Go beyond PV EQE testing

Categories: Solar & Photovoltaics, Material & Chemical, R&D

Benefit from a spectrophotometer mode to determine, and spectrally resolve, how much of the incident light is reflected (and in the case of thin films, transmitted) by your sample.

A measure of total reflectance/ total transmittance enables the characterisation of textured surfaces and diffusing thin films and coatings, including anti-reflective coatings and transparent electrodes.

The measurement of total reflectance requires the collection of the totality of light reflected into the hemisphere above the sample; the measurement of total transmittance requires the collection of the totality of light transmitted into the hemisphere behind the sample.

The sample is illuminated by a wavelength tuneable monochromatic light source whilst an integrating sphere ensures hemispheric light collection. Schemes in which a simple 0°/0° specular reflectance measured in tandem with the EQE cannot account for the diffuse reflected component, and therefore cannot yield accurate reflectance nor IQE data of your sample.

Integrating spheres are coated internally with a highly-, diffusely-reflective material, most commonly BaSO4. Light striking this internal surface is reflected in all directions, to be re-reflected multiple times. What results is an effectively uniform illumination of the sphere walls: a detector may be placed at any location and will receive the same signal.

In practice, a reflectance/ transmittance sphere has a light entrance port, a detector port and a reflectance port. The detector must therefore be positioned in such a manner as not to intercept first pass reflected light.

In reflectance mode, the sphere is used to capture both the specular and diffuse reflected components.

The sample is positioned at a port on the opposite side of the sphere from the light entrance port. To ensure that the specular component is not reflected back out of the sphere, the sample plane is tilted slightly away from normal. The sphere is calibrated using a reflectance standard of known reflectance, having a calibration traceable to a National Metrology Institute.

In transmittance mode, the sphere is used to capture both the direct and diffuse transmitted components with the sample positioned at the sphere entrance port.

When performing a transmission measurement, the monochromatic beam power is first measured with no sample at the sphere port.

The integrating sphere will capture all light reflected or transmitted by the sample and generate in the sphere detector a signal proportional to the total reflectance/ transmittance.

The sphere does not provide information on the origin of the reflected/ transmitted component, whether specular/ direct or diffuse. This is generally not relevant to photovoltaic device research.

In placing samples of different reflectance at the ports of the sphere (compared to the reference measurement), the average reflectance of the sphere wall is modified and the “integrating” properties of the sphere changes. This effect is minimised by limiting the relative area of the ports with respect to that of the sphere.

Doing so, avoids recourse to the use of a comparison sphere which requires that both reference and test samples be mounted and positions exchanged between reference and sample measurements.

The integrating sphere provides a quick and easy way to characterise a wide range of photovoltaic device components, giving you critical information on the performance of components of the device structure, allowing you to drive design, material and process optimisation.

*Measurement data courtesy of CIRIMAT, Toulouse. J. Mater. Chem. C, 2015, 3, 6012.