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Applications

GaAs Quasi-Fermi Level Mapping

Absolute Calibration Module Put to the Test with Gallium Arsenide Samples

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 reache efficiency close to 30%. They have already been studied extensively, and rapidly became a reference system for thin film solar cells.

To investigate the performance of Photon etc.’s hyperspectral imaging platform based on volume Bragg gratings, researchers at IPVF (formerly IRDEP - Institute of Research and Development on Photovoltaic Energy) have characterized GaAs solar cells using 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 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 imagers. The dimension of the recorded images can also reach up to a few square millimeters, depending on the magnification of the objective.

GaAs quasi-Fermi level divided by the charge map (Δμeff/q). This map was calculated from Planck’s law using absolute PL measurements. Adapted from [1].
Fig. 1 - GaAs quasi-Fermi level divided by the charge map (Δμeff/q). This map was calculated from Planck’s law using absolute PL measurements. Adapted from [1].

With the help of a spectral and photometric absolute calibration procedure developed by IPVF, 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 confident regarding the accuracy of the absolute calibration and hyperspectral techniques they used.

Based on these successful results, other materials were studied with this hyperspectral platform. Application notes on the characterization of CIS, CIGS, perovskite and Si can be found in the Photovoltaics section of Photon etc. website.

For more information, contact info@photonetc.com

[1] Delamarre, A., Lombez, L., & Guillemoles, J.-F. (2012). Contactless mapping of saturation currents of solar cells by photoluminescence. Applied Physics Letters, 100(13), 131108.

[2] Delamarre, A. (2012). Characterization of solar cells using electroluminescence and photoluminescence hyperspectral images. Journal of Photonics for Energy, 2(1), 027004.

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