**3. Inverted nanopyramid structures**

### **3.1. Fabrication of inverted nanopyramid structures by UV-NIL**

**Figure 9** illustrates the schematic diagram of overall imprint process steps for the coating of inverted nanopyramid structures on a solar cell front surface. The periodic inverted nanopyramid structures were fabricated on Si master mold by LIL and subsequent pattern transfer process using reactive ion etching followed by KOH wet etching. Details on the fabrication process of the master mold are described in Ref. [9]. The upright nanopyramid patterns were successfully replicated from the Si mold onto the glass substrate using UV nanoimprint process resulting in high fidelity as described in Section 2.1. The upright nanopyramid structured glass was used as a stamp in the second imprint process to produce the inverted nanopyramid patterns. After the UV nanoimprint process, F13-TCS-based SAM was coated onto the upright nanopyramid patterned glass substrate to act as an anti-sticking layer. More details of the UV nanoimprint process parameters and the tools which were used are described in Ref. [11].

was imprinted from the master Si stamp (**Figure 10(a)**). The periodic inverted nanopyramid imprinted onto the surface of the solar cells (**Figure 10(c)**) was obtained from the upright nanopyramid replica stamp. It can be seen that the inverted nanopyramid structures were trans-

**Figure 10.** Top view of SEM image of: (a) the inverted nanopyramid Si master mold, (b) the upright nanopyramid replica stamp and (c) the periodic inverted nanopyramid imprinted from the upright pyramids mold on the front surface of

**Figure 9.** The schematic diagram of the overall fabrication process of inverted nanopyramid structures on the front

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**Figure 11** shows the reflectance of the monocrystalline Si surface with and without the coating of inverted nanopyramid structures measured as a function of wavelength. It can be observed that the surface reflectance of the monocrystalline Si with the inverted nanopyramid layer was significantly decreased over the broad wavelength ranging from 300 nm to

ferred to the surface of the solar cell with high fidelity.

**3.2. Optical properties and device performance**

surface of the solar cells.

the solar cells.

In order to determine and measure the influence of the inverted nanopyramid structure on improving the solar cell conversion efficiency, the inverted nanopyramid structures were printed onto monocrystalline Si solar cells. The monocrystalline Si solar cells were fabricated as described in Section 2.2. **Figure 10(a)** shows the top view SEM images of the periodic inverted nanopyramid Si master stamp. The upright nanopyramid replica stamp (**Figure 10(b)**) Nanopyramid Structures with Light Harvesting and Self-Cleaning Properties for Solar Cells http://dx.doi.org/10.5772/intechopen.75314 35

**Figure 9.** The schematic diagram of the overall fabrication process of inverted nanopyramid structures on the front surface of the solar cells.

**Figure 10.** Top view of SEM image of: (a) the inverted nanopyramid Si master mold, (b) the upright nanopyramid replica stamp and (c) the periodic inverted nanopyramid imprinted from the upright pyramids mold on the front surface of the solar cells.

was imprinted from the master Si stamp (**Figure 10(a)**). The periodic inverted nanopyramid imprinted onto the surface of the solar cells (**Figure 10(c)**) was obtained from the upright nanopyramid replica stamp. It can be seen that the inverted nanopyramid structures were transferred to the surface of the solar cell with high fidelity.

#### **3.2. Optical properties and device performance**

**3. Inverted nanopyramid structures**

34 Emerging Solar Energy Materials

Ref. [11].

**3.1. Fabrication of inverted nanopyramid structures by UV-NIL**

**Figure 9** illustrates the schematic diagram of overall imprint process steps for the coating of inverted nanopyramid structures on a solar cell front surface. The periodic inverted nanopyramid structures were fabricated on Si master mold by LIL and subsequent pattern transfer process using reactive ion etching followed by KOH wet etching. Details on the fabrication process of the master mold are described in Ref. [9]. The upright nanopyramid patterns were successfully replicated from the Si mold onto the glass substrate using UV nanoimprint process resulting in high fidelity as described in Section 2.1. The upright nanopyramid structured glass was used as a stamp in the second imprint process to produce the inverted nanopyramid patterns. After the UV nanoimprint process, F13-TCS-based SAM was coated onto the upright nanopyramid patterned glass substrate to act as an anti-sticking layer. More details of the UV nanoimprint process parameters and the tools which were used are described in

**Figure 8.** Photographs of (a) a water droplet on (I) bare glass, (II) UNP glass and (II) SAM-coated UNP glass and (b) sequential photographs of a self-cleaning process for (I) the bare glass and (II) UNP glass. θCA is the water contact angle.

In order to determine and measure the influence of the inverted nanopyramid structure on improving the solar cell conversion efficiency, the inverted nanopyramid structures were printed onto monocrystalline Si solar cells. The monocrystalline Si solar cells were fabricated as described in Section 2.2. **Figure 10(a)** shows the top view SEM images of the periodic inverted nanopyramid Si master stamp. The upright nanopyramid replica stamp (**Figure 10(b)**)

**Figure 11** shows the reflectance of the monocrystalline Si surface with and without the coating of inverted nanopyramid structures measured as a function of wavelength. It can be observed that the surface reflectance of the monocrystalline Si with the inverted nanopyramid layer was significantly decreased over the broad wavelength ranging from 300 nm to

**Figure 11.** Experimental and FDTD-simulated optical reflectance spectra of Si surface with and without INP structure as a function of wavelength.

1200 nm due to the gradual change in the refractive index between the air and Si surfaces. This is compared with the planar solar cells with no nanopyramid pattern (red curve in **Figure 11**), which resulted in 40% reflections over the visible range.

**Figure 13(a)** and **(b)** shows the J-V characteristics and EQE spectra of the monocrystalline Si solar cell with and without the inverted nanopyramid structures. The photovoltaic parameters of the monocrystalline Si solar cells with and without nanopyramid structures extracted

**Figure 12.** The cross-sectional electric field distribution profiles at different wavelengths employing FDTD analysis (a) INP coated Si with a period of 600 nm, the base size of 500 nm and depth of 400 nm and (b) planar Si subjected to same

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With the use of inverted nanopyramid structures, the J-V characteristics show that there was no significant change in the open circuit voltage (VOC). However, the short-circuit current density (JSC) of planar monocrystalline Si solar cell increased from 29.422 to 32.793 mAcm−2. This

SC increment was mainly due to the reduced reflectance resulting from the inverted nanopyramid structure over a broad wavelength range as shown in **Figure 11**. As a result, the conversion efficiency of the monocrystalline Si solar cell with inverted nanopyramid was increased significantly from 8.122 to 9.075%. This efficiency is 11.73% higher than the one obtained for the planar, not patterned, monocrystalline Si solar cell. The EQE measurement is carried out under a monochromatic illumination with a tungsten-halogen lamp coupled to a monochromator. The EQE values of the tested monocrystalline Si solar cell with inverted nanopyramid layer were significantly higher over the entire wavelength range compared to the non-patterned solar cells. For instance, the EQE value increased by about 8% at wavelength of 450 nm. This higher EQE values for the solar cells with inverted nanopyramid indicate enhanced light trapping and reduced reflections due to the imprinted nanostructures on top of the solar cell surface. This result is precisely matched with the reflectance values obtained in **Figure 11**.

from these J-V curves are summarized in **Table 2**.

J

light conditions.

FDTD simulations were performed with and without inverted nanopyramid structure to verify the reflection attained from the experiments, resulting in simulated reflectance spectra, as also illustrated in **Figure 11**. It is apparent that the theoretical reflection measurement for planar Si substrate is close to the experimental results. The overall trend of simulated reflectance spectra is quite consistent with that of the experiment data, with noticeable decrease of reflectance for inverted nanopyramid structures. Concurrently, the fluctuations of the reflectance can be associated with the limitation of the modeling where the OrmoStamp layer is assumed to have a uniform refractive index over the broad wavelength range under study. Moreover, the cross-sectional electric field intensity distributions at different wavelengths were simulated for incident light propagating from air to the Si substrate with and without inverted nanopyramid structure as shown in **Figure 12**.

As shown in **Figure 12**, the existence of inverted nanopyramid structure results in less intensity and weaker interference of the reflected waves. Hence, these structures are suitable antireflection coatings. In addition, it can be observed from the strong electric field distribution inside the inverted nanopyramid structure that the EM wave energy can be effectively coupled to the inverted nanopyramid structures. This is because more incident photons are coupled to the device due to the formation of a gradual refractive index gradient profile provided by the inverted nanopyramid structure.

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**Figure 12.** The cross-sectional electric field distribution profiles at different wavelengths employing FDTD analysis (a) INP coated Si with a period of 600 nm, the base size of 500 nm and depth of 400 nm and (b) planar Si subjected to same light conditions.

1200 nm due to the gradual change in the refractive index between the air and Si surfaces. This is compared with the planar solar cells with no nanopyramid pattern (red curve in

**Figure 11.** Experimental and FDTD-simulated optical reflectance spectra of Si surface with and without INP structure

FDTD simulations were performed with and without inverted nanopyramid structure to verify the reflection attained from the experiments, resulting in simulated reflectance spectra, as also illustrated in **Figure 11**. It is apparent that the theoretical reflection measurement for planar Si substrate is close to the experimental results. The overall trend of simulated reflectance spectra is quite consistent with that of the experiment data, with noticeable decrease of reflectance for inverted nanopyramid structures. Concurrently, the fluctuations of the reflectance can be associated with the limitation of the modeling where the OrmoStamp layer is assumed to have a uniform refractive index over the broad wavelength range under study. Moreover, the cross-sectional electric field intensity distributions at different wavelengths were simulated for incident light propagating from air to the Si substrate with and without inverted

As shown in **Figure 12**, the existence of inverted nanopyramid structure results in less intensity and weaker interference of the reflected waves. Hence, these structures are suitable antireflection coatings. In addition, it can be observed from the strong electric field distribution inside the inverted nanopyramid structure that the EM wave energy can be effectively coupled to the inverted nanopyramid structures. This is because more incident photons are coupled to the device due to the formation of a gradual refractive index gradient profile

**Figure 11**), which resulted in 40% reflections over the visible range.

nanopyramid structure as shown in **Figure 12**.

as a function of wavelength.

36 Emerging Solar Energy Materials

provided by the inverted nanopyramid structure.

**Figure 13(a)** and **(b)** shows the J-V characteristics and EQE spectra of the monocrystalline Si solar cell with and without the inverted nanopyramid structures. The photovoltaic parameters of the monocrystalline Si solar cells with and without nanopyramid structures extracted from these J-V curves are summarized in **Table 2**.

With the use of inverted nanopyramid structures, the J-V characteristics show that there was no significant change in the open circuit voltage (VOC). However, the short-circuit current density (JSC) of planar monocrystalline Si solar cell increased from 29.422 to 32.793 mAcm−2. This J SC increment was mainly due to the reduced reflectance resulting from the inverted nanopyramid structure over a broad wavelength range as shown in **Figure 11**. As a result, the conversion efficiency of the monocrystalline Si solar cell with inverted nanopyramid was increased significantly from 8.122 to 9.075%. This efficiency is 11.73% higher than the one obtained for the planar, not patterned, monocrystalline Si solar cell. The EQE measurement is carried out under a monochromatic illumination with a tungsten-halogen lamp coupled to a monochromator. The EQE values of the tested monocrystalline Si solar cell with inverted nanopyramid layer were significantly higher over the entire wavelength range compared to the non-patterned solar cells. For instance, the EQE value increased by about 8% at wavelength of 450 nm. This higher EQE values for the solar cells with inverted nanopyramid indicate enhanced light trapping and reduced reflections due to the imprinted nanostructures on top of the solar cell surface. This result is precisely matched with the reflectance values obtained in **Figure 11**.

**Figure 13.** (a) Current density-voltage (J-V) characteristics and (b) EQE spectra of a monocrystalline Si solar cell with and without the inverted nanopyramid structures.

96° after the formation of inverted nanopyramid structure, which exhibited a hydrophobic behavior. Moreover, the hydrophobicity was enhanced with SAM-coated inverted nanopyramid structures. In this case, the contact angle of the SAM-coated patterned solar cell was increased to 125°. As a result, solar cells with inverted nanopyramids can utilize the selfcleaning functionality induced by the high hydrophobic surface properties in addition to the

**Figure 14.** Photographs of a water droplet on: (a) planar solar cell, (b) patterned solar cell with nanopyramids and (c)

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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

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

utilization of their antireflection properties.

SAM-coated patterned solar cell. θ c is the water contact angle.

second imprint process used to form the inverted pyramids.

**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].


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

#### **3.3. Surface wettability**

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 Nanopyramid Structures with Light Harvesting and Self-Cleaning Properties for Solar Cells http://dx.doi.org/10.5772/intechopen.75314 39

**Figure 14.** Photographs of a water droplet on: (a) planar solar cell, (b) patterned solar cell with nanopyramids and (c) SAM-coated patterned solar cell. θ c is the water contact angle.

96° after the formation of inverted nanopyramid structure, which exhibited a hydrophobic behavior. Moreover, the hydrophobicity was enhanced with SAM-coated inverted nanopyramid structures. In this case, the contact angle of the SAM-coated patterned solar cell was increased to 125°. As a result, solar cells with inverted nanopyramids can utilize the selfcleaning functionality induced by the high hydrophobic surface properties in addition to the utilization of their antireflection properties.
