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 [1]. 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 (Δμ_eff) (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.

[1] 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).

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.