1,720,979 research outputs found
Efficient light harvesting with LB films for application in crystalline silicon solar cells
No full textWe have carried out fluorescence lifetime measurements using time correlated single
photon counting (TCSPC) for a cyanine dye near the silicon surface. The measurements have
been carried out for both (100) and (111) crystal orientations of the silicon surface, showing the
dependence of energy transfer rate as a function of the separation between the dye monolayer
and the silicon surface. Langmuir Blodgett fatty acid layers were used to create a multistep
structure and a monolayer of a cyanine dye was deposited on top of the stepped structure.
Spectroscopic ellipsometry has been used to measure the thickness of the fatty acid steps and
provide an accurate estimate of the distance of the dye monolayer to the silicon surface. Time
resolved emission spectra and fluorescence decay curves were measured with a single photon
picosecond time correlated system. We find that the fluorescence lifetime of the dye monolayer
is significantly shortened when present 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 distances close to the silicon surface
(d<5nm). The model can be reconciled with the observed data by modifying the value of the
silicon extinction coefficient which can provide an insight into the energy transfer process in the
near field dye-silicon interactio
Edge-illuminated ultrathin crystalline silicon solar cell
No full textThis paper proposes a radically new solution to enhance the photoexcitation of silicon by directing the illumination onto the edge of the solar cell by means of fluorescence energy collection. This approach makes it possible to consider photovoltaic devices with efficiencies comparable to crystalline silicon thickness of only several micrometers. We will present the first theoretical results and discuss such structures where the path length of light is increased substantially, leading to efficient capture of the near bandgap photon flux. We shall show that using a realistic model for the fluorescent collector, overall device efficiency (collector + solar cell) of 13% maybe be achievable in the near future with a 1?m thick silicon structure. Efficiencies in excess of 17% can be reached with the application of photonics or using thicker silicon layers. We demonstrate that edge illuminated ultrathin silicon solar cells using fluorescent collectors can produce conversion efficiencies close to conventional c-Si solar cell but with greatly reduced material requirement
Ultra-thin silicon solar cell: modelling and characterisation
No full textAn ultra-thin crystalline silicon solar cell with an active silicon
layer of 200 nm has been fabricated and fully characterised
electrically (I-V characteristic, spectral response) and optically
(Variable Angle Spectroscopic Ellipsometry). Interference
effects were observed in the spectral response of the cell
due to multiple reflections from the layers within the cell. A
mathematical model was developed to account for the different
reflections and transmission within the cell which reproduced
excellently the essential features of the experimental
spectral response
Interference effects observed in ultra-thin c-si solar cells: modeling and experiment
No full textA crystalline silicon solar cell with an ultra-thin active silicon layer of 200 nm has been fabricated from SOI wafers. The cell was fully characterised electrically by measuring the I-V and spectral response measurements and optically by ellipsometry. The spectral response of the cell showed interference effects due to multiple reflections from the different layers within the cell structure. A mathematical model was developed based on multilayer film theory to account for the different reflections and transmissions with the cell structure. The theoretical spectral response predictions reproduced excellently the essential features of the measured spectral response
Harvesting sunshine: solar cells, photosynthesis and the thermodynamics of light
No full textEffective capture of sunlight represents one of the grand challenges of photovoltaics today. This paper looks at the opportunities that exist, at the fundamental level, to manage light as the first step of photovoltaic conversion; in particular, how photonics can improve the efficiency and reduce the cost of solar cells. Starting from the current view of light trapping we shall introduce an additional variable: photon frequency. The example of fluorescent collectors will be used to discuss the principal features of frequency management, leading to a novel form of light trapping and, ultimately, the photonic bandgap solar cell. The discussion will be guided by arguments based on thermodynamics to describe photon transformation as part of the absorption / emission / conversion process. By drawing parallels with the capture of light in photosynthetic organisms we shall briefly discuss another important aspect: light harvesting energy collection, and the oportunity this represents for reducing the materials usage in future generations of solar cell
Characterisation of fluorescent collectors based on solid, liquid and Langmuir-Blodgett films
No full textCharacterisation of fluorescent collectors in terms of fluorescence collection efficiency
No full text
Application of concentrating luminescent down-shifting structures to CdS/CdTe solar cells with poor short wavelength response
Full text linkA method to exploit both the wavelength shifting and light concentrating properties of luminescent plates in order to increase the current output of CdS/CdTe solar cells has been investigated. The application of concentrating luminescent down-shifting structures onto CdS/CdTe solar cells in order to combine the benefits of light concentration and wavelength shifting properties of luminescent plates is proposed. This technique is particularly effective in CdS/CdTe solar cells with a poor short wavelength response. An increase in the current output of close to 10% was measured on application of concentrating luminescent down-shifting structures compared to nearly 5% without light concentration to such solar cells. Concentrating luminescent down-shifting structures containing multiple fluorescent species allowed the absorption ability of the device to be extended to a wider range of wavelengths. This combined with strong energy transfer that red-shifts photons to wavelengths with even better solar cell response is seen to result in increases greater than 20% in the short circuit current output of CdS/CdTe solar cells. A general analytical expression that models different methods of applying of luminescent plates to solar cells is proposed which describes the performance of concentrating luminescent down-shifting structures well and can be used to separate the contributions of luminescence and incident excitation photons on the current output. These expressions can also be used to estimate the transport and reflection losses in these device
Solar cell as a waveguide : quantum efficiency of ultrathin crystalline silicon solar cells
No full textAn analysis of an ultra-thin crystalline silicon solar cell with 200 nm thick active layer fabricated in our laboratory shows that the behavior of quantum efficiency as a function of wavelength is primarily determined by the absorption of light in the active layer, showing a series of interference peaks. We have now modeled this solar cell in detail, perceiving the solar cell as a waveguide. Intriguingly we find that the peaks in the quantum efficiency lie close to the frequencies of the trapped (internal) modes of the waveguide. This paper provides a detailed explanation of how these modes, normally inaccessible to external observation, can be detected in external quantum response, in terms of the position of poles of the absorbance in the complex wave number plane
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