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Optimisation of a Protocrystalline Hydrogenated Amorphous Silicon Top Solar Cell for Highest Stabilised Efficiency

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Optimisation of a Protocrystalline Hydrogenated Amorphous Silicon Top Solar Cell for Highest Stabilised Efficiency Optimisation of a protocrystalline hydrogenated amorphous silicon top solar cell for highest stabilised efficiency Jimmy Melskens Cover: A selection of the solar cells discussed in chapter 5 and chapter 6 of this thesis. Photo courtesy of S. M. Klingens. Copyright © 2007 by Jimmy Melskens Optimisation of a protocrystalline hydrogenated amorphous silicon top solar cell for highest stabilised efficiency by Jimmy MELSKENS A thesis written for obtaining the degree of Master of Science in Electrical Engineering (variant Microelectronics) from Delft University of Technology Exam committee: Prof. dr. C. I. M. Beenakker Dr. M. Zeman Dr. M. Bartek Ir. G. van Elzakker Delft, November 2007 Acknowledgements The completion of this M.Sc. thesis project and report would not have been possible without the support of several people. Firstly, I would like to thank dr. Miro Zeman for writing an interesting project proposal for me, guiding me throughout the whole project, and showing me the possibilities of solar cells in everyday life. In addition to this, I am grateful for the opportunity I was given to publish a part of my work in a paper, of which I have presented the contents at the European Materials Research Society 2007 Spring Meeting in Strasbourg, France. Secondly, I am grateful for the support of ir. Gijs van Elzakker and ir. Bas Vet who have advised me during the last year to overcome practical difficulties and for all the times they have made free for me to discuss both the theoretical and practical issues I had to deal with. I am particularly grateful for providing me with the samples I required and the support I received during times of tedious measurements. Also, I am grateful for numerous discussions with dr. René van Swaaij on several theoretical and practical issues. Further, I would like to thank ing. Martijn Tijssen and ing. Kasper Zwetsloot for providing me with the samples I needed for my project and their expertise on the measurement systems in the Solar Cell Group. I am also thankful to Delft ChemTech for making available their spectrophotometer and to dr. Stefan Luxembourg for performing the spectrophotometry measurements I required. Besides the support I received from the above-mentioned and other members of the Solar Cell Group, I am thankful as well for the assistance of ing. Johan Vijftigschild during the outdoor experiment I performed, and for the craftsmanship of the people in the faculty’s mechanical workshop Dienst Elektronische en Mechanische Ontwikkeling (DEMO), without whom improvement of the Fourier Transform Photocurrent Spectroscopy (FTPS) measurement setup would not have been possible. Apart from the technical support I received over the last year, I am grateful for the nice atmosphere in the Solar Cell Group in which I have been working. Whenever I needed some distraction from the work, there was always some time to have a chat with someone. In this respect, I would like thank my fellow M.Sc. students in the Solar Cell Group in particular: Yingge Li (China), Angela Thelen (United States of America), and Ahmad Dagamseh (Jordan). Working in such an international community as the Solar Cell Group and the Delft Institute of Microsystems and Nanoelectronics (DIMES) as a whole has been a very pleasant experience. Last but not least, I would like thank my family and friends for their support during the time I spent in Delft University of Technology to obtain my M.Sc. degree and I hereby apologise for all those times I gave preference to my work in university over social activities. v Abstract In recent decades, the public awareness of the need for renewable energy sources has increased greatly. Considering the current climate changes, it is clear that today’s energy systems have to be radically transformed onto a more sustainable basis to avoid the otherwise very likely adverse effects of global warming on tomorrow’s society and economy. In this framework, solar cells offer an interesting alternative for large-scale electricity generation. The first-generation crystalline silicon (c-Si) solar cells, which still dominate the photovoltaic market, will likely not be used for such large-scale applications, because of the relatively large amounts of material that would be required for the fabrication. Thin- film solar cells, such as hydrogenated amorphous silicon (a-Si:H) solar cells, have a great potential with respect to the costs, because much smaller amounts of material are needed in the fabrication process in comparison to c-Si solar cells. However, the typical conversion efficiency of an a-Si:H silicon solar cell is much lower than the typical conversion efficiency of a c-Si solar cell. Further, the performance of an a-Si:H solar cell degrades over time when the solar cell is exposed to light, which is not the case for c-Si solar cells. In an attempt to increase the conversion efficiency of thin-film solar cells, research interest moved towards multiple-junction solar cells, as opposed to the conventional single-junction solar cells, so a wider range of the solar spectrum could be absorbed. One particularly interesting multiple-junction solar cell is the so-called micromorph tandem solar cell, which consists of an a-Si:H top solar cell and a hydrogenated microcrystalline silicon (μc-Si:H) bottom solar cell. The interesting aspect of this particular multiple-junction solar cell is that both the bottom and the top solar cell can be produced from the same cheap base material: silicon. Both solar cells can be deposited by means of plasma-enhanced chemical vapour deposition (PECVD) from silane gas. To obtain a highly efficient a-Si:H solar cell for possible later use in a micromorph tandem solar cell, it will be attempted in this thesis to optimise the heart of the a-Si:H solar cell: the absorber layer. To obtain a stable material, a hydrogen-to-silane dilution ratio of 20 is used during the PECVD deposition of a-Si:H. The influence of various deposition parameters, such as the pressure, the rf-power, the silane flow, and the substrate temperature on the quality of films and absorber layers is investigated. Fourier Transform Photocurrent Spectroscopy is used to evaluate the quality of Si:H films and solar cell absorber layers, since from the obtained sub-band gap absorption coefficient spectrum the defect concentration can be estimated. It is found that the highest material quality of films and absorber layers is achieved for a high deposition pressure, a low rf-power, and a low substrate temperature. The silane flow does not have a significant influence of the quality of the deposited material. None of these deposition parameters has an influence on the degradation rate in films and absorber layers. An increased stability against light soaking is observed for the films and absorber layers deposited at a hydrogen-to-silane dilution ratio of 20 in comparison to a film and an absorber layer deposited from undiluted silane. vii The Stone Age did not end for lack of stone, and the Oil Age will end long before the world runs out of oil. -- Sheikh Ahmed Zaki Yamani, Saudi Arabia's Minister of Oil and Mineral Resources from 1962 until 1986. Quoted by The Economist, October 23, 2003. Contents Acknowledgements ....................................................................... v Abstract....................................................................................... vii List of figures ............................................................................... xi List of tables............................................................................... xxi Abbreviations ........................................................................... xxiii 1 Introduction............................................................................. 1 1.1 CONTEXT AND PROBLEM STATEMENT.............................................. 1 1.2 OUTLINE.............................................................................. 2 2 The importance of thin-film amorphous silicon solar cells ....... 5 2.1 CLIMATE CHANGE.................................................................... 5 2.1.1 Historical overview and recent findings............................. 5 2.1.2 Kyoto Protocol .............................................................10 2.2 THE GLOBAL ENERGY MARKET.....................................................12 2.2.1 Energy resources and consumption .................................12 2.2.2 The need for renewable energy ......................................14 2.2.3 The importance of photovoltaic technology.......................19 2.3 OVERVIEW OF THE PHOTOVOLTAIC INDUSTRY ..................................20 2.3.1 Historical technological overview ....................................20 2.3.2 The recent growth of the photovoltaic industry .................23 2.4 DEVELOPMENTS IN THIN-FILM AMORPHOUS SILICON SOLAR CELLS ..........25 2.5 FUTURE DEVELOPMENTS IN THIN-FILM SOLAR CELLS ..........................26 2.5.1 Renewable energy policy developments...........................26 2.5.2 Technological developments ..........................................27 3 The properties of amorphous silicon ...................................... 31 3.1 HISTORICAL OVERVIEW............................................................31 3.2 THE DIFFERENT TYPES OF SILICON ...............................................32 3.3 MATERIAL PARAMETERS OF HYDROGENATED AMORPHOUS SILICON ..........34 3.3.1 Density of states distribution..........................................34 3.3.2 Absorption coefficient spectrum......................................37
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