Donor-Acceptor Block Copolymers with Nanoscale Morphology for Photovoltaic Applications

Extensive research activities in synthesis and device engineering have been devoted to the development of donor-acceptor (D-A) bulk heterojunction solar cells in the last few years. Several photophysical processes occur in such devices which have to be optimized for an efficient device operation. First, excitons that are created upon light absorption need to reach the D/A interface within their short exciton diffusion length (10-20 nm), where they may dissociate into holes and electrons. Subsequent charge transport and charge collection can then take place at the electrodes, given that co-continuous pathways of donor and acceptor domains are provided. An active layer thickness of 100-200 nm is required to absorb most of the light, and vertically aligned pathways with a high aspect ratio of either phase should percolate through the film, taking into account the small exciton diffusion lengths. The morphologies resulting from this ideal situation resemble those of vertically oriented microphase separated block copolymer thin films, and hence suggest the exploitation of D-A block copolymers for organic photovoltaics. Furthermore, complex block copolymer architectures are not only desired in order to improve the morphological control but also to enhance the long term stability of the device. The potential of such block copolymers to microphase separate into well-defined structures several tens of nanometers in size thus addresses the morphological requirements mentioned above. This chapter gives an overview of the emerging field of D-A block copolymers. General synthetic efforts that have been undertaken towards this direction are summarized. The D-A block copolymers prepared in our group are reviewed and complemented with recent work on crystalline-crystalline block copolymers.