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«LASER PROCESSING FOR THIN AND HIGHLY EFFICIENT SILICON SOLAR CELLS by Jostein Thorstensen Thesis submitted in partial fulfillment for the degree of ...»

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LASER PROCESSING FOR THIN

AND HIGHLY EFFICIENT

SILICON SOLAR CELLS

by

Jostein Thorstensen

Thesis submitted in partial fulfillment

for the degree of Philosophiae Doctor

Department of Physics

Faculty of Mathematics and Natural Sciences

University of Oslo

March, 2013

© Jostein Thorstensen, 2013

Series of dissertations submitted to the

Faculty of Mathematics and Natural Sciences, University of Oslo No. 1367 ISSN 1501-7710 All rights reserved. No part of this publication may be reproduced or transmitted, in any form or by any means, without permission.

Cover: Inger Sandved Anfinsen.

Printed in Norway: AIT Oslo AS.

Produced in co-operation with Akademika Publishing.

The thesis is produced by Akademika publishing merely in connection with the thesis defence. Kindly direct all inquiries regarding the thesis to the copyright holder or the unit which grants the doctorate.

ACKNOWLEDGEMENT

Although it may seem so from an outside perspective, a Ph. D. thesis is definitely not a solo race. (At least mine haven’t been one.) I can’t even take full credit for the decision to apply for a Ph. D. position at IFE, as this decision was strongly influenced by sensible and good advice from my friend and colleague Trygve Mongstad. Without him, I might not have ended up doing a Ph. D. at all, which would have been a great loss.

I have really appreciated the unique possibility given to me to devote myself for three years to play with cool lasers and stuff while at the same time trying to do my share at saving the world. However, the ride wouldn’t have been nearly as rewarding if I hadn’t been working with some of the best people I have yet come to know. You have always been positive and ready for fruitful discussions on all of the solar related topics that I have needed your help for, thereby doing your share at forming the contents of my thesis. At lunch breaks, coffee breaks, late evenings, conferences and cabin trips, you have been there, making sure that every day has had an enjoyable side. You have given me memories for life, I hope that I have given you something back. I’ll remember you always.

My work on light-trapping structures would not have been the same without my cooperation with Jo Gjessing. Your competence, patience and collaboration on this topic has been greatly appreciated. My semiconductor and passivation expert, Halvard Haug has been a smile full of knowledge throughout my thesis.

I would like to thank my supervisors, Sean Erik Foss, Aasmund Sudbø and Erik Marstein for the valuable input and guidance I have received. Erik, you are always positive and encouraging, emphasizing that cool and important may very well be the same thing.

Aasmund, your experience and knowledge has been invaluable, especially in the process of writing articles and the thesis. You are always patient and thorough, and my work has benefited greatly from your effort. Sean Erik, I probably haven’t been the easiest of Ph. D.

students, demanding quite a lot of space in your busy schedule. But I hope that you agree with me when I say that working together on finding our way through the maze that is laser processing for silicon solar cells has been a great journey. You have somehow always i found time for me, and our many discussions has lead us to some pretty interesting findings (although different findings than what we expected three years ago).

I wish to thank my parents for being there, always interested when I talk about my work (which must be pretty

Abstract

for you by now), and always supportive no matter what. Finally, thank you, Åsa, love of my life. You never doubted that I could do this, even when I sometimes did. You make me stronger than I would be without you.

ii ABSTRACT Solar 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 iii 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 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.

iv

TABLE OF CONTENTS

ACKNOWLEDGEMENT

ABSTRACT

TABLE OF CONTENTS

1 INTRODUCTION

1.1 SILICON SOLAR CELLS

1.2 MOTIVATION AND OBJECTIVE OF THE THESIS

1.3 STRUCTURE OF THE THESIS

1.4 SUMMARY OF THE ARTICLES

2 EXPERIMENTAL TOOLS AND TECHNIQUES

2.1 LASERS

2.2 EXTRACTION OF LASER PARAMETERS

2.3 THIN FILM DEPOSITION

2.4 MICROSCOPY

2.5 WET CHEMICAL PROCESSING

2.6 REFLECTANCE AND TRANSMITTANCE MEASUREMENTS

2.7 MINORITY CARRIER LIFETIME

2.8 SILICON SUBSTRATES

3 LASER PROCESSING FOR SILICON SOLAR CELLS

3.1 STATE OF LASER PROCESSING FOR SILICON SOLAR CELLS

3.2 LASER-MATERIAL INTERACTION

3.3 SIMULATIONS ON LASER-MATERIAL INTERACTION

3.4 LASER INDUCED DAMAGE

4 LIGHT-TRAPPING STRUCTURES IN SILICON SOLAR CELLS

4.1 LIGHT MANAGEMENT IN SILICON SOLAR CELLS

4.2 STATE OF LASER TEXTURING

4.3 MASKED LASER TEXTURING

5 CONCLUSION

6 DISCUSSION AND OUTLOOK

BIBLIOGRAPHY

A. ANALYTICAL EXPRESSION FOR RECOMBINATION BY LASER DAMAGED REGION................ 69 v A.1 ELECTRON DISTRIBUTION

A.2 SURFACE RECOMBINATION VELOCITY

A.3 EFFECTIVE LIFETIME

LIST OF ABBREVIATIONS

PAPER I

PAPER II

PAPER III

PAPER IV

PAPER V

PAPER VI

PAPER VII

PAPER VIII

vi 1 INTRODUCTION Electricity from sunlight. Direct harvesting of the immense and never-ending power brought to us by the sun. Not long ago, this elegant way of generating electricity was associated with satellites and space stations, or remote off-grid locations needing electricity to power a light bulb in a cabin. Today, on the other hand, we can read that Germany generates 50 % of its electric power from photovoltaic (PV) energy during mid-day hours on a sunny day [1]! In 2011, more than 28 GW of new PV generating capacity was installed globally [2]. This corresponds to about 200 km2 of solar panels, or 1.5 times the size of the city of San Francisco! Obviously, our view on PV as a small niche market needs to be reviewed.

In a world where a rapidly increasing demand for energy is ever more strongly conflicting with an urgent need to cut back on greenhouse gas emissions, it seems necessary and inevitable that renewable energy sources will play a major role in our future global energy system. A recent report from the Intergovernmental Panel on Climate Change [3] predicts that wind and PV will account for up to 30 % of the world’s electricity production by 2050, even in the moderate scenarios.

Direct solar energy is a tremendous energy resource, delivering around 4x1024 J of energy to the earth’s surface per year (assuming a solar flux of 1 kW/m2). The world’s total energy consumption was in 2010 around 5.6x1020 J [4], meaning that the solar energy hitting the earth in about one hour is sufficient to cover the energy needs of the humanity for a whole year! This is by far the biggest source of energy available to us, and a great candidate for a transition to a more sustainable energy system. Furthermore, silicon based PV is based on non-toxic, abundant materials, silicon being the second most abundant element in the earth’s crust after oxygen.

PV is currently the fastest growing renewable energy source, with an average growth rate of above 40 % per year since the year 2000 (Figure 1.1). Silicon based solar cells have an 85 % market share [5], and is thereby the absolutely dominant technology in PV. The growth in PV has been linked to economic incentives, and continued growth in

2 CHAPTER 1: INTRODUCTION

installed PV cannot rely on politically driven incentives alone. PV learning curves have, since the 1970’s shown a 20 % reduction in module prices per doubling of cumulative production [6], a quite tremendous price reduction. This trend in price reductions however, has to be continued as incentives are continuously being reduced. This can either happen through reduction of production costs (fewer $ per solar cell), or by an increase in efficiency (more watts per solar cell). A combination of both would of course be ideal. In the current situation, the price for manufacturing of the solar cell and solar module has been dramatically reduced. This leads to a situation where balance of system costs, such as installation costs, the costs of mounting brackets, land usage costs etc. are beginning to dominate the total cost of a PV energy system [7]. Increased efficiency of the solar cell will reduce balance of system costs, e.g. by reducing the number of brackets and land area required for a given output power, meaning that retaining or improving the efficiency of the solar cell is essential for reduction of PV system costs.

The strive towards low cost, high efficiency solar cells has led to the introduction of several new processing tools and techniques that have enabled the impressive cost reductions seen in the PV industry. One group of tools that has the potential to change existing production techniques, and enable new processes and even new solar cell designs are lasers. Lasers have the ability to structure, cut or remove materials, alter the chemical composition of materials through the introduction of impurities, and several other processes. As shall be shown later in this thesis, there exists a range of solar cell related processes for which lasers can be applied. This thesis will focus on the use of lasers as a processing tool for improvement of silicon PV, where lasers have the potential to improve the efficiency of the solar cell and to reduce production costs.

–  –  –

1.1 SILICON SOLAR CELLS Solar cells operate by converting sunlight into electricity. In this section a brief review of the solar cell physics will be given. For a more thorough introduction, see e.g. [10].



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