No abstract

Nine different types of shunt have been found in state-of-the-art mono-and multicrystalline solar cells by lock-in thermography and identified by SEM investigation (including EBIC), TEM and EDX. These shunts differ by the type of their I-V characteristics (linear or nonlinear) and by their physical origin. Six shunt types are process-induced, and three are caused by grown-in defects of the material. The most important process-induced shunts are residues of the emitter at the edge of the cells, cracks, recombination sites at the cell edge, Schottky-type shunts below grid lines, scratches, and aluminum particles at the surface. The material-induced shunts are strong recombination sites at grown-in defects (e.g., metal-decorated small-angle grain boundaries), grown-in macroscopic Si 3 N 4 inclusions, and inversion layers caused by microscopic SiC precipitates on grain boundaries crossing the wafer.

Multicrystalline standard p-type silicon solar cells, which undergo a potential induced degradation, are investigated by different methods to reveal the cause of the degradation. Microscopic local ohmic shunts are detected by electron-beam-induced current measurements, which correlate with the sodium distribution in the nitride layer close to the Si surface imaged by time-of-flight secondary ion mass spectroscopy. The results are compatible with a model of the formation of a charge double layer on or in the nitride, which inverts the emitter

Infrared lock-in thermography allows to image shunts very sensitively in all kinds of solar cells and also to measure dark currents flowing in certain regions of the cell quantitatively. After a summary of the physical basis of lock-in thermography and its practical realization, four types of quantitative measurements are described: local I-V characteristics measured thermally up to a constant factor (LIVT); the quantitative measurement of the current through a local shunt; the evaluation of the influence of shunts on the efficiency of a cell as a function of the illumination intensity; and the mapping of the ideality factor n and the saturation current density J 0 over the whole cell. The investigation of a typical multicrystalline solar cell shows that the shunts are predominantly responsible for deterioration of the low-light-level performance of the cell, and that variations of the injection current density related to crystal defects are predominantly determined by variation of J 0 rather than of n.

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.