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.