Photovoltaic and Photothermal Solar Cell Design Principles: Efficiency/Bandwidth Enhancement and Material Selection

Abstract

There are two main approaches for developing solar cells, including photovoltaic and photothermal technologies. Photovoltaic solar cells benefit from an active region whose performance can be improved by embedding nanoparticles with different shapes and materials. Photothermal solar cells are broadband absorbers, enabling electromagnetic energy absorption in the solar radiation region. Since the solar spectrum is expanded from 120 to 1000 THz, the device bandwidth engineering and its efficiency enhancement through utilizing nanoparticles, multiresonance configurations, and multilayered structures are necessary. Moreover, using chemically inert materials with high thermal conductivities results in stable performance under different environmental conditions. Thus, in this chapter, various photovoltaic and photothermal solar cells will be discussed, emphasizing their design principles. The chapter mainly considers absorption bandwidth enlargement, absorption efficiency enhancement, and material selection considerations. In this regard, solar cells designed with plasmonic materials, transition metals, refractory metals, and carbon materials are presented. Notably, the potential of two-dimensional graphene material in the solar cell design is revealed, and a lightweight graphene-based solar cell with near-perfect coverage of the whole solar spectrum is introduced.

1. Introduction

Solar energy has been known as the most important renewable energy source in the world. Thus, designing devices for solar radiation storage as electricity (photovoltaic solar cell) or thermal energy (photothermal solar cell) is of great interest. Although the latter technology requires a further energy conversion process to produce electricity from harvested thermal energy, the conversion can be achieved with high efficiency. To design solar cells, different types of materials are used in geometrically engineered configurations, each having its pros and cons. The important parameters for evaluating solar cells are their efficiencies, bandwidth, tolerance to environmental conditions, and robustness to the incident angles of incoming waves [1, 2].

The photovoltaic solar cell design can be achieved by employing thin film technology (efficiency of 23.4%), multijunction devices (39.2% efficiency), crystalline silicon (c-Si) based configurations (theoretical efficiency of 26.7%), perovskite (theoretical efficiency of 31%), organic thin films (16.4% efficiency), dye-sensitized method (12.3% efficiency), and perovskite-based quantum dot usage (16.5% efficiency) [3, 4, 5, 6]. Light trapping and confinement capabilities provided by the plasmonic nanoparticles play a crucial role in improving the efficiency of photovoltaic solar cells [7]. Moreover, by using wrinkle-like graphene sheets over the plasmonic nanoparticle, the photocurrent density enhancement can exceed the light trapping limit of the textured screens due to the broadband absorption of bend carbon [8]. Monolayers of semiconducting transition-metal dichalcogenides (TMDs) such as MoS₂ are also promising candidates for absorption efficiency enhancement in Si-based photovoltaics [9].

Photothermal solar cells are electromagnetic wave absorbers, and there is numerous research in the literature dealing with the electromagnetic absorber design in any desired frequency. The design principles of the absorbers obey the same roles, regardless of the selected spectrum. Thus, reviewing the wideband absorber design methods may be beneficial for establishing ideas for efficient solar cell design. The absorption rate of the thermal absorbers is directly connected to their ability to block wave transmission and eliminate wave reflection. This type of solar cell is commonly designed with an array of elements, and the performance of the anti-reflection coating depends on the shape of the constructing elements [10].

Microwave pyramidal absorbers are widely used broadband absorbers, in which the tapered nature of the geometry results in bandwidth enhancement by fulfilling the above-mentioned conditions [11]. The same geometry has been used as an efficient wideband absorber in the millimeter wave spectrum [12]. The pyramidal texture has also applications in solar cell design with high efficiency and industry standards [13]. Moreover, designing a device with fourfold symmetry has a great influence on the polarization in-sensitivity of the solar cell, and solar cells with high tolerance against the incident angle of the incoming wave are desired. Importantly, designing a device that operates under different environmental conditions, including temperature and humidity, increases the reliability of the system. Material selection plays an important role in this regard. In this chapter, photovoltaic and photothermal solar cell technologies will be introduced. Later, different methods of improving their performance (efficiency and bandwidth) are discussed. Due to the importance of material selection in solar cell performance, guidelines for choosing the proper material combinations are presented in detail. Finally, the remarkable properties of two-dimensional graphene material in the full-spectrum solar cell design are revealed.

2. Solar cell design technologies

Two well-known solar cell design technologies employ the photovoltaic or photothermal mechanism for light harvesting. The first method directly converts the absorbed solar energy into electricity. In the second method, the absorbed energy is of the thermal type, and it is later converted into electrical energy. Figure 1a shows a photovoltaic solar cell, in which n and p-type doped semiconductors are used in the design. When the sun’s photons hit this structure, they are absorbed and create energy carriers (electrons and holes) that contribute to producing electrical energy [7]. Moreover, Figure 1b shows the performance of a photothermal solar cell in which mirrors and lenses are used to focus solar energy. This thermal energy can run old steam turbines or Stirling engines to generate electricity. The critical point is that the heat energy, collected by the advanced thermal solar cells, can be stored and used to produce electrical power at the proper time [14].

Figure 1.

(a) Photovoltaic solar cell and (b) electric energy production using photothermal solar cell [14].

It is interesting to mention that photovoltaic solar cells cannot absorb photons at wavelengths higher than their bandgap. Thus, a combination of two technologies can be a solution for designing effective solar cells [14]. On the other hand, a thermophotovoltaic system enhances absorption efficiency during the dark hours, and it consists of a heat source (sun), thermal absorber, and photovoltaic solar cell. The theoretical conversion efficiency of this pollution-free and portable system with a low-maintenance cost is 85.4% [15]. For instance, by using germanium selenide (GeSe) nano-pyramids with optimized heights in the active layer of the perovskite solar cell, the light at a wavelength higher than 800 nm can also be absorbed [16]. As another approach, the exploitation of the different III-V semiconductors separately generates electron-hole pairs in different parts of the incoming solar spectrum for wider absorption coverage [17].

There are three generations of photovoltaic solar cells in the market. The first generation is based on crystalline silicon and has a high conversion efficiency. The high cost of this generation, due to the required large material thickness, is its main drawback. The second generation aims to reduce the device cost without scarifying efficiency. In this category, thin-film cells have gained lots of interest due to the reduction in the utilized raw materials [7, 18, 19]. They usually include a metallic contact layer, an active region, and an indium tin oxide (ITO) top layer. Thin film solar cells collect the carriers effectively, but they suffer from a low absorption rate due to small optical path length. The third generation of solar cells aims to enhance the efficiency of the second generation, and it is an ongoing research topic [20, 21]. An essential parameter of photovoltaic solar cells is their quantum efficiency, which is calculated as the ratio of absorbed power to incident power. Thus [22],

$$\mathit{QE}\left(\lambda \right)=\frac{P_{\mathit{abs}}\left(\lambda \right)}{P_{\mathit{\text{in}}}\left(\lambda \right)}$$E1

The high quantum efficiency of the absorber shows that when the solar cell is exposed to a photon with an arbitrary wavelength, it can generate a significant current.

Photothermal solar cells exploit different patterns on top of the engineered substrates to absorb solar energy. Figure 2a shows an instance of this absorber category. The efficiency of the photo thermal solar cell is strongly dependent on the efficiency of the designed electromagnetic wave absorber [23, 24]. Apart from the high absorption rate, the operating bandwidth coverage (120–1000 THz) is a critical point to tarp the whole solar energy. To evaluate the performance of the solar cell, its absorption spectrum is compared with the AM1.5 solar radiation spectrum, as in Figure 2b [23]. The short circuit current density of the solar energy for AM1.5 is defined as [25]:

Figure 2.

(a) Illustration of the photothermal solar cell and (b) comparison of its spectral radiation with AM1.5 solar spectrum [23].

$$J_{\mathit{SC}}=\frac{e}{\mathit{hc}}\underset{{\lambda }_{\min }}{\overset{{\lambda }_{max}}{\int }}\lambda {\Phi }_{\mathit{AM}1.5}\left(\lambda \right)A\left(\lambda \right)$$E2

where e is the electron charge, h is the Planck’s constant, c is the speed of light, λ is the wavelength, ${\Phi }_{\mathit{AM}1.5}$ is solar radiance at AM 1.5, and A is the absorption.

3. Absorption efficiency/bandwidth enhancement in solar cells

This section aims at introducing different approaches for absorption efficiency/bandwidth enhancement in both photovoltaic and photothermal solar cells.

3.1 Absorption efficiency enhancement

Nanoparticle arrays with different shapes, materials, and the number of layers are usually used for absorption efficiency enhancement [22, 26, 27, 28]. To evaluate the absorption rate in the photothermal solar cells, the unit cell analysis with Floquet port excitation is commonly exploited. The absorption rate (A) is related to the reflection coefficient (R) and transmission coefficient (T) as A = 1-R-T. Figure 3a shows a graphene-based plasmonic nanoparticle array residing on top of the metallic reflector, and Figure 3b shows the absorption rate of the device in terms of substrate height. The perfect absorption rate of the device is due to the excitation of localized surface plasmon resonances in the spherical graphene shells [29].

Figure 3.

(a) Absorption efficiency engineering using a back reflector and (b) absorption rate of the device in terms of substrate height [29].

Although the operating frequency of the provided example is not in the solar spectrum, the method is general and has been widely used for absorption enhancement in different types of solar cells. For instance, silver nanorods are used as the back electrode in a thin film solar cell to achieve a 45% improvement in conversion efficiency [27]. Also, gold and silver spherical nanoparticles are embedded in the rear layer of a perovskite solar cell to improve its efficiency in the red region of the visible spectrum [30]. By investigating the role of dispersion and dissipation of the nanoparticle on the performance, it is proved that dielectric nanoparticles lead to higher enhancement compared to their metallic counterparts [31]. Moreover, the metals are oxidized in various weather conditions, and the structure’s performance is greatly affected by the oxide layer [32]. Thus, high-index dielectric nanoparticles (e.g., titanium dioxide), supporting magnetic Mie resonances, are used to transform the freely propagating sunlight into guided modes (Figure 4a) [33]. Also, silica sphere, hemisphere, moth-eye, and cone nanostructured perovskite solar cells are considered, and photovoltaic performance is investigated under omnidirectional incidence. The moth-eye configuration has led to the best performance, in which short-circuit current density is increased by 8.4% at normal incidence and by 36.4% at 60° incidence compared to the planar reference [34]. The core-shell geometry provides more degrees of freedom for performance manipulation [35]. In this regard, core-shell spheroidal nanoparticles with metal core and modeled oxide shell (Figure 4b), spherical metal-insulator nanoparticles, and metal-metal core-shell nano-cube are integrated, respectively, into the thin film, dye-sensitized, and organic solar cells and plasmon-enhanced light absorption, photocurrent, and efficiency improvement is observed [32, 36].

Figure 4.

Thin film solar cell absorption efficiency improvement using (a) dielectric nanoparticles [33] and core-shell plasmonic particles [32].

3.2 Absorption bandwidth enhancement

To widen the bandwidth of the resonant absorbers, the design of multilayered structures and the combined use of elements with different dimensions to excite multiple resonances are proposed [37, 38, 39, 40]. In the device shown in Figure 5a, the hyperbolic nature of the dispersion band supported by the densely packed graphene strips, along with propagating and localized surface plasmon resonances, respectively, provided by the gap plasmons and plasmonic spherical particles, are effectively used to enhance the absorption bandwidth of the multilayer structures [41, 42]. Considering this idea, Figure 5b shows a thin film amorphous silicon (a-Si) photovoltaic solar cell in which silver nanoparticle arrays are embedded in the surface and active layer, respectively, being responsible for absorption enhancement in the lower and higher wavelengths [22]. In another design, the rear of a thin film crystalline silicon solar cell is decorated with two layers of silver nanoparticles with different dimensions. By optimizing the radii of the particles, a 9.97% and 9.94% increase, respectively, in short-circuit current density and intergraded quantum efficiency is observed in comparison with the same geometry formed by uniform nanoparticles [43]. Also, randomly distributed metallic nanoparticles with optimized filling factors, laying in the photoactive layer of the thin film solar cell, result in the transportation and localization of light in a broad spectrum [44]. Assuming dielectric particles, multilayered silicon nanoparticles with submicron dimensions are stacked in the ultra-thin photovoltaic solar cell for tailoring the absorption efficiency with Mie scatterers [45].

Figure 5.

(a) Absorption bandwidth enhancement using different types of resonances [41] and (b) multiple resonance configuration achieved using multilayered structure [22].

To illustrate the use of multidimensional elements for the bandwidth enhancement, Figure 6a exhibits the wideband absorber design using an oligomer constructed by cylindrical elements. The absorption spectrum, shown in Figure 6b, confirms the presence of multiple resonances, originating from the multidimensional elements. The device performs based on transmission elimination using a reflector and reflection elimination arising from the wideband impedance matching with the free space intrinsic impedance. The real and imaginary parts of the retrieved complex normalized surface impedance are extracted via [46, 47, 48]:

Figure 6.

(a) Absorption bandwidth enhancement using geometrical parameter manipulation and (b)-(c), respectively, absorption spectrum and complex surface impedance of the designed surface [40].

$$Z=\pm \sqrt{\frac{{\left(1+S_{11}\right)}^2-S_{21}^2}{{\left(1-S_{11}\right)}^2-S_{21}^2}}=\pm \frac{1+S_{11}}{1-S_{11}}$$E3

and they are illustrated in Figure 6c. The real and imaginary parts are, respectively, around 1 and 0, confirming the anti-reflection nature of the designed surface. As an instance of the method in the solar cell design, a multiscale metallic fractal nano-carpet configuration is integrated inside the silicon layer of a thin film solar cell for absorption enhancement. The operation of the designed geometry is based on surface plasmon polaritons and localized surface plasmons at different wavelengths, leading to the short circuit current enhancement by a factor of 2.4 [49, 50].

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, 54, 55, 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]:

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].

$$\alpha =\frac{\sqrt{2}\mathit{\pi f}}{c}\sqrt{-{\epsilon }^{\prime }+\sqrt{{{\epsilon }^{\prime }}^2+{{\epsilon }^{″}}^2}}$$E4

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 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]:

$$\alpha =\omega \sqrt{{\epsilon }_0{\mu }_0}{\left(a^2+b^2\right)}^{1/4}sin\left(\frac{1}{2}{tan}^{-1}\left(\frac{a}{b}\right)\right)$$E5

where prime and double prime respectively denote the real and imaginary parts. The parameters a and b in (5) are defined as: $a=\left({\epsilon }_r^{\prime }{\mu }_r^{\prime }-{\epsilon }_r^{″}{\mu }_r^{″}\right)$ and$b=\left({\epsilon }_r^{\prime }{\mu }_r^{″}+{\epsilon }_r^{″}{\mu }_r^{\prime }\right)$. As Figure 8 confirms, the graphene material has a large attenuation constant in the solar spectrum, making it suitable for the absorber design.

Figure 8.

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

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 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.

Material	Thermal conductivity (W m^−1 K^−1)	Density (g cm^−3)	Specific heat (J kg^−1 K^−1)	TMetal/TGraphene
Al	247	2.7	897	0.6503
Au	317.9	19.3	129	0.6326
Ag	428	10.5	235	0.6383
Zn	113	7.1	338	0.5685
Ni	82.9	8.9	444	0.3986
Cu	401	8.96	385	0.4566
Graphene	2000–4000 (suspend) ∼600 (on substrate)	2.25	∼700	1

Table 1.

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

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].

Figure 9.

Entire solar spectrum absorption coverage with (a) graphene-based nano-pillar absorber and (b) its reflection, transmission, and absorption spectra. (c and d) The sensitivity of the absorption rate to the incident angle respectively for the TE and TM waves [1].

5. Conclusion

Photovoltaic and photothermal systems are considered the two main solar cell design technologies, and their design key points are introduced in this chapter. The efficiency and the operating bandwidth are important factors for evaluating the performance of solar cells. To reach efficient solar cells, it is required to optimize the surface geometry in terms of shape, material, and the number of layers. In general, dielectric materials lead to better performance in comparison to noble metals. To design broadband absorbers, multilayered or multiresonance configurations are proposed. Alternatively, pyramidal/conical geometries, supporting gradual impedance matching with the free space intrinsic impedance, can be exploited. In these geometries, the tip truncation and use of hollow cores improve the low-frequency efficiency. Moreover, environmentally friendly and heat-tolerant materials are the best choice for practical solar cell designs and different types of two-dimensional materials can also be beneficial for solar cell performance optimization. Considering all the factors, novel solar cells with improved performance can be proposed for future applications.