**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

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

**Figure 1.** *(a) Photovoltaic solar cell and (b) electric energy production using photothermal solar cell [14].*

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].

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],

$$QE(\lambda) = \frac{P\_{abs}(\lambda)}{P\_{in}(\lambda)}\tag{1}$$

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]:

$$J\_{SC} = \frac{e}{hc} \int\_{\lambda\_{\rm min}}^{\lambda\_{\rm max}} \lambda \Phi\_{AM1.5}(\lambda) A(\lambda) \tag{2}$$

where *e* is the electron charge, *h* is the Planck's constant, *c* is the speed of light, *λ* is the wavelength, *ΦAM*1*:*<sup>5</sup> is solar radiance at AM 1.5, and *A* is the absorption.
