Laser doping from spin-on sources for selective emitter silicon solar cells

Abstract

Development of laser doping process for the formation of a selective emitter (SE) for p-type and n-type silicon solar cells is presented. The SE is formed by laser doping of spin-on dopant sources using an intermediate barrier layer (BL). The BL serves to form shallow emitter and also offers advantage to avoid etch back step. The shallow emitter is formed by applying a controlled thermal diffusion step, which in turn reduces the laser induced defects in the SE. This process has an advantage that the shallow and selective emitters can be formed from a single dopant source. In this investigation, PECVD deposited SiOx was used as the barrier. KrF excimer laser at 248 nm was used for the selective doping. The dopant concentration and depth, as measured by SIMS, were controlled by variation of the laser parameters and barrier thickness. It was found relatively lower thickness PECVD deposited SiOx barrier layer with high dopant content in the spin-on layer at comparably low laser fluences resulted in the best electrical results. The SiOx layers acted as perfect barrier for the boron diffusion. It was also observed that multiple laser annealing above a threshold laser fluence resulted in the redistribution of the dopant along with deepening of selective emitter because of the limitedness of the dopant source. Also, this is attributed to the increase of the total absorbed energy by the successive laser pulses. The results were discussed and presented in detail.