abstract
Since decades, the POCl3 diffusion is the main process technology to form the pn-junction of industrial silicon solar cells. However, the understanding of the detailed phosphorus (P) diffusion mechanism is still an active research topic. Already in 1964 Kooi et al. [1] proposed that during the drive-in in nitrogen (N2) the P2O5 in the PSG reacts with Si from the wafer to SiO2 (which instantaneously dissolves in the PSG) and P which diffuses into the Si wafer, leading to very high P surface concentrations in the Si wafer. In recent years, new POCl3 diffusion recipes have been developed which apply oxygen (O2) during the drive-in, which limits the diffusion of phosphorus into the Si wafer. Hence significantly lower phosphorus surface concentration can be obtained, enabling very low J0e values close to 20 fA/cm2 [2,3,4] which was a major contributor to increased PERC conversion efficiencies in the past years. To explain the limitation of phosphorus diffusion by O2 atmosphere, two contradicting models have been proposed. Werner et al. [5] showed that the O2 from the atmosphere grows an interfacial SiO2 layer between the PSG and the Si wafer, with increasing SiO2 thickness with longer oxidation time. Hence, they propose that the SiO2 acts as a barrier against the diffusion of phosphorus into the Si wafer. In contrast, Li et al. [6] measured a constant SiO2 thickness of around 5 nm and suggested that free phosphorus in the PSG, which is formed by the reaction of Si with P2O5, is oxidized to P2O5 again by the O2 from the atmosphere. Hence, it remains in the PSG rather than diffusing as P into the Si wafer. One systematic difference between the two studies is that Werner et al. apply the O2 flow throughout the complete drive-in time whereas Li et al. shut down the O2 flow during drive-in. In this contribution, we aim at identifying the exact chemical mechanism for the limitation of the diffusion of phosphorus during drive-in in O2 atmosphere. We systematically vary the drive-in time in O2 atmosphere and additionally investigate the effect of a subsequent drive-in in N2.