**4. Material selection for the solar cell design**

Choosing the proper materials for solar cell design is of particular importance. Most common thin film solar cells use Cu (In, Ga) Se2- and CdTe-based photovoltaic technologies, respectively, with module level efficiencies of 19.2% and 18.6%. The high price of In and Ga and the toxicity of Cd is the barrier to the large-scale usage of these solar cells. Metal sulfides and selenides are earth-abundant, highly efficient, environmentally friendly, stable, and cost-effective alternative materials [51]. Also, considering the photothermal absorbers, the absorbers are mainly designed with Au (�1063°C melting point), Ag (�961°C melting point), and Cu metals. The limited bandwidth, disability to tolerate high temperatures, generated in the solar cells with high conversion efficiency, high cost, and oxidation in contact with moisture are the limitations of these materials for developing solar cells [32, 52].

Refractory metals such as titanium (Ti), Tungsten (W) (�3422°C melting point), Chromium (Cr) (�1857°C melting point), Nickel (Ni) (�1453°C melting point), and associated nitrates such as titanium nitride (2930°C melting point) have recently been considered in the design of solar cells due to their high heat tolerance as a result of their high melting point. These corrosion-resistance materials have large imaginary dielectric constants, resulting in a high absorption rate. Moreover, the real part of their dielectric constants is negative, indicating their ability to support surface plasmon resonances [24, 48, 53–56]. In this regard, an all-titanium pyramidal solar cell with a photothermal conversion efficiency of 95.88% in the entire solar spectrum at a temperature of 700°C is designed [57].

Manganese, a transition metal, has the closest constitutive parameters to the ideal metal [58]. **Figure 7a** shows the geometry of truncated pyramidal solar cell design using all-manganese nano-shells [2]. To investigate the absorption capability of manganese material in the solar cell design, the attenuation constant of the wave illuminating the manganese slab is calculated via [59]:

$$a = \frac{\sqrt{2}\mathfrak{af}}{c}\sqrt{-\mathfrak{e}' + \sqrt{\mathfrak{e}'^2 + \mathfrak{e}'^2}}\tag{4}$$

where prime and double prime show the real and imaginary parts of the dielectric constant, respectively. The complex permittivity of the manganese is illustrated in **Figure 7b**. Also, *f* is the operating frequency and *c* is the speed of light in a vacuum. Apart from the influence of the material selection in the absorption efficiency, the use of metallic reflectors for transmission blockage and fulfilling the gradual impedance

*Photovoltaic and Photothermal Solar Cell Design Principles: Efficiency/Bandwidth… DOI: http://dx.doi.org/10.5772/intechopen.110093*

**Figure 7.**

*(a) The geometry of the all-manganese solar cell and (b) the dielectric constant of the manganese in the solar spectrum. (c and d) Respectively show the impact of core material and pyramid tip on the absorption rate [2].*

matching by the pyramidal geometry are the main operation mechanisms. Two other design key points for such a wideband performance is the use of hollow and truncated elements for low-frequency absorption enhancement, as shown in **Figure 7c** and **d** [60, 61].

Finally, the use of two-dimensional graphene material in the photothermal solar cell design is revealed. Carbon-based solar cells are recognized for their broadband absorption nature, excellent chemical stability, high thermal stability, and excellent thermal conductivity [62, 63]. Similarly, graphene sheet has high mobility, large optical transparency, excellent mechanical stability, and chemical inertness. It has been used in the design of different types of photovoltaic solar cells such as dye-sensitized solar cells and quantum dot-sensitized solar cells [64]. To evaluate the absorption capability of the graphene in the solar spectrum for potential use in the photothermal solar cell design, the attenuation constant of the illuminating wave to a graphene slab is studied. The attenuation constant (Neper/meter) is related to the real and imaginary parts of the dispersive material permittivity (*ε*) and permeability (*μ*) of graphene using [65]:

$$a = a\sqrt{\varepsilon\_0 \mu\_0} \left( a^2 + b^2 \right)^{1/4} \sin\left(\frac{1}{2} \tan^{-1}\left(\frac{a}{b}\right)\right) \tag{5}$$

where prime and double prime respectively denote the real and imaginary parts. The parameters *a* and *b* in (5) are defined as: *a* ¼ *ε*<sup>0</sup> *rμ*0 *<sup>r</sup>* � *ε*<sup>00</sup> *<sup>r</sup> μ*<sup>00</sup> *r* � � and *<sup>b</sup>* <sup>¼</sup> *<sup>ε</sup>*<sup>0</sup> *rμ*00 *<sup>r</sup>* þ *ε*<sup>00</sup> *r μ*0 *r* � �. As **Figure 8** confirms, the graphene material has a large attenuation constant in the solar spectrum, making it suitable for the absorber design.

**Table 1** shows the comparison of graphene's thermal conductivity, density, and specific heat with different metals for the same volume and thermal energy. As can be seen, the thermal conductivity of suspended graphene and graphene on the substrate

**Figure 8.** *The attenuation constant of the graphene sheet in the solar spectrum [1].*


#### **Table 1.**

*Comparison of graphene's thermal conductivity, density, and specific heat with different metals [63].*

is 9.3–48.3 and 1.4–7.2 times of that of metals, respectively. As a result, the transfer of heat absorbed by the graphene-based solar cells is more effective than the metallic samples [63]. Thus, graphene material is a promising candidate for solar cell design.

**Figure 9a** shows a highly efficient full-spectrum graphene-based solar cell, designed using hollow nanopillars on top of the titanium nitride (TiN) refractory metal substrate. This material combination guarantees the structure's performance in different weather conditions and at high temperatures. The gradual impedance matching of the surface elements with the free space impedance results in a negligible reflection from the surface, and the metallic substrate ensures the elimination of passing waves. Thus, the incoming wave is efficiently trapped in the device, as shown in **Figure 9b**. Note that the use of the truncated cone improves the low-frequency performance of the device and aids in covering the whole solar spectrum. The device has a robust performance for the incoming wave with a wide range of incident angles, as confirmed by **Figure 9c** and **d** [1].
