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.
At present there are a number of strategies used to control this complicated morphology including the use of solvent additives, mixed solvent systems, side chain tuning and molecular weight control. However, at present there is little understanding as to which strategies are most effective for a given donor:acceptor pair. We have used solution phase small angle x-ray scattering (SAXS) in order to investigate the self-assembly behavior of several different donor systems. We examine how the use of solvent additives, mixed solvent systems, and molecular weight tuning can induce ordering of the donor moiety within the casting solution. Using transmission SAXS on the as-prepared active layers of these BHJ systems, we observe that conditions which induce aggregation and ordering of the polymer chains within the casting solution, promote an ideal degree of phase segregation in the active layer of the device, which leads to enhanced power conversion efficiency (PCE).
However, in cases in which no ordering of the donor polymer is observed in the solution phase, over phase segregated active layers are observed with corresponding low PCEs. This leads us to conclude that aggregation and ordering of the polymer chains in the casting solution is directly related to the formation of an ideal three dimensional interpenetrating network, with phase segregated domains on the order of the exciton diffusion length.