Researchers at the Fraunhofer Center for Silicon Photovoltaics CSP and the Hochschule Anhalt have developed a process, which will perspectively enhance the harvesting of sunlight incident on a solar module. This so-called light management has the goal to redirect that part of light to the active areas of solar cells, which is normally being lost for power generation (e.g., by reflection on the metal contacts or absorption in the spaces between the individual solar cells). For this purpose, femtosecond laser pulses have been used, because these are able to write optically functional structures under the surface of the front glass of solar modules by a non-contact and nondestructive process.
As shown schematically in Figure 1, the laser is being focused into the glass volume such that its refractive index is changed persistently in the focus of the laser beam, while the glass surfaces remain completely unchanged. For correct choice of the laser parameters, the glass remains completely transparent (no absorption losses), and no micro-cracks occur, i.e. the mechanical stability of glass remains intact. Therefore, basically arbitrary optical structures can be generated by scanning the laser over the area to be addressed. As an example, Figure 2 shows the effect of a simple, non-optimized diffraction grating (parallel lines in 6µm distance) on blue light: about 50 % of light under normal incidence (lightest point and highest peak in the middle) are diffracted to different propagation angles.
Currently work is in progress to optimize the structures with respect to their »redirection« efficiency. Basically the procedure comprises the ability to machine even complete (and already installed) solar modules, and thus to improve their power yield: due to the very tight focus of the laser and the consumption of a significant part of the pulse energy for the structural change in the glass, a solar cell below the glass will not be damaged during creation of the light management structures.
These results have been obtained by a cooperation between the Hochschule Anhalt and the Fraunhofer Center for Silicon Photovoltaics (CSP) in the framework of a research program funded by the BMBF (Kooperatives Forschungskolleg »StrukturSolar«). This research project is being continued.
Muchow, M.; Büchner, T.; Seifert, G.: Femtosecond Laser-induced optical microstructures inside glass volume for light management in solar modules. Proc. of 29th European Photovoltaic Solar Energy Conference and Exhibition (29th EU PVSEC), 22-26 September 2014, Amsterdam, The Netherlands. DOI: 10.4229/EUPVSEC20142014-1BV.6.57