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.
The Role of Inhomogeneities for Understanding Current-Voltage Characteristics of Solar Cells
All solar cells show inhomogeneous electronic properties. These inhomogeneities degrade the conversion efficiency of the cells. This holds in particular for multicrystalline (mc) silicon cells, where local differences of the lifetime of more than an order of magnitude exist. This contribution summarizes our research in this field in the last two decades. It explains how these inhomogeneities can be imaged and quantified, and the physical origins and the efficiency degradation potential of J01, J02, and ohmic current inhomogeneities are reviewed. Recent STEM investigations have revealed that the dominant defect-induced recombination in mc material is due to so-called Lomer dislocations, dominating the recombination in small-angle grain boundaries. J02 currents are flowing in positions where extended defects like scratches or the cell edge are crossing the pn-junction. Therefore J02 currents are always highly localized, in contrast to J01 currents. Also the nature of ohmic currents, which are also always localized, is reviewed. Hence, for describing most of the area of silicon solar cells, a one-diode model is sufficient, but J02 and ohmic currents reduce the efficiency in particular at low illumination intensity. Examples for quantitatively estimating the degradation potential of local J01, J02, and ohmic currents are given for two typical solar cells under two illumination intensities. Finally, different pre-breakdown mechanisms are reviewed. Except of the three previously known mechanisms, one new mechanism is described, which is dominant for monocrystalline cells.
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).