**1. Introduction**

Photovoltaic technology, the conversion of solar energy to electricity, can help to solve the energy crisis and reduce the environmental problems induced by the fossil fuels. Worldwide photovoltaic production capacity at the end of 2015 is estimated to be about 60 GW [1] and is expected to keep rising. Yet, there is great demand for increasing the photovoltaic device efficiency and cutting down the cost of materials, manufacturing, and installation. Materials and processing represent a large fraction of the expense. For example, material costs account for 40% of the total module price in the bulk crystalline silicon solar cells. Thin film solar cells have emerged as a means to reduce the material costs. To date, thin film solar cells are made

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

from various active inorganic materials, including amorphous and polycrystalline silicon, GaAs, CuIn*x*Ga1−*x*Se2 and CdTe, hybrid lead halide perovskites, as well as organic semicon‐ ductors. As the thickness of the absorbing semiconductor is decreased, the absorption naturally reduces at energies close to the band gap of the semiconductor. This is particularly a problem for thin film silicon solar cells. Thus, the compromise between enhancing the absorbance of broader solar lights and reducing the usage of narrower band gap semicon‐ ductor materials has to be taken in a thin film solar cell. And novel designs of thin film solar cells in which broadband light can be trapped inside to increase the absorption are highly needed to break the compromise balance.

In the past decade, a few light‐trapping techniques have been investigated, among which a typical example is using a pyramidal surface texture [2]. However, such surface texture is intended for active light‐harvesting layers, which are thicker than the wavelength of sunlight in the visible and near‐infrared regions. The improved light trapping is balanced by the surface roughness that is almost the same order as the film thickness and by the increased surface recombination due to the larger surface area. Recently, the use of metallic nanostructures, which support surface plasmons (SPs) [3], has been regarded as an efficient way for enabling light trapping inside the active layer of a thin film solar cell and has consistently drawn an increasing amount of attention. SPs are coherent electron oscillations that propagate along the interface between a metal and a dielectric or semiconductor material. And SPs cause the electromagnetic field strongly confined at the metal/dielectric or semiconductor interface, with their intensity having an exponential dependence on the distance away from the interface. Thus near‐field electromagnetic field enhancement and the enhanced scattering cross section (SCS) can be obtained through excitation of SPs. The larger electrical field means a stronger absorption, and a larger scattering cross section redirects more incident sunlight into the absorbing layer, resulting in a much larger light absorption in a much thinner semiconductor layer. Hence, both localized surface plasmons (LSPs) [4] excited in metallic nanoparticles and surface plasmon polaritons (SPPs) [5] propagating at the periodic metal/semiconductor interfaces have been so far widely investigated with great interests in designing high‐efficient thin film solar cells [6–9].

In early work using plasmonic structures to improve the light absorption of photovoltaic devices, Au or Ag nanoparticles [5, 10] and nanograting [11] have been introduced into the front side of solar cells [5, 10–12]. Such efforts have common disadvantages that resonances can only occur at certain wavelengths, and the use of metallic nanostructures directly on top of solar cells will block a fairly large amount of total incident light. And then a layer of antireflection coating was combined into the surface metallic grating, to reduce the reflected light and thus to improve sunlight absorption [13]. The fractal‐like pattern of Ag nano cuboids with several feature sizes [14] was employed to simultaneously excite low‐index and high‐ index SP modes along the silicon‐silver interface to achieve broadband absorption. On the other hand, Wang et al. [15] achieved a broadband and polarization‐insensitive absorption enhancement by placing a metallic nanograting at the bottom of the optically active layer. In such design, planar waveguide modes, the Fabry‐Pérot (FP) resonance, and the SPP resonance were effectively coupled, and photons blocking by the surface nanostructures can be avoided. In Ref. [16], Ag nanocone was employed to enhance light trapping, and the simulated results showed that the normalized scattering cross section of the rear located Ag nanocone is higher than that of the front located one. Incorporation of embedded metal nanostructures for light trapping in thin film solar cell has been extensively investigated, such as, with nucleated silver nanoparticles embedded at rear side of amorphous silicon cells [17], using nanosphere [18], silver nanopillars [19], silver triangular [20], gold paired‐strips [21] embedded grating structures, placing metal nanoparticles inside the active layer of solar cells [22], and with a metallic hole array inserted into a tandem solar cell [23]. Furthermore, combination of surface texture with embedded metal nanoparticles was also designed to trap light [24]. More plasmonic structures for light trapping in thin film solar cells will be described in the following sections in detail.

In this chapter, the theoretical formalisms about the SPPs and LSPs will be first described, followed by a summarize of plasmonic structures to improve the performance of thin film solar cell according to their positions of the nanostructures, which can be divided into at least three ways: (1) directly on top of thin film solar cell, (2) embedded at the bottom or middle of the optical absorber layer, and (3) combined with nanowire of optical absorber layer. Finally, conclusions are given.
