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HYPERSPECTRAL STUDY OF HYBRID PEROVSKITE PHOTOVOLTAIC DEVICE

3D perovskite solar cells based on organic-inorganic lead halide have received a great deal of attention in recent years. Their absorption is excellent from UV (200 nm) to NIR (700 nm), they have a high carrier mobility (comparable to silicon) as well as a large diffusion length of carriers. Their quantum efficiency has quintupled in the past five years: from 4% in 2009, they now reach near 20% [1]. Even if this progress is impressive, a deep understanding of the fundamental mechanisms of these systems is still missing.

With this in mind Dr. Henk Bolink from the University of Valencia, in collaboration with researchers at IRDEP (Institute of Research and Development on Photovoltaic Energy, France), investigated the performance of hybrid organic–inorganic methylammonium lead iodide perovskites (CH3NH3PbI3) solar cells. CH3NH3PbI3 perovskite shows efficient photocurrent generation in addition to important photoluminescence, hence it is a great candidate for multi-functional devices [2]. In order to study the main properties of this cell, spectrally and spatially resolved photoluminescence (PL) and electroluminescence (EL) imaging was performed. To carry out such experiment, an hyperspectral imager (IMA™) with a 2 nm spectral resolution and a spatial resolution below 2 μm was used. A sourcemeter was employed for EL and the applied voltage ranged from 1.05 V to 1.2 V. A 532 nm laser (Genesis laser) was used for PL (excitation of ~30 suns) to illuminate the entire field of view under the microscope objective and the PL signal coming from millions of points was collected simultaneously (see global imaging modality section below for more details).

Maps of a few hundreds of micrometers were obtained within minutes. With the help of the spectral and photometric absolute calibration procedure (see section below) developed by IRDEP, it is possible to determine the absolute number of photons emitted from every point of the sample at every wavelength. This unique feature allows researchers to obtain a map of the quasi-Fermi level splitting. This key parameter is of scientific importance since it is directly related to the maximum achievable voltage and saturation currents.

With this instrument, spatial inhomogeneities of optoelectrical properties are rapidly highlighted, allowing researchers to verify their fabrication methods while getting a better understanding of the fundamental properties of their device. 

A peer-reviewed article based on these results will soon be available. If you would like to discuss about the abovementioned results, do not hesitate to contact us: info@photonetc.com.

[1] Zhou H. et al., Interface engineering of highly efficient perovskite solar cells, Photovoltaics, 345(6196), (2014).
[2] Gil-Escrig L., et al, Efficient photovoltaic and electroluminescent perovskite devices, ChemComm, 15(569), (2015).

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.