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Heterojunction Technology (HJT) Solar Panels: Efficiency, Temperature Coefficient, And Manufacturers

Heterojunction Technology (HJT) is a solar cell design that deposits thin films of amorphous silicon onto both sides of a crystalline silicon wafer. This combination creates a cell with 24-26% efficiency, a temperature coefficient of approximately -0.26%/C (the best of any commercial silicon technology), and inherent bifaciality of 90-95%. HJT occupies the premium tier of the residential and commercial panel market.

How HJT cells are made

An HJT cell starts with an n-type crystalline silicon wafer, typically 120-150 micrometers thick. The manufacturing process deposits four key layers:

  1. Intrinsic amorphous silicon (a-Si:H) on the front surface, just 5-10 nanometers thick. This ultra-thin layer passivates the crystalline surface by bonding to dangling silicon atoms, dramatically reducing electron-hole recombination.
  2. Doped amorphous silicon (p-type) on the front, creating the p-n junction that separates charge carriers.
  3. Intrinsic amorphous silicon on the rear surface, providing identical passivation.
  4. Doped amorphous silicon (n-type) on the rear, creating a back surface field that repels minority carriers.

Transparent conductive oxide (TCO) layers and a silver metallization grid complete the cell. The entire deposition process happens at low temperatures, under 200 degrees C, compared to 800-1,000 degrees C for PERC and TOPCon. This low-temperature processing preserves the passivation quality and reduces thermal stress on the wafer.

HJT vs PERC vs TOPCon

ParameterPERC (p-type)TOPCon (n-type)HJT (n-type)
Cell efficiency (commercial)22-24%24-25%24-26%
Cell efficiency (lab record)24.1%26.1%27.09%
Module efficiency20-22%22-24%22-24%
Temperature coefficient (Pmax)-0.34 to -0.37%/C-0.29 to -0.32%/C-0.24 to -0.28%/C
Bifaciality factor65-75%80-85%90-95%
Annual degradation0.45-0.55%/year0.30-0.40%/year0.30-0.40%/year
Year 1 LID1-3%Under 1%Under 1%
Process temperature800-1,000C800-1,000CUnder 200C
Relative manufacturing costBaseline5-10% premium10-20% premium

Why the temperature coefficient matters

The temperature coefficient determines how much power a panel loses when cell temperature rises above the 25 degrees C STC reference. On a sunny summer day, rooftop panel temperatures commonly reach 55-70 degrees C.

Comparison at 65 degrees C cell temperature (40 degrees C above STC):

  • PERC at -0.35%/C: loses 0.35 x 40 = 14.0% of rated power
  • TOPCon at -0.30%/C: loses 0.30 x 40 = 12.0% of rated power
  • HJT at -0.26%/C: loses 0.26 x 40 = 10.4% of rated power

For a 430W HJT panel vs a 430W PERC panel at 65 degrees C, the HJT produces 385.3W while the PERC produces 369.7W, a real-world advantage of 15.6W per panel. Across a 20-panel residential system in a hot climate, this adds up to 312W of additional peak capacity during the hottest hours when electricity demand and rates are highest.

Low-temperature processing advantage

The fact that HJT cells are processed entirely under 200 degrees C has practical benefits beyond passivation quality. Thinner wafers can be used without risk of thermal warping, reducing silicon consumption per cell. HJT manufacturers routinely use 120-130 micrometer wafers versus 150-170 micrometers for PERC and TOPCon. This silicon savings partially offsets the higher cost of the amorphous silicon deposition equipment.

The low-temperature process also means HJT cells have very low residual stress, which contributes to their lower degradation rate and resistance to micro-cracking during thermal cycling.

Where HJT panels excel

Hot climates. The superior temperature coefficient makes HJT the highest-energy-producing technology in locations with high ambient temperatures like Arizona, Texas, Florida, and the Middle East.

Space-constrained installations. When roof area is limited, the combination of high efficiency and good real-world performance means HJT panels extract more energy per square meter than any other mainstream technology.

Bifacial ground mounts. With 90-95% bifaciality, HJT captures nearly as much energy on the rear side as the front. Combined with the temperature coefficient advantage, HJT bifacial panels consistently outperform PERC and TOPCon bifacial in comparative field studies.

Low-light conditions. HJT cells maintain high relative efficiency at reduced irradiance levels, performing well during early morning, late afternoon, and overcast conditions.

Related terms

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Frequently Asked Questions

What is HJT solar cell technology?
HJT (Heterojunction Technology) is a cell architecture that deposits thin layers of amorphous silicon (a-Si) onto both sides of a crystalline silicon (c-Si) wafer. The amorphous layers passivate the wafer surface, reducing recombination losses and enabling cell efficiencies of 24-26%. The term heterojunction refers to the interface between two different semiconductor materials.
What is the efficiency of HJT solar panels?
Commercial HJT cells achieve 24-26% cell efficiency, with the lab record at 27.09% set by LONGi. At the module level, HJT panels typically reach 22-24% efficiency. The gap between cell and module efficiency is 2-3 percentage points due to inactive module area, interconnection losses, and encapsulant absorption.
Why do HJT panels have a better temperature coefficient?
HJT cells have a temperature coefficient of Pmax around -0.26%/C, compared to -0.35%/C for PERC and -0.30%/C for TOPCon. The amorphous silicon layers maintain better surface passivation at elevated temperatures. On a hot day when cells reach 65 degrees C, an HJT panel loses 10.4% of its STC power while a PERC panel loses 14.0%, a meaningful difference in hot climates.
How does HJT compare to TOPCon?
HJT has a better temperature coefficient (-0.26 vs -0.30%/C), higher bifaciality (90-95% vs 80-85%), and lower degradation. TOPCon has slightly lower manufacturing costs because it can partially use existing PERC production lines. In 2026, TOPCon has higher market share due to the cost advantage, while HJT occupies the premium segment.
Which companies make HJT solar panels?
Major HJT manufacturers include REC Group (Alpha Pure-R series), Meyer Burger (European-made), Panasonic (pioneer of the technology), LONGi (Hi-MO 9 series), Huasun, and Maxwell. Panasonic commercialized HJT in the early 2010s, but Chinese manufacturers now produce the majority of HJT capacity worldwide.
Are HJT panels worth the extra cost?
HJT panels typically cost 10-20% more than equivalent PERC panels. The premium is justified in hot climates (better temperature coefficient), space-constrained roofs (higher efficiency), and bifacial ground mounts (higher bifaciality factor). For a standard residential roof in a moderate climate, the extra cost may not be recovered within the warranty period.
Why is HJT better for bifacial applications?
HJT cells achieve a bifaciality factor of 90-95%, meaning the rear side is nearly as efficient as the front. This is because both sides of the wafer have identical amorphous silicon passivation layers. PERC bifacial cells only reach 65-75% bifaciality because the aluminum rear contact partially blocks light. In a bifacial ground-mount installation, HJT panels can capture 15-20% more rear-side energy than PERC bifacial panels.

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.