Analysis of laser-fired local back surface fields using n+np+ cell structures

In order to investigate our new LFC process for the formation of local contacts on a dielectrically passivated rear surface, we have analyzed n/sup +/np/sup +/ structures processed identically to the standard n/sup +/pp/sup +/ cells except for the doping type of the base material. For these devices the p/sup +/-emitter is formed point-like at the rear surface by Al-alloying during the LFC process. The observation of voltages above 600 mV is a clear proof of the creation of a local Al diffusion. Since a pn-junction is much more decisive for a solar cell than a BSF, the n/sup +/np/sup +/ structures were used to understand the physics of the LFC contact and to optimize the laser parameters resulting in efficiencies of 20% for n/sup +/pp/sup +/ LFC cells in a resistivity range between 0.5 Omega cm and 10 Omega cm.