O. Breitenstein, J. Bauer, J. M. Wagner, N. Zakharov, Max Planck Institute of Microstructure Physics, Halle, Germany; A. Lotnyk, Christian-Albrecht-University, Kiel, Germany

Summary: In a solar module about 36 cells are connected in series. Only if they all have the same illuminated current-voltage (I-V) characteristics, their generated voltages simply add up. However, if one of these cells is shadowed or broken, the other cells may bias this one in reverse direction to up to -18 V. If then a large reverse current flows, which may be as large as the short circuit current of the other cells (typically 7 A), this cell dissipates a lot of heat and the module may be destroyed thermally. According to its base doping concentration of 1016 cm-3, a solar cell should undergo avalanche breakdown at -50 V and above. However, in reality breakdown may occur already at -10 to -15 V. Therefore the electric breakdown behavior of solar cells is an important reliability aspect and must be studied in detail. In this investigation the dominant breakdown sites in industrial multicrystalline Si solar cells are localized by lock-in thermography (LIT) under reverse bias. Special LIT techniques allow us to image the temperature coefficient, the slope (steepness) of the breakdown current, and the avalanche multiplication factor locally. The distribution of grown-in recombination-active crystal defects is obtained by electro¬luminescence (EL) imaging. Two dominant breakdown mechanisms are identified, which are breakdown at recombination-active crystal defects, showing a relatively soft breakdown, and avalanche breakdown at dislocation-induced etch pits, which occurs very steep (hard breakdown) and dominates in our cells at high reverse bias.