**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) [3–6]. 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.
