Nickel/Copper Plated Contacts as an Alternative to Silver Screen Printing for the Front Side Metallization of Industrial High Efficiency Silicon Solar Cells

摘要

Solar power generation is largely dominated by photovoltaic (PV) systems which directly convert the incident sun irradiation into electricity. While rapidly declining prices are opening new opportunities for PV, further reductions in manufacturing costs are essential as nearly all PV manufacturers (wafer, cell, modules) experienced losses in 2012. As most of a PV system cost is area related, the highest impact on cost can be achieved by increasing the efficiency of the solar cells in the PV modules while reducing manufacturing costs. This thesis aims at replacing conventional silver (Ag) screen printed (SP) front side contacts by nickel/copper (Ni/Cu) plated contacts in industrial high efficiency silicon solar cells. It is motivated not only by the limitations that SP-Ag front side contacts have regarding solar cell efficiencies (high shading losses, limited line conductivity, and poor contact resistance to moderately doped junctions), but also by the PV industry’s desire to reduce Ag usage to below 50 mg/cell for cost reasons by 2017.Despite the potential advantages of Ni/Cu contacts, their commercialization has so far been limited with the notable exception of BP Solar between the years 1992 and 2008. Reasons for the limitation include the increased process complexity, the availability of suitable low-cost production techniques/tools at that time, and doubts over the cost advantage and long-term reliability. To address these issues, a relatively simple process sequence to define self-aligned Ni/Cu plated front contacts has been developed in this thesis which required to clarify the interactions between front emitter profile, front dielectric(s) patterning, metal deposition, and nickel silicidation. High average solar cell efficiencies ~20.5% (109 cells) with a tight distribution were obtained when applying this sequence to 156x156 mm2 p-type PERC cells and using more industrial plating techniques/tools that were not available to earlier Ni/Cu adopters like BP Solar. First PV modules made from similar cells passed 1.5x thermal cycling and damp heat testing as defined in IEC61215 and accelerated thermal ageing tests indicated that long-term reliability (25+ years at 85˚C) is feasible. The cost to define Ni/Cu plated contacts with this sequence was calculated to be ~4.4€c/cell cheaper than the one for SP-Ag contacts which makes it one of the few technologies that can improve both the efficiency and the cost per cell of the technology it aims to replace.In parallel, Ni/Cu plated contacts were applied to rear emitter n-type PERT cells and a novel silicidation technique based on excimer laser annealing (ELA) was investigated. For the former, efficiencies up to 20.5% were demonstrated in a first trial and a power-loss analysis was conducted which confirmed their higher efficiency potential compared to p-type PERC cells. Even more promising results were obtained when applying ELA to hybrid n-type PERT cells based on a heterojunction rear emitter.