Phononic engineering for hot carrier solar cells

The concept underlying the hot carrier solar cell is to slow the rate of photo-excited carrier cooling to allow time for the carriers to be collected while they are still at elevated energies ("hot"), and thus allowing higher voltages to be achieved from the cell. Significant reduction in carrier cooling has been observed in Quantum Well (QW) nano-structures at very high illumination intensities due to a "phonon bottleneck" mechanism. With the phononic gaps in nano-structures, the optical phonon lifetime can be prolonged by blocking the main phonon decay from optical branches to acoustical branches (such as the Klemens or Ridley decay channels). Si-based hot carrier cell is a very active topic and Si-Ge nanostructures are especially interesting for the application, as their fabrication process is well developed. In this chapter, the authors first analyse the operation of a hot carrier solar cell and lay down the general principles. They then discuss the opportunity of phonon engineering to improve the phonon bottleneck. Finally, they present how these can be modeled in nanostuctures comprising several thousand atoms, where true 3D phonon dispersion relations for Si-Ge nano-structures are obtained using first principles methods. The effects of the nano-structure size and geometry on the phonon dispersion relations are investigated. The possible phonon decay processes in the nano-structures are discussed and compared with the bulk crystal materials. The performance of calculated nano-structures on the hot carrier solar cell is evaluated with the acquired knowledge of phonon modes.