Upgraded metallurgical grade silicon solar cells : a detailed material analysis

Abstract

Multicrystalline p-type wafers from 100% UMG-Si feedstock were used for two cell processes. The best screen printed solar cells reached an efficiency of 16.2% and fill factors up to 79.7%, while the buried contact cells reached an efficiency of 15.5%, caused by a rather low Voc value of 613 mV. Further analysis of the internal quantum efficiency indicated a low effective diffusion length. Detailed analysis of the lifetime variation within the wafer indicates that the passivation of grain boundaries for the screen printed solar cells is better than for the buried contact solar cells. In a second experiment, screen printed solar cells from different ingot heights were characterized with IV, LBIC (Light Induced Beam Current) and EL (electroluminescence) measurements. Cells from the edge and the centre both showed the similar behaviour. The resistivity increases with ingot height which can be explained by the lower net doping. This leads to a decreasing Voc. The IQE and the jsc increase with ingot height which indicates less recombination. In the highest part of the edge brick we observed patterns in the IQE and series resistance maps, possibly caused by localized impurities or doping variations.