Influence of nitride and nitridation on the doping properties of PECVD-deposited BSG layers

High efficiency p-type passivated emitter and rear (PERC) silicon (Si) solar cells are becoming industrial standard with demonstrated efficiencies above 22.6 %. Alternatively to the PERC design there is the possibility to apply a full-area boron back-surface-field (B-BSF) to the solar cell rear side for implementation of passivated emitter and rear totally diffused solar cells (PERT). In this work we investigate plasma enhanced chemical vapor deposited (PECVD) boron silicate glass (BSG) layers employing the precursor gases nitrous oxide (N2O), silane (SiH4) and diborane (B2H6). We experimentally demonstrate the impact of nitride (N) content in the BSG on the boron (B) doping efficiency within a thermal drive-in step, i.e. a subsequent high temperature diffusion step post BSG-deposition. It is found that the N content of the BSG which is mainly controlled by the N2O:SiH4 precursor gas flow ratio has to be kept as low as possible to achieve optimal B doping results of the Si bulk material. In addition an in-situ nitridation process has been developed to prevent build-up of a boron rich layer (BRL) on the highly doped Si surface. PERT and PERC solar cells were processed from material with different base resistivities and equal solar cell efficiencies were achieved.