Influence of laser power on the properties of laser doped solar cells

In recent years, increased attention has been focused on the use of lasers in different fabrication steps of solar cells, in particular laser doping to form emitter and/or selective emitter. In this method the laser energy is used to melt silicon, allowing the diffusion of dopant atoms to occur in the liquid phase. The main advantage of this method is the localised nature of the laser beam, which melts and diffuses a limited area without heating the bulk, therefore reducing the possible degradation associated with high temperature processes. At the University of New South Wales a novel laser doping method was developed, which combines the formation of the selective emitter with a self-aligned metallisation pattern. Despite achieving high efficiencies, concerns arose regarding the adhesion of the metal to the shallow laser doped areas. This issue may be alleviated by increasing the roughness of the surface or even more so by creating holes/grooves in the laser doped areas. One simple way of achieving this is by carrying out the laser doping at higher laser energies to deliberately create some ablation. This paper examines the influence of the laser power on the solar cell electrical parameters to ascertain the relationship and the tradeoff between surface roughness and electrical performance. Efficiencies above 18% on a large area commercial grade p-type CZ substrate were achieved despite some ablation, confirming the potential for using this method to improve adhesion. Efficiency of 18.7% on the same substrate, using lower laser power, demonstrates the capability of the laser doping method.