Quick Facts
What TOPCon is
TOPCon (Tunnel Oxide Passivated Contact) is a high-efficiency solar cell architecture built on n-type monocrystalline silicon. The defining feature is a 1 to 2 nanometre layer of silicon oxide deposited on the rear of the wafer, followed by a doped polysilicon layer that forms the rear contact. The oxide is thin enough for electrons to tunnel through but thick enough to passivate the silicon surface and prevent recombination.
In short, TOPCon is the next step beyond PERC. PERC added a passivation layer to the rear of a p-type cell. TOPCon adds a more sophisticated passivation strategy on top of n-type silicon, recovering both the bulk-material benefits of n-type and the recombination control of a passivated rear contact.
The result is a cell with higher efficiency, lower temperature coefficient, lower degradation, and better bifacial gain than Mono PERC, at modestly higher manufacturing cost.
How TOPCon works
Light enters the cell through the front, just as in any silicon solar cell, and generates electron-hole pairs in the n-type silicon wafer. The front contact carries away holes; the rear contact carries away electrons.
In a traditional cell, electrons recombine before reaching the contact, wasting energy. The thin tunnel oxide layer at the rear of a TOPCon cell creates an interface where electrons can pass through (by quantum tunnelling) but holes cannot. This selectively extracts electrons at the rear contact while suppressing recombination.
The doped polysilicon layer on top of the oxide acts as the actual rear contact and is metallised through a paste pattern. Modern TOPCon cells use silver-poor pastes to reduce metal consumption, an active area of cost reduction.
TOPCon compared with Mono PERC and HJT
| Parameter | Mono PERC | TOPCon | HJT |
|---|---|---|---|
| Substrate | p-type silicon | n-type silicon | n-type silicon |
| Module Efficiency (2026) | 20% to 22% | 21% to 23% | 22% to 24% |
| Temperature Coefficient | minus 0.34% to minus 0.37% per deg C | minus 0.29% to minus 0.32% per deg C | minus 0.24% to minus 0.27% per deg C |
| First-year Degradation | 1% to 2% | around 1% | around 1% |
| Annual Degradation | 0.5% to 0.55% | 0.4% | 0.25% to 0.35% |
| Bifacial Factor | 70% to 75% | 80% to 85% | 85% to 95% |
| Relative CAPEX per Wp | Reference | 5% to 10% higher | 25% to 40% higher |
| Common Use 2026 | Rooftop, ground mount | Utility, premium rooftop | Premium, high-efficiency rooftop |
For utility-scale tenders bid in 2025 and 2026, TOPCon is now the default. Rooftop is migrating more slowly because the CAPEX premium matters more in a price-sensitive segment.
TOPCon in the Indian market
Indian module makers have moved fast on TOPCon. Premier Energies, Adani Solar, Waaree, Vikram, RenewSys, Reliance, and Tata Power Solar all manufacture TOPCon under the PLI scheme’s second tranche. ALMM listings for TOPCon expanded steadily through 2024 to 2026.
Utility-scale tenders awarded in 2025 increasingly require TOPCon modules to meet bid-tariff economics. SECI and NTPC have specified TOPCon in multiple recent tenders. Many corporate PPAs for open-access solar projects also specify TOPCon for the same reason: higher CUF, lower LCOE.
For commercial rooftops above 100 kW, TOPCon is becoming the natural choice when the building has long-term ownership and prefers higher lifecycle energy. For residential rooftops under PM Surya Ghar subsidy, Mono PERC remains common because the upfront premium for TOPCon outweighs the smaller annual savings on a 3 to 5 kW system.
Construction of a TOPCon module
Front glass: 3.2 mm tempered low-iron glass with anti-reflective coating.
Encapsulant: POE on both sides, which is more PID-resistant than EVA and well-suited to glass-glass construction.
Cells: 132, 144, or 156 half-cut TOPCon cells with 16 or more busbars.
Junction box: Three bypass diodes, IP68 rating.
Rear glass or backsheet: Glass-glass is standard for premium bifacial TOPCon. Polymer backsheet for monofacial.
Frame: Anodised aluminium, 30 to 35 mm.
Connectors: MC4 or compatible.
Most premium TOPCon modules sold in 2026 are 580 to 620 Wp at 144-cell format, with sizes optimised for utility-scale racking systems.
Why TOPCon delivers higher energy yield
A TOPCon plant in the same Indian location as a Mono PERC plant typically delivers 3% to 6% more annual energy per kWp installed, for several stacked reasons:
Higher rated efficiency means more Wp on the same module footprint, so more energy for the same area.
Better temperature coefficient means less power loss in Indian summers, where module temperatures reach 55 to 65 deg C.
Lower annual degradation means more energy in later years of the plant’s life.
Better bifacial gain means more rear-side energy on high-albedo surfaces.
For utility projects with 25-year tariff visibility, these stacked gains tilt LCOE meaningfully in favour of TOPCon despite its higher CAPEX.
Limitations and trade-offs
CAPEX is higher than Mono PERC. The premium has narrowed to 5% to 10% per Wp in 2026, but it is still real.
Silver consumption per cell is higher in TOPCon than in Mono PERC, which keeps cost reduction harder than the supply chain would prefer. New silver-poor paste formulations are reducing this gap.
LID (Light Induced Degradation) in the first hours of field exposure can be slightly higher than the most stabilised Mono PERC, but this is well-managed in current designs.
The manufacturing process is more complex than Mono PERC. Yield ramps for new lines take time. This sometimes shows up as longer lead times for premium TOPCon products.
Common mistakes when specifying TOPCon
Treating TOPCon as a generic upgrade without checking the specific module’s I-V curve, bifacial factor, and temperature coefficient. Two TOPCon products from different makers can differ by 0.5% in module efficiency.
Mixing TOPCon and Mono PERC modules in the same string. Different I-V curves cause mismatch losses.
Forgetting that an inverter with negative grounding may not be suited to n-type cells without ungrounding or PID-Box mitigation. Check inverter compatibility.
Comparing only Wp ratings between TOPCon and Mono PERC, ignoring efficiency and temperature behaviour. A 540 Wp TOPCon and a 540 Wp Mono PERC are not the same product.
Assuming bifacial gain equal across all surfaces. Bifacial gain depends on albedo, mounting height, and tilt. Concrete rooftops often deliver 5% to 8% gain; gravel and grass-covered ground gives less.
Best practices
Specify TOPCon clearly in tenders, with required minimum efficiency, temperature coefficient, and bifacial factor.
Use POE-based glass-glass construction for long-term reliability in humid Indian climates.
Pair TOPCon with inverters configured for the cell technology’s polarity. Most modern inverters handle this automatically, but legacy stock may not.
Run lifecycle yield models that account for the lower degradation rate. A flat year-on-year comparison underweights TOPCon’s later-years advantage.
Verify ALMM listing before contracting if the project depends on subsidy or government tender.
Standards and certifications
A TOPCon module sold in India must meet IEC 61215, IEC 61730, IEC 62804 (PID resistance), and BIS certification for the ALMM list. Bankability indicators include BloombergNEF Tier 1, PVEL scorecard ranking, and the manufacturer’s PLI scheme participation.
Related glossary terms
- Mono PERC
- HJT Solar Panel
- Bifacial Solar Panel
- Half-cut Cell
- N-type vs P-type Solar Cells
- Solar Panel Degradation
- PID and Anti-PID
- Tier-1 Solar Panel
- ALMM
Key takeaways
TOPCon is a Tunnel Oxide Passivated Contact n-type solar cell architecture delivering 21% to 23% module efficiency in 2026, with lower temperature coefficient and lower degradation than Mono PERC. It is the fastest-growing mainstream solar technology and the default choice in current Indian utility-scale tenders. Rooftop adoption is moving more slowly because of the CAPEX premium, but the lifecycle yield advantage tilts in TOPCon’s favour for long-term-owned commercial projects.