Bare silicon throws back more than 30% of the light that hits it, so a cell is coated with a thin film whose refractive index sits between that of air and silicon. A cell sealed under glass already loses less, and a coating tuned for air can make it worse, so choose the film for the way the cell will actually be covered. The carrier’s recipe of growing an oxide at 300 to 400°C should be checked by colour and by measured current before it is trusted.
Part of Solar Panel Construction
Why silicon reflects so much light, how a thin coating cuts it, and why the cover glass and encapsulant change which coating is right.
Reflection Depends On The Jump In Refractive Index
When light crosses from a material of refractive index n₁ to one of n₂, the fraction reflected is R = ((n₁ − n₂) ÷ (n₁ + n₂))². The refractive indices used below are general knowledge, not from the other articles: air 1.0, glass or clear encapsulant about 1.5, silicon about 3.9 in visible light.
- Bare silicon in air: R = ((1.0 − 3.9) ÷ (1.0 + 3.9))² = (2.9 ÷ 4.9)² = 0.35, or 35%. The carrier says “30%+”, which agrees.
- Silicon under glass or clear encapsulant (n = 1.5): R = ((1.5 − 3.9) ÷ (1.5 + 3.9))² = (2.4 ÷ 5.4)² = 0.198, or 20%.
- The glass surface itself, air to glass: R = (0.5 ÷ 2.5)² = 0.04 per surface, and about 1 − 0.96² = 7.8% for two (Glass Cover Preparation).
So simply sealing a bare cell under glass cuts its reflection from about 35% to about 20% at the silicon surface, before any coating, because the jump in index is smaller.
A Coating Works Best At A Chosen Index And Thickness
A single film cancels reflection best when its index is the geometric mean of the two sides, n = √(n₁ × n₂), and its thickness is a quarter of the light’s wavelength inside the film, t = λ ÷ (4 × n) (general knowledge). For a cell in air: √(1.0 × 3.9) = 1.97. For a cell under 1.5 glass: √(1.5 × 3.9) = 2.42.
The carrier names three films: silicon dioxide grown by heating the wafer, silicon nitride (“harder to produce”) and titanium dioxide. With the same general-knowledge indices (silicon dioxide 1.46, silicon nitride about 2.0, titanium dioxide about 2.4) the reflection at the design wavelength, from R = ((n_c² − n₁n₂) ÷ (n_c² + n₁n₂))² for a quarter-wave film, comes out:
| Film | In air | Under glass |
|---|---|---|
| None | 35% | 20% |
| Silicon dioxide (1.46) | 8.6% | 21.7% |
| Silicon nitride (about 2.0) | 0.02% | 3.5% |
| Titanium dioxide (about 2.4) | 3.7% | 0.006% |
These are single-wavelength minima; across the whole spectrum the reflection is higher. The pattern is the useful part: silicon dioxide is a good film in air and no help under glass (21.7% is worse than 20% uncoated), while titanium dioxide is the right choice behind glass.
What You Need
- A cleaned wafer or a salvaged cell
- A furnace or hot plate at 300–400°C for the carrier’s oxide, or a titanium dioxide source
- A meter to read short-circuit current in the same light before and after
- A white light and a contrast surface to see the film colour
Method: Coat And Check
Step 1: Decide the covering. Will the cell sit under glass or in air? That sets the film.
Step 2: Apply the film. The carrier’s simplest method is to heat the wafer to 300–400°C in air until a thin silicon dioxide layer forms and the surface turns blue-purple, reducing reflection to about 10–15%. Growth of useful oxide at such a low temperature is very slow (general knowledge, unverified here), so confirm that the colour appears.
Step 3: Check the colour. A blue-purple tint means the film thickness is near the quarter-wave value for visible light, which is about 100 nm for silicon dioxide: t = 600 ÷ (4 × 1.46) = 103 nm at 600 nm wavelength.
Step 4: Measure. Read the short-circuit current of a cell in the same light before and after, and compare under the final glass; the cell that gives more current wins.
Common Mistakes
- Coating for air, then putting glass over the cell — the table shows an oxide film can raise reflection
- Judging by colour alone — thickness and index both matter, so measure
- Heating a finished cell above the limit of its contacts — solder and contact metal melt or diffuse
Safety
A hot plate and a hot wafer
Wafers at 300–400°C burn skin and cause fires on contact with paper or wood, and they crack if cooled quickly or handled with cold tongs (carrier: cells crack easily). Use a stand, tongs and heat-proof gloves, let wafers cool slowly on a heat-resistant surface, and keep titanium-based powders out of the air with a respirator (Dust & Respiratory Protection).
What’s Next
- Learn next: Glass Cover Preparation — the cover that changes which film is right
- Combine with: Cell Interconnection and Panel Wiring — connecting the finished cells