**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.

**Keywords:** photothermal solar cell, photovoltaic solar cell, bandwidth enhancement, efficiency, material selection, 2D materials
