Post annealing to improve the sheet resistance and uniformity of emitter for enhancing conversion efficiency of mass-produced mc-Si solar cells

The influence of annealing on the sheet resistance (Rs) and doping profile of P-N junction emitter and its correlation to the conversion efficiency of mass-produced mc-Si solar cells were investigated. It is found that the Rs of emitter increased reversely after annealing at a low temperature range of 650-750 degrees C, which differs from the conventional phenomenon that the Rs usually decreases with doping amount increasing during phosphorus (P) thermal diffusion process at a relatively high temperature. The doping profiles show that the shallow surface doping concentration was reduced significantly from 6.6 x 1021 cm-3 to 3.9 x 1021 cm-3 after annealing at 700 degrees C while the doping profile remained almost constant below the depth of 20 nm. The decreasing of surface doping concentration indicates that the P atoms distributed in the shallow surface of silicon should uphill-diffuse to the region near to phosphosilicate glass (PSG) layer, which reduces the total doping amount in emitter and consequently increases the Rs. For mass production, the average Rs was improved to -75 omega/sq when the diffused silicon wafers with 66 omega/sq emitter were annealed at 700 degrees C for 20 min, and moreover the Rs uniformity became better than that of the directly as-diffused silicon wafers with same level Rs value. Additionally, the solar cells exhibited a mean conversion efficiency of 18.63%, which was higher by 0.35% and 0.20% than those fabricated from the as-diffused silicon wafers with -66 omega/sq and -75 omega/sq emitter, respectively.