This thesis work demonstrates the feasibility to fabricate high-efficiency crystalline silicon solar cells by using metal stencil printing technique to replace screen printing or electroless plating techniques for implementing crystalline silicon solar cell front metallization. The developed laser-cut stainless steel stencils successfully challenge two of the cell performance limitations associated with commercial screen printing technology: the wide and non-uniform front gridline fingers and low height-to-width aspect ratio of the fingers. These limitations lower the short circuit current density, the fill factor and, in turn, the efficiency of a screen printed solar cell. Metal stencils are capable of printing fine, high and continuous features on the cell front that have a high aspect ratio. Both single-level and double-level structured stainless steel stencils for solar cell front metallization have been developed, with laser-cut double-level stainless steel stencils being demonstrated for the first time worldwide. Both of them are able to print fine, high and continuous gridline pattern to the front surfaces of solar cells in one step, with a certain number of special short bridges being put at the places where fingers meet busbar and along fingers and busbar. The deformation issue of the very thin stainless steel foils due to its thermal expansion in the process of laser cutting is solved by increasing the energy content in each laser pulse that impinges upon the stainless steel foil with changed Q-switch frequencies, while maintaining the laser average output energy in unit time to an optimum value. A chemical etching process has been developed to etch the dross that results from laser cutting, resulting in well formed metal stencils suitable for printing. By a comparison between the metal stencil printed and conventional mesh screen printed silicon solar cells, which are fabricated on similar Cz silicon wafers with a almost identical cell processing sequence except for using different front contact printing masks, the following conclusions are reached: Fired Ag finger lines with 75-??m width on finished solar cells, using a doublelevel stainless steel stencil can be achieved. In contrast, the fired Ag finger line on finished solar cells using a traditional mesh screen is 121-??m wide. The stencil printed finger is smoother and more uniform than by screen printing and the former has a 25-??m fired finger height with a 0.33 height-to-width aspect ratio, compared to a 10-??m fired finger height with a 0.08 height-to-width aspect ratio for the later. With these advantages, the 4-cm2 stencil-printed silicon solar cells has an averaged 1.28 mA/cm2 higher short circuit current and an averaged 5.9% higher efficiency than the 4-cm2 screen printed silicon solar cell, which identifies one of the key advantages of solar cell metallization schemes by using metal stencil printing in place of screen printing. Using a ???feedback alignment??? method for registration of the laser-formed metal stencil printed pattern and the laser-formed groove pattern, Ag paste can be printed and filled into wafer grooves by using a hand-operated without an optical vision system. The fired finger profile is 50-??m wide and 22-??m high. The best metal stencil printed, selective emitter silicon solar cell demonstrates a 34.2 mA/cm2 short circuit current density, 625 mV open circuit voltage, 0.77 fill factor and 16.4% efficiency, with an excellent spectral response at short wavelengths due to its selective emitter cell structure. It is believed that the performance of this type of solar cell can be enhanced with a screen printer that has an optical vision system and an automatic alignment device. The successful development of metal stencil printed silicon solar cells demonstrates the feasibility of the metal stencil printing as a beneficial technology for the PV industry.
| Identifer | oai:union.ndltd.org:ADTP/278926 |
| Date | January 2005 |
| Creators | Yao, Guoxiao, Centre for Photovoltaic Engineering, UNSW |
| Publisher | Awarded by:University of New South Wales. Centre for Photovoltaic Engineering |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Guoxiao Yao, http://unsworks.unsw.edu.au/copyright |
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