What is screen printing?
Screen printing is simply using stencil to reproduce the same print over and over again. PV solar cells are usually metalized by screen printing process. This involves the application of three different metallization paste types onto the c-Si cell. The first paste is the front side silver used on the side that faces the sun; it creates the collector gridlines and silver bus bars, and the second is the rear side tabbing silver or silver-aluminum, and the third is rear-side aluminum paste that reacts with silicon to create the back surface field.
Screen printed solar cells are the finest established, most mature technology of solar cell fabrication, it was first developed in the 1970s. Screen printed solar cells dominate the terrestrial photovoltaic modules market at the moment. The main advantage of screen printing process is the relative simplicity.
Factors affecting screen printing
However, some variables involved in screen printing photovoltaics must be monitored:
- ink composition
- press setup
Below are some processes that affect the screen printed solar cells:
- Phosphorous Diffusion
Screen printed solar cells usually use simple homogeneous diffusion for forming the emitter where doping is the same between the fingers and beneath the metal contacts. To maintain contact resistance at a low level, a high surface concentration of phosphorous is needed beneath the screen printed contact. Yet, the high phosphorous surface concentration produces a dead layer which reduces the cell blue response. Newer cell designs can improve cell blue response by contacting shallower emitters.
- Surface Texturing to Reduce Reflection
Wafers that are cut from a monocrystalline material (monocrystalline material) can be textured easily to reduce reflection through etching pyramids on the wafer surface using a chemical solution. While this etching is perfect for monocrystalline CZ wafers, it depends on the correct crystal orientation, and thus is only marginally effective on randomly orientated grains in the multicrystalline material. Many alternatives have been proposed for texturing multicrystalline materials, using one of the below processes:
- wafer surface mechanical texturing with cutting tools or lasers
- plasma etching
- isotropic chemical etching based on defects instead of crystal orientation
- isotropic chemical etching combined with photolithographic mask
- Antireflection Coatings and Fire Through Contacts
Antireflection coatings are specifically beneficial for multicrystalline material which cannot be textured easily. Two known antireflection coatings are TiO2 (titanium dioxide) and SiNx(silicon nitride). The coatings are applied using simple techniques as chemical vapour deposition or spraying. Adding to optical benefits, dielectric coatings can as well improve the cell’s electrical properties by surface passivation. By screen printing a paste containing cutting agent over the antireflection coating, the metal contacts can bond to the underlying silicon by firing though antireflection coating. This is a very simple process and has an extra advantage of contacting shallower emitters.
- Edge Isolation
There are different techniques for edge isolation like plasma etching, laser cutting, or border masking to prevent any diffusion occurring around the edge.
- Rear Contact
Printing a full aluminum layer on the rear of the cell, with ensuing alloying through firing, produces a BSF (back surface field) and improves the cell bulk. But aluminum is expensive and a second print of Al/Ag is needed for solder able contact. Generally in production, the rear contact is just made by single step printing of an Al/Ag grid printed.
Screen printing is used on a number of substrates. The sequence simplicity makes screen printing perfect for substrates of poorer quality like multicrystalline material and also CZ. The general trend is to replace larger size substrates (up to 15 x 15 cm2) with multicrystalline materials and wafers with thickness as thin as 200 µm.