Series and Contact Resistance Measurements between 80 K and Room Temperature for Industrial Solar Cells

Cells with different front side pastes, processing parameters and substrate orientation were electrically characterized especially with regard to the series and contact resistance. Such cells were used for electrical measurements in an optical cryostat: VOC, dark current and I-V curves under illumination were acquired. Care was taken to provide optimum contacting of the cells and reproducible results could be obtained. Pastes of two different suppliers were processed and yielded significantly different microstructure and electrical properties. A large hysteresis in VOC of about 100 mV at 80 K was found for an overfired cell, the identical cell processed at optimum temperature yielded no hysteresis in VOC. Dark current measurements yielded differences for cells processed with different pastes, these differences increased with decreasing temperature. The temperature dependent series resistance was determined: at low temperatures (80 K) a metallic behaviour was seen that switched to semiconducting close to room temperature. From such measurements conclusions with respect to the current path can be drawn and are discussed. Planar cells were investigated for two different surface orientations, i.e. 〈100〉 and 〈111〉. Pyramidal shaped Ag crystallites form only in the 〈100〉 orientation, not in 〈111〉. The density of Ag colloids in the glass layer is significantly larger in the 〈111〉 orientation. The contact resistance of planar 〈111〉 cells was in the range of textured surfaces and was found to be smaller as compared to 〈100〉 orientation. This indicates that pyramidal Ag crystallites are not necessary for a low contact resistance and a percolative current path along the Ag colloids in the glass layer is proposed.