With their modest cost, compact size and great flexibility, miniature fiber optic spectrometers are attractive analytical tools for photovoltaic materials research and quality control. Typical applications include analysis of the optical properties of solar cell materials, spectroradiometric measurement of solar simulators used in panel testing and quality control in panel production. In this case study, we evaluated NIR spectroscopy as a tool to measure the reflection properties of potential photovoltaic panel materials.

Background
A manufacturer of thin film photovoltaics panels requested near infrared (NIR) reflectivity analysis of several coated glass samples. Measurements were conducted in the NIR range 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 the approaches manufacturers may evaluate in improving panel efficiency.

Experimental Procedure
Five coated glass samples were analyzed using an Ocean Optics NIRQuest Spectrometer (Figure 1), configured with a 100 um slit and optimized for the range from 1200-2100 nm. The sampling setup comprised a high-powered tungsten halogen light source, 400 µm reflection probe and a reflection/transmission optical stage (fixture). A specular reflection standard with ~85-98% reflectivity from 800-2500 nm was used as a reference. SpectraSuite spectrometer operating software, a Java-based spectroscopy platform that operates in Windows, Mac OS and Linux operating systems, completed the setup.

The glass samples were placed on the sample holder uncoated side down, to ensure that the probe was measuring the reflection from the coating through the glass. The optical stage helped to position the probe at 90º to measure specular reflectance. In specular reflection, the angle of incidence is equal to the angle of reflection. Specular reflection increases with the amount of gloss on a surface.

Measurements were taken under overhead lighting conditions, without use of a dark room or box. The high-powered (20 W) 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 cm for each sample, to simulate production conditions.

Ocean Optics NIR Spectrometers use a high-performance Indium Gallium Arsenide (InGaAs)-array detector in a compact optical bench with thermoelectric cooler and low-noise electronics. The particular model used for this setup – the NIRQuest256-2.1 — is a 256-element spectrometer suited to applications involving higher wavelengths (peak responsivity is ~1900 nm). A high gain mode option improves system sensitivity for low light-level and low-concentration measurements. The spectrometer’s rapid integration times – spectral acquisition of 1 millisecond is possible – makes it viable for high volume production environments.

NIRQuest also has external hardware triggering functions, which allow users to capture data when an external event occurs, or to trigger an event after data acquisition. This capability can be especially useful for capturing data from automated processes or from devices such as solar simulators that flash synchronously.

Results
The measurements showed good stability with no averaging and boxcar smoothing; therefore, only one set of spectra was collected. The reflection spectra for the supplied samples (Figure 2) demonstrated that reflection values increased as a function of wavelength comparably across all five samples, peaking at about 2000 nm (2 µm). Also, the gap between the least reflective and most reflective samples was relatively narrow at the lower and upper ranges of the spectrometer setup, with the greatest variation observed at approximately 1700 nm.

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

Conclusions
As developers of photovoltaic materials continue to seek improvement in cell efficiency, the need for analytical tools that are convenient for evaluating glass coatings, dopants and other materials is great. Optical sensing systems such as NIR spectrometers, thin film measurement systems and solar simulator testing units are easily configured for both research lab and process line applications.

In our case study, we demonstrated how NIR spectroscopy can be used to determine the reflectivity of coated glass samples relative to each other and to known reflectance standards.
As a result, the solar light capturing efficiency of the five sample coatings now can be inferred with the utilized Ocean Optics spectrometer and accessories.

The ability to provide UV-NIR coverage in a single miniature spectrometer has always been a challenge. Trade-offs inherent to most diffraction gratings – most noticeably, the effect of blaze angle on the efficiency of the diffraction – can pose challenges for certain applications. While gratings were available that diffracted over a wide range, this came at the expense of decreased optical resolution and increased problems associated with second- and third-order overlap.

Newer gratings such as the XR-1 provide good efficiency over a wider wavelength range (200-1050 nm) than is otherwise possible with standard gratings. What’s more, good optical resolution (<2.0 nm FWHM for most setups) can be maintained, and second- and third-order effects are eliminated by applying proprietary filtering technology to the CCD-array detector window. Transmission efficiency is affected only marginally by this filtering.

Broad spectral response in a single spectrometer offers convenience for those who regularly make measurements in both the UV-Vis and Vis-NIR, yet it also offers a solution for applications where samples are responsive across that same broad range. Examples include certain plasmas, solar irradiance, atomic emission lines and broad-range light sources.

Experimental Conditions

To test the response of the XR-1 grating, we installed the 500 lines/mm groove density grating in the optical bench of our USB2000+ Spectrometer. The spectrometer’s optical bench also included a 25 µm slit and order-sorting detector filter. The grating provides 850 nm of spectral range and is blazed at 250 nm.

The test sample for the experiment was our DH-2000-BAL Deuterium Tungsten Halogen Light Source. The DH-2000-BAL combines the continuous spectrum of deuterium and tungsten halogen light sources in a single optical path to produce a powerful, stable output from 215-2000 nm (we observed only the region from 200-1050 nm). A UV-Vis optical fiber collected the signal from the light source. We recommend our QP450-2-XSR optical fiber, which is a 455 µm core diameter fiber with excellent solarization resistance properties. Integration time of 10 milliseconds is typically sufficient for measuring a light source such as the DH-2000-BAL.

Results

The emission spectrum of the UV-NIR light source measured with the USB2000+XR matched the anticipated spectral output. The XR-1 grating showed good efficiency across the 200-1050 nm spectral range, with the best efficiency in the UV. Optical resolution was calculated at ~1.7 nm (FWHM) with a 25 µm slit (the standard slit option for the USB2000+XR) and at ~1.2 nm (FWHM) with a 5 µm slit. Other expected spectrometer performance characteristics were unaffected by the presence of the grating.

Conclusions

Results demonstrate that an Ocean Optics spectrometer configured with the XR-1 extended-range grating will provide spectral coverage across the 200-1050 nm spectral range without sacrificing optical resolution performance or being subject to second- and third-order diffraction effects. The XR-1 is available in the application-ready USB2000+XR, USB4000-XR1 and JAZ-EL200-XR1 Spectrometers (each has a 25 µm slit and order-sorting filter) or as a custom option in one of our other spectrometers. For applications requiring broad range and sub-nanometer optical resolution (FWHM), our HR2000+CG and HR4000CG-UV-NIR composite-grating spectrometers are recommended.

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- XR-Series Spectrometers