TOPCon solar cell design and performance

When this article first ran in 2021, TOPCon was a prediction. It is not any more. Somewhere between 70 and 80 per cent of the solar cells made today are TOPCon, and if you have bought panels from us recently there is a fair chance the technology this page once forecast is already on your roof. So here is the useful version: what TOPCon is, and why it earns its keep in Perth heat.

TOPCon cell structure

Every solar cell loses some of its electrons at the metal contacts on the back, where they recombine before they can be collected as current. TOPCon, developed at Germany's Fraunhofer Institute for Solar Energy Systems in 2013, puts an oxide layer about one nanometre thick between the silicon and the contact. Electrons can tunnel through it; the recombination losses largely cannot. Pair that with an n-type silicon wafer instead of the p-type used in PERC and you get a cell that converts more of the same sunlight, and keeps doing it for longer.

In practice that means mass-produced modules around 23 to 24 per cent efficient, where PERC topped out near 21. The theoretical ceiling for TOPCon is about 28.7 per cent — close to the roughly 29.4 per cent limit for any single-junction silicon cell — which is why manufacturers backed it as the platform worth scaling.

Panel output in Perth heat

A panel's rated output is measured at a cell temperature of 25 degrees. A panel on a Perth roof on a February afternoon is nowhere near that — cell temperatures of 60 to 70 degrees are normal. Every degree above the test condition costs output: about 0.35 per cent per degree for a typical PERC panel, about 0.29 for the n-type TOPCon panels we install.

Light-induced degradation

PERC panels are built on boron-doped p-type silicon, and the boron-oxygen defect that forms in their first weeks of sun causes light-induced degradation — a real, measurable dip early in the panel's life, with a slower cousin (LeTID) that shows up in heat. N-type silicon is phosphorus-doped, so the boron-oxygen mechanism simply is not there. That is why n-type panels carry tighter degradation warranties than PERC ever did, and it is a physics reason, not a marketing one.

The shift from PERC

In 2021 the open question was whether PERC's successor would be TOPCon, heterojunction or back-contact cells. TOPCon won the volume game because PERC factories could be upgraded to make it, and its early cost problems — it needed roughly half as much silver again per cell as PERC — were engineered down as production scaled. Heterojunction survives in niches. Back-contact became the premium tier: all the metal moved to the rear of the cell, so nothing shades the front. That is what the Aiko panels in our range use, and it is the reason they look the way they do.

One claim from the original article deserves an honest downgrade. TOPCon's higher bifacial factor — around 85 per cent against PERC's 70 — is genuinely valuable on a solar farm with light bouncing off the ground. On a home roof, mounted a hundred-odd millimetres above the tiles, it is close to irrelevant. Buy the panel for its heat behaviour and degradation, not for the rear side you will never use.

Panel choice and roof layout

You no longer choose TOPCon; it comes with the territory. The current panel range — Risen, Jinko Tiger Neo and the back-contact Aiko — is all n-type. The decisions left are the ones a datasheet cannot make: panel count, roof layout, and whether the rest of the system matches the load. If you want those worked through for your address, ask for a written price.