In three-dimensional photovoltaic architectures, heterogeneous optical intensity distributions throughout the structure may generally lead to modifications to the short circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF). In this work an equivalent circuit model has been developed to examine the impact on Voc by heterogeneous and homogeneous internal illumination. The model has been tested against data from planar cell and tube-based solar cells that utilize poly-(3-hexylthiophene): phenyl C61 butyric acid methyl ester (P3HTCBM). This has further been extended to predict optimum optical design for tube-based geometries in which organic photoconversion materials have been applied in both fabrication conditions. The result is that for such geometries to provide the best overall optical confinement and best power conversion performance, aspect ratios must be between 1 and 5. The resulting structure leads to best light capture together with best overall internal partitioning of optical power to achieve the highest possible Voc.