PERC, TOPCon and heterojunction (HJT) are three ways of building a crystalline silicon solar cell. All three convert sunlight into electricity in the same basic way, but each uses a different structure to stop the charge carriers created by light from being lost before they reach the wires. PERC was the mainstream technology for several years, TOPCon has become common in new panels, and heterojunction sits at the premium end. The differences show up as efficiency, behaviour in heat and long-term degradation, and all three appear as labels on panel datasheets.

The problem every cell design is solving

When a photon is absorbed in silicon, it frees an electron and leaves behind a gap called a hole. The cell's job is to separate these carriers and deliver them to the metal contacts before they meet again and recombine, which wastes their energy as heat. Recombination is especially likely at surfaces and where metal touches silicon, because those are places where the regular crystal structure is interrupted.

Passivation is the general term for treating surfaces so fewer carriers are lost there. Much of the progress in cell efficiency over the past decade has come from better passivation and from contacts that collect carriers without creating recombination sites. The layers that surround the cells in a finished module are described in what solar panels are made of.

P-type and n-type wafers

Pure silicon is doped with small amounts of other elements to control its electrical behaviour. P-type wafers have traditionally been doped with boron, giving them an excess of holes. N-type wafers are doped with phosphorus, giving an excess of electrons. For years p-type dominated because it was cheaper and well understood.

N-type wafers have some useful properties. They are not affected by a light-induced degradation mechanism linked to boron and oxygen, which causes early output loss in conventional boron-doped cells, and they tend to be more tolerant of certain impurities. That generally means higher efficiency potential and lower degradation. N-type panels often carry performance warranties with annual degradation towards the lower end of the commonly quoted range of around 0.4-0.55% a year, although terms vary by model.

PERC: passivating the rear

PERC stands for Passivated Emitter and Rear Cell. Earlier cells had an aluminium layer covering the whole back, which conducted current but allowed significant recombination. PERC adds a thin dielectric passivation layer to the rear and connects the aluminium only through small openings, typically made by laser. The layer also reflects light that passed through the silicon back into the cell for a second chance at absorption.

PERC production lines were relatively simple to adapt from older technology, which helped PERC become the industry workhorse. Most PERC cells use p-type wafers, and their efficiency is now close to the practical limits of the design, which is why manufacturers have moved on.

TOPCon: a contact that barely touches

TOPCon stands for Tunnel Oxide Passivated Contact. On the rear of an n-type wafer sits an ultra-thin layer of silicon oxide, only a couple of nanometres thick, covered by heavily doped polycrystalline silicon. The oxide is thin enough for carriers to pass through by quantum tunnelling, yet it keeps the metal and its recombination sites away from the wafer surface. The result is a passivated contact across the entire rear.

A major reason TOPCon spread quickly is that it can be manufactured with equipment and processes closely related to PERC lines. Many TOPCon panels are bifacial, able to use light reflected onto their rear face, and they are commonly paired with glass-glass construction.

Heterojunction: layers of amorphous silicon

A heterojunction cell places very thin layers of amorphous silicon, one undoped and one doped, on both sides of an n-type crystalline wafer. Because the junction forms between two different forms of silicon, it is called a heterojunction. Transparent conductive oxide layers on top carry current to fine metal contacts.

The amorphous layers provide excellent passivation on both faces, giving high cell voltage and the best temperature behaviour of the three. The symmetrical structure also suits bifacial panels. The trade-offs are in manufacturing: the process must run at low temperatures, needs different equipment from PERC and TOPCon lines and uses materials such as indium in the conductive layers, which has kept costs higher.

How the three compare

These are typical published ranges, so always check the datasheet for the specific model. Temperature coefficients matter because panels on a Sydney roof in summer commonly run well above the 25°C test condition. As an illustration, a 400 W panel with cells at 65°C loses about 14% of its rated power at -0.35% per °C, but about 10% at -0.25% per °C.

What buyers see on a datasheet

The cell architecture is rarely the deciding factor on its own; the panel's measured specifications and warranty matter more. When reading a datasheet, look for:

• Cell type: labels such as n-type, TOPCon, HJT or mono PERC.
• Module efficiency: most useful when roof space is limited, since higher efficiency means more watts per square metre.
• Temperature coefficient of Pmax: a smaller negative number means less loss in heat.
• First-year and annual degradation: the warranted output over time, explained further in solar panel degradation explained.
• Bifaciality factor: relevant mainly for ground mounts and carports, not panels mounted flush on a roof.
• Construction and approval: glass-glass or backsheet, and listing on the approved products list for Australian installations.

Each newer architecture also brings long-term questions that independent testing continues to examine, so certification, warranty terms and the manufacturer's capacity to honour them remain as important as the cell label.

Next steps

If you are comparing panels, ask for the datasheet of each proposed model and compare efficiency, temperature coefficient and warranted degradation side by side rather than relying on a technology label. The energy market lists a New Residential Solar System from $5,490 for a 6.6 kW system installed, after STCs; the price is indicative and confirmed after a site assessment. To find out which panel suits your roof space and climate, request a free assessment from Blue Energy Solar.

Frequently asked questions

Are n-type panels worth paying extra for?

Often the question answers itself, because many new panels are now n-type and the price difference has narrowed. Where a premium remains, it tends to be most worthwhile when roof space is limited, the roof gets very hot, or you value lower warranted degradation over a long ownership period. For a large, unshaded roof in a mild location, the benefit may be smaller.

Can new TOPCon panels be added to an older PERC system?

Mixing panels with different electrical characteristics in the same string usually causes mismatch losses, because the string current is limited by the weakest panel. New panels are normally placed on a separate string with its own maximum power point tracker, or on a separate inverter. An accredited designer should check inverter capacity, network approval and whether the added panels are eligible for STCs.

Can you tell the cell type by looking at a panel?

Not reliably. Most modern cells look dark and uniform, and visible differences such as the number of fine wires across each cell, the frame colour or a black or white backing depend on the panel design rather than the cell architecture. The datasheet and the rating label on the back of the panel are the dependable sources for cell type and electrical ratings.