Make Money Printing Solar Cells!!
Harness Energy from the Power of the Sun
Today, solar cell & solar panel manufacturing are one of the hottest and most profitable business opportunities available....add in government start-up grants and contracts, and you'll find it very lucrative, while helping the environment.
With high tech thin film and nano particle technology in the development stage, Silicon Crystal Solar Cells are still the heart of solar energy.
Because of their high yields of energy, long lifespan, and moderate cost, Silicon Crystal is the best choice for alternative energy.
Producing the cells are not much more difficult than screen printing a three color design. The substrate is a doped (treated with boron) silicon wafer.
The most common size wafer being 156mm (6" square). The front of the wafer is printed with a silver paste to create the electrical grid works, or Anode.
The cell back is also printed with the same silver paste in spots to create electrical connections, or Cathode.
The cell back is then printed with an aluminum mask to entrap the collected light energy.
The cell must after each print be sent through a dryer, or sintering oven. The solvents evaporate and the metallic paste fuse to the silicon wafer.
The cell is now ready to be connected to up to 72 other cells to make a full solar panel.
The cells are then laminated into a protetive sleeve before being placed in a glass frame were it is wired and waterproofed.
The Screen Printing and Sintering Technology
The principle of screen printing is show in figure 1. A pattern is photographically defined on a stainless stell screen by means of an emulsion layer.
A paste of the material to be screen printed is pressed through the screen by means of a squeegee. Important screen printing parameters are:
The viscosity of the paste
The mesh count of the screen (Threads/Inch)
The snap-off distance between the screen and the substrate
The pressure and speed of the squeegee
After leveling, the printed wet film is dried (e.g. at 120 C / 60 Min). By then, the film consists of loose conglomerates of very small grains.
The sintering step results in a compact film, where large grains are electronically and optically intimately connected
Sintering is a high temperature step (typically 500 to 800 C), and is usually performed in an oven with a transport belt. In many cases and inert nitrogen atmosphere in the oven is required.
Often a "sinter flux" is added to the paste. At the sintering temperature, this flux is melting, and forms a liquid shell around the grains of the material to be sintered
Sintering is then enhanced by the mechanisms of liquid phase sintering. At first, there is a rearrangment of the grains due to capillary forces; then, small grains are dissolving in the liquid flux, and material is condensed on the larger grains; next, in a coalescence phase, material is dissolved from places with positive curvature, and deposited at places with negative curvature; finally, neck growth can occur, when two grains are seperated by a capillary liquid film. For all these mechanisms to happen, it is necessary that the liquid sinter flux completely wets the solid grains, and that the solubility of these grains in the liquid flux is high enough
Finally, in the materials systems under discussion, the sinter flux should be volatilized completely at the end of the sintering process. The most critical issue when one is to sinter a new semiconductor material is to find a new suitable sinter flux.
Screen printing and sintering of thin film CdTe solar cells was first pioneered by the Japanese company Matshushita; further work was done by the Universities of Gent and of Seoul.
Cadmiumchloride is an almost ideal sinter flux for both CdS and CdTe. It has a low melting temperature, and forms a low melting eutectic with both CdS and with CdTe. The solid solubility of CdTe in liquid CdCl2 at 600 C is high; the same holds for CdS. Finally, the vapor pressure of CdCl2 at 600 C is high enough to allow complete volatilization of CdCl2 after the sinter process.
The basic device structure is shown in Fig 4a. First CdS is screen printed from a (CdS powder + propanediol binder + CdCl2 flux powder) - paste is screen printed at 575-650 C. In both cases, sintering is under nitrogen (<30 ppm of oxygen). As the back contact, a copper doped graphite paste is screen printed and annealed in a N2 atmosphere, which contains a small amount (~1%) of O2.
For small cells, efficiencies up to 12.8% were obtained. When the screens are correctly patterned and positioned relative to each other, the monolithic series integrated structure of Fig 1. is automatically obtained. Module efficiencies up to 8.7% were obtained.
