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NIR Spectroscopy for Photovoltaics Materials Analysis

Our partner Ocean insight evaluated NIR spectroscopy as a method to measure the reflection of photovoltaic panel materials. A manufacturer of thin film photovoltaic panels requested NIR reflectivity analysis of several proprietary coated glass samples. Measurements were conducted from 1200-2100 nm under ambient lab lighting conditions. Because the absorbance of photovoltaic panels is so critical, determining the reflectivity at panel edges and elsewhere is a good indicator of the light loss at those areas. The use of anti-reflective coatings and glass dopants are among many materials manufacturers may evaluate in improving panel efficiency.

Five coated glass samples were analyzed with an Ocean Insight NIR spectrometer that was customized with a 100 um entrance slit and optimized for the range from 1200-2100 nm. (A preconfigured NIRQuest+2.2, which covers 900-2200 nm, would be the spectrometer of choice today.) The sampling setup comprised a tungsten halogen light source, 400 µm reflection probe and an optical stage. A specular reflection standard was used as a reference. Spectrometer operating software completed the setup.

The glass samples were placed on the sample holder uncoated side down, to ensure the probe measured the reflection from the coating through the glass. The optical stage positioned the probe at 90° to measure specular reflectance. Measurements were taken under overhead lighting conditions. The high-powered tungsten halogen light source provided continuous illumination from 360-2000 nm. The distance from the tip of the reflection probe to the surface of the sample was measured at ~7.0 cm for each sample, to simulate production conditions.

Ocean Insight NIRQuest+ spectrometers have an enhanced optical bench design for high-sensitivity performance, allowing for low limits of detection at NIR wavelengths from 900-2500. NIRQuest+ has great thermal stability and can be used in the lab or on the line, such as on a conveyor belt, as its higher sensitivity allows users to capture the necessary spectra over shorter integration times. Also, at longer wavelengths, as in the evaluation of solar panel materials, NIRQuest+ measures diffuse reflection at low noise levels, resulting in cleaner spectra at wavelengths where distinct spectral features can be difficult to capture.

The reflection measurements showed good stability with no averaging and boxcar smoothing. The reflection spectra for the samples demonstrated that reflection values increased as a function of wavelength comparably across all five samples, peaking at about 2000 nm (Figure 1). Also, the gap between the least reflective and most reflective samples was relatively narrow at the lower and upper ranges of the wavelength range, with the greatest variation observed near 1700 nm.

Figure 1. NIR specular reflection measurements eveal spectral differences in coated glass samples evaluated for use in photovoltaic panels.

Reflectance intensity of the coated samples ranged from ~25% at lower wavelengths to as much as 80% at higher wavelengths. These values are relative to the response of the specular reflectance standard, which has nearly “flat” reflectivity across all NIR wavelengths.

As developers of photovoltaic materials seek improvement in cell efficiency, the need for convenient analytical tools to evaluate glass coatings, dopants and other materials is great. Optical sensing systems such as NIR spectrometers can be configured for both research lab and process line applications. NIR spectroscopy can be used to determine the specular reflectivity of coated glass samples relative to each other and to known reflectance standards. As a result, the solar light-capturing efficiency of the sample coatings can be inferred using Ocean Insight spectrometers and accessories.

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