The use of low-dimensional structures such as quantum wells, wires or dots in the absorbing regions of solar cells strongly affects the spectral response of the latter, the spectral properties being drastically modified by quantum confinement effects. Due to the microscopic nature of these effects, a microscopic theory of absorption and transport is required for their quantification. Such a theory can be developed in the framework of the non-equilibrium Green's function approach to semiconductor quantum transport and quantum optics. In this paper, the theory is used to numerically investigate the density of states, non-equilibrium occupation and corresponding excess concentration of both electrons and holes in single quantum well structures embedded in the intrinsic region of a p-i-n semiconductor diode, under illumination with monochromatic light of different energies. Escape of carriers from the quantum well is considered via the inspection of the spectral photocurrent at a given excitation energy. The investigation shows that thermal escape from deep levels may be inefficient even at room temperature. (C) 2010 Elsevier B.V. All rights reserved.