In order to understand the dependence of photoinduced initial processes on thermal annealing, the femtosecond time-resolved fluorescence dynamics of regioregular poly(3-hexylthiophene) (P3HT) in (thermally) annealed P3HT/[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) blend films has been studied by using the fluorescence up-conversion technique. For comparison, a P3HT solution, pristine P3HT, and unannealed P3HT/PCBM blend films have been investigated as well. The fluorescence dynamics of the P3HT solution showed wavelength dependence. Excitation energy transfer between the segments and torsional relaxation possibly occurred in a time scale of several ps in the solution. Observed rise times at longer wavelength emission suggested the formation of these relatively lower emission states (at 650 and 700 nm). Charge transfer (or excitonic quenching) was the dominant process in the fs time scale with emission at 650 nm in the unannealed blend film. In the annealed blend film, the charge transfer (334 fs) and downhill relaxation (942 fs) of self-trapped (dynamic localized) excitons were competitive processes due to the well aligned nanodomains in the P3HT/PCBM blend films. There were different charge transfer rates at different excited states (650 and 700 nm) in the annealed film. The charge transfer process occurred faster at a lower excited state, and a stronger electronic and vibrational coupling in the annealed P3HT/PCBM films was revealed within these measurements as well. The ultrafast anisotropy decays suggested that a strong and ultrafast reorientation of the molecular dipole moments occurred at excited states. The anisotropy decay was mainly determined by the ultrafast process, whereas the energy could continuously migrate along or between P3HT chains in a time scale of ∼100 ps. The ultrafast process suggested that there was an excitation delocalization associated with vibrational modes, as was consistent with the observation from steady-state measurements. On the basis of the understanding of the mechanisms above, the optimized cell performance has been established.

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The photovoltaic performance of organic− inorganic hybrid films based on regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) and ZnO nanorods depends strongly on the quality of the interfaces between the organic polymer and the inorganic nanostructures. A thin, solutionprocessed TiO 2 interlayer was introduced on the surface of ZnO nanorods to improve the interface between ZnO and P3HT. In order to understand the effects of the TiO 2 interlayer on the initial photophysical processes after photoexcitation, femtosecond time-resolved fluorescence dynamics of the hybrid ZnO nanorods/P3HT (ZnO/P3HT) films with or without TiO 2 interlayer coating were comparatively studied using a fluorescence up-conversion technique. Because of the wellaligned P3HT nanodomains in these annealed hybrid films, energetically downhill excitation energy transfer was found to be the dominant process in the subps time scale with an excitation at 400 nm. The fluorescence decay times for the ZnO−TiO 2 /P3HT film were significantly extended, which is likely due to the improved interface quality with fewer traps and defects when TiO 2 interlayer was introduced. Moreover, ultrafast anisotropy decay and a dramatic decrease in residual anisotropy at the emission of 660 nm were observed for the ZnO−TiO 2 /P3HT film in comparison to both P3HT pristine film and the ZnO/P3HT film. This suggests a change in the disorder of energy, the local field, and an efficient energy-transfer process, resulting in a faster exciton reaction (dissociation) within the ZnO−TiO 2 /P3HT film. Solar cells fabricated using TiO 2 -coated ZnO nanorods and P3HT showed efficiencies of up to 0.76%, a 58% improvement over the TiO 2 -free equivalent devices.

The gravitational water vortex power plant (GWVPP) is a new type of low head turbine system in which a channel and basin structure is used to form a vortex, where the rotational energy from the water can be extracted through a runner. This study is focused on the optimization of the runner to improve the efficiency of theGWVPP. Computational fluid dynamics (CFD) analysis iscarried out on three different runner designs with straight,twisted and curved blade profiles. ANSYS CFX was used to analyze the fluid flow through the channel, basin,turbine hub and blade, and results were used to evaluatethe efficiency of each of the runner designs. The CFDanalysis showed curved blade profile to be the mostefficient profile, with a peak efficiency of 82%, comparedto 46% for the straight blade runner and 63% for thetwisted blade version. An experimental test of the turbinesystem was carried out to validate the runner analysis, in ascale version of the GWVPP. The testing showed that therunner behaved as predicted from the CFD analysis, andhad a peak efficiency point of 71% at 0.5m head.