(6db) Composite Conjugated Polymer/Fullerene Dispersions for Organic Photovoltaic Applications

Controlling the structural morphology of conjugated polymer/fullerene composites is an important aspect in improving the performance of polymer solar cells. The efficiency of exciton dissociation and carrier transport to the working electrodes are both a function of the size and distribution of polymer and fullerene domains within the active layer. In the current solution processing paradigm, these characteristics are intrinsically linked to the detailed process history of the film. This fact compromises the promise of polymer solar cells as an inexpensive renewable energy technology because performance gains made in the laboratory may not translate well to scaled-up manufacturing processes. Aqueous dispersions of conjugated polymer/fullerene nanoparticles have the potential to address this challenge. Similar to their thin-film counterparts, the performance of photovoltaic devices derived from conjugated polymer/fullerene composite nanoparticles (CNPs) is a function of the structural morphology within each nanoparticle. However, whereas the structural morphology of an active layer produced using solution phase deposition is a function of the detailed history of the coating process, the structure of CNPs is fixed during their formulation. Because of this fact, the structure can be identified in the dispersed phase and tied, using single particle characterization, to its photovoltaic performance before it is deposited into a device. Therefore, the optimization of devices derived from CNPs can be broken into two discrete problems: improving the intrinsic properties of a nanoparticle as a function of how it is produced and optimizing the extrinsic properties of devices through the deposition of optimized CNPs. Our work has addressed both of these challenges. First, using P3HT/PCBM nanoparticles, we show that internal phase segregation is a complex function of the way the particles are produced and their composition. We also show that the resulting structure can be identified and optimized for photovoltaic performance. Second, we outline the principles that affect the deposition of these CNPs onto device relevant electrodes, and provide insight into the production of high-quality active layers from CNP dispersions. The combination of these two separate approaches opens up the potential for novel roll-to-roll processes to produce high performance photovoltaic modules of arbitrary complexity with flexible form factors.