Copper indium gallium selenide (Cu(In,Ga)Se2 or CIGS) is one of the favorite candidates for thin films solar cells. CIGS possesses a high absorbance and a direct bandgap in addition to being stable under long-term illumination. An efficiency exceeding 20% from typical polycrystalline CIGS devices has been repeatedly reached by several research groups. Even if encouraging, this efficiency is still below the Shockley-Queisser theoretical limit. This can in part be attributed to the cells inhomogeneities coming from its polycrystalline nature that also blurs the relationship between global performances and materials properties. To quantify the impact of the morphology on the cell efficiency, studying the spatial variations of its different properties is paramount.
With this in mind, researchers at IRDEP (Institute of Research and Development on Photovoltaic Energy) investigated a CIGS microcell (diameter of 35 μm) through spectrally and spatially resolved photoluminescence (PL) and electroluminescence (EL) imaging . To carry out such experiments they used an hyperspectral imager (IMA™) with a 2 nm spectral resolution and a spatial resolution below 2 μm. A sourcemeter was employed for EL with Vapp = 0.95 V. A 532 nm laser (Genesis laser, Coherent) was used for PL (excitation of 580 suns). The entire field of view under the microscope objective was excited and the PL signal coming from a million point was collected simultaneously (see global imaging modality section below for more details).
Fig 1 (a) and (b) present PL and EL images of the CIGS microcell. By combining their spectral and photometric absolute calibration method (see section below), with the generalized Planck’s law, researchers at IRDEP were able to extract the quasi-fermi level splitting (Δμeff) (see FIG. 1 (c) and (d)) which is directly related to the maximum voltage of the cell. With the help of the reciprocity relation between solar cells and LEDs, the external quantum efficiency (EQE) can be deduced from the EL images.
These results show that fundamental properties of microcells such as quasi-fermi level splitting and, potentially, external quantum efficiency are accessible on a micrometer scale over the entire surface of the sample.
 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, 10, 1002, (2014).