Previous simulations of photovoltaic devices are based on classical models,which neglect the atomistic details and quantum mechanical effects besides the dependence on many empirical parameters.Here,within the non-equilibrium Greens function formalism,a quantum mechanical method for the simulation of the performance of inorganic nanowire-based photovoltaic devices is presented [1].Based on the density-functional tight-binding theory,the method allows simulation of current-voltage characteristics and optical properties of photovoltaic devices without relying on empirical parameters.Numerical studies of silicon nanowire-based devices of realistic sizes with more than ten thousand atoms are performed and the results indicate that atomistic details and non-equilibrium conditions have clear impact on the photoresponse of the devices.