Thermoelectric conversion in tandem thermoelectric-photovoltaic applications

An efficient use of solar energy as a renewable power source has been the subject of an enduring research effort, still challenging scientists and technologists worldwide. Beyond standard photovoltaic (PV) and thermodynamic solar power plants, the increase of conversion efficiency achieved in thermoelectric generators (TEGs) has opened a new, lively research front aimed at the direct conversion of solar power into electricity. Although the efficiency of thermoelectric (TE) materials cannot rival that of PV materials yet, TEGs allow concentrating solar power without requiring bulky and expensive optical devices. Also, TEGs may be deployed in harsh environments (e.g. deserts) where maintenance of PV cells is often unaffordable. However, it is realistic to consider today TEGs as tools to improve, not to replace, PV generators. PV efficiency losses will be analyzed in details by decoupling source- and absorber-dependent losses, showing that the implementation of a tandem PV-TE device might lead to a substantial increase of available power densities. Specifically, it will be shown that the proper choice of the TEG geometry and layout remarkably depends on the PV material. In addition, since the efficiency of the tandem device depends on the energy gap and then on the working temperature of its PV stage, a major increase of the conversion efficiency can be achieved by adding an intermediate layer between the PV an the TE stage capable of absorbing the under-the-gap fraction of the solar spectrum. This increase of efficiency will be discussed and commented upon also in view of the constraints they impose to the TEG layout and to its material characteristics. The critical importance of effective heat dissipation will also be addressed. The conclusion will be reached that not only could tandem PV-TE cells improve the conversion rate of existing solar cells but also that they could enable the use of lower cost PV materials, currently not considered because of their marginal PV efficiency.