Solar Panel Cell Types Explained

If you have been reading solar panel spec sheets, you will have seen a run of technology labels: monocrystalline, PERC, TOPCon, N-type, HJT. These are not just marketing terms — they describe how the silicon cells inside the panel are made, and the differences translate into real performance outcomes over the 25-year life of a system. Here is what each means in plain English.

Monocrystalline vs polycrystalline

The first distinction is whether the silicon is monocrystalline (grown as a single crystal) or polycrystalline (cast from multiple crystals). Polycrystalline panels were common until the mid-2010s but have largely disappeared from the residential market — they are cheaper to manufacture but noticeably less efficient, and the price gap has closed to the point where they are rarely competitive. Almost every panel sold in the UK residential market today is monocrystalline. If a quote offers polycrystalline panels, it is worth asking why.

PERC: the technology that dominated 2016–2023

PERC stands for Passivated Emitter and Rear Cell. It adds a passivation layer to the back of a standard monocrystalline cell, which reduces the recombination of electrons at the rear surface — a significant source of efficiency loss. PERC cells typically achieve 20–21.5% module efficiency and carry performance warranties guaranteeing less than 0.5–0.55% annual degradation. They are a P-type technology (boron-doped silicon), which is relevant because P-type cells are more susceptible to light-induced degradation (LID) in their first months of operation.

PERC is a mature, well-proven technology. It is still a perfectly sound choice, particularly if cost is a priority — PERC panels typically cost a little less per watt than TOPCon equivalents. Several reputable manufacturers (Jinko, LONGi, Canadian Solar) still offer PERC lines alongside their TOPCon ranges.

TOPCon and N-type: the 2026 default

TOPCon (Tunnel Oxide Passivated Contact) takes rear passivation further, using an ultra-thin tunnel oxide layer and a polysilicon rear layer to reduce recombination losses more aggressively. More importantly, TOPCon panels use N-type silicon (phosphorus-doped rather than boron-doped), which is inherently more resistant to LID and to light and elevated temperature-induced degradation (LeTID).

The practical result: TOPCon panels typically achieve 22–23% efficiency and warrant less than 0.4% annual degradation. Over 25 years, that degradation difference means a TOPCon panel will produce roughly 8–10% more energy than an equivalent-wattage PERC panel by the end of its warranted life. For a roof that will be generating for 25+ years, that is a meaningful gap.

N-type cells also perform better under diffuse light — relevant in the UK where overcast conditions are common. The improvement in low-irradiance performance (at 200 W/m² versus the standard 1,000 W/m² test condition) is typically 1–2 percentage points in favour of N-type, which adds up over a UK year where many generation hours are at lower irradiance levels.

HJT: premium technology worth knowing about

Heterojunction (HJT) panels layer amorphous silicon on crystalline silicon, achieving some of the lowest temperature coefficients and best low-light performance on the market. Efficiencies of 23–24%+ are achievable. They are currently more expensive than TOPCon and less widely available in the UK residential market, but worth considering if you are optimising for a constrained north-facing or east-west roof.

Which technology is right for your roof

For most UK homes in 2026, TOPCon is the default sensible choice — better degradation, better low-light performance, and the price premium over PERC has narrowed to around 5–10%. PERC remains a good option if budget is tight and roof space is not constrained. HJT is worth a look for difficult orientations or very space-limited roofs where every watt matters.

To understand how efficiency figures compare between these technologies, see our explainer on solar panel efficiency. To see how these different cell types are reflected in the panels currently available, browse solar panels on SmartSolarHomes.