TOPCon Solar Cells Explained: The Next-Generation Technology Replacing PERC

TOPCon (Tunnel Oxide Passivated Contact) is the solar cell technology rapidly replacing PERC as the industry standard, using an ultra-thin 1-2 nanometer silicon dioxide tunnel oxide layer paired with doped polysilicon to achieve commercial module efficiencies of 22-24%. With a laboratory cell record of 26.89% and a better temperature coefficient than PERC, TOPCon is projected to capture over 60% of global cell production by 2027. JinkoSolar, Trina Solar, JA Solar, and LONGi are leading the transition.

How TOPCon works

TOPCon's key innovation is the passivating contact on the rear surface of the cell. In a PERC cell, the rear passivation layer (Al2O3/SiNx) is an insulator, so current can only flow through small laser-opened holes where aluminum makes direct contact with the silicon. At these contact points, electron recombination occurs because the metal-silicon interface is rich in defects.

TOPCon eliminates this problem by replacing the insulating passivation with a conductive passivating contact. The structure consists of two layers:

Tunnel oxide layer: An ultra-thin film of silicon dioxide (SiO2), typically only 1-2 nanometers thick. This is thin enough for electrons to pass through via quantum mechanical tunneling but thick enough to chemically passivate the silicon surface, reducing recombination dramatically. The tunnel oxide is grown by thermal oxidation or wet chemical methods.

Doped polysilicon layer: A 100-200 nanometer layer of heavily phosphorus-doped polycrystalline silicon deposited on top of the tunnel oxide, typically by low-pressure chemical vapor deposition (LPCVD). This polysilicon serves as both the electrical contact and a field-effect passivation layer. Its heavy doping creates a strong electric field that repels minority carriers (holes) from the rear surface.

The result is a full-area rear contact — current flows through the entire rear surface rather than through small contact holes. This eliminates the recombination at metal-silicon contacts that limits PERC efficiency and provides a pathway to cell efficiencies above 25%.

TOPCon vs PERC: quantified comparison

Parameter	PERC	TOPCon	Advantage
Cell efficiency (production)	23-24%	25-26%	TOPCon +1.5-2% absolute
Module efficiency (commercial)	21-23%	22-24%	TOPCon +1-1.5% absolute
Lab record (cell)	24.06%	26.89%	TOPCon +2.8% absolute
Temperature coefficient	-0.34 to -0.38%/°C	-0.29 to -0.34%/°C	TOPCon runs cooler
Bifacial factor	70-75%	80-85%	TOPCon captures more rear light
First-year degradation (LID)	1-2%	0.5-1%	TOPCon degrades less
Annual degradation	0.45-0.55%/year	0.40-0.45%/year	TOPCon degrades slower

The bifacial factor improvement deserves attention. TOPCon cells are n-type (phosphorus-doped base) rather than p-type (boron-doped base) like PERC. N-type silicon is not susceptible to boron-oxygen light-induced degradation (LID), which is why TOPCon has lower first-year degradation. The n-type base also provides a more symmetric cell structure that captures rear-side light more efficiently.

The PERC-to-TOPCon upgrade path

TOPCon's rapid adoption is not just about performance — it is about manufacturing economics. A PERC production line consists of roughly 8-10 major processing steps: texturing, diffusion, etching, passivation (PECVD), anti-reflective coating, laser contact opening, metallization (screen printing), and testing.

To upgrade to TOPCon, a manufacturer adds approximately 3-4 additional steps after the front-side processing: rear polishing/etching, tunnel oxide growth, polysilicon deposition (LPCVD), and polysilicon doping activation. Most of the existing PERC equipment — texturing, front-side diffusion, PECVD, metallization, testing — can be reused.

The estimated upgrade cost is $30-50 million per GW of production capacity. For comparison, building a new HJT line from scratch costs $100-150 million per GW because HJT requires entirely different equipment (PECVD for amorphous silicon, specialized metallization for low-temperature processing).

This conversion advantage has driven a massive industry shift. Chinese manufacturers with hundreds of GW of PERC capacity have been systematically upgrading to TOPCon since 2023. By the end of 2025, global TOPCon production capacity is estimated to exceed 500 GW per year.

Market share trajectory

The ITRPV 2024 roadmap projects the following technology market share evolution:

Year	PERC	TOPCon	HJT	Other
2023	~90%	~8%	~2%	~0%
2024	~60%	~30%	~5%	~5%
2025	~40%	~48%	~7%	~5%
2027 (projected)	~20%	~62%	~10%	~8%

TOPCon's growth rate is remarkable — from under 10% to a projected majority in just four years. This is one of the fastest technology transitions in solar history, driven by the relatively low conversion cost and the meaningful efficiency gain over PERC.

Real-world performance advantage

The combination of higher efficiency, better temperature coefficient, and lower degradation adds up to a significant lifetime energy advantage over PERC.

Consider two identical 8kW systems in Dallas, Texas — one using PERC panels (22% efficiency, -0.36%/°C coefficient, 0.50%/year degradation) and one using TOPCon (23.5% efficiency, -0.30%/°C coefficient, 0.40%/year degradation):

Year 1 energy: The TOPCon system produces roughly 3-4% more energy due to its higher base efficiency and better temperature coefficient in the Texas heat.

Year 25 energy: The TOPCon system has degraded less (10% cumulative vs 12.5% for PERC), widening the performance gap to about 5-6%.

Lifetime total: Over 25 years, the TOPCon system produces approximately 4-5% more total energy. On an 8kW system generating roughly 12,000 kWh/year, that is about 12,000-15,000 kWh of additional lifetime production — worth $1,500-2,500 at typical residential electricity rates.

Whether this justifies a price premium depends on the actual cost difference at the time of purchase. In 2025-2026, TOPCon panels are typically priced just 5-10% above equivalent PERC models, making the economics favorable in most scenarios.

Related terms

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Frequently Asked Questions

What is TOPCon in solar panels?

TOPCon stands for Tunnel Oxide Passivated Contact. It is a solar cell architecture that places an ultra-thin layer of silicon dioxide (SiO2), typically 1-2 nanometers thick, on the rear surface of the cell, topped with a layer of doped polycrystalline silicon. The tunnel oxide is thin enough for electrons to quantum-mechanically tunnel through it, allowing current to flow while providing excellent surface passivation that reduces electron recombination. This combination pushes cell efficiency above 25%, compared to 23-24% for PERC.

How efficient are TOPCon solar panels?

Commercial TOPCon modules in 2025-2026 typically achieve 22-24% module efficiency, with premium models reaching 24.5%. At the cell level, production TOPCon cells achieve 25-26% efficiency, with the laboratory record at 26.89% (set by LONGi in 2024). For comparison, the best PERC cells top out around 24%, and the practical PERC module efficiency ceiling is about 23%. TOPCon's higher efficiency means more watts per square meter of roof space.

What is the difference between TOPCon and PERC?

Both technologies add passivation layers to reduce recombination, but they differ in approach. PERC uses a dielectric layer (Al2O3/SiNx) on the rear with laser-opened contact holes through which aluminum makes direct contact with silicon. TOPCon uses a tunnel oxide plus doped polysilicon layer that provides full-area passivated contact — current flows through the entire rear surface via quantum tunneling rather than through small contact holes. This full-area contact eliminates the recombination that occurs at PERC's metal-silicon contact points, enabling about 1-2% higher absolute efficiency.

Why is TOPCon replacing PERC?

Two reasons: higher efficiency and manufacturing compatibility. TOPCon achieves 1-2 percentage points higher module efficiency than PERC, and it has a better temperature coefficient (-0.29 to -0.34%/°C vs -0.34 to -0.38%/°C). Crucially, existing PERC production lines can be upgraded to TOPCon by adding a few processing steps (tunnel oxide growth and polysilicon deposition) at an estimated cost of $30-50 million per GW — much cheaper than building new HJT lines from scratch at $100-150 million per GW. This conversion economics makes TOPCon the natural successor.

Which manufacturers make TOPCon solar panels?

All of the top-5 global solar manufacturers now produce TOPCon modules. JinkoSolar's Tiger Neo series was among the first mass-market TOPCon products. Trina Solar produces the Vertex N series. JA Solar offers the DeepBlue 4.0 N-type. LONGi launched the Hi-MO X6. Canadian Solar has the TOPBiHiKu series. Dozens of smaller Chinese manufacturers also produce TOPCon panels. As of 2025, global TOPCon production capacity exceeds 400 GW per year and is growing rapidly.

Is TOPCon better than HJT?

It depends on your priorities. TOPCon offers higher efficiency at lower manufacturing cost, which makes it the volume leader. HJT has a better temperature coefficient (-0.24 to -0.26%/°C vs -0.29 to -0.34%/°C for TOPCon), making it superior in very hot climates. HJT also has inherently better bifaciality and lower degradation. However, HJT manufacturing costs are significantly higher (requiring entirely new production equipment), which is why TOPCon has won the market share battle. For most residential installations, the performance difference between TOPCon and HJT is small — typically 1-2% annual energy difference.

What is the temperature coefficient of TOPCon panels?

TOPCon panels typically have a temperature coefficient of Pmax between -0.29 and -0.34%/°C, compared to -0.34 to -0.38%/°C for PERC and -0.24 to -0.26%/°C for HJT. The improvement over PERC means TOPCon panels retain more power in hot conditions. At 65°C cell temperature (a typical summer afternoon), a TOPCon panel with -0.30%/°C coefficient loses 12% of its rated power, while a PERC panel with -0.36%/°C loses 14.4%. That 2.4 percentage point difference compounds over the system's 25-year lifetime.

Sources

Marko Visic

Physicist and solar energy enthusiast. After installing solar panels on my own house, I built TheGreenWatt to share what I learned. All calculators use NREL PVWatts v8 data and peer-reviewed formulas.