Brian A. Gregg

Flexible Solid‐State Photoelectrochromic Windows

Photoelectrochromic smart window technology is extended to include the use of flexible substrates and solid-state electrolytes. This should facilitate their application as retrofit modifications of office windows, where, by blocking incoming solar irradiation, they could substantially lower air-conditioning costs. These devices are based on a dye-sensitized TiO{sub 2} electrode coupled with a 500 nm thick WO{sub 3} electrochromic counter electrode, separated by a cross-linked polymer electrolyte containing LiI. A novel method for preparing conducting nanoporous TiO{sub 2} films is described that allows for the construction of these devices on flexible organic substrates. Colloidal solutions of TiO{sub 2} free of surfactants were spin-coated onto indium-tin oxide coated polyester substrates, resulting in highly transparent films ranging from 100 nm to 1 {micro}m in thickness. Upon annealing at 100 C, these films were strongly adherent and displayed excellent photoconductivity as shown by their current-voltage characteristics. The devices typically transmit 75% of visible light in the bleached state. After a few minutes of exposure to white light (75 mW/cm{sup 2}), the windows turn dark blue, transmitting only 30% of visible light. They spontaneously bleach back to their initial noncolored state upon removal of the light source.

DYE SENSITIZED SOLAR CELLS : ENERGETIC CONSIDERATIONS AND APPLICATIONS

The high concentration of electrolyte ions permeating the nanoporous films of dye sensitized solar cells eliminates all but nanoscopic electric fields in the solution and in the Ti02. The only substantial electric field is expected to occur at the Ti02/solution interface, and it is primarily across this interface that the photopotential drops in operating cells. In the dark, the low conductivity of the TiO, and the high conductivity of the solution ensure that applied potentials drop over only a small fraction of the nanoporous TiO, film near the substrate electrode. Therefore, measurements in the dark cannot be directly compared to measurements under illumination because the latter access most or all of the Ti02 film. The sensitizing dye is located partially inside the electrochemical double layer at the TiO,/solution interface and so its redox potential is not fixed relative to either the Ti02 or the solution. If the dye is mostly inside the double layer, its potential will tend to follow that of the Ti02 ; if it is mostly outside, it will be almost independent of the Ti02. Different photovoltage-limiting kinetic steps can result in these two cases. The narrow absorption spectra of many dyes provides the dye cells with a natural advantage over conventional solar cells in applications such as photoelectrochromic windows and power windows.

Surfactant porphyrins linked to ruthenium oxide microcolloids: Amicroheterogeneous photoreactor

Abstract The synthesis of a series of porphyrins linked through bipyridines of RuO2 microcolloids is described. These compounds were designed for use in water-in-oil microemulsions where the porphyrin, a one-electron photocatalyst localized in the organic phase, can transfer charge across the phase boundary to the RuO2, a multi-electron dark catalyst localized in the aqueous phase. The redox behavior of these compounds has been characterized by cyclic voltammetry and their diffusion coefficients were measured by chronocoulometry. Fluorescence and phosphorescence measurements on these compounds are consistent with efficient photoelectron transfer from the excited singlet state of the porphyrin.

Porphyrin octaesters: new discotic liquid crystals

Six liquid crystalline prophyrins are described which exhibit discotic mesophases over a broad temperature range before melting to stable, low viscosity, isotropic liquids suitable for capillary filling into photovoltaic cells.

Photochemical solar cells based on dye-sensitization of nanocrystalline TiO2

A photoelectrochemical solar cell that is based on the dye-sensitization of thin nanocrystalline films of TiO2 (anatase) nanoparticles in contact with a non-aqueous liquid electrolyte is described. The cell, fabricated at NREL, shows a conversion efficiency of ∼9.2% at AM1.5, which approaches the best reported value of 10%-11% by Gratzel at EPFL in Lausanne, Switzerland. Femtosecond (fs) pump-probe spectroscopy has been used to time resolve the injection of electrons into the conduction band of nanocrystalline TiO2 films under ambient conditions following photoexcitation of the adsorbed Ru(II)-complex dye. The measurement indicates an instrument-limited −50 fs upper limit on the electron injection time. We also report the sensitization of nanocrystalline TiO2 by a novel iron-based dye. cis-[FeII(2,2′-bipyridine-4,4′-dicarboxylicu200aacid)2(CN)2], a chromophore with an extremely short-lived, nonemissive excited state. The dye also exhibits a unique “band selective” sensitization through one of its two absorpti...

Discotic Liquid Crystalline Porphyrins: Photophysical and Photoelectrical Properties of Large-Area Crystalline Films

Porphyrins exhibiting discotic liquid crystalline phases have been developed in order to prepare thin, large-area, crystalline films of molecular conductors. A series of octaalkylporphyrins bearing different side chains have been synthesized including some with electron-withdrawing substituents at the meso positions. The photophysical properties of thin films of these compounds are a strong function of the film order (crystallinity). A substantial and persistent photovoltaic effect was achieved (e.g., V oc = 0.3 V, j sc = 0.4 mA/cm 2 under white light, 150 mW/cm 2 ) in capillary-filled symmetrical cells with indium-tin oxide electrodes. A model based on kinetically-controlled asymmetric exciton dissociation leading to photoinjection at the illuminated interface is presented to explain these results. This appears to be the first unambiguous example of a photovoltaic cell controlled entirely by interfacial kinetics. The predominance of the photoinjection process in these cells is attributed to the single-crystal-like character of the porphyrin films.