Christopher Tassone, SSRL
Polymer bulk heterojunction (BHJ) solar cells have attracted significant attention in industry and academia because of their potential for achieving large-area, light-weight, and flexible photovoltaic devices through cost-effective solution deposition techniques. These devices consist of a blend of an absorbing polymer and an electron accepting fullerene, the molecular packing and phase segregation of which heavily influence power conversion efficiency by effecting important processes such as exciton splitting, charge transport, and recombination. Understanding and utilization of molecular interactions to predicatively control the morphology across multiple length scales, ranging from the nano to the mesoscale, is critical for the future design of devices with increased efficiencies.
In this talk I will show how solution phase self-assembly of the donor polymer can develop ideal bulk heterojunction morphologies which lead to enhanced device power conversion efficiencies. We have used solution phase small angle x-ray scattering (SAXS) in order to investigate the self-assembly behavior of several different donor and acceptor systems. We have also used a combination of transmission small angle x-ray scattering (SAXS), utilizing both hard and soft x-ray techniques, as well as grazing incidence wide angle x-ray scattering (GIWAXS) to characterize the solid state film morphology across all device relevant length scales. We find that solution phase self-assembly of the donor polymer leads to the development of an “ideal” degree of phase segregation, which in turn leads to enhanced device performance over non solution phase assembled counterparts.
