During the last decade the field of polymer photovoltaics has seen a tremendous improvement in both device efficiency and understanding of the underlying physical processes. One has come to a point in which the prototypical large bandgap material system P3HT:PCBM is nearing optimal device performance. In order to enhance efficiencies even further, research activities for new materials are needed with better aligned energy levels. One interesting approach is by narrowing the donor bandgap to enhance light absorption. Recent developments on small band gap (2.0eV) materials for photovoltaic applications are reviewed. First, an introduction is given regarding the processes governing the exciton dissociation, charge transport requirements, energy level engineering of both donor and acceptor materials, and other parameters determining the photovoltaic performance. The focus is on polymeric donor materials, which are subdivided by the type of monomeric units that constitute the backbone. Finally, the synthetic methods and conditions, processing of the devices, and the device performances are summarized.