Hot carrier solar cells aim to reduce one of the major loss mechanisms present in photovoltaics, namely thermalization of charge carriers excited well above the conduction band edge of the absorber material. These highly energetic carriers scatter some of their energy to atomic vibrations know as phonons. The optical phonons generated by this process decay into two acoustic phonons via the Klemens mechanism, which emits two acoustic phonons of half the energy of the original optical phonon. If this decay mechanism were inhibited much of the thermalization loss might be recovered through optical phonons scattering their energy back to conduction band electrons.

One possible way to inhibit the Klemens decay mechanism is by engineering phononic bandgaps in a nanostructured material. This can be done by periodically modulating the acoustic impedance of the absorber material such that the Klemens phonon energy is the same as that which satisfies the Bragg condition for coherent reflection. Under these conditions the specific Klemens mode phonon energies are excluded. This is likely to require fine tuning of a periodic high quality three dimensional superlattice.

Preliminary calculations of the spectrum of phonon energies of a model coherent cubic quantum dot array have been carried out using harmonic and adiabatic approximations. Calculation of the phonon dispersion was performed by direct diagonalization of the system’s dynamic matrix.

Further results of these calculations will be presented together with an assessment of the potential of quantum dot superlattices as the absorber material in a hot carrier solar cell