Electron transfer kinetics plays a key role in determining the energy conversion efficiency of dye-sensitized photoelectrochemical solar cells. Photoinduced charge separation in such cells results in oxidation of the sensitizer dye. The resulting dye cation may be rereduced by recombination with injected electrons or by electron transfer from iodide ions in the redox electrolyte, often referred to as the regeneration reaction. In this paper, we employ transient absorption spectroscopy to investigate the kinetic competition between these two pathways in Ru(dcbpy)2(NCS)2-sensitized nanocrystalline film TiO2 electrodes immersed in a propylene carbonate electrolyte. The experiments monitored both the dye cation decay kinetics and the yield of product species, assigned to I2 - radicals generated by electron transfer from iodide ions to the dye cation. The kinetic competition between the recombination and the regeneration processes is found to be dependent upon both the iodide concentration and the electrical bias applied to the dye-sensitized electrode. Similar regeneration kinetics were observed when Zn-tetra-p-carboxy-phenyl-porphyrin was used as sensitizer dye. In contrast to the recombination reaction, the rate of the dye cation regeneration reaction by iodide is found to be independent of applied bias. At high iodide concentrations, the cation regeneration reaction is sufficiently fast to compete successfully with the recombination reaction for all biases studied. When an intermediate iodide concentration is used, the acceleration of the recombination kinetics at negative biases results in a reduction in the dye cation lifetime and a loss of I2 - yield. This bias dependence is found to be in good agreement with a numerical modeling of the data employing a continuous time random walk model for the charge recombination dynamics, assuming the iodide reaction to be first-order in iodide concentration. We conclude by discussing the implications of these observations for solar cell function.

The recombination kinetics of photogenerated charge carriers in a composite of poly[2-methoxy-5- (3′,7′-dimethyloctyloxy)-1-4-phenylene vinylene], (MDMO–PPV) and the functionalised fullerene 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 are investigated at room temperature by transient absorption spectroscopy. The decay dynamics of positively charged MDMO–PPV polarons were found to be either monophasic or biphasic, depending upon the laser excitation density employed. The slower, power law, decay phase (100 ns–10 ms) is attributed to recombination dynamics of localized polarons, while the fast decay component (<20 ns) is attributed to recombination of relatively mobile polarons observed when the density of localized states is exceeded by the density of photogenerated polarons (∼1017 cm−3). The implications of these observations are discussed in relation to polymer/C60 photovoltaic cells.

The recombination dynamics of long-lived photogenerated charge carriers are investigated by transient absorption spectroscopy (TAS) on micro- to millisecond time scales in a blend of poly[2-methoxy-5-(3‘,7‘-dimethyloctyloxy)-1-4-phenylene vinylene], (MDMO-PPV) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61 (PCBM) at room temperature. In this work we focus on the physical origins of these recombination dynamics. Studies are conducted as a function of blend composition (including consideration of pristine MDMO-PPV films), solvent, temperature, and the effect of white light bias. We conclude that following low-intensity excitation of the MDMO-PPV/PCBM blend, MDMO-PPV polarons are trapped in an inhomogeneous distribution of localized states (∼1017 cm-3) above the mobility edge for the polymer valence band. The recombination dynamics of these polarons with PCBM anions are limited by the thermally activated detrapping of these polarons, being rather insensitive to both PCBM anion dynamics and MDMO-PPV/PCBM phase segregation. Steady-state white light illumination results in the continuous occupancy of the tail of these localized states, causing an acceleration of the recombination dynamics. These observations are discussed in terms of their relevance to device function.

Transient absorption spectroscopy was employed to study electron-transfer dynamics in dye sensitized nanocrystalline solar cells incorporating a polymer electrolyte, poly(epichlorohydrin-co-ethylene oxide) containing NaI and I 2 . Solar cells employing this solid-state electrolyte have yielded solar to electrical energy conversion efficiencies of up to 2.6%. Electron-transfer kinetics were collected as a function of electrolyte composition, white light illumination, and device voltage and correlated with current/voltage characterization of the cell. The yield of electron injection from the dye excited state into the TiO 2 electrode was found to be insensitive to electrolyte composition or cell operating conditions. Regeneration of the dye ground state by electron transfer from Iions in the polymer electrolyte exhibited half times of 4-200 µs, depending upon the concentration of NaI in the polymer electrolyte. A long-lived product of the regeneration reaction was observed and assigned to the I 2radical. At low NaI concentrations, kinetic competition was observed between this regeneration reaction and charge recombination of the oxidized dye with electrons injected into the semiconductor. The decay kinetics of the dye cation, and the yield of I 2 -, were found to be unchanged by illumination of the cell under either short circuit or open circuit (V oc ) 0.75 V) conditions. From these observations, we conclude that the charge recombination dynamics in this cell are not strongly dependent upon the TiO 2 Fermi level over this voltage range. Analogy with studies of recombination dynamics in three electrode photoelectrochemical cells employing a redox inactive liquid electrolyte suggest this observation may be related to the Lewis base nature of the polymer employed.

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Cured and uncured scraps from manufacturing of epoxy based carbon fiber reinforced composites were treated with a pyrolytic process to provide, as solid residue, carbon fibers to be re-used in new composites production. The industrial scraps were pyrolyzed at different temperatures in a 70 kg batch pilot plant and the pyrolysis products (gas, oil, and solid) were fully characterized. The solid residue (carbon fibers covered by a carbonaceous layer) was subjected to a further oxidative step at 500 and 600 C for different residence times to provide fibers devoid of any organic residue that did not volatilize during pyrolysis. The effects of both pyrolysis and oxidative process on the recovered fibers were evaluated by scanning electron microscopy and Raman Spectroscopy. The reinforcement behavior of pyrolyzed and pyrolyzed/oxidized chopped fibers, compared to virgin fibers, was tested in the production of new Chopped Carbon Fiber Reinforced Composites. The optimized double pyrolysis/ oxidation process was found to provide fibers whose performance in the composites were comparable to the virgin ones. POLYM. COMPOS., 36:1084-1095, 2015

Glass fiber-reinforced polyester composite (GFRPs)

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