Luminescent waveguides. From fluorophore to matrix

The waveguide is an optical device commonly used in laser technology. Its physical properties enable it to guide and concentrate the luminous flux passing through it. This ability to propagate light, for example within a laser fiber, is based on a difference in refractive index, resulting in total internal reflection. Different geometric configurations were studied: fiber or plate. Geometric parameters were analyzed to highlight their influence on optical properties. For example, it is easy to see that internal waveguide interactions are different in macroscopic configurations (thickness in the centimetre range) and microscopic configurations (thickness in the micrometre range).

Interest in these waveguides is growing daily in the field of photovoltaics. At present, all solar technologies have absorption gaps in the solar spectrum. This physical limitation is intrinsic to the properties of the materials making up the solar cell. For example, the absorber material, whose role is to convert light energy into an electron-hole pair, performs well over a specific light range. Typically, these materials absorb visible light, which accounts for 46% of the total energy emitted by the Sun. The rest of the energy is found in the infrared and ultraviolet. These areas are not efficiently exploited by solar cells. Nowadays, only heterojunction cells can broaden the absorption range of the solar spectrum by combining different absorbers. Luminescent waveguides are therefore an alternative to overcome the problem of photon absorption in areas not accessible by absorber materials.

Luminescent waveguides are mainly used to harness photons in the ultraviolet range. The use of fluorophore materials enables waveguides to transfer energy from the UV to the absorption range of the solar cell, thanks to the spectral shifting effect. Many fluorophores have this ability to transfer energy in different modes: down shifting, down conversion, up conversion. Downsifting involves different physical mechanisms depending on the type of material. The choice of support matrix is also an important factor in waveguide design.