Identification of factors reducing Voc in MC silicon solar cells

Dynamical Precision Contact Thermography has been used to map the forward current in the dark of 10×10 cm² sized multicrystalline solar cells made of block-cast silicon material. Moreover, local I-V-characteristics have been measured thermally. Extended regions of increased current density as well as local shunts at the edges, under, and between grid lines have been observed. In shunt positions the cells have been investigated in detail using scanning electron microscope techniques. Only some of the local shunts are related to accumulations of grain boundaries, others are pn-junction defects. The dominant shunts often have been found at the edges of the cells. The dependence of the shunt strength on elastic deformation of the cells, which is sometimes observed, indicates that mechanical stress may influence certain shunts. The quantitative influence of shunts on the efficiency is shown to increase to above 30% for illuminations below 0.2 suns