Cost-effective and Reliable Copperplated Metallisation for Silicon Solar Cells: A Development Path

Abstract

Copper (Cu) plating can reduce the consumption of silver for silicon (Si) photovoltaic manufacturing, whilst also offering the potential to increase cell efficiency by way of reduced shading due to very narrow fingers and contacting Si surfaces without requiring high dopant concentrations. A cost-effective and reliable plating method for both n-type and p-type Si surfaces can make possible low-cost Cu metallisation for a range of different cell structures. A single side plating method called field-induced plating (FIP), also known as forwardbias plating, was developed to plate metal contacts to p-type Si surfaces. This process was used in combination with light-induced plating (LIP) to fabricate Cu-plated bifacial laser-doped Si solar cells with efficiencies of up to 19.2%. However two key performance limiting factors were identified: (i) increased recombination due to p-type laser-doping (which limited the fill factor of cells to 75%; and (ii) poor adhesion of the plated metal. The introduced recombination was analysed using injection-dependent lifetime analyses of quasi-steady-state photoluminescence and photoconductance (QSSPL and QSSPC) measurements. Although boron laser doping resulted in the local ideality factor increasing to ~1.5, the introduced recombination could not be analysed assuming a single Shockley-Reed Hall defect. Since some activation of the deep-level boron-oxygen (B-O) defect was suspected during cell processing, the recombination properties of this defect were analysed. The electron/hole capture cross-section ratio of the B-O defect was estimated to be 9.7±1.7 and 9.7±1.9 using QSSPL and QSSPC measurements, respectively, performed after belt furnace annealing, light soaking and regeneration of wafers that were symmetricallypassivated with SiNx and rapidly annealed after deposition. Surface roughness was identified to be a key factor for contact adhesion. It was shown that use of a UV ps laser to ablate the SiNx antireflection coating to form openings for a plated metal grid can result in average 180º pull test forces of 2.1 N/mm, substantially exceeding the industry benchmark of 1 N/mm. Pull forces were too low to be measured for laser-doped and ns UV laser ablated openings. However, due to different plating rates occurring across a cell in LIP, busbar pull forces were shown to not always be a good indicator for plated finger adhesion. Stylus-based finger adhesion measurements were therefore used to demonstrate how contact formation, plating rate and chemistry, grid geometry and annealing can all affect finger adhesion. This thesis presents a compelling case for metallisation metrology if reliable plated metallisation is to be achieved in an industrial environment.