Enhanced efficiency in organic photovoltaic cells using engineered plasmonic nanostructures

Institution: University of Minnesota-Twin CitiesTitle: Enhanced efficiency in organic photovoltaic cells using engineered plasmonic nanostructuresIntellectual MeritOrganic photovoltaic cells (OPVs) have the potential to redefine the cost of solar energy conversion. Organic semiconductors are attractive due to their compatibility with high throughput processing methods, but demonstrated power conversion efficiencies are only around 7%. The proposed research outlines a new approach to overcome the low absorption efficiency typical in OPVs by exploiting surface plasmons in nanostructured metallic electrodes. Since absorption in OPV materials leads to exciton formation, photocurrent generation requires the dissociation of excitons into their constituent charge carriers. This process usually occurs at a hetero-junction between electron donating (D) and accepting (A) materials. The challenge lies in the fact that the exciton diffusion length is typically shorter than the optical absorption length, necessitating the use of thin active layers to efficiently collect and dissociate excitons. This work integrates thin film OPVs with plasmonic electrodes, permitting sub-wavelength confinement and resonant enhancement of the optical field, to increase the absorption and power conversion efficiencies. The combination of a thin OPV with engineered plasmonic electrodes offers the potential for a high level of tunability and control over the exciton-plasmon coupling to realize high efficiency. In the proposed research, novel architectures that offer optical field enhancement from nanostructured plasmonic electrodes will be introduced to maximize optical absorption in thin OPVs and permit the efficient harvesting of excitons and charge carriers. Extensive computational modeling will be performed to identify optimal design rules for plasmonic OPVs. The use of continuous, nanopatterned metal films is attractive, since the field enhancement is longer range (~200 nm) compared with metallic nanoparticles, enhancing absorption throughout the active OPV layers. Furthermore, these continuous films can concurrently function as electrodes, which is not possible with metallic nanoparticles. For high-throughput fabrication of plasmonic electrodes, a template-stripping method will be used. Using mature silicon fabrication technology, a variety of nano-patterned templates will be fabricated. A metal film deposited on the template will form a smooth surface at the interface, which it can be peeled off of the substrate using an elastomeric stamp and directly transferred to the top of a completed OPV cell using cold welding. This scheme will provide unprecedented flexibility in the fabrication of plasmonic OPVs, since the top and bottom electrodes can be independently nano-patterned with high throughput over large areas. The OPVs proposed in this work will also utilize graded donor-acceptor heterojunctions (GHJs). The use of a GHJ is attractive because it balances the need for efficient exciton diffusion (large interface area) with efficient charge collection (graded pathways for transport). The growth of GHJs is also tunable, enabling a range of compositions through which to examine how the design of plasmonic OPVs is impacted by the spatial D-A composition.Broader ImpactsGraduate students associated with this project will acquire an interdisciplinary spectrum of knowledge in nanofabrication, plasmonics, and molecular photophysics in the context of organic photovoltaic devices for renewable energy. Existing undergraduate research opportunities will be enhanced through continuing relationships with the University of Minnesota UROP program and the NSF REU program. For K-12 education, multiple high school researchers will be hosted and mentored in the PIs laboratory during the summer through the Summer Research Cluster in Renewable Energy program. These activities will be complemented by plans to disseminate results from the proposed work to industry via on-campus workshops and annual hands-on lab short courses for industry participants. Finally, ?Sit with a Scientist? sessions at the Science Museum of Minnesota will be organized through the proposed outreach plan during the one week-long NanoDays event in April of each year.