Through-the-glass optical metrology for mapping 60 cm × 120 cm CdTe photovoltaic panels in off-line and on-line configurations

Through-the-glass spectroscopic ellipsometry (SE) has been applied in mapping analyses of 60 cm × 120 cm panels consisting of soda-lime glass coated with a four-layer transparent conducting oxide (TCO) stack and a two-layer CdS/CdTe photovoltaic (PV) heterojunction. Both off-line and on-line measurement configurations have been evaluated, using a rotating compensator for modulation of the incident polarization state and a multichannel detection system for high speed spectroscopy. Because of the longer time available for signal averaging during off-line measurements, the off-line capability assists in model development and qualification of the on-line capability, which operates at a higher speed and a limited data point density over the panel area. For on-line analysis, the panel is lying film side up on a linear conveyer and is moved past an SE mapping station. In this case, the ellipsometer heads traverse from side to side beneath the panel for through-the-glass measurements. Comparisons of off-line and on-line analyses indicate good agreement in the variations of the CdS layer effective thickness over the panel. The deduced CdS effective thickness, which includes components of the bulk and interface layers, is found to be a robust parameter in both analyses, and is also critical for predicting light collection in CdTe PV modules.