Large-Area Diode Laser Defect Annealing of Polycrystalline Silicon Solar Cells

A new process was presented to anneal crystallographic defects in solid-phase crystallized silicon that produces higher Suns-V_oc voltages than the conventional belt-furnace annealing (BFA) process. A few-millisecond continuous-wave diode laser treatment anneals defects present in the polycrystalline silicon and activates dopants. It is shown that the silicon/glass-interface system reaches an effective steady state at every point during the laser treatment, and the relative temperature profile is determined by the laser beam profile rather than its power or scanning speed. The peak temperature that is reached during a few-millisecond laser exposure is shown to increase and then level off with laser dose, indicating partial recrystallization of the film. A reduction in boron-doped p-type sheet resistance and phosphorus-doped emitter resistance is reported for a wide range of laser dose and scanning speed. The highest substrate temperature required during the process sequence is reduced from 960°C to 620°C. A peak 1-sun voltage of 492 mV is achieved using a 4-ms exposure at 500-J·cm^-2 laser dose, which is an improvement of 32 mV over the voltage after optimized BFA.