**4. Conclusions**

.

when the Al2O3 spacer thickness is 30 nm. And the ratio increases to three as the Al2O3 spacer

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

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

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

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 hemisphere (41%). The photocurrent is as large as 33.4 mA/cm2

Ag particle [78]. Wang et al. [75] patterned the entire CuIn*x*Ga(1−*<sup>x</sup>*)

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

is obtained with a 1‐μm thick c‐Si cell. Similarly, Zhang

, which is higher than these

Se2 (CIGS) thin film cell into

thickness is 10 nm.

152 Nanostructured Solar Cells

cell.

*3.2.5. Combination of front and rear nanostructures*

current density as high as 29.7 mA/cm2

in most structures.

In this chapter, we have summarized the advances in the research of light trapping and the plasmonic enhancement of thin film solar cells. Metallic nanoparticles can excite localized surface plasmons, which benefit for light scattering and light concentration to enhance the light absorption in thin film cell. A corrugated metallic film like grating on the absorber layer can couple sunlight into surface plasmon polaritons to enhance light absorption too. The light enhancement by these plasmonic modes is strongly related to the material, size, shape, refractive index of the medium, position in the absorber layer of metallic nanoparticles, and metallic corrugated nanostructures.

The position of metal nanostructures has significant effect on the plasmonic solar cells. The metallic nanoparticles like hemispheres and cylinders on the front side of thin film solar cell have larger normalized scattering cross section than that for the rear located particles when the dielectric spacer between metallic particles and absorbing layer is relatively thick. How‐ ever, 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. The rear located nanoparticles can avoid such Fano effect. Therefore, careful designs like using rear located Ag nanocone can achieve larger SCS than that at front side. On the other hand, adopting the embedded metallic nanostructures can obtain broadband and polarization insensitive absorption enhancement benefiting from the effective coupling of planar waveguide modes, FP resonance and SPPs resonance. Further‐ more, dual‐interface hybrid structure based on the nanostructures of front surface absorber combined with rear located nanoparticles or nanograting may be an excellent way to enhance light absorption in thin film solar cells. Anyway, new plasmonic nanostructures still need to be investigated to further achieve broadband and polarization insensitive absorption enhance‐ ment at wide incident angles.
