*3.2.3. Other effect in the embedded plasmonic nanostructure*

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

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

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

particle absorption.

150 Nanostructured Solar Cells

and simulation.

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

Similar to plasmonic nanoparticles at the front side of solar cell, refractive index of the surrounding medium has effect on the plasmonic resonance and hence the absorption [57, 58]. Park et al. [57] applied the Ag nanoparticles on the rear side of p‐Si tin film and investigated the optimum surface condition for plasmonic enhanced light absorption through experiment and simulation. It is found that the existence of SiO2 layer between Si and Ag particles has a major effect on the SCS and hence the absorption in the cells. Peak of SCS with nanoparticles on the thermal SiO2 is located at the light wavelengths <700 nm, while nanoparticles on the native SiO2 layer and directly on Si show peaks of SCS at wavelengths >700 nm. The sample with nanoparticles on the native SiO2 has the highest short‐circuit current density enhance‐ ment. The simulation in Ref. [59] reveals that by tuning the thickness of Si and transparent conductive oxide layers in the ITO/a‐Si:H/Ag nanoparticles/ZnO:Al/Ag mirror structure, the driving field intensity experienced by the embedded plasmonic particles can be enhanced up to a factor of 14. The effect of light losses induced by the rear located Ag nanoparticle on the light trapping of Si wafer was studied by Zhang et al. [60]. The light losses include the intrinsic absorption loss from Ag particles and the additional absorption loss induced by the void plasmons in the Al reflectors. The study reveals that Ag particles are effective to enhance the photocurrent in cells with planar front surface, while the absorption enhancement is substan‐ tially influenced by the plasmonics in the textured cells.

#### *3.2.4. Comparison of plasmonic nanostructures located at different positions*

Although the absorption enhancement can be achieved more or less by plasmonic nanostruc‐ ture located at various positions of solar cell active layer, it is meaningful to compare the enhancement among the different locations of nanostructures [16, 61–64]. In early reports, metallic particle shapes such as spheres, hemispheres, and cylinders are commonly employed. It is usually seen that the SCS for the front located particles is obviously larger than that for the rear located particles when the dielectric spacer between metallic particles and absorbing layer is relatively thick [63, 64]. However, the front located particles involve a detrimental Fano effect resulting from the interference effects between the scattered light and the incident light, which reduces light absorption below the plasmon resonance wavelength. Using the rear located nanoparticles can avoid the Fano effect. Hence, the ideal design is to make the SCS value of the rear located particles higher than that of the front located particles for a wide range of the electric spacer thickness. Yan et al. [16] adopted an Ag nanocone as the plasmonic particles in the c‐Si/Al2O3/Ag nanocone solar cell structure. The particles are treated as the front located particles when light is incident on the particles from air, while it is described as the rear ones if light is illuminated from the Si side. The FDTD simulation shows that the ratio of SCS between the rear and the front located particles at the resonance wavelength is near two when the Al2O3 spacer thickness is 30 nm. And the ratio increases to three as the Al2O3 spacer thickness is 10 nm.

## *3.2.5. Combination of front and rear nanostructures*

Dual‐interface nanostructures [65–72] including the rear metal plasmonic grating/nanoparti‐ cles and the front dielectric grating or metal particles have also been adopted to enhance absorption efficiency in thin film solar cell. For example, in Ref. [65], an Ag triangular grating at the back side of cell and a triangular ITO grating at the front a‐Si are combined (as shown in **Figure 2(e)**). The simulation presents that the ITO triangles focus on the short‐wavelength incident light and subsequently spread it inside the active layer, and a combination of dielectric waveguide mode and FP resonance arises at 660 nm. Additionally, an absorption peak at 760 nm resulting from the waveguide mode and a peak at 810 nm corresponding to the plasmonic mode are observed. An integrated absorption for TM illumination of the AM 1.5 G spectrum in the 400–950 nm region is 83.1%. Having different periods at specific interfaces provides more efficient diffraction into both plasmonic and dielectric guide modes. Other similar structures like rear located Ag strip grating combined with silicon front surface trapezoidal [70] or strip [69] texture are reported too to enhance light trapping in Si thin film cell.

In Ref. [66], the Ag nanohemisphere is deposited on the rear of a nanohole‐textured Si thin film. The role of Si nanohole is to absorb the light at the short wavelength, due to antireflection effect and light trapping properties. Long‐wavelength light absorption benefits from the excitation of the LSP induced by the rear located Ag hemisphere. By adjusting parameters of Si nanohole and Ag hemisphere, the short‐circuit current density can reach to 25.4 mA/cm2 . Shi et al. [71] combined silicon front surface grating and the rear‐located bilayer Ag nanohe‐ mispheres. In that case, the grating and metal nanoparticles are optimized, and a short‐circuit current density as high as 29.7 mA/cm2 is obtained with a 1‐μm thick c‐Si cell. Similarly, Zhang et al. [24] propose a hybrid structure based on the biomimetic silicon moth‐eye structure combined with rear located Ag hemispherical particles in the 2 μm thick c‐Si cells (as shown in **Figure 2(f)**). The FDTD simulation results present the integrated light absorption enhance‐ ment over the solar spectrum is 69% compared with the cells with the conventional light trapping design, which is larger than these in only silicon moth‐eye structure (58%) and only Ag hemisphere (41%). The photocurrent is as large as 33.4 mA/cm2 , which is higher than these in most structures.

#### **3.3. Hybrid of metallic nanostructures with nanowire of optical absorber layer**

Reducing the use of both active absorbing and non‐earth abundant materials in thin film solar cell is pursued. A nanowire optical antenna absorber was proposed in Ref. [73], which demonstrated that the absorption of sunlight in Si nanowires can be significantly enhanced over the bulk Si. The active layer in the thin film solar cell structures was designed into nanostructures in quite a few researches [73–78], such as Si nanowire array with a wire‐ embedded Ag back reflector [76] and silicon nanocone hole solar cell with back located square Ag particle [78]. Wang et al. [75] patterned the entire CuIn*x*Ga(1−*<sup>x</sup>*) Se2 (CIGS) thin film cell into

an 1D nanograting array, with Ag strips on top of CIGS gratings and Ag planar acting as the surface and rear electrodes, respectively (as shown in **Figure 3(a)** and **(b)**). For TE illuminations, the observed larger absorption enhancement compared to the conventional cell mainly results from the scattering by the top electrode as well as the coupling with the multiple internal reflection resonance. As thickness of the CIGS layer increases, the light absorption enhance‐ ment shows a red shift and becomes broader and stronger simultaneously. In the case of TM light, SPPs are excited and lead to that most incoming light is transmitted through the Ag nanowire array. The transmitted light is confined in the CIGS wires, which results in a broadband absorption enhancement. Similar to the TE case, the SPP‐related enhancement presents a red shift as the CIGS thickness increases. As the nanograting period and width increase, both magnitude and bandwidth of the enhancement first increase and then reduce beyond certain points. Such CIGS nanograting solar cell can enhance the overall current density over 250% compared to the bare thin film cells. In Ref. [77], the structure of Si nanowire/ Si layer/ZnO:Al/Ag nanohemispheres back reflector (as shown in **Figure 3(c)**) is investigated through simulations. The nanohemispheres Ag back reflector can excite LSP resonance and cause scattering of light, giving rise to light absorption in Si. The short‐circuit current density reaches to 28.4 mA/cm2 , demonstrating an enhancement of 22% compared with a Si nanowire solar cell with a planar back reflector.

**Figure 3.** (a) The entire thin film cell CuIn*x*Ga(1−*<sup>x</sup>*) Se2(CIGS) is patterned into a 1D nanograting array, with Ag strips on top of CIGS gratings and Ag planar acting as surface and rear electrodes [75]; (b) cross‐sectional view of (a); (c) Ag nanohemispheres at the bottom of Si nanowire cells [77]. Figures reproduced with permission: (a)–(c) © 2012 OSA.
