Thin Ti adhesion layer breaks bottleneck to hot hole relaxation in Au films.

Abstract

Slow relaxation of highly excited (hot) charge carriers can be used to increase efficiencies of solar cells and related devices as it allows hot carriers to be extracted and utilized before they relax and lose energy. Using a combination of real-time density functional theory and nonadiabatic molecular dynamics, we demonstrate that nonradiative relaxation of excited holes in an Au film slows down 30-fold as holes relax across the energy range -2 to -1.5 eV below the Fermi level. This effect arises due to sharp decreases in density of states (DOS) and reduced hole-phonon coupling in this energy range. Furthermore, to improve adhesion, a thin film of transition metal, such as Ti, is often inserted between the noble metal layer and its underlying substrate; we demonstrate that this adhesion layer completely eliminates the hot-hole bottleneck because it significantly, 7-fold per atom, increases the DOS in the critical energy region between -1.5 eV and the Fermi level, and because Ti atoms are 4-times lighter than Au atoms, high frequency phonons are introduced and increase the charge-phonon coupling. The detailed ab initio analysis of the charge-phonon scattering emphasizes the nonequilibrium nature of the relaxation processes and provides important insights into the energy flow in metal films. The study suggests that energy losses to heat can be greatly reduced by judicious selection of adhesion layers that do not involve light atoms and have relatively low DOS in the relevant energy range. Inversely, narrow Ti adhesion layers assist heat dissipation needed in electronics applications.