**3. Plasmonic light trapping in thin film solar cells**

Metallic nanoparticles placed on the top of a solar cell will scatter the incident sunlight to couple and trap freely propagating plane waves into the active absorbing thin film, by folding the light into the thin film. When metallic nanoparticles are embedded inside of active layer, they can be treated as subwavelength antennas in which the plasmonic near-field is coupled to the absorbing layer, which will increase the effective absorption cross section. A patterned metallic structure on the backside of a thin absorber layer can couple sunlight into SPP modes, as well as the planar waveguide modes. Taking these light trapping effects into considerations, various structures have been designed to increase the light absorption of thin film solar cells.

#### **3.1. Metallic nanostructures on top of thin film solar cell**

When small metal nanoparticles are placed close to the surface of solar cell, light will mainly scatter into the dielectric with a larger permittivity [27]. The optical path length, thus, can be increased due to the scattered light obtains an angular spread in the dielectric. With a metallic reflector on the backside of the solar cell, the reflected light from the backside can couple to the surface nanoparticles and will reradiate into the active layer partly. In 2006, Yu et al. [12] deposited Au nanoparticles above the amorphous silicon film with thickness of only 240 nm.

They observed an 8.1% increase in short‐circuit current density and an 8.3% increase in energy conversion efficiency with a modest density of Au particles. Moreover, finite‐element electro‐ magnetic simulations showed that substantially larger improvements should be attainable for higher nanoparticle densities. Ag nanoparticles were placed on top of a 1.25 μm thick Si‐on‐ insulator solar cell; a broadband absorption enhancement and a 16‐fold enhancement at 1050 nm were reported [10]. Atwater et al. [28] demonstrated that on GaAs thin cells with Ag nanoparticles located at the surface yielded a significant enhancement for wavelengths longer than 600 nm and exhibited an improvement of 8% for the short circuit current density. In addition, Au nanopillar [29] or Ag nanoparticle [30] was also used to excite the localized particle plasmon to enhance the light trapping in the organic solar cells, like P3HT: PCBM.

#### *3.1.1. Effect of nanoparticle material, size, shape, and surrounding conditions*

( ) 0

<sup>0</sup> ( 1) *l p*

e

e w

w w

reaches that of SPPs at a semi-infinite metal-dielectric interface.

**3. Plasmonic light trapping in thin film solar cells**

**3.1. Metallic nanostructures on top of thin film solar cell**

py (TERS).

144 Nanostructured Solar Cells

Assuming *ε*(*ωLSP*) has a Drude form, i.e., = 1 <sup>−</sup>

e

Re <sup>1</sup> <sup>0</sup> *LSP <sup>l</sup>*

*l*

*l l l*

where *l* stands for the angular momentum index. For a small enough sphere where the static field approximation is well satisfied, *l* thus equals to 1, which means that the dipole excitation is mainly responsible for LSP. With the increase in radius, the interaction between multipoles is becoming more and more important. Eventually at infinite radius, the frequency of LSP

Similar to SPPs, LSPs exhibit an impressive local field enhancement as all energy is bounded. However, LSPs exist much more common than SPPs, as LSP does not require a specific polarization direction of the incident electromagnetic wave. A lot of applied techniques thus are developed on the basis of LSP enhancement, such as surface enhanced Raman spectroscopy (SERS), tip enhanced fluorescence spectroscopy (TEFS), and tip enhanced Raman spectrosco-

Metallic nanoparticles placed on the top of a solar cell will scatter the incident sunlight to couple and trap freely propagating plane waves into the active absorbing thin film, by folding the light into the thin film. When metallic nanoparticles are embedded inside of active layer, they can be treated as subwavelength antennas in which the plasmonic near-field is coupled to the absorbing layer, which will increase the effective absorption cross section. A patterned metallic structure on the backside of a thin absorber layer can couple sunlight into SPP modes, as well as the planar waveguide modes. Taking these light trapping effects into considerations, various

structures have been designed to increase the light absorption of thin film solar cells.

When small metal nanoparticles are placed close to the surface of solar cell, light will mainly scatter into the dielectric with a larger permittivity [27]. The optical path length, thus, can be increased due to the scattered light obtains an angular spread in the dielectric. With a metallic reflector on the backside of the solar cell, the reflected light from the backside can couple to the surface nanoparticles and will reradiate into the active layer partly. In 2006, Yu et al. [12] deposited Au nanoparticles above the amorphous silicon film with thickness of only 240 nm.

é ù <sup>=</sup> ê ú ê ú + + ë û  2

+

+ = (14)

2 thus Eq. (14) turns into:

(15)

The surface nanoparticle material, size, shape, refractive index of the medium and distance from the active layer are key factors determining the scattering and coupling effect [6, 31–33]. In Ref. [33], the relationship between the normalized scattering cross section (SCS) and the spherical particle size in both air and silicon was investigated. The plasmonic resonance exhibits an obvious red‐shift effect in Si compared to that in air. Additionally, the resonance peaks red shift and broaden with increasing particle size, which will significantly enhance the light trapping in the red and near‐IR region. Typically, a relatively high scattering efficiency can be obtained with a particle size of ~100 nm. The particle shape also has effect on the efficiency. As shown in **Figure 1(a)**, cylindrical and hemispherical particles bring about much stronger light absorption enhancements than spherical particles [31], which may due to the fact that the average spacing to the substrate is smaller for these geometries than for spheres, and this allows efficient coupling of the scattered light into semiconductor substrates. The plasmon resonance peak always corresponds to the best light harvesting effect, and such resonance peak can be changed with the refractive index of the surrounding material. Plasmon resonances of Ag or Au nanoparticles locate at 350 and 480 nm, respectively, which can be red shifted over the entire 500–1500 nm by placing them in SiO2, Si3N4, or Si [34–36]. When the distance between the nanoparticles and the absorbing layer increase, the light scattered into the absorbing layer will decrease [31]. However, such decrease is not significant. The carrier recombination occurs at the metal can be avoided due to separation by the dielectric layer. Moreover, increasing the distance between the nanoparticles and the absorbing layer can increase the SCS because large distance prevents destructive interference effects between the incident and reflected fields. Besides, it is demonstrated that Ag particles are better choice than Au, because Ag not only offers a lower price but also leads to much higher light absorption enhancement. Besides the noble metal, Al nanoparticles on front side of silicon solar cell show a 28.7% photon absorption enhancement in Si wafers, which is much larger than that induced by Ag or Au [37]. By combined with SiN*<sup>x</sup>* anti‐reflection coating, Al nanoparticles can even produce a 42.5% enhancement, which provides a low cost and high efficiency solution for practical larger‐scale implementation of plasmonic nanoparticles for solar cell performance enhancement. Furthermore, it is experimentally demonstrated [38] that the use of periodic arrays of Al nanoparticles placed in the front of a thin Si film (shown in **Figure 1(b)**) causes a broadband photocurrent enhancement ranging from the ultraviolet to the infrared with respect to the reference cell. Single particle resonances contribute to the enhancement in the infrared spectral range, and the collective resonances lead to an efficient coupling of light in the ultraviolet‐blue range [38], thus a broadband enhancement can be realized.

**Figure 1.** Various metallic nanostructures on top of thin film solar cells. (a) Fraction of light scattered into the substrate for different sizes and shapes of Ag particles on Si [31]; (b) Al nanoparticles placed on front of a thin Si film [38]; (c) Ag strips on the top of Si thin film cell [39]; (d) 2D quasiperiodic gold disks on top of the Si layer [43]; (e) a fractal‐like pattern of silver nano cuboids on top of Si thin film cell [14]; (f) combination of AR coatings and gratings on top of ultrathin Si cell [44]. Figures reproduced with permission: (a) © 2008 AIP; (b) © 2015 OSA; (d), (e) © 2013 OSA; (f) © 2011 ACS.

#### *3.1.2. Other surface nanostructures*

In addition to metal nanoparticles, other structures such as gratings have also been employed for light trapping. Pala et al. [39] investigated the effect of periodic array of Ag strips (with the structure shown in **Figure 1(c)**) on the absorption enhancement in the Si thin film cell based on the finite‐difference frequency‐domain (FDFD) method. The simulation results show that the broadband light absorption benefits from the high near‐fields surrounding the nanostruc‐ ture and the effective coupling to waveguide supported by the thin Si film. Light absorption enhancement can be obtained both in the transverse electric (TE) and transverse magnetic (TM) polarized plane wave. For TM illumination, the Ag strips can effectively concentrate light in their vicinity at frequencies near their surface plasmon resonance, which depends on the strip geometry and its dielectric environment. And the sizes of Ag strips in the range from 50 to 100 nm are optimal. The lateral spacing of the strips governs the excitation of waveguide modes, while the number of allowed waveguide modes is determined by the thickness of the Si layer. For TE illumination, only waveguide modes are excited, and the absorption enhance‐ ment directly results from an increased interaction length of the light with the Si film. Other structures like plasmonic cavity with subwavelength hole arrays [40] and nanotoroid arrays [41] were also employed in the P3HT: PCBM and silicon thin film solar cells, respectively.
