Ab initio study of the photocurrent at the Au/Si metal-semiconductor nanointerface

Photo-induced charge transfer at the interface of two materials is a fundamental process in photovoltaic applications. In this study, we have considered a model of a simplified photovoltaic cell composed of a Si nanocrystal co-doped with Al and P, interfacing with Au electrodes. The photo-induced time-dependent electric currents were computed from a combination of ab initio electronic structure and time-dependent density matrix methodology, and using the continuity equation for electronic currents. A dissipative equation of motion for the reduced density matrix for electronic degrees of freedom is used to study the phonon-induced relaxation of hot electrons in the simulated system. Equations are solved in a basis set of orbitals generated ab initio from a density functional. Non-adiabatic couplings between electronic orbitals are computed on-the-fly along nuclear trajectories. Charge carrier dynamics induced by selected photoexcitations show that hole relaxation in energy and in space is much faster than electron relaxation. The overall net charge transfer across the slab is small; however, local currents at the Si/Au interfaces are substantial. It is also shown that the relaxation of the induced current can be used to parameterise the dynamical conductivity by means of a fluctuation-dissipation relation.