Use of X-Ray Diffraction, Molecular Simulations, and Spectroscopy to Determine the Molecular Packing in a Polymer-Fullerene Bimolecular Crystal

摘要

Bulk-heterojunction (BHJ) solar cells made from blends of semiconducting polymers and fullerenes are attractive because they can be printed at low cost and have demonstrated efficiencies greater than 8%. For these solar cells to produce current, excitons created during light absorption must reach a conjugated polymer (donor): fullerene (acceptor) interface and dissociate by charge transfer. Electrons and holes must then travel through the fullerenes and polymer, respectively, to reach the electrodes before recombination occurs. The forward and back charge-transfer processes and charge transport depend critically on the molecular packing. Slightly modifying the distance between the donor and acceptor can change the charge-transfer rates by more than an order of magnitude and consequently have an enormous impact on exciton dissociation and recombination. Likewise, small changes in the molecular packing can significantly change charge-carrier mobilities. Although important processes, such as charge separation and recombination, can depend on the polymer-fullerene wavefunction overlap, very little is known about how polymers and fullerenes pack on the molecular level since it is very difficult to accurately characterize molecular packing in BHJs. To design materials for BHJs rationally, it is therefore critical to develop an understanding of the molecular packing and its effects on key electronic processes. Until now, it has not been possible to establish the detailed molecular organization of BHJ blends using X-ray diffraction (XRD), because the blends were either amorphous or too disordered to yield enough XRD reflections to accurately determine the unit cell and molecular packing. The best BHJ material combinations have largely been discovered by trial and error.