Theory and simulation of photogeneration and transport in Si-SiOx superlattice absorbers

Si-SiOx superlattices are among the candidates that have been proposed as high band gap absorber material in all-Si tandem solar cell devices. Due to the large potential barriers for photoexited charge carriers, transport in these devices is restricted to quantum confined superlattice states. As a consequence of the finite number of wells, large built-in fields and any kind of disorder, the electronic spectrum can deviate considerably from the minibands of a regular superlattice. In this paper, a quantum-kinetic theory based on the non-equilibrium Green's function formalism for an effective mass Hamiltonian is used to investigate photogeneration and transport in such devices for arbitrary geometry and operating conditions. By including the coupling of electrons to both photons and phonons, the theory is able to provide a microscopic picture of indirect generation, carrier relaxation and inter-well transport mechanisms beyond the ballistic regime.