The Making of Crystalline Silicon Solar Panels
There is a precise step-by-step process for the making of solar panels. Within each of these steps there are processes that must be strictly followed. Manufacturers used by Homeland Solar (such as SolarWorld) are certified to have followed these standards – and offer an extended warranty on performance as a generator of POWER (wattage).
Step 1: The Growing of the Crystal
Silicon rock is heated and melted until a white hot liquid is formed in a vessel or pot for molten metal (crucible). This liquid of molten silicon is fused into a single giant crystal. The atoms within this crystal are perfectly aligned in a particular orientation and desired structure.
Charging the Crystal
Stacked carefully within a quartz pot (crucible) is approximately 250 pounds of polysilicon rock. This is where some would say “the magic starts”. A small amount of boron, the only other ingredient used in the process, is impregnated into the silicon disk.
By using the boron as a doping agent the resulting crystal is assured of having a positive potential electrical orientation. A thick wall of insulating graphite surrounds the vessel which is then placed and locked inside the cylindrical furnace.
Melting the Polysilicon Rock
The silicon contents within the crucible begin to melt into a shimmering slurry, as the crystal furnace heats to a temperature of approximately 2500 degrees Fahrenheit (1371 Celsius). A silicon seed crystal, which is hung from a narrow cable, is lowered slowly into the slurry by the use of a rotary device on top of the furnace.
Growing the Crystal
The seed crystal is rotated in the opposite direction of the crucible pot as it turns. The seed's crystalline structure is matched by the silicon as it freezes onto the seed crystal. As the crystal grows, the cable and crystal seed are slowly raised, allowing the crystal to elongate at a controlled width.
Cooling the Crystal
By this time, we are about 2 1/2 days from the time the crucible was first charged with polysilicon. After several hours of cooling, the temperature reduces to about 300° F (149º C). Now the furnace hood and shaft move away from the encasement. A completed cylindrical crystal is revealed and slowly swung to one side. This new crystal is now ready to be moved to the next step in the process - and to a new production room.
Step 2: Wafering of the Crystal
Cutting the Crystal
In order to have a uniform crystal with a consistent width, the top and tail of the crystal must first be sawed off. The saw used for this is known as a wafering saw. This kind of saw typically draws a thin wire bearing a liquid abrasive across the surface of the crystal. The wire saw cuts the crystal into “ingots” no more than two feet in length.
Squaring the Crystal
Inside another wire slicing machine, 16 ingots are stood on-end on a rack. The cutting wire within the slicing machine maintains a lattice configuration as it descends through the ingots. Four rounded edges are sheared off, leaving the four sides flat and resulting in ingots with a square cross-section (except for the corners which are still rounded).
Slicing the Crystal
Once the crystal has been squared, each is placed in a even more intricate wire saw. This saw has two cylindrical drums, with a wire winding between them hundreds of times, creating a tightly-spaced web of parallel segments. The ingots are mounted sideways on glass and metal holders – and pressed, two at a time, through the wire web, as the wire unspools through the machine. This slices the crystal into the thickness of a piece of paper, approximately 2.5 wafers per millimeter (of crystal). After this slicing, they are detached from their holders and loaded into carriers (called “boats”) for transport to the next step in the manufacturing process where they are converted into photovoltaic (PV) cells.
Step 3: Production Phase Intro
At this point, the wafer-cells cannot produce any more electricity than a piece of rock picked out of the river. But, the wafer is the primary building block for the PV cell. As the cell enters the multi-step production phase of manufacturing, its important characteristic is its crystalline structure and “positive” electrical potential.
Etching the Wafer
This phase of production is the only phase that requires a dedicated clean room. Here, the blank grey wafers are converted into productive cells, blue (if from multiple crystals and black if sliced from the same cell). The conversion is completed by a series of intricate heat and chemical treatments. An extremely small layer of silicone is removed from the wafer by a process called “texture etching” of the underlying crystal structure – and reveals an irregular pattern of pyramids, invisible to the naked eye. These pyramids have the ability to absorb more visible light (photons).
Diffusing the PV Cell
The next step in the production phase is too diffuse a thin layer of phosphorus onto the surface of each wafer. This is accomplished by placing the wafers in long cylindrical cartridges and placing them in an chamber-like oven. As the surface of the wafer is exposed at high heat to phosphorus gas, impregnation of the wafer occurs at a molecular level. Once completed, the surface has a negative potential electrical orientation. In combination with the boron doped layer below, the new negative layer on the surface creates a positive-negative junction. This P/N junction is the essential partition for the electrical functionality of each PV cell.
Coloring and Printing the Cells
Next, the newly made yet still gray cells are placed in trays and moved into heavy vacuum chambers. Here, blue-purple silicon nitrate is deposited on the tops of the cells. This coating of silicon nitrate (nitride) reduces reflection even further, in the energy dense blue end of the light spectrum, increasing absorption. With this process, the cells take on their final dark color, distinctive of solar (PV) cells. The cells can now produce electricity by optimally gathering photons. However, they still lack the ability to collect and transmit that power. This is accomplished through a series of silkscreen like steps. Printed in metal on both sides of the cell, therefore, are pinstriped "fingers" and bus-bar circuitry to control electrical flow of photon-sourced power. For the new functioning cell to work properly, the only thing now needed is light, itself. Sunshine!
Step 4: Assembling the Solar Panel
Each phase of the production must adhere to very specific processes, as listed. During crystal growth, careful control of heating and cooling was the dominant concern. In wafer making, there was precise abrasion and cutting. Finally, production of PV cells concentrated on chemistry. With any factory-produced item, a final assembly step is always needed. This final step in PV is known as moduleling – or the making of the solar module.
Soldering the Cells
Module manufacturing is a highly automated process at most panel manufacturers, and specifically at SolarWorld. The finished modules each way around 45 pounds (20+ kilograms). Housed behind safety fencing, steel robots handle the lifting of the assembled lightweight PV cells as they become finished products – PV modules. For 60-cell modules, the PV wafer-cells are first soldered into strings of ten (10) in an over-under-over-under pattern of metal connectors linking each cell. Laid out in six (6) strings, a rectangular matrix of 60 cells is created. Each of these rectangular matrixes is then laminated onto glass.
Final Framing, Inspection and Shipping
To become a complete module, a frame is added. This frame protects against weather and other impacts. Each module also must have a junction box added to enable inter-connection to other modules or to an inverter or charge controlling circuit (often via conduit). At SolarWorld, to be palletized for delivery to the end user, inspection and careful cleaning is final step.