Emitter profile tailoring by gas flux variation in tube furnace PoCl3-diffusion and analysis of the phosphosilicate glass

We analyze possibilities and difficulties of forming POCl3-diffused emitters for industrial application with low recombination by reducing the maximum phosphorus concentration in the emitter. For this purpose we vary the concentration of reactants in the process tube furnace. Both life time samples and solar cells are processed with reference emitters and emitters of the proposed variation. During processing we take special care of a detailed characterisation of the phosphosilicate glass and assess its basic properties. We observe a gain in cell efficiency of 0.2% absolute on standard screen-printed silicon solar cells fabricated with the improved emitters. Diffusions with low concentration of reactants provide higher open circuit voltages and efficiencies. The fill factor level is maintained, since the contact resistance does not increase although the peak doping reduces from ~8 to ~5x1020cm-3. A correlation between peak doping and saturation current is observed. Increasing deposition time in expense of drive-in time allows mitigating the negative impact on sheet resistance homogeneity and homogeneity along the processing boat (suitable for 200 Wafers).