In order to minimize efficiency losses due to reflection of sun light on the front glass of PV modules, high-end solar glass is often equipped with an antireflective coating. The coating is deposited as a liquid by spray-coating or by printing techniques. Thereafter it is dried and annealed in consecutive production steps. The properties of the coating must be optimized in a way that the minimum of the reflectance matches the spectral absorption and conversion efficiency of the solar cells. Hence, spectral reflectance measurements directly deliver the most relevant information to characterize the coating in the production line. Fig. 1 shows the reflectance of uncoated glass and of glass coated by two different ARCs.
Fig. 1. Reflectance spectra of uncoated glass and of AR coated glass with reflection minima at 665 nm and 760 nm.
As the reflectance of an ARC on glass is very low (this is the very purpose of the ARC) and it is mostly deposited on rolled glass which is used in this field of application, in-line measurements are quite challenging. However, optimisation of the optical metrology system’s design allows measurements with a repetition rate of up to 100 Hz even under these conditions.
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Fig. 1+2: a-Si thickness on two substrates A and B: in-line Flames is compared to ex-situ reference (Horiba)
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Combining two Flames metrology systems of different spectral range gives access to measuring the properties and thickness of all layers throughout thin-film solar cell manufacturing processes, including transparent conducting oxide (TCO) and absorber and buffer layers. Especially, with the IR spectral range included, the position and number of interference fringes of the thick absorber layers can be automatically analyzed and fitted to determine the absorber’s film thickness on-line. By measuring the reflectance after each deposition step, the thickness of each layer can be determined with high precision.
While TCO and CdS film thicknesses are detected with a spectrometer operating in the visible to near-infrared spectral range (500–1000 nm), a determination of the film thickness of the absorber layer requires an infrared reflectance measurement, as these materials are designed to completely absorb visible light. The thickness measurement accuracy is typically 1–2 %. LayTec has built upon its extensive expertise in fitting optical data and accurately measuring optical properties (n and k values) in multi-layer processes to create the most precise automated thickness analysis available for thin-film PV.
To find out how Flames can optimize your process please contact firstname.lastname@example.org or simply call +49(0)30 89 00 55-0.
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