These 9BB, 12BB, etc figures are the number of BusBars (BB) across the surface of a solar cell. These are the thin silver colored lines you see on the dark colored cells of a solar panel. They gather the electricity output from the cell surface and bring it to a place where it can be connected to the other cells in the panel.
To work efficiently, busbars need to contact as much of the top silicon surface of the solar cell as possible. The photovoltaic silicon material that generates electricity has a high internal resistance. That is, it generates electrical current but cannot transfer it over a distance. We have to collect this produced power from evenly distributed points across the surface. The more contact points, the lower the electrical resistance, the greater the current (amperage) output from the cell. There is balance point however – a solar cell needs to have enough busbars to efficiently connect its output, but not so many that the busbars block light from reaching the solar cell surface. Typically busbars are made as narrow as possible using highly conductive metals.
Basically, a solar cell is like a silicon “sandwich” that turns light into electricity. Electrical output is collected from bottom and top sides. The connection to one side of the cell (or both sides for bifacial solar panels) has to collect the electrical output without blocking light from reaching the silicon surface of the cell.
When you’re shopping for solar panels you’ll see terms like 9BB, 12BB, 16BB, 18BB, etc. These “nnBB” figures are shorthand specifications for the number (n) of busbars (BB) across each solar cell. For example “12BB” would mean that each cell has 12 busbars (12 silver lines) across its surface. Newer solar panels like many of the N-type and bifacial panels available today feature wider rectangular cells that have many busbars, like 16BB or 18BB. These designs allow panels to deliver higher current and more wattage per square foot than previously possible.
Some types of solar cells have tree-like busbar configurations – this is common on older low cost polycrystalline cells.
I hope this give you a better understanding of this “nnBB” thing!
