A single cell gives under 0.7 V and is as fragile as a papad. A module wires many cells together and seals them in glass and plastic so they survive 25–30 Indian summers, monsoons and hailstorms. This lesson opens up the module and then decodes the technology names on every quotation: PERC, TOPCon, HJT, bifacial and half-cut.
What is inside a module
| Layer | Job |
|---|---|
| Front glass (3.2 mm tempered, anti-reflective coated; 2 mm in glass-glass designs) | Lets light in, takes hail and foot traffic during cleaning (but never walk on modules) |
| Encapsulant (EVA or POE film) | Glues the stack, cushions cells, keeps moisture out. POE resists moisture and PID better |
| Cells and ribbons | Make the power; ribbons (busbars) connect cells in series |
| Backsheet or rear glass | Insulates and protects the rear; glass on bifacial modules |
| Aluminium frame | Stiffness and the mounting points for clamps |
| Junction box | Terminations, bypass diodes, output cables and connectors |
Bypass diodes
A typical module has three bypass diodes, each across one-third of the cells. If a leaf or bird dropping shades a cell, its section stops conducting well. Without a diode the shaded cell would block the whole string's current and heat up (a hot spot). The diode lets current go around that section, so the module loses about one-third of its voltage instead of nearly all its power.
Half-cut cells
Most modules sold in India today use half-cut cells: each full cell is laser-cut in two, giving 144 half-cells instead of 72 full cells (or 132, 120, 108 in other formats).
Why bother?
- Lower resistive loss. Each half-cell carries half the current. Resistive loss goes with current squared, so loss in each cell's ribbons drops to about a quarter.
- Better shade tolerance. The module is wired as top and bottom halves in parallel. Shade on the bottom row (dust collects there on low-tilt roofs) costs less.
- Cooler hot spots, because less current flows through a shaded cell.
The module's voltage and current stay similar to a full-cell module of the same size.
Cell technologies compared
| Mono PERC | TOPCon | HJT | |
|---|---|---|---|
| Wafer type | p-type | n-type | n-type |
| Typical module efficiency (2026) | 21–21.5% | 22–23% | 22.5–23.5% |
| Pmax temperature coefficient | about −0.35 %/°C | about −0.29 to −0.30 %/°C | about −0.24 to −0.26 %/°C |
| First-year / yearly degradation (typical warranty) | 2% / 0.55% | 1% / 0.4% | 1% / 0.35–0.4% |
| Bifaciality (rear vs front) | about 70% | about 80% | about 85–90% |
| Indian DCR supply (2026) | Widest | Growing quickly | Limited |
Values are typical datasheet figures; always read the actual datasheet.
PERC (passivated emitter and rear contact) adds a reflective, passivating layer on the back of the cell so light that passes through gets a second chance. It made mono-crystalline modules the standard and is still the most common DCR module in India.
TOPCon (tunnel oxide passivated contact) uses n-type wafers with an ultra-thin oxide and doped polysilicon layer at the rear contact. It gives higher efficiency, lower degradation and better heat performance than PERC, at a modest premium. Indian factories have been switching lines to TOPCon.
HJT (heterojunction) sandwiches an n-type wafer between thin amorphous silicon layers. It has the best temperature coefficient, which suits hot rooftops, but costs more and has fewer Indian suppliers.
Older polycrystalline (blue, speckled) modules are no longer a mainstream choice for new projects.
Worked example: same roof, two technologies
A south-facing roof in Ahmedabad fits exactly 10 modules of 2,279 × 1,134 mm (2.584 m² each).
- PERC 550 Wp: efficiency = 550 ÷ (2.584 × 1,000) = 21.3%. System = 10 × 550 = 5.50 kWp.
- TOPCon 585 Wp, same size: efficiency = 585 ÷ 2,584 = 22.6%. System = 5.85 kWp.
Extra generation per year: 0.35 kWp × 1,616 kWh/kWp ≈ 566 kWh, before even counting TOPCon's better heat behaviour. On a small roof where area is the limit, that is a real gain. On a large roof where you can simply add a module, compare cost per Wp instead.
Bifacial modules
A bifacial module has glass (or a transparent backsheet) at the rear and cells that collect light from both sides. The rear picks up light reflected from the roof.
How much extra? It depends almost entirely on the installation:
- Rear gain needs height and a bright surface. On an elevated RCC structure 1 m or more above a white-coated roof, rear gain can be several percent. On a grey, dusty slab it is smaller.
- On a flush tin-shed mount the rear sees almost no light, so the gain is close to zero.
- Rear shading from purlins, rails and the structure reduces gain. Rails should not run across the cells.
Bifacial glass-glass modules are also heavier (often 30–33 kg for this size) and need the right clamps. They are excellent on carports, elevated structures and ground mounts; on low flush roofs, buy a monofacial module and save money.
Multi-busbar and other details
You will see "MBB", "16BB" or "0BB" on datasheets. More, thinner busbars shorten the path electrons travel and reduce shading by metal, gaining a little efficiency and tolerating micro-cracks better. It is a refinement, not a reason to choose one module over another on its own.
Key points
- A module is glass, encapsulant, cells, backsheet or rear glass, a frame and a junction box with three bypass diodes.
- Half-cut cells cut resistive loss and improve shade tolerance without changing module voltage much.
- TOPCon and HJT beat PERC on efficiency, heat and degradation; PERC is still the most common DCR module.
- Higher efficiency matters most where roof area is the limit.
- Bifacial gain depends on height, roof reflectance and rear shading; on flush roofs it is negligible.
Check yourself
- Why does a half-cut cell module lose less power in its ribbons than a full-cell module?
- A 2.58 m² module is rated 570 Wp. What is its efficiency?
- A customer has a tin shed with modules mounted flush 100 mm above the sheet. Should you sell bifacial?
Answers
- Each half-cell carries half the current, and resistive loss goes with current squared, so the loss per cell falls to about a quarter.
- 570 ÷ 2,580 ≈ 22.1%.
- No. With the rear only 100 mm above a dark sheet, almost no light reaches it. A monofacial module gives the same output for less money and weight.