What TOPCon is
TOPCon stands for Tunnel Oxide Passivated Contact. It is a refinement of the standard silicon solar cell rather than a wholly new device, which is one reason it spread so quickly. Where the previous mainstream design (PERC) used p-type silicon and contacted the cell directly, TOPCon is built on n-type silicon and adds a thin passivating contact on the rear of the cell. By 2024 this architecture had overtaken PERC to become the dominant technology in new module production, and industry roadmaps expect its share to keep growing for years yet.
How the passivating contact works
The core problem in any solar cell is recombination: charge carriers that should flow out as current are instead lost, especially where the silicon meets a metal contact. The metal is a very "leaky" surface, so direct contact wastes voltage.
TOPCon tackles this with two stacked layers on the back of the cell. First comes an extremely thin layer of silicon oxide, roughly 1 to 2 nanometres thick, sitting on the silicon. On top of that sits a doped polysilicon layer that connects to the metal. The oxide is thin enough that electrons can tunnel through it (a quantum-mechanical effect), so current still flows freely. But the layer also passivates the silicon surface and is carrier-selective: it lets the wanted carriers pass while strongly suppressing recombination. The result is a contact that collects current without the heavy voltage penalty of touching bare silicon.
That higher voltage is where the efficiency gain comes from. It is the same passivated-contact idea behind heterojunction cells, just achieved with a different material stack.
How it compares with PERC and HJT
Efficiency. Commercial TOPCon modules typically convert around 21 to 23 per cent of incoming light, a step above mainstream PERC. In the laboratory, certified TOPCon cells have reached the high 26 to 27 per cent range, edging towards the roughly 29.4 per cent practical ceiling for single-junction silicon set by Auger recombination. Heterojunction (HJT) cells sit in a similar high band; PERC is generally a percentage point or so lower at module level.
Temperature coefficient. Panels lose output as they heat up. PERC modules commonly lose around 0.34 to 0.35 per cent of power per degree Celsius above the rating temperature. TOPCon is usually a little better, with many datasheets quoting around -0.29 to -0.30 per cent per degree. HJT is typically the strongest of the three on this measure. In hot Australian conditions the difference is real but modest, so it is worth checking the exact figure on the datasheet rather than assuming.
Bifaciality. TOPCon cells collect light from the rear as well as the front. Their bifaciality factor (rear output as a share of front) is often quoted around 80 to 90 per cent, higher than typical PERC and useful for ground-mount and elevated installs with reflective surfaces. HJT's symmetric structure can push this higher still.
Degradation and LID/LeTID. Because TOPCon uses n-type silicon, it sidesteps the boron-oxygen light-induced degradation (LID) that affects many p-type PERC products, and is generally less prone to light- and elevated-temperature-induced degradation (LeTID), though it is not wholly immune to the latter. Manufacturers commonly warrant TOPCon at around 1 per cent loss in the first year and roughly 0.4 per cent per year thereafter, often over a 30-year term. PERC warranties are usually a touch steeper; HJT is generally similar to or slightly better than TOPCon.
Cost and maturity. A major practical advantage is that existing PERC factory lines can be upgraded to TOPCon with extra steps (such as the oxide and polysilicon deposition) rather than rebuilt from scratch. That kept manufacturing costs close to PERC and is a big part of why TOPCon scaled so fast. HJT uses a more distinct, lower-temperature process and has historically carried a cost premium.
What it means for buyers
For most Australian rooftops, TOPCon is now simply what a good mainstream panel is made of. The sensible takeaway is not to chase the cell acronym but to read the numbers it produces: rated efficiency, the temperature coefficient, the bifaciality factor (if relevant to your install), and the performance warranty's year-one and annual degradation figures. Two panels both labelled "TOPCon" can still differ meaningfully on these. If you want to understand the line items on a quote, see reading a solar datasheet; and for how TOPCon's passivated-contact cousin differs, see HJT solar technology.
The headline efficiency records make good marketing, but the gap between a laboratory cell and a panel on your roof is large, and real-world output depends far more on orientation, shading, temperature and installation quality than on which high-efficiency n-type architecture you chose.