modes contribute to light trapping and enhanced absorption. Calculations show 43% enhancement in the short circuit current as compared to a cell without metallic stripes. If the stripes are placed directly on the absorber layer, then they can serve as an electrode at the same time.
Another promising in-coupling geometry is the random arrangement of metallic wires, which is an inexpensive alternative that serves both as an electrode and a scatterer. It can be fabricated over large areas through economic chemical synthesis approaches. Fig. 2 shows an example of such a film that has an optical transparency equivalent or better than that of metal-oxide thin films for the same sheet resistance. 
The amount of generated photocurrent in a solar cell is directly related to the light intensity inside the absorber layer. By embedding nanoparticles in the absorber region, one can benefit from strong light localization around metal nanoaparticles through the excitation of surface plasmons particularly when they are in resonant with the incident light. For efficient energy conversion from sunlight into photocuurent, absorption of metallic nanoparticles must be much less than the absorber layer. This is the case for many of the organic and inorganic semiconductors. For example, colloidal Ag and Au nanoparticles are used as intermediate reflectors in organic solar cells.  The conversion efficiency of dye-sensitized solar cells has been enhanced by embedding gold nanodisks.  Other works have shown that embedding nanoparticles can also enhance the efficiency of many inorganic solar cells such as silicon cells. 
|Fig. 2: Averaged transmissivity versus sheet resistance over the wavelength range 400-800 nm for Ag gratings, ITO, CNT meshes and Ag nanowire meshes.  (Reprinted with permission © 2008 American Chemical Society.)|
In the third plasmonic light trapping technique, the plasmonic structure is placed on the back surface of a solar cell where light is partially converted into surface plasmon polaritons at the metal-absorber interface and partially coupled to waveguide modes of the absorber (Fig. 1c). Light travels along a much longer path in comparison to the physical thickness of the absorber, which results in enormous absorption enhancemnets. In addition to light trapping, the plasmonic layer on the back surface of a cell can play the role of an electrode to collect charge carriers. Using SPPs to enhance absorption has been investigated in several organic and inorganic solar cell designs. For example, using a one-step direct imprinting process plasmonic silver nanodome arrays are incorporated in the back reflector of a dye-sensitized solar cell that results in 16% enhancement in short circuit current.  In another study, thin-film amorphous silicon is deposited on a textured metal back reflector, which shows a 26% photocurrent enhancement mainly in the near- infrared regime.  It should be noted that the effect of scattering and localization are simultaneously present with coupling into SPP and waveguide modes and good engineering is required to extract the maximum efficiency from these light-trapping mechanisms.
© Farzaneh Afshinmanesh. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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