**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, 26–28]. 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**

**Figure 2.** *(a) Illustration of the photothermal solar cell and (b) comparison of its spectral radiation with AM1.5 solar spectrum [23].*

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

#### **Figure 3.**

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

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].

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, dyesensitized, and organic solar cells and plasmon-enhanced light absorption, photocurrent, and efficiency improvement is observed [32, 36].

#### **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

**Figure 4.**

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

**Figure 5.**

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

multiple resonances are proposed [37–40]. 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].

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

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 [46–48]:

#### **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].*

$$Z = \pm \sqrt{\frac{\left(\mathbf{1} + \mathbf{S}\_{11}\right)^2 - \mathbf{S}\_{21}^2}{\left(\mathbf{1} - \mathbf{S}\_{11}\right)^2 - \mathbf{S}\_{21}^2}} = \pm \frac{\mathbf{1} + \mathbf{S}\_{11}}{\mathbf{1} - \mathbf{S}\_{11}} \tag{3}$$

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 nanocarpet 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].
