Characterizing the light guiding of fluorescent concentrators

Abstract

Fluorescent concentrators have gained new research interest recently. The development of new material systems for this type of solar concentrator requires the testing of a wide range of materials. The most important characteristic to be tested is the ability of the concentrators to guide light to their edges where the solar cells are mounted. This ability depends on a large set of parameters and cannot be determined with one simple measurement. We present a method to determine this light guiding efficiency spectrally resolved with transmission, reflection, and centermount measurements using a photospectrometer and an integrating sphere. This method represents considerable progress in comparison to external quantum efficiency (EQE) measurements on systems with attached solar cells. The method is fast and flexible. Because no optical coupling and no solar cells with varying properties are involved, accuracy and reproducibility are higher. The method gives an area average of the light guiding efficiency, rather than the value for the point of excitation as does the EQE measurement. Reasonably similar samples can be compared without requiring corrections. For fully quantitative results on an absolute scale, for samples with large Stokes shifts and/or very different properties, additional corrections must be applied. These corrections use data from samples without luminescent material incorporated, which are measured as references. The information obtained for this correction can be also used to derive additional relevant data, such as the absorption of the dyes used and the fraction of light lost into the escape cone. For the tested materials, the measured spectral collection efficiency reached up to 60% at certain wavelengths.