Enhancing conversion efficiency in Si-based hybrid photovoltaic-thermoelectric solar cells

Photovoltaic (PV) solar cells play a central role among renewable energy sources, which currently account for 33% of electric power generation worldwide. PV cell efficiency is known to be limited by their incapability of converting the portion of the solar spectrum whose energy is lower than the PV material energy gap. However, such low-frequency part of the solar spectrum can be converted into heat and then, in turn, transformed into electric power by thermoelectric generators (TEGs). Considering that 90% of PV modules are based on Si, either single-crystalline or polycrystalline, the development of Si-based hybrid solar harvesters with enhanced conversion efficiencies would provide a remarkable contribution to the overall renewable power generation. This review will analyse recent advancements in the PV-TEG hybridisation focusing on Si-based PV cells at low solar concentration. Two well-established approaches will be mainly discussed, namely tandem hybridization, where TEGs are in a thermal series with the PV cell, and spectral-split hybridization, where instead suitable optics splits the solar spectrum, conveying its high-frequency part to the PV cell and the remaining part toward the TEG. Account will be given to both energetic and economic profitability, elaborating on the strategies to evade physical and economic constraints that are still limiting the exploitation of such technologies in the most relevant and diffused class of solar harvesters.