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Abstract
This thesis focuses on the development of laser processes to be used for advancements in solar cell fabrication. Two different types of laser processing applications are investigated. The first is the development of an innovative laser doping with grooving technique. Both buried contact solar cells (BCSC) and laser doped selective emitter (LDSE) solar cells have achieved considerable success in large-scale manufacturing. Both of these technologies are based on plated contacts. The high metal aspect ratios achieved by BCSC allow for low shading loss while the buried metal contacts in the grooves provide good contact adhesion strength, which has in some cases been shown to exceed that of conventional screen-printed contacts. In comparison, although the LDSE cell achieves significantly higher efficiencies and is a much simpler approach for forming the selective emitter region and self-aligned metal plating, the metal adhesion strength falls well short of that achieved by the BCSC. The first part of this thesis involved the development of a new concept of laser doping with grooving to form narrow grooves with heavily doped walls in a simultaneous step, with the self-aligned metal contact subsequently formed by plating. This process capitalizes on the benefits of both BCSC and LDSE cells. The laser-doped grooves are only 3-5 um wide and 10-15 um deep; the very steep walls of these grooves remain exposed even after the subsequent deposition of the antireflection coating (ARC). This unique feature significantly reduces the formation of laser-induced defects since the stress due to the thermal expansion mismatch between the ARC and silicon is avoided. Furthermore, the exposed walls allow for nucleation of the subsequent metal plating. This novel structure also benefits from greatly enhanced adhesion of the plated contact due to it being buried underneath the silicon surface in the same way as the BCSC. Cell efficiencies of over 19% are achieved by using this technology on p-type Czochralski (Cz) wafers with a full area aluminium (Al) back surface field (BSF) rear contact. Test structures indicate that much higher voltages and consequently higher efficiencies could be achieved if this technology is combined with a passivated rear approach.
The second part of this project is focused on using laser illumination to control the charge state of hydrogen atoms within the silicon. The diffusivity of hydrogen in silicon can vary by up to a few orders of magnitude depending on the charge state of the interstitial atoms. Recent studies show that illumination or carrier injection plays an important role in controlling the charge state of hydrogen. A laser hydrogenation tool was developed together with a colleague to study this effect. By carefully controlling the charge state of hydrogen, defects and recombination centres can be passivated within the silicon wafer. This has been shown to be able to potentially transform low quality UMG silicon wafers into ones similar in quality to monocrystalline Cz silicon wafers, and this approach has been shown to improve cell efficiency and eliminate light induced degradation (LID) in p-type monocrystalline solar cells. A prototype tool based on LEDs was developed to demonstrate this technology on 6-inch commercial solar cells. A laser equipment manufacturer has also developed a commercial tool based on this laser hydrogenation technology, showing that this technology not only works in the research lab but is ready to be transferred to the industry.
Finally the laser doping and grooving technology is incorporated with the advanced laser hydrogenation with the goal of passivating laser induced defects. Over 1 % absolute efficiency improvement was achieved on cells with a relatively large amount of laser induced defects, resulting in the defected cells achieving similar performance to those with less laser damage introduced during fabrication. This suggests that the laser doping and grooving cell technology could be carried out with a wider processing window if advanced hydrogenation technology is incorporated.