Metallization is a critical process in silicon solar cell manufacturing, forming the conductive pathways that collect and transport generated current. The primary metallization techniques include screen printing, plating (Ni/Cu/Ag), and advanced finger grid optimization. Each method has distinct advantages and challenges related to contact resistance, shading losses, and material costs, particularly with silver (Ag) as a dominant cost driver.
Screen printing remains the most widely used metallization technique due to its simplicity and scalability. A silver paste is deposited through a patterned screen onto the silicon wafer, followed by a firing step to form ohmic contacts. The paste typically contains glass frits that aid in etching the anti-reflective coating (ARC) and promoting adhesion. Contact resistance is influenced by paste composition, firing profile, and wafer surface doping. Excessive firing temperatures can lead to shunt paths, while insufficient firing results in high contact resistance. Shading losses arise from the trade-off between grid line width and coverage—narrower fingers reduce shading but increase series resistance. Advanced screen printing techniques, such as dual printing or stencil printing, enable finer line widths below 40 µm while maintaining low resistivity.
Silver consumption is a major cost factor, accounting for up to 10% of total module expenses. Strategies to reduce silver usage include:
- Low-ag paste formulations with higher conductivity fillers.
- Multi-wire designs that replace busbars with thinner wires.
- Partial plating, where screen-printed seed layers are thickened by plating.
Plating techniques, particularly nickel/copper/silver (Ni/Cu/Ag) stacks, offer lower material costs and improved conductivity compared to screen printing. The process begins with a thin seed layer (often screen-printed or deposited via physical vapor deposition) followed by electroless nickel plating to form a diffusion barrier. Copper is then electroplated for bulk conductivity, and a thin silver cap prevents oxidation. Plating achieves finer line widths (20–30 µm) with lower resistivity, reducing both shading and series resistance. However, challenges include potential copper diffusion into silicon, which degrades cell performance. The nickel barrier layer must be optimized to prevent this while maintaining low contact resistance.
Finger grid optimization is essential for minimizing power losses. Key parameters include finger width, spacing, and height. Narrower fingers reduce shading but require higher aspect ratios (height/width) to maintain conductivity. Advanced laser patterning or aerosol jet printing enables high-aspect-ratio grids with widths below 20 µm. Grid spacing must balance resistive losses (closer spacing) and shading losses (wider spacing). Computational modeling and light-beam-induced current (LBIC) mapping help optimize these parameters for specific cell architectures.
Contact resistance is a critical performance metric, influenced by interfacial properties between the metal and silicon. Heavy doping (e.g., via phosphorus diffusion for n+ contacts) reduces contact resistivity but can increase recombination losses. Laser-doped selective emitter (LDSE) structures localize high doping under metal contacts, improving contact resistance without compromising bulk passivation.
Shading losses are minimized by reducing finger width while maintaining conductivity. Light-trapping designs, such as bifacial cells or rear-contact architectures, further mitigate shading by moving contacts to the rear side. However, these require more complex metallization schemes.
Cost drivers extend beyond silver usage. Plating processes involve additional steps (seed layer deposition, plating baths, waste treatment), increasing complexity. Screen printing, while simpler, faces paste cost volatility. Copper-based solutions offer cost savings but require robust barrier layers to prevent contamination. Emerging techniques like copper electroplating with self-aligned barriers or nickel silicide contacts aim to balance cost and reliability.
In summary, metallization for silicon solar cells involves trade-offs between conductivity, shading, and cost. Screen printing dominates for its scalability, while plating offers superior performance at reduced material costs. Finger grid optimization and advanced doping techniques further enhance efficiency. Continued innovation in materials and processes is essential to drive down costs while maintaining high performance in silicon photovoltaics.