Alloying from Screen-printed Aluminum Pastes for Silicon Solar Cell Applications

Abstract

Alloying from screen-printed aluminum pastes on silicon is a simple, reliable and cost-effective metallization technique, which is standardly applied for the rear contact formation of silicon solar cells in today’s production lines. Despite its long history and widespread utilization, however, there have still been open questions on the fundamentals of the alloying process, on the interaction of paste additives with the Al-Si system during alloying, and on the recombination characteristics of Al-alloyed contacts. This work has therefore dealt with investigating and quantifying alloying from screen-printed Al pastes on Si for solar cell applications in detail.The main achievements of this work are summarized in brief in the following:(1) Derivation of a comprehensive analytical model for alloying from screen-printed Al pastes which allows for the accurate calculation of the Al contact structure and the doping profiles of the Al-doped p+ Si (Al-p+) regions for a broad range of printing and firing conditions.(see chapter 2)(2) Detailed theoretical and experimental investigation of the recombination characteristics of Al-p+ regions with and without surface passivation. The influence of the printing and firing conditions on the recombination characteristics of the Al-p+ regions has been clarified and optimal conditions have been determined. Implied open-circuit voltages of up to 651 mV for non-passivated and 699 mV for surface-passivated Al-p+ regions have been realized.(see chapter 3)(3) Deduction of a comprehensive analytical model for alloying from screen-printed Al pastes containing boron additives. This model enables the precise calculation of the acceptor profiles of the Al- and B-co-doped p+ Si (Al-B-p+) regions and the investigation and optimization of their recombination characteristics. Excellent implied open-circuit voltages of 665 mV have been achieved for full-area Al-B-p+ regions without surface passivation for the optimal effective B percentage of 0.03 wt%.(see chapter 4)(4) Investigation of the structural and electrical properties of local contacts formed by full-area screen-printing and firing of Al pastes on locally opened dielectric layers. By intentionally adding Si to the Al paste, the recombination characteristics of the local contacts were improved, so that the effective rear surface recombination velocity was reduced down to less than a third of the value of a conventional Al paste.(see chapter 5)(5) Application of these results to improve the conversion efficiencies of n-type Si solar cells with screen-printed full-area or local Al-p+ rear emitters. Nickel plating has been demonstrated to be a promising technique for the front contact formation of these solar cells. Etching of highly phosphorus-doped n+ Si regions was implemented as an industrially feasible approach for the formation of the n+ front surface fields. Conversion efficiencies in the range of the highest values so far reported have been achieved.(see chapter 6)