- 9:40 AM

Nanostructured Materials for Solar-to-Electric Energy Conversion

Eray S. Aydil, Department of Chemical Engineering and Materials Science, University of Minnesota, 151 Amundson Hall, 421 Washington Avenue SE, Minneapolis, MN 55455-0132

Harnessing the solar energy incident on earth to provide clean energy to approximately 10 billion people is the single most important challenge for humans. The availability of economical and sustainable energy sources strongly influences the global society, and for this reason, new developments in solar-to-electric conversion methods would impact everyone's lives. We need high-efficiency low-cost solar cells for solar to electric energy conversion and we need efficient and practical methods to convert solar energy to chemical energy. While steady progress has been made in Si based solar cells, the cost of producing electricity from sunlight is still 4-5 times more expensive than the competitive technologies. The correlation between reduction in cost and increase in usage is undisputable and high cost electricity production per kW-hr is the single most important barrier that must be reduced to make solar energy use competitive with fossil fuels. Two key factors that contribute to the high energy cost are the low solar cell efficiencies and expensive manufacturing methods that rely on high temperature synthesis in vacuum. For example, commercial amorphous and crystalline Si based solar cells convert ~8% and ~15% of incident radiation to electrical energy, respectively, but electricity produced from such devices cost more than $0.35/kW-hr. For unconcentrated solar radiation incident onto a solar cell made of a single threshold energy absorber, such as Si, the maximum attainable thermodynamic efficiency is approximately 31%. This fundamental limitation arises because the excess photon energy above the threshold is first converted into carrier kinetic energies and then ultimately lost as heat through electron-phonon scattering and phonon emission. This limitation can be overcome either by collecting the hot carriers before they have a chance to relax or by generating additional carriers through impact ionization. Several novel concepts, including tandem and hot carrier cells and solar cells that generate multiple electron-hole pairs per absorbed photon have been proposed to surpass this limit. However, compared to the traditional PV technologies, these new concepts are barely at the beginning stages of their technological evolution and fundamental understanding of their operation and assembly is limited. Significant improvements in the cost and efficiency of solar-to-electric energy conversion systems require a fundamental understanding of the synthesis, structure and properties of the materials used in their assembly. This talk will review some of the recently proposed solar cell architectures that are based on nanostructured materials and the scientific and engineering problems that must be overcome to bring them closer to commercialization. Nanotechnology enables novel approaches to solar-to-electric energy conversion that may provide both high efficiencies and simpler manufacturing methods. Large interfacial areas in nanostructured materials present significant advantages both for light absorption and charge separation, the two critical steps for solar-to-electric energy conversion. Furthermore, the ability to engineer the energy states of electrons through size reduction and quantum confinement effects provides additional flexibility for designing novel large area heterojunctions. While semiconductor heterojunctions created through sophisticated thin film deposition methods such as molecular beam epitaxy have been studied extensively, synthesis and characterization of heterojunctions between nanostructured semiconductor materials is in its infancy. A fundamental understanding of the synthesis, structure and properties of heterojunctions between dissimilar semiconductor nanoparticles can lead to novel devices including efficient, reliable and inexpensive solar cells. Last decade has produced a number of new ideas and solar cell designs that are now at beginning stages of their technological evolution curves. Foremost amongst these new cell designs are the dye sensitized solar cells (DSSCs) and polymer heterojunction polymer solar cells. DSSCs separate charge carrier generation from carrier transport into different materials within the cell. In a DSSC, a monolayer of a photosensitive dye is adsorbed on a mesoporous nanocrystalline semiconductor, usually TiO2, in the presence of an electrolyte. Photons are absorbed by the dye to excite electrons from the HOMO to the LUMO level and the excited electrons are rapidly injected into the TiO2 . The charged dye is reduced through an electrochemical reaction with a redox couple (I3-/I-) in an electrolyte; I- is oxidized to I3- at the dye-electrolyte interface and I3- is reduced back to I- at the cathode to complete the solar electrochemical cell. To overcome the shortcomings of the present design, increase the cell efficiency and reduce manufacturing cost we replaced TiO2 nanoparticles with ZnO nanowires produced by low temperature solution growth methods. ZnO nanowires, grown on transparent conducting oxide (TCO) substrates from aqueous solutions of Zn(NO3)2 and methenamine at temperatures <100 C provide a morphology suitable for use as the photocathode in dye-sensitized and quantum-dot-sensitized solar cells. ZnO nanowires grow vertically on the TCO and provide both high surface area for dye adsorption and direct conduction path for the photogenerated electrons. We use 1-10 Ám long ~50-100 nm diameter ZnO nanowires to assemble both dye- and quantum-dot-sensitized solar cells where the wide-band-gap ZnO acts as an electron acceptor. In dye-sensitized solar cells, a monolayer of cis bis(sothiocyanato)bis(2,2'bipyridyl-4,4'-dicarboxylato) ruthenium(II)bistetrabutylammonium dye is adsorbed onto the ZnO nanowires and act as the sensitizer. While, nanoparticle-based dye sensitized solar cells have been studied for 15 years, use of nanowires in dye-sensitized solar cells is recent. It is presumed that the electron transport in the nanowires will be faster than that in nanoparticles. Indeed, using intensity modulated photocurrent and photovoltage spectroscopies we have shown that the electron transport in the nanowire solar cells is at least two orders of magnitude faster than that in nanoparticle-based dye sensitized solar cells. While increased transport provides an advantage for the nanowire-based dye sensitized solar cells, the overall conversion efficiencies (~1%) are still an order of magnitude lower than those obtained with nanoparticle-based dye-sensitized solar cells (~10%). The nanowire-based solar cells are limited by their surface area and recent work is focused towards strategies that increase the nanowire surface area. Such strategies include hybrid nanowire-nanoparticle solar cells and growth methods that maximize nanowire nucleation density, orientation and growth rate.