APPLICATIONS  |  CIGS MICROCELLS

RESULTS          ABSOLUTE CALIBRATION          GLOBAL IMAGING          RELATED PRODUCTS


 

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 (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.


[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).

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 cellsPhysical 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 cellsProgress 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.

RELATED PRODUCTS

IMAPL

From solar cells to advanced materials, our fast and all-in-one hyperspectral microscope IMA PL offers unmatched image and data quality.

IMAEL

Perfectly suited for the analysis of photovoltaic cells and semiconductors, IMA EL is a fast hyperspectral microscope for the characterization of materials by means of electroluminescence.

HYPERCUBE

The HyperCube™ will transform your microscope into a high resolution spectral imaging system, opening new research perspectives in biological imaging. Designed to fit commercial microscopes, cameras and a vast variety of excitation modules, The HyperCube™ gives access to the detailed composition of your sample.

TUNABLE LASER SOURCE

Our turn-key sources unite the flexibility of supercontinuum light sources to the incomparable out-of-band rejection of our optical filters, allowing easy and precise sample excitation or instrument calibration.