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 cells, Science, 345(6196), 2014.
[2] Zhao J et al., 19.8 % efficient ‘honeycomb’ textured multicrystalline and 24.4 % monocrystalline silicon solar cells, Applied 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 cells, Progress in Photovoltaics, 2014.
[4] A. Delamarre et al., Contactless mapping of saturation currents of solar cells by photoluminescence, Applied Physics Letters, 100, 2012.
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 cells, Physical 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 cells, Progress in Photovoltaics, (2014).
Hyperspectral characterization of next-generation solar cells and LEDs
This video shows how spectrally and spatially resolved PL and EL maps can help identify defects, losses, and uniformity in advanced materials. A hyperspectral photoluminescence demonstration is performed on large grain perovskite crystals.
