Extensive investigations on industrial multicrystalline silicon solar cells have shown that, for standard 1 X cm material, acid-etched texturization, and in absence of strong ohmic shunts, there are three different types of breakdown appearing in different reverse bias ranges. Between À4 and À9 V there is early breakdown (type 1), which is due to Al contamination of the surface. Between À9 and À13 V defect-induced breakdown (type 2) dominates, which is due to metal-containing precipitates lying within recombination-active grain boundaries. Beyond À13 V we may find in addition avalanche breakdown (type 3) at etch pits, which is characterized by a steep slope of the I-V characteristic, avalanche carrier multiplication by impact ionization, and a negative temperature coefficient of the reverse current. If instead of acid-etching alkaline-etching is used, all these breakdown classes also appear, but their onset voltage is enlarged by several volts. Also for cells made from upgraded metallurgical grade material these classes can be distinguished. However, due to the higher net doping concentration of this material, their onset voltage is considerably reduced here.

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Electroluminescence spectra of different types of crystalline silicon solar cells obtained under reverse bias are analysed on a macroscopic as well as on a microscopic scale. The calibrated spectra of all samples exhibit a dominant peak at the band-gap energy E g % 1.1 eV. Although the fraction of visible light may vary over one order of magnitude, the shapes of the calibrated spectra are qualitatively similar. Single emission sites have been investigated with respect to the externally applied voltage V and their onset voltage V O by a microscope attached to a spectrometer. While the fraction of visible light within the spectra of individual sites increases with higher absolute onset voltage V O of the respective site, the actual applied voltage V has no influence on the shape of the spectra. Furthermore, we find that the emission intensity of all investigated single sites increases linearly with the applied voltage V. A nonlinear increase of local emission intensities as reported in the literature results from consecutive appearance of distinct breakdown sites in close distance to each other. Such series of breakdown events are not resolved without using microscopic measurements and therefore mistaken as the illumination intensity of a single site.

This work presents a method for extracting the absolute local junction voltage of a‐Si:H thin‐film solar cells and modules from electroluminescence (EL) images. It is shown that the electroluminescent emission of a‐Si:H devices follows a diode law with a radiative ideality factor nr larger than one. We introduce an evaluation method that allows us to determine the absolute local junction voltage in cases of nr > 1, while existing approaches rely on the assumption of nr = 1. Furthermore, we find that the experimentally determined values of nr vary from sample to sample. It is also explained why the derived radiative ideality factor is influenced by the spectral sensitivity of the camera system used in the experiment. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Approach to the physical origin of breakdown in silicon solar cells by optical spectroscopy J. Appl. Phys. 108, 123703 (2010) Calibrated microscopic measurements of electroluminescent emission spectra of reverse biased multi-crystalline silicon solar cells in a wide range of photon energies E (0.8 eV E 4 eV) are reported. The observed spectra originating directly from point-like sources exhibit a broad maximum around 0.8 eV followed by a high photon energy tail. A model for intraband emission accurately fits microscopically measured spectra obtained from single point sources. Furthermore, we do not find significant features from interband recombination. From the fits to the intraband transition model, we extract an effective charge carrier temperature of around 4000 K for all investigated spots. The analysis also yields the different depths of the sources, which are shown to be consistent with the dimension of the space charge region. From the areas around the point sources, we observe indirect emission of internally reflected light. Due to the multiple paths through the wafer, this indirect emission exhibits a maximum at a photon energy slightly lower than the band gap energy E g . We demonstrate that global, non-microscopic measurements are strongly influenced by this indirect radiation and therefore prone to misinterpretation. V C 2013 AIP Publishing LLC.[http://dx