Investigations on laser-fired contacts for passivated rear solar cells

Scope of this work was to investigate the properties of 'Laser-fired Contacts (LFC)', a technique developed at Fraunhofer ISE for implementing the point or line contact pattern of passivated rear silicon solar cells. Prior to cell processing test structures have been used to optimize the laser process. The solar cell results achieved using this optimized process on 0.5 omega cm and 2.0 omega cm 250 µm thick float zone wafers clearly indicate that a local back surface field is generated by laser alloying aluminum into the silicon. In comparison to photolithographically processed references with ohmic point contacts the LFC cells on 2 omega cm material show on average a 17 mV higher open circuit voltage and more than 3% higher fill factor. In addition to cell processing, LBIC measurements have been performed in order to control and to illustrate the laser-fired local aluminum back surface field.