The front-contact trade-off
The metal grid on the front of a cell has to do two opposing things: let light into the silicon, and carry electricity out of it. Silicon is far less conductive than metal, so without a metal grid the current would have to travel too far through the cell and be lost to series resistance. But every bit of metal on the front also shades the cell. Cell design is the search for the best compromise between shading losses (which cut the current) and contact resistance (which cuts the fill factor).
Three terms worth knowing
- Fill factor (FF) — the ratio of the cell's maximum power to the product of its open-circuit voltage and short-circuit current. A higher fill factor means a better-performing cell.
- Open-circuit voltage (Voc) — the maximum voltage the cell produces with no load connected (and therefore no current flowing).
- Short-circuit current (Isc) — the current that flows when the cell's terminals are shorted (zero voltage across the cell). The rated value is quoted at standard test conditions, but Isc exists at any level of illumination.
Busbars and fingers
Fingers are the many fine metallisation lines that collect current across the cell surface. Busbars are the larger strips that gather current from the fingers and connect to the external leads (the ribbons that join cells together). The whole grid is laid out to minimise both resistive loss and reflection from excess metal.
Why multi-busbar (MBB) won
Adding more busbars gives the current more, shorter paths to reach a collector — which shortens the distance it has to travel through the resistive fingers, one of the largest contributors to a cell's series resistance (alongside the emitter, the metal–silicon contact, and the interconnect). That's why designs moved from 3 and 5 busbars (5BB) to multi-busbar layouts with nine or more thin round wires. The advantages compound:
- Cuts front-side silver use — the savings vary widely by baseline, commonly cited around 10–30% per watt, and up to ~50%+ against older 3BB or full-ribbon designs.
- Less metal on the front means less shading.
- Round wires reflect some light back into the cell rather than straight out.
- Shorter finger paths reduce resistive losses and improve tolerance to micro-cracks.
- Better suited to bifacial light harvesting.
Rear-contact cells
Rear- (or back-) contact designs move some or all of the front grid to the back of the cell. With the front largely clear of metal, shading losses fall and cells are easier to interconnect — at the cost of more demanding manufacturing. The approach pays off most in high-current, high-efficiency cells.
Passivation enhancements: PERC, PERT, PERL
Separate from the front grid, the back of the cell is where the last decade of efficiency gains came from — by adding passivation layers that stop charge carriers recombining at the rear surface.
PERC — Passivated Emitter and Rear Contact
Invented at the University of New South Wales in the 1980s and adopted commercially from around 2012, PERC adds an insulating passivation layer to the rear surface. It lifts cell efficiency by roughly one percentage point over the older design — a meaningful system-level gain — and became the mainstream baseline.
PERT — Passivated Emitter Rear Totally-diffused
PERT diffuses the entire rear surface instead of using an aluminium back-surface field. It supports both monofacial and bifacial builds, is typically built on n-type silicon, and avoids the light-induced degradation that affected early PERC — at broadly comparable cost.
PERL — Passivated Emitter Rear Locally-diffused
PERL combines ideas from PERC and PERT: both surfaces passivated, with selective rear diffusion only at the metal contacts. The original UNSW PERL cell reached around 25% efficiency on p-type float-zone silicon — a crystalline-silicon record that stood for years. The locally-diffused rear approach also pairs naturally with n-type wafers and bifaciality for long-lived, high-efficiency modules.
Bifacial cells
A bifacial cell generates from both faces, capturing light reflected off the ground and surroundings onto the rear. The real-world gain depends heavily on the surface below (its albedo) and the mounting height — typically in the range of 5–20% extra yield, occasionally more over bright surfaces like white gravel or snow, and close to zero on a dark, flush-mounted roof. The trade-offs are a modest manufacturing-cost premium, heavier glass-glass construction, and the need to mount them where the back can actually see reflected light.
When the back face is well-exposed, glass-glass bifacial modules also tend to carry longer (often 30-year) performance warranties and resist potential-induced degradation. Whether that premium is worth it for a given roof is exactly the kind of question independent ratings should answer — see how we score build and cell technology in the methodology.