1,721,017 research outputs found
Non-linear spectroscopic studies at interfaces : experiment and theory
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Excitation energy transfer rate from langmuir blodgett (LB) dye monolayers to silicon: effect of aggregate formation
No full textTime-resolved emission spectra (TRES) and decay curves have been recorded from mixed LB oxacarbocyaninedye monolayers and stearic acid at different distances to the silicon surface. We observe interlayerenergy transfer between monomers and dimers present in the monolayer competing directly with energytransfer to silicon at close distances. We resolve these competing processes by studying the TRES spectraand decompose them into their emission components. We found the energy transfer rate for the monomerto silicon to be double than that of the dimer at a distance of d ~ 5 nm to the silicon surface. TheFörster radius for the energy transfer to silicon was estimated at 5.5 ± 0.5 nm
Efficient fluorescence quenching near crystalline silicon from Langmuir–Blodgett dye films
No full textThe distance dependence of the fluorescence efficiency from Langmuir Blodgett (LB) dye layers deposited on top of silicon substrates was studied for different silicon crystal orientations (<100> and <111>). The distance to the silicon surface was varied with stearic acid LB layers (SA). Spectroscopic Ellipsometry (SE) provided accurate measurements of the thickness of the separation steps and the refractive index of the dye layer. It was found that the fluorescence efficiency of the overlying LB dye layers was quenched significantly by the presence of the semiconductor at close distances to the surface. No significant difference in the fluorescence quenching for the LB dye between the two crystal orientations was observed. The results obtained are compared with previous work from fluorescence time and intensity measurements. The importance of interference effects is stressed at high semiconductor-dye distances but the fluorescence quenching observed at small distances is due to efficient energy transfer to the semiconductor
Modelling the performance of fluorescent solar collectors
No full textThe theoretical power conversion efficiency of a silicon solar cell with a fluorescent solar collector is believed to reach 90% of the maximum efficiency of an ideal silicon solar given by the Shockley-Queisser detailed balance limit, but the practical efficiencies are significantly lower due to several loss mechanisms. This work presents an analytic model which take the non-ideal coupling between the collector and the solar cell mounted at the edge into consideration and it is shown in good agreement with experimental results
Photon frequency management for trapping & concentration of sunlight
No full textThis paper considers a range of techniques which – within the realm of classical optics – can be used to enhance light capture as a first step in photovoltaic energy conversion. Examples include a simple case of downshifting, fluorescent collectors which reduce the size of a light beam, and a novel form of light trapping to increase the path length of light within the solar cell. The results are discussed using a thermodynamic framework where the energy exchange with an absorbing/fluorescent medium allows the entropy of the captured photon gas to be lowered, reducing the étendue of the emitted beam. We show that frequency management represents a powerful tool, allowing enhancement in light trapping above the Yablononovitch limit, leading to potentially highly efficient but very thin crystalline silicon solar cells
Comparison of fluorescent down-shifting layers for increasing the efficiency of CdS/CdTe solar cells
No full textThe poor spectral response of some Cadmium Sulfide/Cadmium Telluride (CdTe/CdS) solar cells in the UV/blue part of the spectrum can be improved through down-shifting of light using fluorescent dyes. A fluorescent down-shifting structure absorbs UV/blue light and emits it at a longer wavelength where the External Quantum Efficiency (EQE) of the solar cell is significantly higher. This study compares fluorescent structures with different dyes and different concentrations, some benefiting from resonance energy transfer. The results indicate that the application of a down-shifting structure is effective in increasing the EQE of the solar cell in the UV/blue light region, and can also be used to concentrate light from a larger area onto the solar cell. A theoretical model is being developed and the first results have been compared with experimental spectroscopic results, with a satisfactory agreemen
Design of grid computing infrastructure to aid second harmonic generation studies at the liquid/air interface
No full textInterfacial Second Harmonic Generation can be used to study surface orientation and aggregation properties of molecules with respect to concentration. A study of the laser dye Rhodamine 6G at the air/water and oil/water interfaces probed the orientation of the dye molecules as a function of the bulk dye concentration and revealed the formation of surface aggregates at higher concentration. This study was used to aid the design of a grid computing service to enable the collection of quality data in an efficient fashion, thus enabling a scale up to high-throughput or parallel operation.Active involvement in the experiment revealed the requirement for a system that encompasses data throughout the laboratory process. This starts with the collection and storage of the raw data from the apparatus, and continues through the analysis, ending with the presentation in published works. The use of semantically rich techniques ensures that data can be exchanged between different programmes, helping to overcome the problems of file-type compatibility. The flexibility of the analysis path in the work results in traditional relational databases being inappropriate, so a more flexible data storage scheme was found using RDF. By using Publish and Subscribe technologies we are able to provide remote real-time monitoring of laboratory events. This allows for the detection of errors without being present during data collection
Hot photons and open-circuit voltage in molecular absorbers
Full text linkHot carrier solar cells have attracted interest for many years. Although no working exemplars exist today, the challenges to overcome have become clearer and a substantial research effort has been underway with a focus on inorganic semiconductors, including quantum wells. In this paper we propose a novel strategy to potentially exploit hot photons, based on organic absorbers. Our approach, when combined with photon management structures similar to photonic fluorescent collectors, can potentially enhance the efficiency of complete photovoltaic devices. We present a characterisation method of fluorescent collectors by evaluating the chemical potential and temperature of the emitted fluorescence photon flux. We report on observation of temperatures of the emitted photon flux well above the ambient temperature, indicating the presence of hot photons. We propose a theoretical background to describe how excess thermal energy carried by hot photons can be exploited to increase the chemical potential of the photon flux which is closely related to the open-circuit voltage of the solar cell
Silicon sensitisation using light harvesting layers
Full text linkLangmuir–Blodgett monolayers of a cyanine dye mixed with stearic acid were deposited on glass and silicon substrates with spacer layers of pure stearic acid monolayers or silicon dioxide films deposited by PECVD. By using the time correlated single photon counting technique, time resolved emission spectra (TRES) and decay curves were measured to characterise the dependence of energy transfer rate on the separation between the dye monolayer and the silicon surface and also for the dye concentrations in the monolayers. We observe interlayer energy transfer between monomers, dimers and higher aggregates present in the monolayer deposited on glass but also competing directly with energy transfer to silicon at close distances. We find that the fluorescence lifetime of the dye monolayer is significantly shortened when deposited close to the silicon surface signifying efficient energy transfer. The dissipation of the excitation energy near silicon is explained using the classical theory developed for metals and a deviation is observed for monolayers deposited at distances close to the silicon surfac
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