Reflectance Spectroscopy
In a fraction of a second, the photovoltaic (PV) reflectometer measures the reflectance spectrum of a wafer or cell that is dimensionally within 6 in. × 6 in.
The measured reflectance plots are deconvolved to derive physical parameters, including surface roughness and texture, antireflective coating thickness, metallization area and height, and backside metallization properties.
![Pair of drawings showing how direct normal incident light reflects in a scatter from a rough surface and some scattered incident light to a spot reflects directly normal.](/pv/assets/images/chart_reflectance_measure.gif)
The reflectance measurement uses a principle of reciprocity
![Schematic of the PV Reflectometer system; the Reflectometer itself at the left consists of a hollow sphere with light sources around the upper circumference, sample stage at the bottom and sensing equipment at the top; a diode array spectrometer at lower right and computer at upper right](/pv/assets/images/chart_schematic_reflectomet.jpg)
A schematic of the reflectometer
![PV Reflectometer-generated graph of reflectance (Y-axis) versus wavelength (X-axis) for silicon PV wafers at three processing stages: texture-etched (highest reflectance), sawed and cleaned (slightly lower reflectance) and anti-reflection coated (significantly lower reflectance, with reflectance lowest for the coated stage in the 500 to 800 nanometer wavelength range.](/pv/assets/images/chart_reflectometer_results.gif)
Reflectometer results on three groups of commercial PV-Si wafers (4.5 in. × 4.5 in.)
at different stages of solar cell processing.
antireflective (AR) coating thickness: 794 to 858 Å
![PV Reflectometer-generated graph of reflectance (Y-axis) versus wavelength (X-axis) for a different set of silicon PV wafers at four processing stages: after sawing (highest reflectance), after texturing (significantly lower reflectance), with conducting oxide coating (slight additionally lowered reflectance) and with anti-reflection coating (slight further lowered reflectance, more significant at the shorter wavelength range).](/pv/assets/images/chart_reflectance_curves.gif)
Reflectance curves (in arbitrary units) of 5-in. × 5-in. wafers taken with the PV-Reflectometer.
These data yield information on the surface roughness, texture height, and oxide and
AR coating thicknesses.
AR coating thickness: ˜600 Å
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