APPLICATIONS  |  GaAs SOLAR CELLS

RESULTS          ABSOLUTE CALIBRATION          GLOBAL IMAGING          RELATED PRODUCTS


 

Gallium arsenide (GaAs) is one of the most commonly used III-V semiconductor compounds for photovoltaic applications. This can be attributed to its high electron mobility, its direct bandgap and its well handled growth mechanisms. GaAs single junction devices now reaches an efficiency close to 30%. They have already been studied extensively, and rapidly became a reference system for thin film solar cells. To investigate the pros and cons of a novel hyperspectral imaging platform based on volume Bragg gratings, researchers at IRDEP (Institute of Research and Development on Photovoltaic Energy) have characterized GaAs solar cells using the platform (IMA).

They successfully obtained spectrally and spatially resolved photoluminescence (PL) images of a standard GaAs solar cell from the Fraunhofer Institute for Solar Energy Systems (ISE).  A 532
 nm laser was used to homogeneously illuminate the entire field of view under a microscope objective, allowing the PL signal coming from a million points to be collected simultaneously. This global illumination modality (see section below for more details) is an effective solution against the problem of lateral carriers’ diffusion and to avoid artefacts from the sample roughness, both observed in point by point hyperspectral imager. The dimension of the recorded images can also reach up to a few square millimeters, depending on the magnification of the objective.

With the help of a spectral and photometric absolute calibration procedure 
(see section below) developed by IRDEP, it is possible to determine the absolute number of photons emitted from every point of the surface of a sample, at every wavelength. This unique feature allows researchers to obtain a map of the cell’s quasi-Fermi level splitting (Δμeff) directly from the PL images. The quasi-Fermi level splitting is of great interest since it is directly related to the maximum achievable voltage of a cell and to the saturation currents. FIG. 1 presents the obtained map of Δμeff/q [1,2]. The quasi-fermi level splitting measured is Δμeff = 1.1676 ± 0.010 eV, with a small drop near the electrical contact (vertical blue line in the middle of FIG. 1) and the external boundaries of the cell. The results being in agreement with the studies found in the literature on GaAs, researchers are now confident regarding the accuracy of the absolute calibration and hyperspectral techniques they used.

Based on these successful results, the next step is to study with the same platform materials with unknown spatial properties. Application notes on the characterization of CIS, perovskite and CIGS can be found in the Photovoltaic section of Photon etc website.


[1] Delamarre A., Lombez L. and Guillemoles J.-F., Contactless mapping of saturation currents of solar cells by photoluminescence, Appplied Physics Letters, 100, 131108, (2012).
[2] Delamarre A., Lombez L., Guillemoles J.-F., Characterization of solar cells using electroluminescence and photoluminescence hyperspectral images, Journal of Photonics for Energy, 2, 027004 (2012).

SPECTRAL AND PHOTOMETRIC ABSOLUTE CALIBRATION

Most if not all luminescence characterisation techniques provide data in arbitrary units. A deep interpretation of such results is often limited by this lack of information. With this in mind, researchers at IRDEP developed a powerful method for spectral and photometric calibration. With this technique, they are able to determine the absolute number of photons of a given energy emitted from every point of the surface of their sample. By performing this calibration, researchers can further investigate Planck’s law and the reciprocity relations between a solar cell EQE and the EL emitted at a given voltage [1]. Hence, the absolute calibration of the hyperspectral data provides a direct way to extract spatial variations of several properties such as open circuit voltage (Voc), saturation currents and external quantum efficiency (EQE).

In order to perform an absolute calibration and measure the signal to get the number of photons, two steps are needed [2]. First, for each wavelength of the spectral region of interest, a relative calibration is achieved on a given area by coupling a calibrated halogen lamp to an integrating sphere. This setup, providing a spectrally and spatially homogeneous output, allows the correction of sensitivity fluctuations. Then, an absolute calibration is carried out for a given wavelength on a single point of the sample. To do so, the output of a fibered coupled laser is imaged and compared with the intensity measured with a power meter. Finally, combining the relative calibration of the whole sample and spectral range to the absolute calibration at a given wavelength and point,  the absolute calibration of the whole sample can be extrapolated for every wavelength of interest.

[1] Rau, U., Reciprocity relation between photovoltaic quantum efficiency and electroluminescent emission of solar cells, Physical Review B 76, (2007).
[2] Delamarre A. , Paire M., Guillemoles J.-F.  and Lombez L., Quantitative luminescence mapping of Cu(In,Ga)Se2 thin-film solar cells, Progress in Photovoltaics, (2014).

GLOBAL IMAGING MODALITY

As previously stated, this hyperspectral platform allows the acquisition of the entire field of view under a microscope, wavelength after wavelength. Using a megapixel sensor, the acquisition of filtered images will provide spectral information from million of points at the surface of the sample. By design, this modality requires uniform illumination over the entire field of view. When compared to typical confocal PL setups where the excitation is done at only one point (~1 μm2), thus leaving the surrounding area at rest, global illumination avoids the recombination of carriers due to localized illumination. Indeed, the isopotential created when using global illumination prevents the above mentioned charge diffusion. In confocal setups, lateral diffusion of carriers towards the darker regions of a sample has the effect of reducing the PL signal so the excitation power needs to be increased considerably in order to observe PL signal. This high power density is far from what the PV material will ever experience in real conditions. In fact, the power density used in confocal microscopy usually reaches 104 suns, far from the operating conditions of a photovoltaic device, which is a serious complication for the interpretation of the results. Homogeneous illumination used for the global imaging modality allows carrying PL experiments in the range of 1 - 500 suns which is within the range of realistic operating mode of concentrated PV.

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