**4. Vapor-liquid-solid growth mechanism**

The main challenge regarding the performance of thin-film photovoltaic cell structure is the sunshine reflection losses, for instance, with no treatment materials, around 30% of the light illuminated at the Si surface can be lost due to the reflection at the interface between air and Si [78]. During the illumination of sunlight to the surface thin-film cell, some of the light is converted into energy, others will be transmitted, whereas some parts reflect [79]. The loss due to reflectance can be reduced by using techniques, such as coating with anti-reflection and light trapping materials [80]. To reduce this loss, the most commonly used is dielectric antireflection coatings; however, it is difficult to hide the whole absorption wavelength range. Broadband antireflection methods will be achieved by light trapping schemes, such as inverted pyramid structures, but these boost the cost due to their complicated fabrication method. Contrary, NW arrays have a strong antireflection ability with superior wavelength, polarization, and angle-dependent properties compared to planar structures because NWs can form graded-refractive index layers [80]. Consequently, it will reduce the light reflectance at the interface of the two media by avoiding abrupt changes in the refractive index [81].

Wu et al. [82] have presented a model for effective and fast design of both squarely and hexagonal InP NW arrays to achieve the highest light-harvesting for PV application, achieving the maximal short-circuit current density of 33.13 mA/cm2 . They have investigated the geometrical dimensions for vertically aligned single, double, and multiple diameters of NW arrays. NWs and nanorods have almost the same properties and solar cells can also grow as nanorods morphology to harvest highly efficient sunlight by reducing reflection. Diedenhofen et al. [50] grew layers of GaP nanorods on AlInP/GaAs substrates. They found that nanorods can greatly reduce the reflection and increase the sunshine transmission into the substrate overbroad spectral and angular ranges due to the graded index of refraction. Strudley et al. [58] studied the sunshine transport inside an NW mat. They found that due to mesoscopic transport the high-density semiconductor NW mats exhibit huge interference contributions. From their statistical analysis of intensity oscillations, they linked that transport for focused illumination is governed by a minimum of around three open transmission modes, which is a record low value for light in a 3D medium.

Additionally, semiconductor NW is a 1D nanostructure, which is usually on the order of the sunshine wavelength. Due to their high refractive index, they behave as optical antennae that can modify the absorption and emission properties [80]. The absorption properties of NWs when they are vertically standing are determined by the waveguide modes [82]. InP NW arrays, which are vertically aligned and grown on a semi-infinite SiO2 substrate are schematically shown in **Figure 5** with either squarely or hexagonal arrangement. Repeatable unit cells in **Figure 6a** and **b** insets

#### **Figure 5.**

*Schematics of vertically aligned InP NW arrays. (a) Squarely, and (b) hexagonal NW arrays with insets explaining their respective unit cells [82].*

**Figure 6.**

*(a) Schematic drawing of the periodic GaAs NWs structure, (b) absorptance, (c) reflectance, and (d) transmittance of GaAs NW array with different fill factors [83].*

show respective characterization dimensions for each arrangement. Such morphology and topology of the NW arrays are in accord with the majority of the InP NW-based photovoltaic cell structures. Within each of the unit cells, the NWs own identical or different diameters as Di (where i = 1, 2, 3,…). Periodicity P is the core to core spacing of a pair of adjacent NWs that has an analogous value for squarely arranged NWs, whereas fully different values for hexagonal NW clusters.

*Solar Energy Conversion Efficiency, Growth Mechanism and Design of III–V Nanowire-Based… DOI: http://dx.doi.org/10.5772/intechopen.105985*

NWs are more efficient in light absorption compared to thin-film materials of an equivalent volume. Krogstrup et al. [84] have observed a remarkable increase in absorption in single-NW solar cells, which is related to the vertical configuration of the NWs and to a resonant increase in the absorption cross-section, and the results obtained opened a new route to third-generation PVs cells. Their short-circuit current result of 180 mAcm−2 is higher than that predicted by the Lambert-Beer law.

On other hand, when the NW is lying horizontally, the absorption properties are determined by leaky-mode resonances, which provide a chance to engineer the light absorption in NWs by controlling their physical dimensions [85]. Once the resonant modes are supported by the NWs leaky, the overlap between the incident electromagnetic attraction field and the guided mode profile is maximized, facilitating enough coupling with incident light.

Due to their outstanding advantages, NW arrays have advanced light trapping ability and hence strongly enhanced optical absorption in comparison with the thinfilm [86]. This can significantly enhance the broadband light absorption over a good range of incident angles, especially the near and below bandgap absorption [81, 87]. With the same thickness as thin-film layers, the NWs short-circuit current can reach high results [88].

### **4.1 Effect of NW diameter and period on absorption**

Nanowire diameter and separation are typically on the order of the wavelength of sunlight, where interference effects are dominant, therefore, the reflectance, absorptance, and transmittance of nanowire arrays must be determined using wave optics [89]. Long Wen et al. [83] have simulated to evaluate the efficiency limits of GaAs NW array solar cells and determined the requirements of the optical design for improving the efficiency **Figures 6a–d**. They have suggested that the optimized design NW might absorb 90% of above bandgap sunlight. Their combined optoelectronic simulation results reveal that optimization of optical geometry can lead to an attainable photovoltaic efficiency of 22%.

By fixing the filling factor, which is given by D/P, where D is the diameter of the nanowire and P is the separation between the grown nanowires, the effect of NWs diameter can be determined by varying it. **Figure 7a** shows the optical characteristics of the GaAs NW array with different diameters at a fixed D/P of 0.5 is plotted. The absorptance of a 2.2 µ m GaAs thin-film is also plotted for comparison. In short wavelengths, it can be observed that the absorptance spectra for all NW arrays are kept above 90%, which is much higher than the planar case due to the lower effective refraction index, and thus lower reflection at the top of NW arrays. In the longwavelength regime, the absorption spectra show a significant increase when increasing diameter from 60 to 180 nm. The electromagnetic field can be coupled efficiently into the NWs at resonances, due to the large refractive index contrast between the NWs and surrounding air. For small diameter GaAs NW arrays, with fewer supporting modes, most of the incident light cannot be guided into the NWs. The absorbance of the NW array with D = 180 nm is high above the band gap wavelength, whereas when D is increased to 240 nm the absorbance of the NW array decreased (**Figure 7a**) due to the increased reflection and the insufficient field concentration at longer wavelength.

**Figure 7b** shows plots of the vertical cross-section of the photogeneration profiles. The NWs with D = 60, 180 nm and D/P = 0.5 under 1mWcm−2 sunshines at different

**Figure 7.** *(a) Absorptance of NWA with different diameters, and (b) Photogeneration profiles calculated by FDTD simulations [83].*

wavelengths for photogeneration rates are revealed. At λ = 400 nm, it is concentrated near the highest sides of the NW for both diameters. Only a little fraction of the incident wave is transmitted onto the substrate, this will be explained by the short absorption length of GaAs at this wavelength. At 600 nm and above, for a NW, the photogeneration rates are focused on several lobes that form along the NWs for a NW array with 180 nm diameter, indicating strong guided modes confined within the NWs. In contrast, for the case of D = 60 nm, the optical generation becomes more homogeneously covered by the NWs with a longer wavelength. Clearly, the 180 nm diameter NW array induces a much larger optical concentration than the 60 nm diameter one. From both **Figures 6** and **7** one can easily understand the effect of NW diameter on photon energy harvesting.

### **4.2 Effect of NW length on absorption**

Photocurrent density is often further bettered by adding nanowires length (L). **Figure 8** shows the reckoned donation to all photocurrent from the NWs and the substrate during a GaAs NW PV device for an NW periphery of 180 nm and a period of 350 nm (90). Due to the proliferation of NW length, the donation from the nanowire to all photocurrent rises, while the GaAs substrate donation similarly decreases. The uttermost photocurrent of 27.3 mAcm−2 is obtained at 5 μm length, is on the brink of the perfect photocurrent density of 29.9 mAcm−2 (calculated by integrating the AM1.5G spectrum above the GaAs bandgap). At the optimum NW diameter, spacing, and length of the harvesting properties of III–V NWs can be improved.

In 2015 Nicklas Anttu [28] compared the effectiveness of InP NW assemblage solar cells with the classical InP bulk solar cells. They accounted an NW assemblage of 400 nm periods, 4 μm length, and 170 nm periphery, which may produce 96 of the short-circuit current accessible within the impeccably taking up InP bulk cell. Also, the NW solar cells cast smaller photons than the bulk cell at the identical occasion, which allows for a more open-circuit voltage. They consequently found that NWs longer than 4 μm can really show, despite producing a lower short-circuit current, an efficiency limit of up to 32.5% that is above the bulk cells.

They have predicted the unborn capabilities in affecting both the emission and absorption characteristics of the NW assemblages, for instance, by (1) varying NWs shape, (2) varying the period of NWs, (3) sheeting the NWs with a nonabsorbing

*Solar Energy Conversion Efficiency, Growth Mechanism and Design of III–V Nanowire-Based… DOI: http://dx.doi.org/10.5772/intechopen.105985*

#### **Figure 8.**

*Theoretical contributions from a GaAs nanowire array and the GaAs substrate to the total photocurrent density in a PV device versus nanowire length obtained at a nanowire diameter of 180 nm and period of 350 nm [90].*

dielectric shell, (4) fitting a dielectric material between the NWs, and (5) by introducing optical antireflection layers on top of the NW. Such improvement of the NW array could conceivably further accelerate its effectiveness limit.
