Photovoltaic (PV) module manufacturing involves several critical steps that transform individual solar cells into durable, high-performance panels ready for installation. The process includes tabbing and stringing, lamination, framing, and junction box integration, each requiring precision and stringent quality control. Industrial-scale production also emphasizes sustainability, particularly in material selection and end-of-life recycling. Below is a detailed breakdown of these processes.
Tabbing and Stringing
The first step in module assembly is interconnecting solar cells into strings through tabbing and stringing. Solar cells arrive with front and back contacts, typically made of silver for the front grid and aluminum or silver for the back. Copper ribbons coated with solder are used to bridge adjacent cells.
In tabbing, thin ribbons are soldered onto the front side of one cell and the back side of the next, creating a series connection. Automated tabbing machines apply precise heat and pressure to ensure strong electrical bonds without damaging the cells. Infrared heating or hot air is commonly used to melt the solder.
Stringing involves arranging multiple tabbed cells into a linear sequence, usually 10 to 12 cells per string for standard 60- or 72-cell modules. The strings are then laid out in a matrix on a transparent front sheet, typically tempered glass. Proper alignment is critical to avoid microcracks or electrical mismatches that degrade performance.
Lamination (EVA and POE Encapsulation)
Lamination encapsulates the cell matrix within protective layers to shield against moisture, mechanical stress, and UV degradation. The most common encapsulants are ethylene-vinyl acetate (EVA) and polyolefin elastomers (POE).
The layered structure consists of:
- Front glass (3-4 mm tempered, low-iron)
- Encapsulant (EVA or POE, 0.4-0.6 mm)
- Solar cell matrix
- Encapsulant (EVA or POE, 0.4-0.6 mm)
- Backsheet (polyester-based or glass)
EVA dominates the market due to its low cost and proven reliability, but POE is gaining traction for its superior resistance to potential-induced degradation (PID) and hydrolysis. The stack is loaded into a laminator, where vacuum pressure removes air bubbles, and heating (140-160°C) crosslinks the encapsulant. The lamination cycle lasts 15-20 minutes, ensuring complete adhesion and durability.
Framing
Framing provides structural rigidity and facilitates mounting. Aluminum frames are standard due to their lightweight, corrosion resistance, and strength. The frame is attached to the laminated panel using mechanical clips or adhesive sealants, often silicone-based for weatherproofing.
Corner keys or screws secure the frame’s edges, and a drainage gap may be incorporated to prevent water accumulation. Anodized or powder-coated finishes enhance corrosion resistance, particularly for harsh environments.
Junction Box Integration
The junction box houses electrical connections and bypass diodes that mitigate shading losses. It is typically mounted on the backsheet using thermally conductive adhesive or mechanical clips.
Cables from the cell strings are soldered or plugged into the junction box terminals. Bypass diodes (usually 1-3 per module) are integrated to allow current flow around shaded or faulty cells. The box is sealed with silicone or polyurethane to prevent moisture ingress, and some designs include snap-on lids for serviceability.
Quality Control: Electroluminescence (EL) Imaging
EL imaging is a non-destructive inspection method to detect microcracks, broken contacts, or material defects. After lamination, modules are subjected to a forward bias current, causing the cells to emit infrared light.
A high-resolution camera captures the emission pattern:
- Uniform glow indicates healthy cells.
- Dark spots or lines reveal cracks or electrical discontinuities.
EL testing identifies defects that reduce efficiency or cause long-term failure. Automated systems classify modules based on defect severity, with rejects sent for repair or recycling.
Sustainability and Recycling
PV module recycling addresses growing waste from decommissioned panels. Industrial recycling processes focus on material recovery:
1. Mechanical Processing:
- Removal of frames and junction boxes.
- Shredding to separate glass, encapsulant, and cells.
2. Thermal Processing:
- Pyrolysis at 450-600°C decomposes EVA/POE, releasing glass and silicon wafers.
3. Chemical Processing:
- Acid leaching recovers silver and lead from cell contacts.
- Etching removes anti-reflective coatings for silicon reuse.
Current recycling rates exceed 90% for glass and aluminum, but silicon and silver recovery remain costly. Innovations in solvent-based encapsulant removal and direct wafer reuse aim to improve economics.
Industrial Process Optimization
Large-scale manufacturers prioritize throughput and consistency. Key metrics include:
- Lamination yield (>99% defect-free).
- Stringing speed (up to 1,200 cells per hour).
- EL inspection throughput (one module every 10-20 seconds).
Automation reduces human error, with robotic arms handling tabbing, stringing, and framing. Predictive maintenance on laminators and soldering stations minimizes downtime.
Environmental regulations increasingly mandate recycling programs, driving adoption of greener encapsulants like POE and halogen-free backsheets. Lifecycle assessments guide material choices, balancing performance and sustainability.
In summary, PV module manufacturing integrates advanced materials, precision engineering, and rigorous quality control to deliver reliable solar panels. The industry’s shift toward sustainable practices, particularly in recycling and material innovation, underscores its commitment to long-term environmental stewardship.