In this thesis, we have demonstrated a new generation of 'all-polymer' three-dimensional photovoltaic cells made of a large array of high-aspect ratio charge-collecting carbon electrodes surrounded by a matrix of polymer photoactive material. This application essentially takes the technology of Organic-MEMS one step further where the unique advantages of carbon coupled with patterneability through micro and nanofabrication technologies offer a unique opportunity for meshing it with organic PV cell technology. The ensuing most fundamental significance enabled by this marriage of two technologies is the 3-D architecture involving high aspect-ratio carbon posts where the diffusion length for charge carriers is minimized while offering large enough optical depth for absorbing incident light. This numerical model for this device investigates a number of physical phenomena and mass equilibrium reactions that govern the transportation and accumulation of charge carriers and also provides an insight to the losses occurred. This comprehensive numerical model is expected to offer a useful tool in understanding the complex electrostatic and mass equilibrium reactions that occur in the 3D configuration of the organic solar cell.