User:Strataga/Fluorescent solar collector

Background:
Sunlight provides by far the largest of all carbon-neutral energy sources. More energy from sunlight strikes the Earth in one hour (4.3×1020 J) than all the energy consumed on the planet in a year (4.1×1020 J).

Yet, in 2007, solar electricity from photovoltaics provided less than 0.1% of the world's electricity. The huge gap between our present use of solar energy and its enormous undeveloped potential is simply due to the high cost of solar cells when compared to other type of energy resource.

The challenge nowadays faced by the solar industry is clearly set, and consist in the cost reduction of photovoltaic systems. One way to significantly reduce the cost of solar cells is to reduce the use of the base material (silicon or other common semiconductors).

Fluorescent Solar Collectors (FSC) are attractive devices since: (i) have the potential to significantly reduce the amount of semiconductor material currently used in solar cells, (ii) theoretically are as-efficient as traditional solar cells.

Fluorescent Solar Collectors (FSC)
Fluorescent solar collectors were introduced for the first time in 1976 by Weber and Lambe, and usually consist of a clear material sheet doped with fluorescent dyes bordered by PV cells.

The fluorophore absorbs the incident sunlight and re-emits photons at a weakly reabsorbed wavelength. The re-emitted photons are trapped by total internal reflection (TIR) and directed towards the PV cells.

Using this conformation, the collectors concentrate the solar radiance towards a small solar cells resulting in a greatly reduced usage of the bulk semiconductor (Fig 1).

Fig 1 : A schematic diagram of fluorescent solar collector.

Photon managment
The key factor for making a good FSC device resides in a broad absorption of the incident light by using a judiciously chosen set of dyes while ensuring that the mix of fluorophores emits in a absorption free region.

The emission region is preferably as narrow as possible and as close as possible to the excitation limit of the semiconductor (800nm to 1000nm); in order to  preserve an optimal absorption of the daylight and to ensure an adequate excitation of the edge PV cell.

Fig 2: Absorbance spectra of different fluorophores compared to the irradiance of the sun.

Theoretical efficiency limitations
FSC efficiencies are theoretically slightly less than that of pure silicon solar cells, they can not absorb the full available solar spectra since an absorption free area allowance has to be made for the re-emitted light.

The models developed at the University of Southampton have shown that the efficiency of fluorescent collectors can theoretically reach 90% of the limitation efficiency of standard silicon photovoltaic structures.

In other words, assuming a constant standard solar illumination of 1000W/m2 and an ideal collector of 1 m2; the energy output of this FSC would reach 270W (a standard silicon solar cell would output 300W) and this using material 10 to 20 times cheaper than silicon.

However, in practice, the observed overall efficiencies are less than about 5% mainly because of the fluorescence escaping through the front face of the device (Fig 3). In addition, FSCs may suffer from short lifetime due to the weak photostability of the dyes used in the manufacture process.

Fig 3: Spectroscopic study of the luminescence escaping from the front face of the collector.

Future development
FSCs have great potential as the next generation of cheap and efficient PV systems and the reduction of losses by using enhanced designs could lead to a major breakthrough in this area.

A promising avenue is to confine fluorescence by using state of the art dielectric photonic mirrors (hot mirrors), whilst improvement in photostability can be achieved by using quantum dots.

Hot mirrors allow the high energy photons to go through (i.e. 200-800nm) and reflect light with a longer wavelength (800 -1200 nm). By placing a hot mirror on the top of a collector, the fluorescence escaping from the front of the device is greatly allowing progress to be made towards the maximum theoretical efficiency.

Quantum dots (QD) are the most advanced concept in the field of wavelength conversion for use with solar cells. One of the optical features of small quantum dots is a tuneable fluorescence emission wavelength.

Qualitatively speaking, the band gap energy that determines the frequency (and hence colour) of the fluorescent light is inversely proportional to the diameter of the quantum dot. The larger the dot, the redder the fluorescence; and the smaller the dot, the bluer it is.

The photostability of QDs is excellent and the wavelength emission can be tuned to the nm precision.

We believe that the optimal use of these new ideas and concepts can lead to the developments of a new, cheap, generation of photovoltaic devices.

For more informations, visit : (www.soton.ac.uk/~solar) --T.J.J. Meyer (talk) 15:37, 20 January 2008 (UTC)