Electroplating of plasma doped back surface contact silicon solar ells

Crystalline silicon photovoltaic (PV) industry is growing at an average rate of ~ 15%. Continuing carbon-based fuel depletion in combination with increasing green house effects will continue to add to this robust growth trend. Conservative estimates indicate that PV market will reach ~ 100 GW_p/year before the year 2020. In order to sustain such production levels, impact on materials and supplies supporting industrial manufacturing practices must be examined. For example, the industry has steadily moved to thinner (~ 150 ?m) wafers in order to save on Si costs. For these thin wafers, conventional screen printing process to form Al-alloyed back surface fields will become unsuitable due to thermal expansion mismatch leading to wafer warpage and breakage; the cost of Al will also become a factor. Therefore, alternate low-cost and low temperature processes will be required to sustain cost-effective manufacturing. Low-energy plasma implantation processes with electroless Ni as seed layer followed by electroplating Cu are attractive alternatives for replacement of water and chemical intensive semiconductor processing and screen printing respectively. While the transition to front surface in a typical n/p/p+ solar cell is relatively straightforward, the formation of low-cost, back surface field represents a technical challenge in terms of p⁺⁺ doping. We report on our efforts at developing of low-energy, low-cost plasma implantation methods to form boron-doped back surface fields. We report on replacement of Al BSF with localized boron-BSF Al contacts using conventional screen printing and firing processes. Performance of the boron-doped solar cells was superior to the conventional Al BSF solar cells. Preliminary electroless Ni was deposited followed by annealing and Cu electroplating; electrical characterization work is in progress with promising initial results. All solar cell work was carried out on 6-inch diameter p-Si wafers with POCl₃ d- oping and PECVD SiN passivation; typical efficiencies were ~ 15 %.