Screen-printed solar cells were first developed in the 1970's. As such, they are the best established, most mature solar cell fabrication technology, and screen-printed solar cells currently dominate the market for terrestrial photovoltaic modules. The key advantage of screen-printing is the relative simplicity of the process.
There are a variety of processes for manufacturing screen-printed solar cells. The production technique given in the animation below is one of the simplest techniques and has since been improved upon by many manufacturers and research laboratories.
There are many variations to the scheme shown above which give higher efficiencies, lower costs or both. Some techniques have already been introduced into commercial production while others are making progress from the labs to the production lines.
- Phosphorus Diffusion
Screen-printed solar cells typically use a simple homogeneous diffusion to form the emitter where the doping is the same beneath the metal contacts and between the fingers. To maintain low contact resistance, a high surface concentration of phosphorus is required below the screen-printed contact. However, the high surface concentration of phosphorus produces a "dead layer" that reduces the cell blue response. Newer cell designs can contact shallower emitters, thus improving the cell blue response. Selective emitters with higher doping below the metal contacts have also been proposed 1, 2 - but none have yet been introduced into commercial production.
- Surface Texturing to Reduce Reflection
Wafers cut from a single crystal of silicon (monocrystalline material) are easily textured to reduce reflection by etching pyramids on the wafer surface with a chemical solution. While such etching is ideal for monocrystalline CZ wafers, it relies on the correct crystal orientation, and so is only marginally effective on the randomly orientated grains of multicrystalline material. Various schemes have been proposed to texture multicrystalline materials by using one of the following processes:
- Antireflection Coatings and Fire Through Contacts
Antireflection coatings are particularly beneficial for multicrystalline material that cannot be easily textured. Two common antireflection coatings are titanium dioxide (TiO2) and silicon nitride (SiNx). The coatings are applied through simple techniques like spraying or chemical vapour deposition. In addition to the optical benefits, dielectric coatings can also improve the electrical properties of the cell by surface passivation. By screen-printing over the antireflection coating with a paste containing cutting agents, the metal contacts can fire though the antireflection coating and bond to the underlying silicon. This process is very simple and has the added advantage of contacting shallower emitters 10.
- Edge Isolation
There are various techniques for edge isolation such as plasma etching, laser cutting, or masking the border to prevent a diffusion from occurring around the edge in the first place.
- Rear Contact
A full aluminium layer printed on the rear on the cell, with subsequent alloying through firing, produces a back surface field (BSF) and improves the cell bulk through gettering. However, the aluminium is expensive and a second print of Al/Ag is required for solderable contact. In most production, the rear contact is simply made using an Al/Ag grid printed in a single step.
Screen-printing has been used on a variety of substrates. The simplicity of the sequence makes screen-printing ideal for poorer quality substrates such as multicrystalline material as well as CZ. The general trend is to move to larger size substrates - up to 15 x 15 cm2 for multicrystalline materials and wafers as thin as 200 µm.
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