Influence of copper contamination on the illuminated forward and dark reverse current-voltage characteristics of multicrystalline p-type silicon solar cells

We studied the influence of copper (Cu) on the performances of conventional photovoltaic (PV) solar cells by growing two multicrystalline (mc) boron‐doped silicon (Si) ingots from ultra‐pure feedstocks, one of these feedstocks being deliberately contaminated with 90 ppm wt of Cu. Industrial‐like solar cells were fabricated and the associated external gettering and hydrogenation effects were studied. An originality of our approach consisted in evaluating the forward but also reverse current‐voltage (I‐V) characteristics of the fabricated cells. Furthermore we assessed the stability under illumination of the PV parameters as Cu is known to be responsible for light‐induced degradations (LID) of the carrier lifetime. On the one hand we unexpectedly showed that the PV conversion efficiency (η) was not affected by the initially large Cu concentrations. We demonstrated that it was due to the complementary actions of the external gettering effect developed by the phosphorus‐diffusion and the bulk hydrogenation. The Cu‐addition slightly enhanced the pn junction hard breakdown, however the extracted junction breakdown voltages fulfilled the common industrial requirements for this parameter. On the other hand we highlighted significant decreases under illumination of the PV performances for the Cu‐contaminated solar cells fabricated from wafers coming from the upper part of the ingot (i.e., samples with the highest Cu concentration). These decreases could be explained by the previously proposed mechanisms in the literature, which argue that the excess charge carriers could reduce the electrostatic repulsion between interstitial Cu ions and Cu precipitates, this effect enhancing the Cu precipitation. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)