  Solar cell thermal recovery is recently attracting more and more attention in
the research community as a viable solution to increase photovoltaic
efficiency. However the convenience of the implementation of such strategy is
bound to the precise evaluation of the recoverable thermal power, and to a
proper definition of the losses occurring within the solar device. In this work
we establish a framework in which all the solar cell losses are defined and
described. Aim is to determine the components of the thermal fraction. We
therefore describe an experimental method to precisely compute these components
from the measurement of the external quantum efficiency, the current-voltage
characteristics, and the reflectivity of the solar cell. Applying this method
to three different types of devices (bulk, thin film, and multi-junction) we
could exploit the relationships among losses for the main three generations of
PV cells available nowadays. In addition, since the model is explicitly
wavelength-dependent, we could show how thermal losses in all cells occur over
the whole solar spectrum, and not only in the infrared region. This
demonstrates that profitable thermal harvesting technologies should enable heat
recovery over the whole solar spectral range.
