Heterojunction (HJT) solar cell
A cell that sandwiches an n-type crystalline silicon wafer between ultra-thin amorphous silicon layers, giving very high voltage and a low temperature coefficient.
Heterojunction technology combines two different forms of silicon. A crystalline n-type wafer is coated on both sides with a few nanometres of intrinsic hydrogenated amorphous silicon (for passivation) and then doped amorphous silicon layers that form the junction and back-surface field, topped with transparent conductive oxide. Sanyo commercialised the concept as HIT in the 1990s; the patents later passed to Panasonic.
The amorphous layers passivate the surfaces almost perfectly, giving open-circuit voltages of 740–750 mV, the highest of mainstream silicon cells. The process uses only four main steps and stays below about 200–250 °C, which suits thin wafers.
- Mass-production cell efficiency: about 25–26.5%.
- Temperature coefficient: about −0.24 to −0.26 %/°C, an advantage in hot climates (see temperature coefficient).
- Bifaciality: 85–95% thanks to its symmetrical structure.
- Low degradation, often warranted at 0.25–0.4% per year.
The main cost penalty is low-temperature silver paste and indium in the conductive oxide; copper plating and silver-coated copper are being adopted to reduce it. HJT is also the preferred bottom cell for perovskite-silicon tandems.
Related terms
All terms →- TOPCon
Tunnel Oxide Passivated Contact: an n-type silicon cell whose rear uses an ultra-thin oxide and doped polysilicon layer to extract current with minimal recombination.
- PERC (Passivated Emitter and Rear Cell)
A silicon cell design that adds a passivating dielectric layer to the rear surface, reducing recombination and reflecting light back into the cell.
- Perovskite solar cell
A solar cell whose absorber is a metal-halide compound with the perovskite crystal structure, offering tunable bandgap, cheap processing and very fast efficiency gains.
- Temperature coefficient
The percentage change in a PV module power, voltage or current per degree Celsius of cell temperature away from 25 °C.
- Bifacial module
A PV module that collects light on both its front and rear faces, using a transparent back sheet or glass to harvest ground-reflected and diffuse light.