*3.1.3. Random nanostructures*

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

**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) ©

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

2011 ACS.

146 Nanostructured Solar Cells

*3.1.2. Other surface nanostructures*

ultraviolet‐blue range [38], thus a broadband enhancement can be realized.

Most plasmonic nanostructures designed on the front surface of thin film cells are always periodically distributed. Nishijima et al. [42] numerically and experimentally investigated the effect of periodic and random particle patterns on the plasmonic resonance. For the periodic arrays, the extinction peak (corresponding to the plasmonic resonance) value decreases with increasing periodicity, and the peak wavelength is red‐shifted. The increasing disorder results in increasing extinction and a broader plasmon resonance, which may be due to grating‐like diffraction losses. Light absorption is enhanced by more than two orders of magnitude for the random configuration of nanodiscs, demonstrated by the FDTD (finite‐difference time‐ domain) simulations. The random structures applied to solar cell have advantages of simple and low‐resolution fabrication. In Ref. [43], absorption enhancement by introducing 2D quasiperiodic and 2D periodic gold disks on top of the Si layer (with structure presented in **Figure 1(d)**) was compared. The simulation results present that, due to much more isotropic than the square lattice, the Penrose tiling structure can excite much more waveguide modes for absorbance spectra dependent on the azimuthal angle as well as the angle of incidence. Therefore, enhancement factor throughout the day as well as over the year may vary little. For the quasiperiodic lattice, the enhancement factor varies from 15.8 to 16.2 during summertime, whereas the variation ranges between 15.4 and 16.6 for the periodic lattice. Consequently, the performance of the solar cell with a quasicrystalline arrangement is expected to be more stable than that with a periodic one. An ultra‐thin silicon solar cell coated by a fractal‐like pattern of silver nanocuboids (as shown in **Figure 1(e)**) was investigated through FDTD simulation in Ref. [14], in which a broadband absorption is achieved due to the exists of multiple cavity modes and SP modes in the structure. The cavity modes come from Fabry‐Pérot resonances at the longitudinal and transverse cavities, respectively. Low‐index and high‐index SP modes, which propagate along the silicon‐silver interface, are simultaneously excited due to several feature sizes distributed in the fractal‐like structure.

#### *3.1.4. Combination of AR coating and metal nanostructures*

Surface metal nanostructures have also been integrated with the antireflection (AR) coatings to enhance performance of plasmonic solar cells [44–46]. A detailed comparison between traditional AR coatings, plasmonic gratings, and structures that combine AR coatings and gratings on ultrathin Si absorbing layers (as shown in **Figure 1(f)**) was presented by FDTD simulations in Ref. [44]. Plasmonic gratings exhibit strong, narrow‐band light absorption enhancement, while the traditional AR coatings result in more modest, broadband light absorption enhancement. The AR coatings even lead to stronger absorption than the gratings alone for the thicker films. However, the combination of AR coatings and gratings surpasses enhancements of either of these structures individually. The reason for this improvement originates mainly from the enhanced absorption within the propagating periodic modes rather than the localized resonances for the structures. Al nanoparticles [45] located above a 75 nm silicon nitride antireflection coating in a 1 μm silicon film also provide a strong broadband light absorption enhancement.

## **3.2. Metallic nanostructures embedded inside the active layer**

Designing the metallic nanostructures on top of solar cells has the problem of blocking a fairly large amount of total incident solar photons. While the metal nanostructures are placed at the bottom of optically active layer, they can be treated as back reflector and support SPP modes at the metal/semiconductor interface, as well as couple light into the photonic modes. Ferry et al. demonstrated [47] by numerical simulations that a single subwavelength scattering objects on the metallic back surface of a 200 nm Si thin film can enhance absorption by a factor of 2.5 over a large area for the portion of the solar spectrum near the Si band gap. The incident light is coupled into an SPP mode as well as a photonic mode that propagates inside the Si wave‐ guide. And the coupling effect of each mode can be controlled by the height of the scatter. The photonic modes suffer from only very small losses in the metal. While for the ultrathin Si solar cell with thickness smaller than 100 nm, the photonic mode approaches its cutoff frequency and all the scattered light is converted into SPPs. The fraction of light coupled to both SPP and photonic modes increases with increasing wavelength because the incoming light at shorter wavelengths is directly absorbed in the Si layer.

Furthermore, Wang et al. [15] demonstrated that a broadband and polarization insensitive absorption enhancement was achieved in an α‐Si cell with hybrid gratings of Ag and indium tin oxide (ITO) at the bottom of the a‐Si layer (as presented in **Figure 2(a)**), which were originated from the effective coupling of planar waveguide modes, the Fabry‐Pérot (FP) resonance and the SPP resonance. Similar to Ref. [47], the improvement mainly occurs at certain long wavelengths. The FP resonance inside the α‐Si layer is excited both for TE or TM polari‐ zations when both layer thickness and the incidence wavelength satisfy the resonance condition. Since the FP resonance is typically dependent on the thickness of α‐Si layer, the added Ag nanograting will change the cavity thickness and cause the resonance red shift. Therefore, adjusting the Ag nanograting will be a practical means to tune the FP resonance to betterfit the solar spectrum without changing the α‐Si layer thickness. The excitation of guiding modes in the planar α‐Si layer slab waveguide can lead to strong enhancement in the near‐ infrared regime for both TE and TE illuminations by adding the Ag nanograting. Such enhancement region red shifts with a thicker Si layer, a larger grating period, and a thicker Ag nanograting. The presence of the Ag nanograting provides more absorption enhancement for TM illumination, as SPPs are excited at both Si/Ag and ITO/Ag interfaces. Plasmonic wave‐ length is less dependent on α‐Si thickness and red shifts as the grating's period increases. The absorption over the solar spectrum by such design shows an up to 30% broadband absorption enhancement when comparing to bare thin film cells, and the total enhancement under an unpolarized illumination varies little between 24 and 31% with an incident angle changing from 0 to 82°.

**Figure 2.** (a) Ag gratings at the bottom of thin film Si [15]; (b) nucleated Ag particles is embedded inside the ZnO layer at the rear side of the a‐Si:H cell [17]; (c) Ag triangular corrugations at the bottom of a 100 nm c‐Si cell [20]; (d) an Ag hole‐array is inserted into a PCBM/CIGS tandem solar cell [53]; (e) Ag triangular gratings at the back side of cell and triangular ITO gratings at the front a‐Si cell [65]; (f) the biomimetic silicon moth‐eye structure combined with rear lo‐ cated Ag hemispherical particles in 2 μm thick c‐Si cell [24]. Figures reproduced with permission: (a) © 2010 ACS; (b) © 2012 ACS; (c) © 2012 OSA; (d) © 2014 OSA; (e) © 2012 APS; (f) © 2016 OSA.

#### *3.2.1. Various nanostructures at the bottom of active layer*

originates mainly from the enhanced absorption within the propagating periodic modes rather than the localized resonances for the structures. Al nanoparticles [45] located above a 75 nm silicon nitride antireflection coating in a 1 μm silicon film also provide a strong broadband

Designing the metallic nanostructures on top of solar cells has the problem of blocking a fairly large amount of total incident solar photons. While the metal nanostructures are placed at the bottom of optically active layer, they can be treated as back reflector and support SPP modes at the metal/semiconductor interface, as well as couple light into the photonic modes. Ferry et al. demonstrated [47] by numerical simulations that a single subwavelength scattering objects on the metallic back surface of a 200 nm Si thin film can enhance absorption by a factor of 2.5 over a large area for the portion of the solar spectrum near the Si band gap. The incident light is coupled into an SPP mode as well as a photonic mode that propagates inside the Si wave‐ guide. And the coupling effect of each mode can be controlled by the height of the scatter. The photonic modes suffer from only very small losses in the metal. While for the ultrathin Si solar cell with thickness smaller than 100 nm, the photonic mode approaches its cutoff frequency and all the scattered light is converted into SPPs. The fraction of light coupled to both SPP and photonic modes increases with increasing wavelength because the incoming light at shorter

Furthermore, Wang et al. [15] demonstrated that a broadband and polarization insensitive absorption enhancement was achieved in an α‐Si cell with hybrid gratings of Ag and indium tin oxide (ITO) at the bottom of the a‐Si layer (as presented in **Figure 2(a)**), which were originated from the effective coupling of planar waveguide modes, the Fabry‐Pérot (FP) resonance and the SPP resonance. Similar to Ref. [47], the improvement mainly occurs at certain long wavelengths. The FP resonance inside the α‐Si layer is excited both for TE or TM polari‐ zations when both layer thickness and the incidence wavelength satisfy the resonance condition. Since the FP resonance is typically dependent on the thickness of α‐Si layer, the added Ag nanograting will change the cavity thickness and cause the resonance red shift. Therefore, adjusting the Ag nanograting will be a practical means to tune the FP resonance to betterfit the solar spectrum without changing the α‐Si layer thickness. The excitation of guiding modes in the planar α‐Si layer slab waveguide can lead to strong enhancement in the near‐ infrared regime for both TE and TE illuminations by adding the Ag nanograting. Such enhancement region red shifts with a thicker Si layer, a larger grating period, and a thicker Ag nanograting. The presence of the Ag nanograting provides more absorption enhancement for TM illumination, as SPPs are excited at both Si/Ag and ITO/Ag interfaces. Plasmonic wave‐ length is less dependent on α‐Si thickness and red shifts as the grating's period increases. The absorption over the solar spectrum by such design shows an up to 30% broadband absorption enhancement when comparing to bare thin film cells, and the total enhancement under an unpolarized illumination varies little between 24 and 31% with an incident angle changing

light absorption enhancement.

148 Nanostructured Solar Cells

**3.2. Metallic nanostructures embedded inside the active layer**

wavelengths is directly absorbed in the Si layer.

from 0 to 82°.

Various plasmonic structures embedded at the back of active layer were proposed and investigated to enhance the solar cell absorption. Metal nanoparticles like nanospheres are widely employed [17–19, 48–50]. In Ref. [17], Chen et al. proposed a novel structure of glass/ transparent conductive oxide (TCO)/p‐i‐n a‐Si:H/ZnO:Ag, in which the nucleated Ag particles were embedded inside the ZnO layer at the rear side of the cell (as shown in **Figure 2(b)**). The simulations showed clearly that smaller nanoparticles (20–100 nm) have smaller scattering/ absorption cross‐sectional ratio but more equivalence in their scattering intensity versus angle distribution, while larger nanoparticles possess dominant scattering but focused primarily in limited scattering angles. Therefore, the proposed particle in Ref. [17] has a larger core with the surface simultaneously covered evenly with half‐truncated small particles (1/5 size of the large particle). The larger particle brings about a larger scattering coefficient in the longer wavelength because of the excitation of the dipolar and quadrupolar plasmonic modes. And the smaller particle provides larger angle scattering for shorter wavelength light. The experi‐ mental results demonstrated that, compared with these of the randomly textured solar cells without nanoparticles, a broadband light absorption enhancement and prominent perform‐ ance including a 23% improve of the energy conversion efficiency and a 14.3% increase in the short‐circuit photocurrent density were achieved in the solar cells designed with 200 nm nucleated Ag nanoparticles with 10% coverage density. The surface coverage of the nucleated Ag particles at the rear side of cell should be set at the optimal range because too small coverage seems to be insufficient to cause significant light scattering, while too large may lead to obvious particle absorption.

In addition, an 80 nm thick Ag layer with triangular corrugations was designed on the bottom of a 100 nm thick c‐Si layer, and the triangular corrugations (as shown in **Figure 2(c)**) were penetrated into the active Si layer [20]. Similar to Ref. [15], SPP mode and optical resonances contributed to the absorption enhancement. The FDTD simulation demonstrates that SPPs are excited at the Ag/Si interface of the triangular gratings, and the coupling of the excited SPP resonance with the FP resonance modes is observed for the TM illumination. For TE light, the excited cavity mode is demonstrated, and the coupling between waveguide and cavity modes is observed. Such triangular corrugations sustain large absorption enhancement factors (33.8– 43.3%) up to relatively wide incidence angles, with enhancement higher than that using the Ag strip grating [15]. In Ref. [21], the FEM (finite element method) simulations demonstrated that the absorption enhancement with Au paired‐strip grating embedded at back side of solar cell is higher than that with the Au single‐strip grating. And the enhancement depends on the absorber layer thickness and the refractive index of the surrounding medium. Other structures like dual plasmonic nanostructures [51] are employed in single organic solar cells, in which Au nanoparticles are embedded in the active layer, and an Ag nanograting is used as the plasmonic back reflector. Through the collective excitation of Floquet modes, SPP, LSP, and their hybridizations, broadband absorption enhancement was observed both by experiment and simulation.

#### *3.2.2. Metal nanostructures embedded in the middle of active layer*

Absorption enhancement by plasmonic nanostructures embedded at the back side of active absorbing layer mainly focused on the long wavelength range, due to that the short wavelength part has mostly been absorbed by the solar cell. Therefore, plasmonic nanostructures embed‐ ded in the middle of the cell were also investigated [52–56]. Zhang et al. [53] proposed a structure that an Ag hole‐array was inserted into a PCBM/CIGS tandem solar cell (as shown in **Figure 2(d)**). Such metallic hole array is expected to reflect the short‐wavelength photons back to the top cell and transmit long‐wavelength photons to the bottom cell through the extraordinary optical transmission (EOT) effect, as well as act as an intermediate electrode to allow a fabrication of hybrid organic‐inorganic tandem solar cell. The simulation by FEM method demonstrates that for the cell including 100 nm PCBM/50 nm Ag hole‐array/100 nm ITO/100 nm CIGS, the absorption inside the top subcell is always enhanced with different Ag hole array period, due to the back reflection of the Ag array, and for the bottom subcell, the absorption for the longer wavelength range (>650 nm) is greatly enhanced, originating from the EOT effect. When varying the period in the range from 200 nm to 1.5 μm, five resonant mechanisms are identified to participate in the EOT, including SPP mode, magnetic plasmon polaritons, LSP, and optical waveguide modes. It is shown that the thickness of Ag array or

that of ITO, the duty cycle of the holes also affects the optical performance of the cell. More than 40% integrated power enhancement can be achieved in a variable structural parameter range. Recently, metal nanoparticles are also placed at the middle of Si active layer [56] or the perovskite solar cell layer [55] to provide absorption enhancement.
