Phonon Lifetimes in Model Quantum Dot Superlattices

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 222-228
It has been proposed that slowed phonon decay and correspondingly long “hot” carrier lifetimes could be obtained through the careful engineering of quantum dot (QD) superlattices (SLs). These QD superlattices would have applications to Hot Carrier Solar Cells (HCSCs). Here, two QD superlattice geometries, FCC and HCP, are investigated numerically to see if these geometries yield the desired properties for long phonon lifetimes and slowed carrier cooling. The result is that the HCP system yields quite large phonon bandgaps for features of a reasonable size. Typical phonon lifetimes in HCP superlattices are on the order of 22 ps for a 1.8 nm diameter QD. The FCC system, for extremely small feature sizes and precise spacing, appears to yield the required phonon bandgap to inhibit first order phonon decay processes almost entirely. For the FCC SL geometry, high energy phonon lifetimes should be greatly enhanced.