  The endeavor of the scientific community to maximize the possibility to
harvest Sun irradiation for energy production is mainly devoted to the
improvement of the power conversion efficiency of devices and to the extension
of the spectral range in which solar devices operate. Considering that a
significant portion of the Sun irradiation at the ground level is in the
infrared, the research on materials and systems that operate in such region is
gaining increasing attention. In this review, we will report recent
advancements in multijunction solar cells, inorganic-organic perovskite solar
cells, organic solar cells, colloidal quantum dot solar cells focusing on the
absorption of such devices in the infrared. In addition, the use of
upconverting nanostructures will be introduced as a way to indirectly exploit
infrared radiation to increase power conversion efficiency of photovoltaic
devices. Moreover, we will describe plasmon induced hot electron extraction
based solar cells, that are particularly promising in absorbing the infrared
portion of the Sun irradiation when the active materials are doped
semiconductors, which show intense plasmonic resonances in the infrared. The
review includes the optical spectroscopy tools to study the hot electron
extraction from doped semiconductor-based heterojunctions.
