  Hot Carrier solar cells have long been recognized as an attractive contender
in the search for high efficiency photovoltaic devices but their fabrication
requires solution of two important material challenges: finding materials with
drastically reduced carrier cooling rates and realization of selective energy
contacts to extract the photogenerated carriers. This paper is concerned with
the problem of absorber phononic engineering to reduce carrier cooling rates.
The physics of carrier cooling is explored and experimental data of other
authors are discussed with a view to assessing the potential of state-of-the
art nanostructured materials for PV conversion. A tentative initial calculation
based on the thermalisation in these nanostructures and assuming all other
aspects as ideal, gives 54% efficiency at 2500suns as compared to 54% for no
thermalisation at all. Phononic band gap engineering to further reduce carrier
cooling or bring down the threshold concentration is discussed.
