Screen Printed Solar Cells

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.

Animation showing a basic technique for fabricating screen printed solar cells.

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.

1. 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 - and are being introduced into commercial production.
2. 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:
   1. mechanical texturing of the wafer surface with cutting tools or lasers 3, 4, 5;
   2. isotropic chemical etching based on defects rather than crystal orientation 6;
   3. isotropic chemical etching in combination with a photolithographic mask 7, 8;
   4. plasma etching 9.
3. 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 (SiN_x). 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.
4. 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.
5. 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.
6. Substrate
   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 20 x 20 cm² for multicrystalline materials and wafers as thin as 150 µm.

Close up of a screen used for printing the front contact of a solar cell. During printing, metal paste is forced through the wire mesh in unmasked areas. The size of the wire mesh determines the minimum width of the fingers. Finger widths are typically 100 to 200 µm.

Close up of a finished screen-printed solar cell. The fingers have a spacing of approximately 3 mm. An extra metal contact strip is soldered to the busbar during encapsulation to lower the cell series resistance.

Front view of a completed screen-printed solar cell. As the cell is manufactured from an older multicrystalline substrate, the different grain orientations can be clearly seen. Newer multicrystaline cells have finer grains that are not easily seen. The square shape of a multicrystalline substrate simplifies the packing of cells into a module.

Rear view of a finished screen-printed solar cell. The cell can either have a grid from a single print of Al/Ag paste with no BSF, or a coverage of aluminium that gives a BSF but requires a second print for solderable contacts. Click on the image to switch between the two views.

• 1. J. Horzel, Szlufcik, J., Nijs, J., and Mertens, R., “A simple processing sequence for selective emitters”, Twenty Sixth IEEE Photovoltaic Specialists Conference. New York, NY, USA, pp. 139-142, 1997.
• 2. D. S. Ruby, Yang, P., Zaidi, S., Brueck, S., Roy, M., and Narayanan, S., “Improved Performance of Self-Aligned, Selective-Emitter Silicon Solar Cells”, 2nd World Conference and Exhibition on Photovoltaic Solar Energy Conversion. Vienna, Austria, 1998.
• 3. S. Narayanan et al., “18% efficient polycrystalline silicon solar cells”, Twenty First IEEE Photovoltaic Specialists Conference, vol. 1. pp. 678-680, 1990.
• 4. G. Willeke, Nussbaumer, H., Bender, H., and Bucher, E., “A simple and effective light trapping technique for polycrystalline silicon solar cells”, Solar Energy Materials and Solar Cells, vol. 26, pp. 345 - 356, 1992.
• 5. R. Hezel, “Recent progress in MIS solar cells”, Progress in Photovoltaics: Research and Applications, vol. 5, pp. 109-120, 1997.
• 6. R. Einhaus, Vazsonyi, E., Szlufcik, J., Nijs, J., and Mertens, R., “Isotropic texturing of multicrystalline silicon wafers with acidic texturing solutions”, Twenty Sixth IEEE Photovoltaic Specialists Conference. New York, NY, USA, pp. 167-170, 1451, 1997.
• 7. M. J. Stocks, Carr, A. J., and Blakers, A. W., “Texturing of polycrystalline silicon”, Solar Energy Materials and Solar Cells, vol. 40, pp. 33 - 42, 1996.
• 8. J. Zhao, Wang, A., and Green, M. A., “19.8% Efficient Multicrystalline Silicon Solar Cells with Honeycomb Textured Front Surface”, 2nd World Conference and Exhibition on Photovoltaic Solar Energy Conversion. Vienna, Austria, 1998.
• 9. K. Fukui, Inomata, Y., and Shirasawa, K., “Surface texturing using reactive ion etching for multicrystalline silicon solar cells”, Twenty Sixth IEEE Photovoltaic Specialists Conference. New York, NY, USA, pp. 1451, 47-50, 1997.
• 10. J. Szlufcik, Sivoththaman, S., Nlis, J. F., Mertens, R. P., and Van-Overstraeten, R., “Low-cost industrial technologies of crystalline silicon solar cells”, Proceedings-of-the-IEEE, vol. 85. pp. 711-730, 1997.