What is a heterojunction (HJT) cell?
An HJT cell is built around a crystalline-silicon wafer that is sandwiched between two ultra-thin layers of amorphous ("thin-film") silicon. Those amorphous layers passivate the wafer surfaces — they tie up the defects where charge carriers would otherwise be lost. The result is an exceptionally high open-circuit voltage, which is the foundation of HJT's efficiency advantage.
HJT cells are naturally bifacial (they generate from both faces) and are made with a low-temperature process — typically below 200 °C — which is gentler on the wafer than the high-temperature firing steps used by other cell types.
HJT vs PERC vs TOPCon
PERC (Passivated Emitter and Rear Contact) is the technology HJT and TOPCon are displacing. PERC added a rear passivation layer to the older aluminium back-surface-field design, lifting cell efficiency by roughly a percentage point. TOPCon and HJT are both "next-generation" passivated-contact approaches that push higher. In practice, the differences that matter to a buyer are:
- Efficiency ceiling. Mainstream PERC modules top out around 21–22%; TOPCon and HJT modules commonly reach 22–23%+ at the module level, with HJT holding the higher cell-efficiency records.
- Temperature coefficient. HJT typically has the lowest (best) temperature coefficient of power — often around −0.24 to −0.26%/°C, versus roughly −0.34%/°C for PERC. Lower means less output lost as the panel heats up.
- Bifaciality. HJT's bifaciality factor is the highest of the three, frequently above 90%, so it captures more reflected light on suitable mounting.
- Degradation. HJT and TOPCon are largely free of the light-induced degradation (LID) that affected early PERC, so first-year and long-term degradation figures are usually better.
Why HJT performs well in the real world
Two properties do most of the work. The low temperature coefficient means an HJT array holds onto more of its rated power on hot afternoons — exactly when panels are hottest and, in many climates, when generation matters. The strong low-light response means it starts earlier and finishes later in the day. Combined with high bifaciality on reflective surfaces, these traits raise real-world yield relative to the nameplate rating.
The trade-offs
HJT is not automatically the right choice. It generally costs more to manufacture, uses more silver (and historically indium) than PERC, and demands careful handling because the thin amorphous layers are sensitive. Whether the efficiency and temperature advantages justify the price depends on roof space, climate, and budget — which is the kind of comparison our product ratings are designed to make objective.
What to check on the datasheet
Independent of the marketing, four numbers tell you most of what you need:
- Module efficiency (%) at standard test conditions.
- Temperature coefficient of Pmax (%/°C) — closer to zero is better.
- Bifaciality factor (%) — only relevant if the back face will see reflected light.
- Year-one and annual degradation (%), plus the performance-warranty end point.
Those are four of the criteria Solar Analytica scores for every panel — see the methodology for how they're weighted.