TOPCon Cell

What is a TOPCon cell

TOPCon, Tunnel Oxide Passivated Contact, is a next-generation crystalline silicon solar cell architecture that uses an ultra-thin tunnel oxide layer (1 to 2 nanometres of silicon dioxide) plus a thin doped polysilicon layer at the rear surface. The combined structure provides excellent electrical passivation of the rear contact while still allowing current to tunnel through to the metallisation.

TOPCon delivers 24 to 25 percent cell efficiency, with module efficiency landing at 22 to 23.5 percent. Compared to PERC, TOPCon offers 1 to 2 percentage points higher efficiency, a better temperature coefficient, lower LeTID and a higher bifaciality factor (80 to 85 percent against 70 to 80 for PERC).

From 2024 onward, TOPCon has become the dominant cell technology for new module manufacturing capacity globally and in India. Major Indian manufacturers have transitioned or are transitioning from PERC to TOPCon in their new gigawatt-scale plants. See How Solar Power Systems Work in India for where cell technology fits in the rest of a rooftop system.

TOPCon explained simply

A standard PERC cell has a passivated rear with metal contacts that touch the silicon directly. TOPCon adds a layer of ultra-thin glass on the back, then a thin layer of doped silicon, then the metal contact. Current still flows through (electrons tunnel through the thin oxide), but electron recombination at the metal contact is reduced.

The result is higher voltage and current per cell, leading to 1 to 2 percent more conversion efficiency at the same module footprint.

Why TOPCon matters

Efficiency leadership. 24 to 25 percent cell efficiency is the new commodity benchmark.

Module-area savings. More watts per square metre for the same rooftop or land area.

Better temperature performance. Indian summer sites lose less yield to heat.

Lower degradation. Annual losses of 0.40 to 0.50 percent against 0.45 to 0.55 for PERC.

Bifacial leadership. 80 to 85 percent bifaciality factor improves yield gain.

Industry transition. Major capacity additions in 2024 to 2026 are TOPCon-first.

How TOPCon works

1. Light absorbed in n-type silicon substrate.
2. P-N junction (front emitter, n-base) separates carriers.
3. Electrons collect at front contact through standard silver paste fingers.
4. Holes travel toward rear, encounter the tunnel oxide.
5. Tunnel oxide is thin enough (1 to 2 nm) for quantum tunnelling.
6. Doped polysilicon layer beneath collects holes.
7. Metal contact on polysilicon completes the circuit.

The tunnel oxide passivates the silicon-metal interface electronically, reducing recombination by an order of magnitude compared to direct metal contact.

Technical deep dive

Cell structure

Efficiency progression

Temperature coefficient

About 0.04 to 0.05 percentage points better than PERC. This temperature advantage matters most in Indian summers; see Solar Yield in India, kWh-per-kW Benchmarks by State for how module temperature derating affects real-world generation in hot states like Rajasthan.

Degradation

Bifaciality

TOPCon bifaciality factor: 80 to 85 percent. Combined with rear-side power generation typical of a bifacial module, total annual yield gain over monofacial PERC can reach 25 to 30 percent.

Real-world applications

Utility-scale solar plants. SECI tender bids increasingly assume TOPCon.

C&I rooftop solar.

Residential rooftop solar.

Solar carports.

Floating solar.

Agrivoltaics.

Solar pumping under PM-KUSUM.

Advantages

Highest mainstream efficiency.

Lower temperature loss in hot climates.

Lower degradation.

Higher bifaciality.

Compatible with existing module manufacturing equipment with modest retooling.

Cost premium over PERC is narrowing.

Limitations

Manufacturing complexity adds 5 to 10 percent cost.

n-type silicon supply chain is still scaling.

Polysilicon deposition is sensitive to process control.

Boron diffusion challenges in emitter formation.

Common problems

Process variability between manufacturers. Quality matters more for TOPCon than for mature PERC.

PID risk for systems with grounded inverter without PID-resistant materials.

Hot spots from cracked cells (same as any silicon cell).

Soiling losses similar to PERC, 5 to 25 percent annually.

Junction box reliability.

Best practices

Choose TOPCon from established Tier-1 manufacturers with proven yields.

Verify PID test certification.

Specify positive power tolerance.

Confirm bifaciality factor in the datasheet.

Use proper torque on MC4 connectors.

Plan annual cleaning schedule.

Document module serial numbers at commissioning.

Comparison: TOPCon vs PERC vs HJT

Indian market context

Indian PLI tranches incentivise transition from PERC to TOPCon. Tranche II includes specific allocations for n-type cell capacity.

Major Indian TOPCon manufacturers in 2026: Waaree, Adani Green Solar, Tata Power Solar, Vikram Solar, Premier Energies, Goldi Solar, Saatvik Green Energy.

ALMM List I additions in 2025 to 2026 are predominantly TOPCon.

SECI utility tender bid documents increasingly specify TOPCon performance assumptions, often modelled with solar simulation and PV modelling software during project design.

PM Surya Ghar accepts ALMM-listed TOPCon modules.

Standards and certifications

Common mistakes

Buying TOPCon from unproven manufacturers. Quality consistency is key.

Mixing TOPCon and PERC in the same string.

Underestimating the bifacial gain in plant design.

Ignoring temperature coefficient differences when modelling yield.

Treating TOPCon as a marketing label without verifying actual cell architecture.

Key takeaways

TOPCon is the dominant new-build cell technology from 2024 onward. Higher efficiency, lower temperature loss, lower degradation and better bifaciality make it the rational choice for any new Indian solar project where the 5 to 10 percent capex premium is acceptable. The Indian manufacturing transition is rapidly underway, with most ALMM additions now TOPCon.

Further reading

For how TOPCon Cell plays out in real projects, these guides go deeper:

• ALMM List-II Exemption Window for Net Metering & Open Access
• Solar Shading Analysis Software 2026: Guide
• Solar Panel ki Kimat 2026: Poora Rate Guide

Sources

• Feldmann F., Bivour M., Reichel C., Hermle M., Glunz S. W., Passivated rear contacts for high-efficiency n-type Si solar cells, Solar Energy Materials and Solar Cells, 2014.
• IEC 61215, Module qualification.
• IEC 61730-1, -2, Module safety.
• IEC 62804, PID test.
• IEC TS 63209, Extended thermal cycling.
• IS 14286, Indian Standard for crystalline modules.
• MNRE Approved List of Models and Manufacturers (ALMM).
• NREL Best Research-Cell Efficiency Chart.
• ITRPV Roadmap, latest edition, VDMA.
• IEA PVPS Trends in Photovoltaic Applications 2024.