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 ( R s ) 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 R s of emitter increased reversely after annealing at a low temperature range of 650–750 °C, which differs from the conventional phenomenon that the R s 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 × 10 21 cm −3 to 3.9 × 10 21 cm −3 after annealing at 700 °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 R s . For mass production, the average R s was improved to ∼75 Ω/sq when the diffused silicon wafers with 66 Ω/sq emitter were annealed at 700 °C for 20 min, and moreover the R s uniformity became better than that of the directly as-diffused silicon wafers with same level R s 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 Ω/sq and ∼75 Ω/sq emitter, respectively.