**4. Conclusions**

These results demonstrate that the periodic inverted nanopyramid structures reduced the reflections, increased the short-circuit current and improved the efficiency of the monocrystalline silicon solar cells under this study. This is due to the formation of a gradual refractive index gradient between air and the solar cell, which can reduce the Fresnel reflectance and direct more incident light inside the solar cell active layer. The combined light trapping and antireflection effect have been improved, and the optical path length has been prolonged by the inverted nanopyramid structures resulting in increasing the overall conversion efficiency of the monocrystalline Si solar cells. In addition, the nanopyramid coating can be applied after the solar cell fabrication is completed to eliminate any losses due to surface damage by the etching processes for example [45].

**Figure 13.** (a) Current density-voltage (J-V) characteristics and (b) EQE spectra of a monocrystalline Si solar cell with and

In outdoor environments, solar cells are exposed to the elements and can be easily contaminated by dust particles which interfere with incident light affecting the cell light absorption and thus, reducing the device performance. Therefore, self-cleaning properties at the front surface of the solar cell would maintain the cell performance when exposed to dusty environments [40, 41]. **Figure 14** shows the contact angle values of water droplets measured on the planar solar cell, inverted nanopyramid patterned solar cell and SAM-coated inverted nanopyramid patterned solar cell. As shown in **Figure 14**, the contact angle of the solar cell was increased from 55 to

**Table 2.** Device characteristics of monocrystalline Si solar cells coated with glass with and without the inverted

**Monocrystalline Si solar cells VOC (V) JSC (mAcm−2) FF (%) PCE (%)** Without nanopyramid 0.525 29.442 52.55 8.122 With nanopyramid 0.58 32.793 52.71 9.075

**3.3. Surface wettability**

without the inverted nanopyramid structures.

38 Emerging Solar Energy Materials

nanopyramid structures.

In this chapter, periodic upright and inverted nanopyramid structures were utilized as lighttrapping and self-cleaning nanostructures. Low-cost, high-resolution LIL and UV-NIL technologies were used to fabricate the master mold and form these structures. The performance of the solar cells was improved in terms of overall efficiency and reduced reflections. In addition, a superhydrophobic property of the nanopyramids was explored in terms of adding a self-cleaning functionality to the front side encapsulation. The inverted nanopyramid structures were fabricated on Si substrate by LIL and subsequent pattern transfer process using reactive ion etching followed by KOH wet etching. The periodic inverted nanopyramid structures on a silicon substrate were used as a master mold for the imprint process. During the first nanoimprint process, the upright nanopyramid structures were fabricated on the glass substrate by simple, high-throughput and low-cost UV-NIL using Si master mold with inverted nanopyramid structures. The upright nanopyramid structured glass substrates were tested for protective cover glass for solar cell applications and were utilized as a mold for the second imprint process used to form the inverted pyramids.

The diffuse transmittance and haze ratio values were significantly increased for the upright nanopyramid patterned glass, especially, in the wavelength range 300–600 nm compared to the bare glass. This indicates that antireflection and strong light-scattering functions are obtained due to the upright nanopyramid graded refraction index structures. The use of upright nanopyramid structured glass as a cover glass lead to improve the power conversion efficiency of the encapsulated monocrystalline Si solar cell by about 10.97%. This is mainly due to the increased light scattering and prolongs the optical path length caused by the upright nanopyramid structures compared to the reference cells with bare glass. In addition, the fluorinated upright nanopyramid structured cover glass exhibited larger contact angle (θCA ~ 132°) and excellent self-cleaning properties.

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Nanopyramid Structures with Light Harvesting and Self-Cleaning Properties for Solar Cells

http://dx.doi.org/10.5772/intechopen.75314

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In the second nanoimprint process, the periodic inverted nanopyramid structures were fabricated on the monocrystalline solar cell front surface using a UV-NIL. The pyramid coating can be applied after cell fabrication to eliminate any losses due to surface damage by the etching processes. The inverted nanopyramid structures decreased the reflectance and increased the external quantum efficiency over a broad wavelength range. The periodic inverted nanopyramid structure has successfully reduced the Fresnel reflection and led to directing and trapping more incident light into the monocrystalline Si solar cells, thereby improving the short-circuit current density and enhancing the power conversion efficiency. The power conversion efficiency of the monocrystalline Si solar cell with inverted nanopyramid structures was improved by 11.73% compared to the planar solar cell. Moreover, the surface of the solar cells exhibited hydrophobic properties due to increased contact angle caused by the nanostructure patterns and the self-assembled monolayer coating. The enhanced hydrophobicity provided the solar cells with an added self-cleaning functionality. These results suggest that the periodic inverted nanopyramid and upright nanopyramid structures with light-harvesting and self-cleaning properties have considerable potential for various types of solar cells and optical systems in real outdoor environments.
