Investigation of phonon scattering in superlattices for design of efficient multiple quantum-well hot carrier solar cells

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The hot carrier solar cell is a type of solar energy converter that captures the excess thermal energy of photo-generated electrons and holes in a semiconductor to produce electric power. Hot carrier solar cells hold the promise of yielding significantly higher efficiency beyond traditional limits. A promising material system for achieving high efficiency hot carrier solar cells involves multiple quantum wells, comprised of semiconductor materials arranged in alternating layers known as superlattices, to effectively diminish energy loss from hot electrons. Energy loss from electrons occurs by dissipation of energy to high energy lattice vibrations, which further dissipate to low energy lattice vibrations by scattering. Decoupling the high energy from low energy lattice vibrations by minimizing scattering can ultimately enhance the solar cell efficiency. In the proposed research, the project team will tackle this challenge and design high-efficiency hot carrier solar cells through engineering the superlattice composition and by straining the semiconductor crystal. A dominant phonon scattering mechanism in semiconductors is the Klemens channel, which involves decay of an optical phonon into two acoustic phonons. By modifying superlattice composition, the energy gap in the phonon dispersion can be modified providing avenues to suppress Klemens like channels in phonon scattering. This can enable longer phonon lifetimes, resulting in non-equilibrium phonon populations, thus facilitating hot phonon bottleneck in the thermalization of electrons. Strain can similarly modify phonon dispersion, again allowing for the possibility to diminish phonon scattering. The role of superlattice composition and strain will be studied in two superlattice systems - InAs/AlSb and AlAs/GaAs. Analysis will be performed through a first-principles approach by using harmonic and anharmonic force interactions derived from density-functional theory along with a solution of the phonon Boltzmann transport equation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.