Advanced metallization for screen-printed solar cells

Solar cells with screen printed metallization have been dominating the PV market for decades due to thewell understood and robust fabrication process and the readily available production equipment. However, the efficiency of this type of solar cells is limited by some fundamental weaknesses such as 1) high metal shading; 2) high recombination at front contact regions and rear surface; and 3) parasitic light absorption at the rear surface. This thesis investigates methods to improve front and rear metallization to overcome these limitations and increase the cell efficiency. On the front side, the development of a novel screen-printing based metallization scheme is presented. An efficiency gain of 1.1% absolute is demonstrated on this advanced semiconductor finger structure compared to a conventional screen printed solar cell fabricated under the same environment. This is achieved by incorporating laser doping and metal plating to reduce shading and recombination losses, while maintaining the large current carry capacity and solderability of screen printed metal. Further investigation on the voltage potential of the advanced semiconductor finger front shows over 700 mV implied VOC and 680 mV real VOC can be achieved on test structures using solar grade p-type CZ wafer. On the rear side, the thesis investigates a passivated and locally contacted structure using laser to open the dielectric layer followed by screen printing to form contacts. Optimization of the process conditions to minimize the Kirkendall void formation while maintaining an adequate thickness of localized back surface field is presented. Cell efficiency of over 20% is demonstrated on solar grade p-type CZ substrate. An alternative approach is investigated in which deep boron laser doping is incorporated to pre-form the localized back surface field to avoid the trade-off between Kirkendall voids and adequate localized back surface field formation. Using this process, it is shown that lower firing temperature and conventional Al paste can be used without any efficiency drop