Converting wafers into cells
At this point, a wafer is no more capable of producing electricity than a sliver of river rock. The wafer is the main building block of a PV cell, but so far its only notable characteristics are its crystal structure and positive potential orientation. All of that changes in the third, multistep, cell-production phase of PV manufacturing.
In the only phase requiring a designated clean room, a series of intricate chemical and heat treatments converts the blank, grey wafers into productive, blue cells. A so-called texture etch, for instance, removes a tiny layer of silicon, relying on the underlying crystal structure to reveal an irregular pattern of pyramids. The surface of pyramids – so small they’re invisible to the naked eye – absorbs more light.
Next, wafers are moved in cartridges into long, cylindrical, ovenlike chambers in which phosphorus is diffused into a thin layer of the wafer surface. The molecular-level impregnation occurs as the wafer surface is exposed to phosphorus gas at a high heat, a step that gives the surface a negative potential electrical orientation. The combination of that layer and the boron-doped layer below creates a positive-negative, or P/N, junction – a critical partition in the functioning of a PV cell.
Coloring and Printing
The burgeoning, still-grey cells move in trays into heavy vacuum chambers where blue-purple silicon nitride is deposited onto their tops. The coating with silicon nitride – yet another member of the silicon family of materials – is designed to reduce reflection even further in the energy-dense blue end of the light spectrum. It leaves the cells with their final, dark color. Now, the cells can optimally gather photons and produce electricity. They lack, however, any mechanism to collect and forward the power. So, in a series of silkscreen- like steps, metals are printed on both sides of the cell, adding pin-stripe "fingers" and bus-bar circuitry. A functioning cell is born – only sunshine needed.