Quick Facts
What anti-reflective coating is
Anti-Reflective Coating (ARC) is a thin film applied to optical surfaces that reduces light reflection. In solar panels, ARC is applied to two interfaces: the front of the silicon cells and the front of the cover glass. Each interface would otherwise reflect a meaningful fraction of incident sunlight, reducing the light available for conversion to electricity.
Without ARC:
Uncoated glass reflects approximately 4% of light (per surface; total 8% for two glass surfaces).
Uncoated silicon (with its high refractive index) reflects 30% to 40% of incident light.
Total uncoated reflection loss can exceed 35% of incident light, severely limiting cell output.
With ARC:
Coated glass reflects approximately 1% to 2%.
Coated silicon cells reflect 5% to 10%.
Total reflection loss drops to under 12%, allowing 88% of incident light to reach the active cell area.
The improvement from ARC is one of the most significant single contributions to solar panel efficiency, accounting for several percentage points of cell efficiency.
How ARC works
ARC works through thin-film interference. The coating is designed with specific thickness and refractive index to cause destructive interference between light reflected from the coating’s top surface and light reflected from the coating-substrate interface.
The mathematics:
A quarter-wavelength thickness causes a 180-degree phase shift between the two reflections.
The reflected waves destructively interfere, cancelling each other out.
The transmitted wave continues into the substrate (silicon or glass).
For silicon nitride ARC on a silicon cell with target wavelength of 600 nm (green light, in the middle of silicon’s response range), the optimal coating thickness is about 75 nanometres.
The thin-film interference works perfectly only at the target wavelength. At other wavelengths, the cancellation is less effective. ARC is tuned to maximise reduction across the spectrum where silicon is most sensitive (roughly 400 to 1000 nm).
The remaining reflection occurs at wavelengths poorly matched to the ARC. Silicon nitride ARC gives cells their characteristic blue colour because the longer (red, infrared) wavelengths are well-absorbed but shorter (blue, ultraviolet) wavelengths reflect more.
ARC on solar cells
Silicon nitride (SiNx) is the standard ARC for crystalline silicon cells. It serves dual purposes:
Anti-reflective: Reduces light reflection from the silicon surface.
Front passivation: Reduces electron recombination at the cell’s front surface.
The SiNx layer is deposited by PECVD (Plasma-Enhanced Chemical Vapour Deposition) at about 400 deg C. The thickness (about 75 nm) and refractive index are precisely controlled.
Premium cells may use multi-layer ARCs (SiNx plus thin SiO2 layer) for broader-spectrum reduction. These provide marginal additional benefit (less than 0.5% efficiency gain) at higher manufacturing cost.
ARC on solar glass
Glass ARC is applied to the front of the cover glass to reduce reflection from the glass-air interface.
Common ARC materials and processes:
Sol-gel porous silica (SiO2): Most common for solar glass. A sol-gel solution is applied to the glass surface, forming a porous coating that has an effective refractive index between air and glass. The intermediate refractive index reduces the abrupt change in refractive index that causes reflection.
Sputtered MgF2: Less common for solar glass but used in some premium products.
Chemical vapour deposition: For specific high-end glass.
The ARC is applied during glass manufacturing, before the panel is laminated. The coating is on the outside (sun-facing) surface of the glass.
Premium versus budget ARC
ARC quality varies significantly between manufacturers:
Premium ARC:
Multi-layer for broader spectrum reduction.
Durable formulations resistant to UV and weathering.
25-year warranty equivalent for coating durability.
Budget ARC:
Single-layer for cost reduction.
Less weatherproof.
Faster degradation; reflection increases over years.
The quality difference shows up over the panel’s life. Premium ARC maintains effectiveness for 25 years; budget ARC may show 1% to 2% additional reflection by year 10 to 15.
ARC degradation
ARC degrades through several mechanisms:
UV exposure: Long-term UV can chemically alter the coating material.
Weathering: Rain, dust, and thermal cycling erode the coating.
Cleaning: Aggressive cleaning with abrasives or harsh chemicals can damage ARC.
Mechanical abrasion: Sand and dust particles can scratch the coating surface over years.
Premium ARC formulations resist these degradation mechanisms better, maintaining effectiveness for the full panel life.
Best practices for ARC preservation
For solar panel operation:
Use soft brushes and clean water for cleaning. Avoid abrasive scrubbers.
Avoid harsh chemicals (acidic or alkaline cleaners) that can attack ARC.
Avoid pressure jets at high pressure, which can erode ARC.
Avoid manual scraping, even for stubborn dirt. Soaking and gentle wiping is preferable.
For module procurement:
Verify ARC quality. Some manufacturers publish coating durability test results.
For long-term installations, premium ARC justifies modest cost premium.
For high-soiling environments, ARC durability matters more.
Common mistakes regarding ARC
Treating all ARCs as equivalent. Quality varies significantly.
Ignoring ARC durability. Cheap ARC degrades faster.
Using harsh cleaning methods. Damaged ARC reduces panel output.
Mismatching ARC and cell technology. Some specialised cells (HJT, perovskite) may use different ARC strategies.
Underestimating ARC’s impact. The 4% to 5% efficiency contribution is substantial over 25 years.
Standards and references
ARC is part of cell and glass manufacturing. Cell-level ARC is verified through IEC 61215 design qualification. Glass-level ARC is tested per glass manufacturer specifications and IEC 61215. Reflection measurements use spectrophotometry per ASTM E1175 and similar standards.
Related glossary terms
- Solar AR Glass
- Mono PERC
- TOPCon Solar Panel
- HJT Solar Panel
- PERC Cell Architecture
- Passivated Emitter
- IEC 61215 Standard
Key takeaways
Anti-Reflective Coating (ARC) is a thin film applied to solar panel surfaces (both glass and cells) that reduces light reflection through thin-film interference. ARC reduces total reflection losses from over 35% (uncoated) to under 12% (coated), translating to several percentage points of cell efficiency. Silicon nitride (SiNx) is the standard cell ARC; porous silica is common on glass. Premium ARC formulations maintain effectiveness for 25+ years; budget ARC may degrade faster. ARC quality is a key but often overlooked aspect of solar panel quality.