Photovoltaic and optoelectronic device group

Abstract:

Dye-sensitized solar cells are composed of mesostructured electron transporting electrodes, typically fabricated from metal oxides. These electrodes are sensitized with a light absorbing dye and filled with a redox active electrolyte or hole-transporter. There has been considerable interest in solid-state DSCs and hybrid solar cells which use a molecular hole-transporter or light absorbing semiconducting polymer in place of the corrosive redox active electrolyte typically employed. Highest reported efficiencies for solid-state DSC are around 5%, and two major aspects of this device require significant improvement for this to be increased to values greater than the electrolyte based counterpart (c.a. 12%). The first is to enhance the light absorption in the active layer. The second is to improve the conversion of absorbed photons to collected charge. The nano to meso-structure of the metal oxide electrodes critically influences the end performance of the solar cell. Diblock copolymers exhibit well defined morphological structures on the nano to meso scale. Typical phases of interest for photovoltaic applications are the cylindrical phase or the 3D gyroid morphology. Replicating these morphologies into electronically active materials can be done a number of ways --a one-pot synthesis, where the block copolymer acts as a structure-directing agent for a semiconductor precursor, or a sequential templating process where one of the phases of a structured polymer film is removed and replaced with a semiconductor. We converted well-defined diblock copolymer structures into anataze TiO both ways and incorporated these films into solid-state DSCs. By comparing both cylindrical and the bi-continuous double gyroyd phases, we present the benefits and disadvantages of 1D vs 3D nanostructures in this solar cell. 2

Abstract:

Hybrid dye-sensitized solar cells are typically composed of mesoporous titania (TiO2), light-harvesting dyes, and organic molecular hole-transporters. Correctly matching the electronic properties of the materials is critical to ensure efficient device operation. In this study, TiO 2 is synthesized in a welldefined morphological confinement that arises from the self-assembly of a diblock copolymer - poly(isoprene-b-ethylene oxide) (Pl-b-PEO). The crystallization environment, tuned by the inorganic (TiO2 mass) to organic (polymer) ratio, is shown to be a decisive factor in determining the distribution of sub-bandgap electronic states and the associated electronic function in solid-state dye-sensitized solar cells. Interestingly, the tuning of the sub-bandgap states does not appear to strongly influence the charge transport and recombination in the devices. However, increasing the depth and breadth of the density of sub-bandgap states correlates well with an increase in photocurrent generation, suggesting that a high density of these sub-bandgap states is critical for efficient photo-induced electron transfer and charge separation. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

We report on the development of solution-processed ZnO-based dye-sensitized solar cells. We fabricate mesoporous ZnO electrodes from sol-gel processed nanoparticles, which are subsequently sensitized with conventional ruthenium complexes and infiltrated with the solid-state hole transporter medium 2, 2', 7, 7'-tetrakis-(N, N-di-p-methoxyphenylamine)-9, 9'-spirobifluorene (spiro-OMeTAD). Starting from ZnO nanorods synthesized from solution, we investigate the porous ZnO film morphology using various precursor formulations. The nature of the polymeric additive used in the initial ZnO formulation, as well as the ZnO electrode sintering treatment, is varied and its influence on device performance and charge dynamics, probed by transient perturbation techniques, is discussed. We show that using ethyl-cellulose in the initial ZnO formulation is responsible for an improved dye loading on the ZnO porous electrode, while a gradual sintering step at 350 degrees C is suitable for the proper removal of the organic phases that can be found in the ZnO films after their deposition by spin-coating. Using only 800 nm thick porous ZnO electrodes sensitized by N719, the best performing device exhibits a short-circuit current density of 2.43 mA cm(-2) under simulated solar emission of (100 mW cm(-2)), associated with an overall power conversion efficiency of 0.50%.

Abstract:

Improving the solar light harvesting and photon-to-electron conversion efficiency for hybrid, organic-inorganic photovoltaics are critical challenges. Titania based solid-state hybrid solar cells are moderately efficient at converting visible photons to electrons, but major electrical losses still remain. A material based paradigm shift is required to dramatically enhance the performance of these devices. Here, we present an investigation into solid-state dye-sensitized solar cells (SDSCs) incorporating a molecular hole-transporter and mesoporous tin oxide electrodes, in place of titania usually employed. We investigate the influence of treating the surface of the SnO(2) with different oxides and find that MgO "passivated" SnO(2) electrodes demonstrate an unprecedented absorbed photon-to-electron conversion efficiency of near unity across a broad spectral range. A dual surface treatment of TiO(2) followed by MgO enables tuning of the solar cell photovoltage, fill factor, and efficiency with visible light absorbing cells delivering 3% solar-to-electrical full sun power conversion efficiency.