LPRL

Light Cascade Technology

LPRL’s patented ‘light-cascade’ technology is based on impregnating optically active molecules or crystal particles into a host material.  The optically active materials are chosen according to their absorption bands and red-shifted emission bands. 

Figure 1 shows a schematic representation of the light-cascade concept.  For a given application, dopant A is selected to have an absorption band in the region of interest (e.g. 500 nm for solar cells).  Dopant B has an absorption band that overlaps the red-shifted emission band of dopant A, and in turn re-emits at a higher wavelength. This process continues until the final radiation emitted is in a desired wavelength range (for example, in the 700 to 1000 nm range).  The result is that the transmitted radiation spectrum is significantly red-shifted in comparison with the incident radiation spectrum.

 Figure 1 : Schematic diagram illustrating the light cascade concept.

Depending on the application, the active dopants may be added to the host in a uniform manner so that they all occupy the same volume, or a in a sandwich structure in which dopants A occupy layer A, dopants B occupy layer B, and so forth.  Below is a figure showing an example of the spectral shift that is obtainable using this technology.  In this example, four dopants (labeled a, b, c, d) are used and a sketch is shown of their absorption and emission spectra.  The corresponding peaks in the final transmitted spectrum are clearly seen.

Figure 2 : Solar spectrum transformed by light cascade panel.

 

Shifting the solar spectrum

 

Using light-cascade technology, solar cell encapsulaters may be fabricated not only to provide chemical and mechanical protection, but also to red shift the solar spectrum that is transmitted to the solar cells.  The transmitted spectrum better matches the peak conversion efficiency range of Si or GaAs solar cells (700 to 1000 nm), resulting in an overall increase in energy conversion efficiency of 25 to 50 %, depending on the particular geometry used and the climatic conditions.

Applications

The light-cascade technology may be applied to the encapsulation of solar cells, making the encapsulation an active participant in the conversion of solar energy into electrical energy.  Two possible geometries are discussed below.

Direct transmission  

The geometry used for this technique is shown in the figure below.  The solar cells are aligned on a plane surface and covered with a sheet of host material (e.g. glass or PMMA).  The host material is doped during its fabrication so that the spectrum transmitted is red-shifted by the light-cascade process.

 

Figure 3 : Solar spectrum transformed by light cascade panel.

According to independent measurements, solar cell modules fabricated according to this concept have a short-circuit current over 40 % higher than identical modules encapsulated with traditional glass, and 25 % higher than the same modules non-encapsulated.

Power modules using this technology have seen uses in many fields, ranging from public lighting and signaling facilities to marine markers.  A few examples are shown below.

 

Figure 4 :  Several applications of photovoltaic modules incorporating light cascade technology.  From left to right is shown 1) public lighting, 2) road signal, 3) marine signal, 4) marine communications power supply.

Photon trap

Solar cell modules integrated into a photon trap have even larger short-circuit currents than can be obtained using the direct transmission geometry.  Several possible geometries for constructing a photon trap solar cell module are shown below.

One such geometry is shown schematically below in Figure 4.  In this configuration the doped top sheet is simply sandwiched between a dichroic mirror on the top surface and a white mirror on the bottom surface and the sides.  The dichroic mirror is transparent for wavelengths between 350 and 950 nm, allowing the solar spectrum to penetrate into the doped layer.  The light cascade process in the doped layer shifts the spectrum to longer wavelengths, and a significant portion of the photons become trapped in the doped layer due to

1. total internal reflection at the top surface, or
2. reflection at the top surface off the dichroic mirror (for photons with energies < 1.33 eV) or
3. reflection off the bottom or side mirrors.

 

Figure 5 :  Photovoltaic module exploiting a 2D photon trap.  

The trapped photons eventually impinge upon the solar cells that cover one end of the doped layer. 

Another possible geometry is shown in Figure 5.  In this configuration the surface of the solar cells are oriented parallel to the solar rays and are encapsulated in a photon trap consisting of a dichroic mirror on the top surface, and totally reflecting mirrors on the bottom and sides.