Evaluation of Micrometer Scale Lateral Fluctuations of Transport Properties in CIGS Solar Cells

CIGS is a material showing high efficiencies in photovoltaic devices although numerous questions remain about its physical mechanisms. Among them is the influence of the polycrystalline nature on global efficiencies. In order to study the spatial fluctuations of the optoelectronic parameters, two original setups are developed. Firstly a Hyperspectral Imager is used to investigate spectrally resolved luminescence images. It is also possible to calibrate it in absolute values, which means that the signal is measured in number of photons. From photoluminescence measurement we deduce maps of the quasi-Fermi level splitting with variation of 30 meV. These results are compared with a more common confocal microscope, which shows that the carrier transport has to be taken into account for the interpretation of these experiments. Using electroluminescence and reciprocity relations, we calculate map of the External Quantum Efficiency with the Hyperspectral Imager. For this investigation a second setup is developed, using Light Beam Induced Current with different excitation wavelengths. Therefore mapping of the diffusion lengths is possible, exhibiting a distribution around 1.09 µm with standard deviation of 0.10 µm.