Evaluating the Aluminum-Alloyed $\hbox {p}^{+}$ -Layer of Silicon Solar Cells by Emitter Saturation Current Density and Optical Microspectroscopy Measurements

Surface-passivated and surface-unpassivated aluminum-alloyed p⁺-layers are characterized. By varying the firing conditions and the thickness of the screen-printed aluminum paste, different sheet resistances R_sh of the p⁺-layer were fabricated. The emitter saturation current density J_0e plotted versus R_sh follows distinctly different trends for the passivated and unpassivated samples. An aluminum paste with a boron additive achieves a much higher doping concentration and a lower sheet resistance but nevertheless follows the same J_0e curves as the pure Al paste. The aluminum p⁺-layer was quantitatively analyzed with microphotoluminescence and Fano-Raman measurements. The latter shows an increased defect recombination at the interface between the p⁺-layer and the moderately doped Si bulk. The lower Shockley-Read-Hall lifetime in this region can be attributed to a high defect concentration in the most highly doped layer, represents an impediment to the reduction of J_0e for Al-doped emitter regions, and needs to be optimized in future investigations.