Solar Panel Cell Types Explained: Mono, PERC, TOPCon, and N-Type

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 modules typically achieve around 20–21.5% efficiency and carry performance warranties guaranteeing roughly 0.45–0.55% annual degradation source. 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) source. For a detailed comparison, see our TOPCon vs PERC solar panels guide.

The practical result: TOPCon panels typically achieve around 22–23.8% efficiency and warrant roughly 0.4% or lower annual degradation source. Lower annual degradation plus a smaller first-year drop means a TOPCon panel will typically produce several per cent more energy than an equivalent-wattage PERC panel by the end of its warranted life — commonly cited as a gap of a few to several per cent over 25 years source. 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 (measured at 200 W/m² versus the standard 1,000 W/m² test condition) is typically around 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. Module efficiencies of around 23–24% are achievable source. 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 5–15% source. 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.

Half-cut cells: the configuration that changed shade tolerance

Half-cut cell technology is not a separate cell chemistry — it applies to PERC, TOPCon, and HJT alike. The idea is simple: each cell is laser-cut in half, giving 120 or 144 smaller cells instead of 60 or 72 standard ones. The benefits are real and measurable. Smaller cells carry lower current, which reduces resistive losses (heat generated by electron flow through the cell). More importantly, a half-cut panel wires its cells in parallel strings across the top and bottom halves of the panel, so shading one half reduces output from that half only — the unshaded half continues generating at full rate. On a standard full-cell panel, shade on any cell in a string can drag down the whole string.

In practice, half-cut cells improve real-world output by roughly 2–3% compared with equivalent full-cell panels under typical UK conditions with intermittent cloud and partial shading. Almost every mainstream panel sold in the UK in 2026 uses half-cut cells, so it is rarely a differentiator — but if a quote offers older full-cell panels at a lower price, the real-world output difference is worth factoring in.

Bifacial panels: generating from both sides

Bifacial panels have a transparent rear surface (glass-glass construction or a transparent backsheet) that allows light reflected from the ground or roof surface to hit the rear of the cells. The additional generation from the rear — the bifacial gain — typically adds 5–15% to real-world output depending on the albedo of the surface below. A white PVC membrane, light gravel, or sand gives a high albedo and good bifacial gain; dark roof tiles give almost none.

Most TOPCon panels in 2026 are bifacial by design — the N-type cell structure lends itself to bifacial manufacture, and the glass-glass construction also improves long-term durability and moisture resistance. For a standard UK pitched roof with dark clay or concrete tiles, bifacial gain is minimal and should not be the primary selection criterion. Where bifacial genuinely matters is ground-mounted systems, flat roofs with light membranes, or east-west ballasted installations. For a detailed look at when bifacial panels earn their premium, see our guide to bifacial solar panels.

Perovskite and tandem cells: what is coming next

Perovskite cells have achieved laboratory efficiencies above 33% in tandem configurations (a perovskite cell stacked on a silicon cell), far above anything available commercially. The appeal is obvious: higher efficiency per square metre, potentially at lower manufacturing cost than silicon. Several manufacturers — including Oxford PV, Saule Technologies, and REC Group — are working toward commercial production.

The barrier is durability. Silicon cells are stable for 25+ years in outdoor conditions; perovskite materials have historically degraded more quickly under heat, humidity, and UV exposure. Encapsulation improvements have extended perovskite cell lifetimes significantly in lab conditions, but independent long-term outdoor validation at commercial scale is still thin. As of mid-2026, no perovskite-silicon tandem panel is certified for residential installation in the UK under the IEC 61215 standard required for MCS-certified systems.

For practical purposes: perovskite tandem panels are not available for UK residential installation now, and are unlikely to be within the next two to three years at a price point that makes sense for most homeowners. For more on where the efficiency frontier sits today, see our perovskite solar cells explainer.