APPLICATIONS  |  PHOTOVOLTAIC

APPLICATION NOTES          ABSOLUTE CALIBRATION          GLOBAL IMAGING          RELATED PRODUCTS


 

The field of photovoltaic keeps expanding and the variety of solar cells has never been that broad: from dye-sensitized and organic cells to thin films and multi-junctions cells, photovoltaic materials have seen their efficiencies grow considerably with the latest progress. Perovskite solar cells now reach near 20% [1] efficiency, approaching monocrystalline silicon [2]. Despite the important advances in this field, large scale commercialization of most of the newcomers proves to be challenging. This can be in part attributed to non-uniformities in materials and difficulties to economically process cells over large scale. In order to bring to the market the next generations of solar cells, researchers require performant and specialized measurements systems. 

Researchers have to be able to see the big picture and study the spatial distribution of their materials’ properties. To answer that need, Photon etc has developed a hyperspectral imaging platform (IMA™) for solar cells analysis in collaboration with IRDEP (Institute of Research and Development on Photovoltaic Energy, France). This platform provides rapid electroluminescence (EL) and photoluminescence (PL) maps allowing spatial study of defects, constraints and optoelectronic properties. The system was compared to a classical confocal microscope, showing significant gains in acquisition time. 


Photoluminescence and electroluminescence mapping provide rapid quality control and allow studies of fundamental properties of photovoltaic films or devices. Those techniques have already been successfully used to characterize inhomogeneities in CIS and perovskite solar cells, and to obtain spatial distribution of the quasi-fermi level splitting (Δμ) and the external quantum efficiency (EQE) of CIGS [3] and GaAs [4] solar cells. To obtain the latter, a patented method of absolute spectral and photometric calibration was developed.

For more details and results from our collaborators, see the application notes below.

REFERENCES

[1] Zhou HP, Chen Q, Li G, Luo S, Song TB, Duan HS, Hong ZR, You JB, Liu YS and Yang Y, Interface engineering of highly efficient perovskite solar cellsScience, 345(6196), 2014.
[2] Zhao J et al., 19.8 % efficient ‘honeycomb’ textured multicrystalline and 24.4 % monocrystalline silicon solar cellsApplied Physics Letters, 73, 1998.
[3] Delamarre A. , Paire M., Guillemoles J.-F.  and Lombez L., Quantitative luminescence mapping of Cu(In,Ga)Se2 thin-film solar cellsProgress in Photovoltaics2014.
[4] A. Delamarre et al., Contactless mapping of saturation currents of solar cells by photoluminescenceApplied Physics Letters, 100, 2012

MATERIALS

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 (~1μm2), 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.

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.

 

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.

PHySpec™ SOFTWARE

PHySpec is our proprieraty software that controls all Photon etc’s devices and supported cameras. It is easy to install and presents a user friendly interface. Its multithreaded architecture is a key feature when processing in parallel complex algorithms, acquiring, visualizing, importing and exporting data in real time.

GRAND-EOS

GRAND-EOS combines a hyperspectral microscopy system with a hyperspectral wide-field imaging platform, giving access to micro and macro modalities with both VNIR (400-100 nm) and SWIR (900-1700 nm) spectral ranges.