The fiber-based solar cell is an example of a three dimensional photovoltaic architecture that uses the natural mode structure of optical fibers to enhance light capture. In this work we explore the spectral response of such cells when the thickness of the absorbing layer is varied. We demonstrate two important consequences associated with this architecture. The first is that fiber-based devices generally require a thinner active layer than the analogous planar structure. This helps to avoid exciton recombination and reflection loss in the geometry. Secondly, the geometry exhibits a broader absorption and external quantum efficiency than its planar counterpart. We interpret this as being due to enhanced absorption of charge transfer excitons. (C) 2011 Elsevier B.V. All rights reserved.