Exciton annihilation in dye-sensitizednanocrystalline semiconductor (Al 2 O 3) films has been studied through laser-induced fluorescence spectroscopy. The relative quantum yield of the fluorescence decreases with increasing excitation light intensity, the indication being that exciton annihilation occurred. The rate constants of the annihilation were estimated for three dyes, N719, D149, and MK2, that are known to be sensitizing dyes for efficient dye-sensitized solar cells. The hopping time between dye molecules and the diffusion length of excitons within their lifetime were also estimated to facilitate discussion of the relevance of exciton annihilation to primary processes in dye-sensitized solar cells.
a b s t r a c tThe effect of Li ion on the light-harvesting and electron injection efficiencies of D149-sensitized nanocrystalline TiO 2 films was studied. Absorption spectra shifted toward the long-wavelength region when films were immersed in a Li ion solution, indicating that Li ion enhanced light-harvesting efficiency. We also found that electron injection efficiency was dramatically enhanced by Li ion. These observations indicate that it is essential to incorporate Li ion in the electrolyte of D149-sensitized solar cell devices to improve their performance.
Chemical etching or electropolishing of Nb superconducting radio-frequency (SRF) cavities to achieve a clean and smooth internal surface require the removal of the passive Nb2O5 film. Typically, an aggressive chemical, such as hydrofluoric acid is included in the solution to accomplish this. The problem with using HF is that it is dangerous both to people and to the environment. As an alternative, in order to depassivate the surface, cathodic pulses may be interspersed between anodic pulses which passivate the Nb surface forming Nb2O5 film, in place of or in conjunction with off-times. Use of such cathodic pulses eliminates the need for HF and/or fluoride salts or other chemicals to actively remove the surface oxide. A present understanding of a pulse reverse electropolishing of Nb in alkaline electrolyte-sodium hydroxide will be reported. In this study, 10~30 % concentration of NaOH has been used as the electrolyte. In order to better understand the mechanism of a pulse reverse electropolishing of Nb with sodium hydroxide, and optimize the process parameters for Nb cavity process, the effect of cathodic pulse duration and amplitude, off time, and anodic pulse duration have been systematically studied, the surface of Nb samples have been characterized chemically and morphologically. We expect the results will enable application of this safer and greener process to future accelerator projects using Nb SRF cavities.
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