On Monday 20 August and Tuesday 21 August, Dr. Otwin Breitenstein from the Max Planck Institute of Microstructure Physics in Germany will host three talks on his work on implications of inhomogeneities on the IV characteristics of solar cells and luminescence and lock-in thermography imaging techniques for quantitative and qualitative solar cell characterisation.
Session 1: Luminescence Imaging of Solar Cells - New Developments
In this talk our contributions in the last 4 years to further developing quantitative luminescence imaging methods for solar cells are reviewed. First it is explained by 2D device simulations and a simple model why previous methods for J01 imaging by PL have led to erroneous results. This is due to the model of independent diodes applied in these methods, which does not consider horizontal balancing currents and leads to erroneous local current distributions. On the other hand, DLIT-based "Local I-V" investigations, which also rely on the model of independent diodes, lead to a correct but blurred J01 distribution, since here the current is measured more directly. Two alternative PL based J01 imaging methods will be introduced, which are the Laplacian evaluation and the nonlinear Fuyuki evaluation. These methods lead to high-resolution J01 images, which are quantitatively equivalent to DLIT-based J01 images. This, however, can only be realized if some more improvements for evaluating luminescence images are applied, which will also be introduced here. These are the correct imaging of the calibration constant and correctly regarding photon scattering in the detector, for which appropriate methods will be introduced. For nonlinear Fuyuki evaluation moreover band-pass light filtering and vignetting correction must be applied. A new method combining Laplacian and nonlinear Fuyuki evaluation will be proposed. Most recent investigations of illumination intensity-dependent recombination at certain grain boundaries will be introduced, which lead to an apparent ideality factor of the luminescence at these defects smaller than 1. Finally, it is described how a Griddler model of an inhomogeneous solar cell can be constructed, including an inhomogeneous distribution of J01, Rgrid, and Rcontact, solely based on luminescence imaging results.
Session 2: Lock-in Thermography - Special Investigation Techniques
Lock-in thermography (LIT) is the technique of choice for imaging inhomogeneous dark current densities. Therefore dark LIT (DLIT) is mostly used for qualitative shunt imaging, both under forward and reverse bias. However, both DLIT and illuminated LIT (ILIT) can be used for a number of other quantitative imaging and analysis methods on solar cells, which will be introduced in this talk. This is first the DLIT-based 'Local I-V' method. Here DLIT images taken at several biases are converted into current density images, and each image pixel is fitted to a two-diode model. This leads to images of J01, J02, n2, and Gp = 1/Rp, which are converted by the software into images of efficiency-relevant cell parameters. Dark and illuminated I-V characteristics can be simulated for certain regions and for the whole cell, leading to predictions of local and global efficiency parameters. By these investigations the influence of certain regions, like the cell edge, shunts, or crystal defect regions, on the efficiency can be estimated realistically for various illumination intensities. An alternative efficiency imaging method is based on ILIT investigations. This method has certain advantages but also certain limitations compared to DLIT-based 'Local I-V', which will be discussed. Both ILIT and DLIT can be used to image the short circuit current density Jsc, if the global short circuit current is known. This Jsc distribution is needed for performing a realistic efficiency simulation of mc cells, a corresponding option is included in the 'Local I-V' software. Finally, a new DLIT method for performing depth-dependent investigations will be introduced.
Dr. Otwin Breitenstein received his Ph.D. in physics from University of Leipzig (Germany) in 1980 with a work on Deep Level Transient Spectroscopy (DLTS). Since 1992 he is with Max Planck Institute of Microstructure Physics, Halle, Germany, where his group investigates defects in semiconductors. Since 1999 he has been using lock-in thermography, optical imaging methods, and electron microscopy, for characterizing efficiency-limiting defects in crystalline silicon solar cells. He is mainly interested in detecting internal shunts and generally evaluating the local efficiency of inhomogeneous silicon solar cells. Besides lock-in thermography he is also using and further developing luminescence imaging methods (electro- and photoluminescence) for solar cell characterization. In his group many new mechanisms being responsible for locally reduced efficiency and local pre-breakdown in solar cells have been identified. He is Assistant Professor at University of Halle, Germany, giving lectures on the physics of solar cells and on advanced solar cell and material characterization techniques. He is author of several hundred publications in journals and at conferences and author of a book on "Lock-in Thermography" (Springer 2003, second edition 2010, third edition will appear in 2019).