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Nanostructured Metal-Organic Composite Solar Cells

This project aims at realizing a high efficiency organic photovoltaic device using the multijunction concept and metal nanoscale features to enhance the overall cell performance. In particular, transparent high-sheet-conductivity nanopatterned metal films are being developed for use as transparent conductors allowing parallel subcell connection, and metal nanostructures are being embedded in the active layers to enhance the photon absorption and charge separation efficiency. This year we discovered that by concentrically filling nano-scale holes in metal contacts with different dielectric materials, it is possible to significantly extend the bandwidth of an optical mode (HE11) that enables efficient light transmission through the metal. We have also made significant progress in the design and optical analysis of metallic nano-antenna\field concentrating structures. A darkfield microscope was successfully constructed to determine and subsequently optimize the resonance properties of individual nanoscale antennas. Introduction Organic-based solar cells have high potential to reduce the cost of photovoltaics. Low-cost active materials, high-throughput reel-to-reel deposition technologies, and application versatility makes them very likely to become competitive against inorganic thin-film devices. However the power conversion efficiency of organic photovoltaics (OPV) is still too low. This project aims at realizing a high efficiency organic devices by exploiting the unique optical properties of metal to realize high-sheet-conductivity and virtually transparent contacts. It also aims at optimizing the use of metal nanostructures in the active layers to enhance the photon absorption and charge separation efficiency. Background The performance of organic photovoltaics has been improving relatively quickly in the last few years. However this technology is still facing major fundamental limitations towards higher efficiency and stability that need to be overcome for it to be competitive with inorganic thin-film solar cells. This project proposes an innovative cell design to increase the efficiency of organic photovoltaics: a stack of organic/inorganic heterojunctions with embedded nanostructured metal features to improve the overall cell performance. The stack design is a high potential route to increase the light absorption efficiency of photovoltaics. The splitting of the solar spectrum through complementary absorption by different cells with specifically designed bandgaps minimizes thermal losses and increases the overall photon conversion efficiency. Results During the last year, progress has been made three different research directions outlined below. Two new approaches for transparent contacts were investigated and advances were made in the synthesis and characterization of metallic antenna structures. First, we describe our most recent results pertaining to developing nanoengineered transparent metal contacts. The replacement of transparent metal oxides such as indiumtin-oxide (ITO), by a cheaper and higher performance alternative is important for many thin-film photovoltaic cells. Metal oxides have a number of disadvantages. The cost of sputtered ITO thin films may be too high for applications in solar cells and large-area light-emitting diodes (LEDs) for lighting. Moreover, the cost of ITO is likely to increase further due to the potential shortage of indium. On flexible substrates, the brittleness of metal oxides leads to film cracking under bending, ultimately leading to device failure. In cases where a transparent electrode is required on top of an organic layer, the sputter deposition of ITO onto an organic material is known to cause damage to the underlying organic layers that leads to decrease in device performance. Lastly, there is an inherent trade-off between optical transparency and sheet resistance. Thicker films and a high doping concentration increases the film conductivity, but also decreases the optical transparency, as shown in Fig. 1. For a sheet resistance of Rsh=10Ω/sq, the solar photon flux-weighted optical transparency is TSW<80%. 10 10