新能源9,10班翻译材料费下载.pdf

Source Material In the past it was common for the source material for solar cell production to be off cuts from the integrated circuit industry The source material has to be checked for impurities and to make sure that it has the right dopant type and resistivity Almost all cells are presently made on p type substrates Notable exceptions are the rear contact cells made by Sunpower and amorphous crystalline silicon heterojunction cells manufactured by Sanyo The solar cell industry has grown very rapidly in the past few years and now uses more silicon than the entire integrated circuit industry The rapid expansion has pushed up the price of raw crystalline silicon feed stock from under 20 kg to over 200 kg Silicon scrap comes from a wide variety of sources One source is off cuts and scrap material from the semiconductor industry Wafer scraps from the production line are recycled for growing new ingots Growing Ingots The ingot growth for Multicrystalline silicon is quite simple melt the silicon in a large crucible and let it cool slowly to form large crystal The specifics of furnace design allows the ingot to cool slowly so that very large grains 1 cm are formed Animation of the growth of a multicrystalline ingot Tub used for growing silicon The dimensions are about 50 cm x 50 cm x 25 cm deep The tub has to withstand the melting point of silicon at 1415 C For comparison iron melts at 1538 C Crystal growing furnaces The system is loaded and unloaded using the hoists at the bottom Loading the growth tub into the furnace Finished silicon ingot Sawing the Ingot into Bricks The large silicon ingot is sawn into more managable bricks The individual bricks are now ready to saw up into wafers Wafer Slicing Slicing up the bricks into wafers is a delicate operation Each wafer is up to 15 x 15 cm2 and under a third of a mm 300 m thick Modern solar cell factories use wire saws rather than the internal diameter blade saws previously used for the semiconductor industry In fact the semiconductor industry is now moving to the wire saw due to their superior technology A large industrial wire saw The wire sawing is done on the right hand side but the bricks are not loaded yet The left hand side holds the wire and the control circuitry Close up of the wires The wires are covered with slurry and are wound around the drums When running the drums spin at high speed and the silicon bricks are pushed down from the top Move the mouse over the picture for a closer view The feed wire is just visible coming in the top left of the spool Close up of the wire saw with the bricks loaded and ready to go Texturing Cutting silicon into wafers leaves the surface covered with cutting slurry and the surface is damaged due to the action of the saw Wafers are cleaned in a hot solution of sodium hydroxide that removes the surface contamination and the first 10 m of damaged silicon The wafers are then textured in a more dilute solution of sodium hydroxide with isopropanol as a wetting agent For multicrystalline wafers acidic texturing is often used as it gives a more uniform etch rate across gain boundaries After the wafers are cut into slices they are then put into cassettes for saw damage removal and texturing The cassettes are further loaded into holders for moving through the production line During the texturing process hydrogen is released and bubbles adhere to the wafer The mesh place on top of the cassettes stops the wafers from floating out of the cassettes Wafers during the texturing process Normally there is a protective blind in place to prevent sodium hydroxide splashes The protective blind rolls back and the wafers are removed from the texturing bath Wafers are put through several rinse cycles and an acid rinse to neutralise the sodium hydroxide Finally the wafers are loaded into a centrifuge for a final rinse and spin dry Emitter Diffusion The emitter diffusion process is performed in a variety of ways In this case a phosphorous containing coating is applied to the surface The wafers are then put in a belt furnace to diffuse a small amount of phosphorous into the silicon surface Loading the phosphorous coating The trend in production lines is to have as much automation as possible and to move the wafers through the lines on a belt The loading and unloading of cassettes is a major source of yield loss Wafers moving from the diffusion coating furnace to the high temperature diffusion furnace The diffusion coating is clear so the wafer do not look markedly different from the previous video The wafers travel through the diffusion furnace for roughly an hour Finally the wafers are loaded into a centrifuge for a final rinse and spin dry After the diffusion process the wafers are loaded back into casettes for an acid etch to remove the diffusion glass The edge isolation process removes the phosphorous diffusion around the edge of the cell so that the front emitter is electrically isolated from the cell rear A common way to achieve this is to stack the wafers on top of each other then plasma etch using CF4 and O2 Loading the plasma etching system and then etching the wafers Anti Reflection Coatings An antireflection of silicon nitride is typically deposited using chemical vapour deposition process CVD Precursor gases of silane SiH4 and ammonia NH3 are fed into a chamber and break down due to temperature LPCVD or due to a plasma enhancement PECVD Other systems use microwaves to cause the silane ammonia reaction to take place The complete reaction is 3SiH4 4NH3 Si3N4 12H2 but the usual reaction to produce a non stoichiometric film with the incoporation of large amounts of hydrogen SixNy H Older cell designs use titanium dioxide TiO2 which provides a good antireflection coating and is simpler to apply but does not provide surface or bulk passivation Wafers being deposited with silicon nitride antireflection coating giving a blue color Screen Print Front Silver paste is forced through a patterned screen Those areas with gaps in the pattern leaves a metal pattern on the surface Screen printer in operation The cells move along a conveyor belt Here they enter on the right and exit on the left Close up of the screen printing operation The cells are undernearth the printer so are not visible from the top After printing the paste is still wet and is easily smudged Here the wafers are loaded into a drier to evaporate off the organic binders in the paste Driers operate at a low temperature of around 200 C After the wafers are dried they are loaded back onto the conveyor belt to continue their movement in through the production line Zooming in on a cell after the front screen print is finished At this stage the silver still exists as a powder resting on the cell A later firing process at high temperature bonds the silver to the silicon Screen Print Rear Aluminium The rear is printed in two parts A thick layer of aluminium paste covers most of the cell and provides a back surface field BSF Silver strips are also printed on the cell for later soldering to the interconnect metal tabs After the front print is complete the cells are flipped over to print on the rear Printing the aluminium paste on the rear The cell is just visible through the screen as it moves through the line The aluminium printing is an expensive step since it is very thick and covers almost the whole cell A cell immediately after printing Small gaps are left in the aluminium to print silver strips later in the process Note how wet the paste is Drying the aluminium