Invited Paper EN-TuM11
Architectures for Enhanced Exciton Harvesting in Organic Photovoltaic Cells

Tuesday, October 19, 2010, 11:20 am, Room Mesilla

Organic semiconductors are attractive for application in photovoltaic cells due to their compatibility with lightweight, flexible substrates, and high-throughput processing techniques. Optical absorption in these materials leads to the creation of tightly-bound, mobile excitons. In order to generate photocurrent in an organic photovoltaic cell (OPV), excitons must diffuse to a dissociating, electron donor-acceptor (D-A) interface. Most organic semiconductors are characterized by exciton diffusion lengths that are considerably smaller than the optical absorption length. This trade-off between diffusion and absorption often necessitates the use of thin active layers to maximize exciton harvesting. Among the approaches that have been demonstrated to mitigate the short exciton diffusion length, the use of a D-A bulk heterojunction has been widely studied. In these structures, the D-A materials are blended to realize a large interface area for exciton dissociation. The film morphology is typically optimized by thermal annealing, which results in the formation of pathways for charge carrier collection. This talk will explore two alternate OPV architectures designed to overcome the exciton diffusion bottleneck. The first involves the use of composite donor layers that contain both a fluorescent host and a phosphorescent guest sensitizer. The inclusion of the phosphor sensitizer in the donor layer enables the population of the long-lived triplet exciton state of the fluorescent host. Diffusion via the host triplet leads to a near-doubling in the exciton diffusion length and an increase in device efficiency. The second architecture relies on the use of OPVs containing a continuously graded D-A film composition as a means to simultaneously optimize the exciton diffusion and charge collection efficiencies. In these graded heterojunction OPVs, the power conversion efficiency is observed to exceed that of comparable devices containing either planar or uniformly mixed heterojunctions. In both of these approaches, improved performance is realized by utilizing architectures that enable an increased level of control over the exciton diffusion and charge collection efficiencies.