Production of crystalline silicon solar cells, one of the most critical step is made in front and back of a very fine circuit, electronic light students derived cells.The metal coating process is usually performed by screen printing technology, conductive paste containing a metal wire net through pore pressure on circuit or electrode is formed on silicon wafers.Typical of crystalline silicon solar cells in the whole production process from beginning to end the need for multiple screen printing steps.Usually, there are two different process for the battery positive (contact line and bus) and the back (screen printing electrode/passivation and bus).The solar cell is the development and utilization of new energy, low carbon environmental protection of trademark products.Silk screen printing on the solar cell is, the making process of the back electrode and the back field plays a very important role, silk screen printing technology is now quite mature solar electrodes, has become the mainstream technology.
1. Basic solar screen printing
Start printing process from silicon placed to the stage.Very fine silk screen plate fixed on the net frame, placed above the silicon wafer, silk screen closed other areas other than the grid line, so that the conductive paste to pass.Silicon and silk screen distance to strictly control (called printing clearance, or network distance).Because positive need to be more slender wire, used in positive printing screen its grid usually are much smaller than used to print on the back.Placed the right amount of slurry on the screen, with scraper daub pulp, evenly fill in mesh, blade in the process of moving the pulp through wire net hole extrusion on silicon wafers.
The process of temperature, pressure, speed and other variables must be strictly controlled.Every time after the completion of the printing, silicon chips are placed in baking furnace, the conductive paste solidification.Then the silicon was sent to a different printer, printed on the back of their positive or more lines.After completion of all printing steps, put the wafer in high temperature furnace sintering.
2. The silicon printing front and back
The front and back of each piece of solar cells has through the conductor screen printing, their functions are different.Positive line finer than the back;Some manufacturers to print on the back of the first conductive wire, silicon chip and then turn it over to the printing front line, so as to maximize reduce possible damage in the machining process.(face to the sun on the front side), the design of most of the crystalline silicon solar cells are very fine circuit (" fingers line "), the effective area collected photoproduction electron transfer to the larger gathering wire - "bus", then passed to the components of the circuit in the system.The fingers of the positive line are much finer than on the back of the line (narrow to 80 microns), and because of that, positive printing steps need higher precision and accuracy.The back of the silicon wafer and positive printing requirements are different, technology is not so strict.The first step in the printing process is printing on the back a layer of aluminum based conductive material, and not very thin conductive grid.At the same time, can not capture the pool of light reflected back.This layer can also "passivation" solar cells, closed redundant molecular path, avoid flow electronic capture by the gap.Printed on the back of the second step is to create bus, connected with an external circuit system.
3. The related technological parameters
As far as possible in order to improve the conversion rate of solar cells, reduce the shade of silver wires with panels on the silk screen version of grid line width should be narrow, could cause and grid line is too fine conductive silver pulp thickness is too thin, it may even be broken.Therefore, the grid lines on the screen version generally between 80 ~ 80 microns wide.Due to the conductive silver pulp after printing than the width of the silk screen version increased, after printing a silver pulp after sintering width between 110-150 microns.In order to reduce the series resistance, on the panels of try to lower the total weight of the silver wires and basic in 0.01 ~ 0.02 g.Printing pressure in 75 ~ 80 Newton.
Industry's latest research suggests that if you use a dedicated emulsion such as TieFo dragon multilayer film screen, the conversion rate and the stability of the solar cell has a good performance.The web version of the specification and related technical parameters for:
4. Other requirements
Screen tension of the web tension is commonly 28 + 2 n.Back to the life of the electrode wire mesh should be more than 15000 times, back to the life of the electric wire mesh should be more than 15000 times, is the life of the electrode wire mesh should be greater than 10000 times.
Screen should be stored in the condition of constant temperature, constant humidity, temperature of dramatic change to cause a decline in tension, and humidity will cause degeneration of the latex, which effect use effect and even scrapped.
Specific storage conditions: temperature 22 + / - 3 ℃, humidity of 50% plus or minus 10%.
Production workshop before use should be silk screen in advance, in plant stand for more than 24 hours.The plant should ensure when in urgent need of let stand for 2 hours.
Screen in front of the factory has always been stored in a clean environment, therefore before use should be kept clean, quality inspection process should be performed in a relatively clean environment, after the inspection should be in the original sealed plastic bags in warehouse, to avoid dust contamination before use.
Screen is very sensitive to vibrations, any collision led to the decrease of the tension and even collapse net scrap, therefore in the process of mobile, use to should take put down gently, to avoid any collisions.
Any blunt objects on the surface of the screen is very fragile, net yarn section may cause damage, decrease tension, graphics, damage, shorten service life even scrapped.So in the mobile, should be avoided in the process of using net yarn surface contact with any blunt objects.
Is the best way to store with dedicated vertical storage shelf, if the flat is not more than 5 layers, and layer between the separated at net frame with a soft cushion.
Silk screen of the emulsion is not a very stable material, and thickness is small, in the process of storage some complicated physical and chemical changes will happen, so after screen into the factory should be used within a month.