Investigation of carrier lifetime in p-type Cz-silicon: specific limitations and realistic prediction of cell performance

Recent studies have revealed that the metastable defect causing the lifetime degradation in standard boron-doped Czochralski silicon (B-Cz-Si) is correlated with boron and oxygen. This is confirmed by the authors´ results from a comprehensive investigation of carrier lifetime in p-type Cz-Si. While no degradation and excellent lifetimes close to the theoretical limit are observed for all gallium-doped samples, the stable degraded lifetime of the B-Cz-Si samples is strongly reduced by the Cz-specific defect. In order to allow realistic simulations of cells from B-Cz-Si, the measured doping dependence of the bulk lifetime in B-Cz-Si is modeled by a simple empirical expression. It is demonstrated that the optimal doping concentration leading to maximum efficiency is strongly shifted with the used solar cell structure and the actual degradation state. While the maximum stable efficiency for a high efficiency RP-PERC solar cell is predicted for a base doping of 5.10¹⁵ cm^-3 using B-Cz-Si, the optimal value for an industrial cell with screen printing emitter varies between 10¹⁵ cm^-3 for a cell with boron BSF and 10¹⁶ cm ^-3 for a cell without BSF. The physical background leading to these optima is discussed in detail. Finally, the authors verified the theoretical predictions for the high efficiency cell structure fabricating RP-PERC cells on B-Cz-Si