Selective emitter by laser doping from phosphosilicate glass

In the industrial fabrication of silicon solar cells, lowly doped emitters offer an opportunity to increase cell efficiency by improving blue response as well as lowering the saturation current density of the device. However, conventional approaches of metallization, like screen-printed contacts, combined with shallow emitters result in the problem of firing through the emitter and thus shunting the cell. Employing a highly doped region underneath the front contacts, a selective emitter, can avoid such shunting. Laser doping offers the possibility to tailor a diffusion profile by a simple laser process and create a selective emitter. J0E values of lowly doped emitters are presented and their impact on cell performance is shown. Two solid-state laser systems have been used to fabricate a laser doped selective emitter. Two different kinds of metallization are employed in the solar cell process and compared. The internal quantum efficiency shows an improved response in the short wavelength region. Cell efficiencies of >17.7 % on 50x50 mm² cells with FF over 80 % using a shallow 120 W/sq emitter and a laser doped selective emitter have been achieved in an industrially feasible process.