We propose a lateral-tandem organic photovoltaic system consisting of a dispersive-focusing element and continuously-tuned, series-connected sub-cells. The proposed system overcomes the efficiency limitation of organic photovoltaic devices by spectral re-distribution of incoming solar photons and their delivery to the wavelength-matched, resonant sub-cells. By numerical simulations, we demonstrate that optical resonance in a microcavity sub-cell with a metal/organic multilayer/metal structure can be tuned over a broad spectrum by varying the thickness of the organic multilayer. We show that the power-conversion efficiency exceeding 18% can be obtained in a lateral-tandem system employing an ideal dispersive-focusing element and the microcavity sub-cells.