Aluminium back-surface field doping profiles with surface recombination velocities below 200 cm/s

In this paper, the authors show that a Si solar cell Al back-surface field (BSF) doping profile is formed by a fast alloying and regrowth process in accordance with the Al-Si phase diagram, and not by solid-state Al diffusion. Al was screenprinted on 3 Ωcm monocrystalline CZ wafers. Subsequently a temperature treatment of 30 seconds was given at 850°C. The Al doping profiles were measured by SIMS, CV and Stripping Hall. The doping profiles show a peak concentration at a depth of around 10 μm and are clearly different from diffusion profiles. The recorded profiles are in good agreement with each other. The peak and surface concentrations are close to the solid solubilities of Al in Si at 850°C and at the eutectic temperature, respectively. The thickness of the profile correlates with the composition of the alloy liquid at 850°C and the amount of deposited Al. Surface recombination velocities between 130 cm/s and 170 cm/s were measured with photoconductivity decay (PCD), probed by microwaves. These results agree with numerical calculations using the measured profiles. The minority carrier lifetime and mobility in the Al doped BSF appear to be lower than in borium doped Si