Reduction of the phosphorous cross-contamination in nip solar cells prepared in a single-chamber PECVD reactor

A new approach to reduce phosphorous contamination in the intrinsic layer during the deposition of amorphous silicon (a-Si:H) nip solar cells prepared in single-chamber reactors is presented. This novel process consists of a hydrogen etching plasma performed after the n-layer deposition, which prevents a recycling of phosphorous from the reactor walls when exposed to a hydrogen-rich plasma during the subsequent i-layer deposition. The implemented process reduces the phosphorous cross-contamination in the i-layer, as corroborated by secondary ion mass spectroscopy measurements. Furthermore, the end of the etching process can be easily monitored by measuring the DC bias voltage at the powered electrode. By applying this process, we were able to improve the fill factor from 70% up to 75%, without degradation in the other parameters of the cell, neither in the initial nor in the stabilized state. Finally, by implementing this process in a-Si:H/a-Si:H tandem solar cells we obtained an initial efficiency of 10.3% (Voc=1.76 V, FF=74.5%, Jsc=7.8 mA cm-2); light soaking test resulted in a stabilized efficiency of 8.5%.