Shunt defects influence the output performance of a solar cell or a module negatively. Extensive studies of their origins are necessary to reduce parasitic power dissipation in solar cells. Simple measurement techniques are desirable to distinguish between fabrication-induced shunts and shunts caused by electrical breakdowns, e.g. due to opacity. We investigated shunts in a micromorph silicon thin film solar module by means of dark (DLIT) and illuminated lock-in thermography (ILIT). We derived their electrical characteristics from thermography images by varying the bias voltage. A classification of the defects indicated several types of shunts with different electrical properties. For optimisation processes of solar cells or quality management issues it is important to know the exact location of a shunt or defect. State-of-the-art thermography approaches enable only a lateral localisation of shunts. This is sufficient for single junction solar cells, but not for multi junction solar cells like micromorph tandem solar cells, where a shunt can occur in the amorphous or microcrystalline layer or both. We present a measurement technique based on ILIT which relates a shunt to its original layer in a tandem solar cell. To reveal the actual structure of the investigated shunts, we milled trenches through them by using a Focused Ion Beam (FIB) and imaged their cross-sections by scanning electron microscopy (SEM).
展开▼
