Laser processing for thin and highly efficient silicon solar cells
Appears in the following Collection
- Fysisk institutt 
AbstractSolar energy is rapidly becoming one of the most promising renewable energy sources available to us. Its abundant availability greatly surpasses any other energy source, and with the immense progress seen in production technology for photovoltaics (PV) over the last decade, the price for converting solar energy into electricity is rapidly decreasing. However, further price reductions are still required for solar energy to be directly cost competitive with conventional energy sources in the majority of the world.
This thesis focuses on the use of lasers as a processing tool for silicon based PV. Lasers may perform a range of solar cell processes, such as edge isolation, doping, removal of dielectrics, structuring and contact formation, and have the potential to enable processes required for advanced, high efficiency solar cell concepts.
Two objectives were formulated for this thesis. The first objective focuses on acquiring new fundamental knowledge on the interaction between ultrashort pulse lasers and silicon and dielectrics used for solar cells. Such knowledge is valuable in itself, and is important for process understanding and development. The second objective focuses on the development of laser based techniques for the production of light-trapping textures. This as light trapping gets increasingly important as the wafer thickness used in industry is constantly being reduced and as new wafering techniques may render traditional texturing methods obsolete.
On the interaction between pulsed lasers and silicon or dielectric layers, emphasis has been put on ultrashort laser pulses. Mechanisms causing ablation and the process result after ablation have been the main focus. The most investigated dielectric has been silicon nitride thin films. Through experiments and simulations it has been found that the dense electron-hole plasma created during the leading edge of an ultrashort laser pulse, either through linear or two-photon absorption, will play a prominent role in the ablation behavior of both silicon and silicon nitride using such ultrashort laser pulses. It has been shown that this plasma formation causes optical confinement of the laser energy which in silicon greatly reduces the optical penetration depth, and as such reduces the depth of the laser induced damage. Using lasers at a wavelength of 532 nm, the depth of the laser induced damage is reduced from approx. 3 µm to around 0.25 µm when going from nanosecond to picosecond pulse duration. Knowledge about the depth of laser damage as function of pulse duration is valuable when seeking the right laser for a given process. In silicon nitrides, the plasma formation causes significant energy deposition into normally transparent films and may open for direct ablation of the dielectrics. It has also been shown that the ablation threshold on silicon is dependent on the temperature of the silicon substrate. In production, this would mean that the use of slightly elevated substrate temperatures would reduce the laser power required for a given throughput, or correspondingly increase throughput achievable with a given laser power.
On the topic of light-trapping structures fabricated by the use of lasers, two processes have been developed, and the performance of the textures has been measured. The patch texture, a geometric light-trapping texture for <100>-oriented monocrystalline silicon, showed a simulated increase in Jsc of 0.5 mA/cm2 when compared with the random pyramids texture, being the current industry standard. New wafering techniques provide thin silicon wafers for which the patch and random pyramids textures may not be applicable, and for which no industry standard texturing process exists. With this in mind, a diffractive honeycomb texture was developed. The use of microspheres on the wafer surface as focusing elements enabled the production of features with sizes well below 1 µm. The diffractive honeycomb texture shows a photogenerated current of 38 mA/cm2 on 21 µm thick silicon wafers.
The results summarized above shows that both fundamental understanding of the laser-material interaction and results that are directly applicable have come from the investigation of laser-material interaction. The texturing processes that have been developed show that laser based texturing processes are capable of delivering high quality textures suitable for a range of different substrates.
List of papers. Papers I, II, IV and VI are removed from the thesis due to publisher restrictions.
Paper I: J. Thorstensen and S. E. Foss, “Laser assisted texturing for thin and highly efficient monocrystalline silicon solar cells,” in Proceedings of the 26th European Photovoltaic Energy Conference, pp. 1628 – 1631, 2011. doi:10.4229/26thEUPVSEC2011-2BV.3.11
Paper II: J. Thorstensen, S. E. Foss, and J. Gjessing, “Light-trapping properties of patch textures created using Laser Assisted Texturing,” Progress in Photovoltaics: Research and Applications, Early view. Article first published online: 16 JAN 2013. doi:10.1002/pip.2335
Paper III: J. Thorstensen, J. Gjessing, E. Haugan, and S. E. Foss, “2D periodic gratings by laser processing,” Energy Procedia, vol. 27, pp. 343–348, 2012. doi:10.1016/j.egypro.2012.07.074 Published Open Access. © 2012 Published by Elsevier Ltd.Selection and/or peer review under responsibility of the Centro de Micro Análisis de Materiales, Universidad Autónoma de Madrid.
Paper IV: J. Thorstensen, J. Gjessing, E. S. Marstein, and S. E. Foss, “Light-trapping Properties of a Diffractive Honeycomb Structure in Silicon,” IEEE Journal of Photovoltaics, vol.3, no. 2, pp. 709 – 715, 2013. doi:10.1109/JPHOTOV.2013.2240563
Paper V: J. Thorstensen and S. E. Foss, “Temperature dependent ablation threshold in silicon using ultrashort laser pulses,” Journal of Applied Physics, vol. 112, no. 10, p. 103514, 2012. doi:10.1063/1.4766380 Copyright 2012 American Institute of Physics
Paper VI: Jostein Thorstensen, Ragnhild Sæterli and Sean Erik Foss, “Laser ablation mechanisms in thin silicon nitride films on a silicon substrate,” submitted to IEEE Journal of Photovoltaics, April 2013.
Paper VII: Jostein Thorstensen and Sean Erik Foss, “New approach for the ablation of dielectrics from silicon using long wavelength lasers,” Energy Procedia, vol 38, pp. 787-793, 2013. doi:10.1016/j.egypro.2013.07.347 Published Open Access. Copyright 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the scientific committee of the SiliconPV 2013 conference
Paper VIII: Jostein Thorstensen and Sean Erik Foss, “Investigation of depth of laser damage to silicon as function of wavelength and pulse duration,” Energy Procedia, vol 38, pp. 794-800, 2013. doi:10.1016/j.egypro.2013.07.348 Published Open Access. Copyright 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the scientific committee of the SiliconPV 2013 conference