Effect of material inhomogeneity on the open-circuit voltage of string ribbon Si solar cells

The effect of material inhomogeneity on open-circuit voltage (V_OC) of string ribbon Si solar cells is investigated by a combination of experimental results and a simple analytical model. Light beam-induced current (LBIC) measurements showed that a cell with no detectable defective region gave an efficiency of 15.9% with a V_OC of 616 mV. However, a neighboring cell with highly defective regions covering 38% of its area, as determined by LBIC measurements, gave an efficiency of 14.1% with V_OC of 578 mV. Another cell with 19% highly defective regions gave an efficiency of 15.0% with V_OC of 592 mV. A simple and approximate analytical model was developed to quantify the loss in V_OC on the basis of recombination intensity and the area fraction of defective regions. This model showed that the majority of the loss in V_OC is associated with the most defective region, even if its area fraction is relatively small.