Reflectance and Transmittance Measurements: Take Your PV Research to the Next Level

Shift your interest away from the device level to characterize at the material level: active or component layers. Reap the rewards of a spectrophotometer methodology to establish, and spectrally resolve, the extent to which the sample reflects (and transmits, in the case of thin films) the incident light.

Characterization of Thin Films and Coatings

The characterization of textured surfaces and diffusing thin films and coatings, including anti-reflective coatings and transparent electrodes, is enabled by a calculation of total reflectance/total transmittance.

Aggregate transmittance spectra of 100 nm Mg-doped CuCrO2 thin films deposited on quartz substrate and annealed at divergent temperaturesAggregate transmittance spectra of 100 nm Mg-doped CuCrO2 thin films deposited on quartz substrate and annealed at divergent temperatures. Measurement data courtesy of CIRIMAT, Toulouse. J. Mater. Chem. C, 2015, 3, 6012.

Total transmittance spectra of 100 nm Mg-doped CuCrO2 thin films deposited on quartz substrate and annealed at various temperatures. Measurement data courtesy of CIRIMAT, Toulouse. J. Mater. Chem. C, 2015, 3, 6012.

Collecting the totality of light reflected into the hemisphere above the sample is the prerequisite for the measurement of total reflectance; the collection of the totality of light transmitted into the hemisphere behind the sample is the prerequisite for the measurement of total transmittance.

Hemispheric light collection is enabled by an integrating sphere, whilst a wavelength tunable monochromatic light permits sample illumination. Systems in which a simple 0°/0° specular reflectance measurement alongside the EQE cannot correctly account for the diffuse reflected component and thus cannot generate correct IQE data of your sample.

Integrating spheres are internally coated with a material possessing high- and diffuse-reflective properties, ordinarily BaSO4. When light strikes this internal surface, its reflection encompasses all directions, and it is subsequently re-reflected multiple times. The consequence is a functionally constant sphere wall illumination: a detector can be located at any position, and the same signal will be received.

Reflectance/Transmittance Sphere

Theoretically speaking, a reflectance/ transmittance sphere comprises a reflectance port, a detector port, and a light entrance port. It is therefore imperative that the detector is located in such a manner as to prevent the interception of first pass reflected light. The sphere captures both the diffuse and specular reflected components in reflectance mode.

The sample is located at a port on the obverse side of the sphere from the light entrance port. The sample plane is marginally pitched away from normal, to prevent the specular component from reflecting back out of the sphere. The sphere is calibrated using a reflectance standard of known reflectance, having a calibration traceable to a National Metrology Institute.

Integrating sphere configured for the measurement of total reflectance (left) and total transmittance (right).

Integrating sphere configured for the measurement of total reflectance (left) and total transmittance (right).

With the sample located at the sphere entrance port, transmittance mode enables the sphere to capture both the direct and diffuse transmitted components. In order to execute a transmission determination, the monochromatic beam power is first determined without sample at the sphere port.

All light transmitted or reflected by the sample will be captured by the integrating sphere, and produce a signal correlative to the total reflectance/ transmittance in the sphere detector. No information is provided regarding the reflected/ transmitted component source, whether specular/ direct or diffuse. This is generally not relevant to photovoltaic device research.

Through the placement of samples of varying reflectance at the ports of the sphere (juxtaposed with the reference measurement), the sphere wall's average reflectance is adjusted and the “integrating” attributes of the sphere alters.

Reducing the relative area of the sphere with respect to that of the ports will minimize this effect. Doing so bypasses the utilization of a comparison sphere, which would entail the mounting of both reference and test samples, and would necessitate the exchange of positions between sample and reference measurements.

Conclusion

Total reflectance spectra of bare and anti-reflective coated silicon.

Total reflectance spectra of bare and anti-reflective coated silicon.

Characterizing a wide range of photovoltaic device components is rendered quick and easy by the integrating sphere, which equips you with essential data on the way in which device structure components are performing, which in turn empowers you to stimulate design, material and process optimization.

This information has been sourced, reviewed and adapted from materials provided by Bentham Instruments Limited.

For more information on this source, please visit Bentham Instruments Limited.

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