Durham E-Theses CdTe solar cells: growth phenomena and device performance Major, Jonathan How to cite: Major, Jonathan (2008) CdTe solar cells: growth phenomena and device performance, Durham theses, Durham University. Available at Durham E-Theses Online: http://etheses.dur.ac.uk/605/ Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in Durham E-Theses • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full Durham E-Theses policy for further details. Academic Support Oce, Durham University, University Oce, Old Elvet, Durham DH1 3HP e-mail: [email protected] Tel: +44 0191 334 6107 http://etheses.dur.ac.uk CdTe solar cells: growth phenomena and device performance By Jonathan Major, MPhys The copyright of this thesis rests with the author or the university to which it was submitted. No quotation from it, or information derived from it may be published without the prior written consent of the author or university, and any information derived from it should be acknowledged. 1 8 DEC 2008 A thesis presented in candidature for the degree of Doctor of Philosophy in the University of Durham Department of Physics June 2008 Abstract A systematic study is presented on the control of CdTe and CdS layers during their growth, with the understanding gained being implemented in the production of solar cells with enhanced performance. In particular the growth mechanisms for close space sublimation (CSS) — grown CdTe were evaluated as a function of processing gas (N2, 02 and H2) and nitrogen pressure. Films were shown to form via the Volmer-Weber growth mode with films deposited under nitrogen showing well defined crystal facets. Inclusion of oxygen in the deposition ambient produced islands of a rounded morphology, reduced size and increased number density, whilst hydrogen was shown to increase the island number density and the level of substrate coverage. Growth mechanisms were deduced from the morphologies observed at different stages of growth by ex-situ AFM and SEM and by comparison with growth literature, especially the work of P. Barna. Nucleation density, step flow and impurity incorporation are all invoked in the discussion.Factors influencing the cell performance were evaluated with the aid of a optical beam induced current (OBIC) and external quantum efficiency (EQE) system built as part of this work and having the capacity to measure EQE or OBIC maps with a resolution of 12.5pm. The system was used to evaluate the photovoltaic response of CdTe/CdS devices as a function of wavelength with the impact of the nitric-phosphoric acid (NP) etch on the back surface, the uniformity of CdTe/CdS devices deposited by different methods and the effect of absorber layer thickness of PV uniformity being assessed. The performance of CdTe/CdS devices was evaluated as a function of variables that could be influenced by growth of the CdTe and CdS layers. The use of lower substrate temperature and the incorporation oxygen in CdS increased V„ from 0.51 to 0.65V is discussed. Oxygen in the CdTe was also shown to influence the junction position and hence efficiency, while oxygen in the CdS layer was also shown to be vital for the formation of hetero-junctions. The CdTe grain size was shown to be significantly increased for deposition under higher nitrogen pressures (Grain diameter = [0.027P + 0.9]gm, where P is the pressure in Torr), with the average grain diameters being 0.94pm at 2Torr and 5.63pm at 200Torr. Device performance was improved as a result with the peak device efficiency being increased from 2.1% at 2Torr to 14.1% at 100Torr. The series resistance was shown to be minimised for larger grain size, owing to the reduced contribution of grain boundaries. Suggestions for the fabrication of high efficiency solar cells are given with reference to the efficiency limiting factor. II Acknowledgement I would like to offer my heartfelt thanks to my supervisor Professor Ken Durose for his support and expert guidance throughout my time here. I would also like to thank him for the countless hours of proof reading and his valiant attempt to teach me the difference between Cumbrian and English. My thanks also to the long suffering technical staff of the Physics Department, in particular Norman Thompson and David Pattinson. A large number of people have worked within, or around, the research group during my time here who I would like to thank for there friendship and for making this an enjoyable place to work; Guillaume Zoppi, Kenya "K" Mam, Chris Hodgson, Martin Archbold, Mahieddine Emziane, Ben Cantwell, Yuri Proskuryakov, Tom Hindmarch, Justus Oldenburg, Ben Taele, Mohammed Al Turkestani, Jiang, Rob Treharne, Aktar Rind and Paul Scott. This work was carried out as part of the Supergen PV21 project and my thanks are offered to the all the team members who have worked on the project for their collective wisdom and insight. In particular thanks go to consortium members Vincent Barrioz, for the growth of MOCVD devices featured in this work, and to Guillaume Zoppi, for his help with the XRD characterisation of devices. My thanks also to the Engineering and Physical Sciences Research Council who funded this work. Finally, this thesis is dedicated to my parents, who have supported me in everything I've ever done, and to Claire, who has always encouraged, loved and mocked me in equal measure. Without you all this thesis wouldn't have been written. IV CONTENTS 1 Introduction 1 1.1 References for Chapter 1 4 2 Photovoltaic devices 5 2.1 Introduction 5 2.2 Semicondutors and the photovoltaic effect 5 2.2.1 Types of semiconductor 5 2.2.2 The photovoltaic effect 6 7 2.3 Semiconductor junctions 2.3.1 Homo-junction 8 2.3.2 Schottky junction 9 2.3.3 Hetero-junction 10 11 2.4 J- V characteristics of solar cell devices 2.4.1 The ideal device 11 2.4.2 The solar spectrum and non-ideal device losses 14 14 2.4.2.1 The solar spectrum 15 2.4.2.2 Fundamental losses 2.4.2.3 Resistance losses 16 2.5 Solar cell device materials 18 18 2.5.1 Silicon 2.5.2 III-V materials 18 2.5.3 Thin film material 19 19 2.5.3.1 Hydrogenated amorphous silicon 19 2.5.3.2 Copper indium diselenide 20 2.5.3.3 Cadmium telluride 2.6 References for Chapter 2 21 V 22 3 CdTe/CdS solar cells 22 3.1 Introduction 22 3.2 Device structure 23 3.2.1 Glass substrate 24 3.2.2 Transparent conducting oxide front contact 25 3.2.3 Cadmium sulphide window layer 26 3.2.4 Cadmium telluride absorber layer 26 3.2.4.1 CdTe grain size and grain boundaries 27 3.2.4.2 Doping of CdTe 29 3.2.4.3 Un-optimised device structures 30 3.2.5 Cadmium chloride activation step 31 3.2.6 Intermixing between CdTe and CdS layers 31 3.2.7 Back contact 33 3.3 High efficiency CdTe/CdS devices 34 3.4 References for Chapter 3 38 4 Growth mechanisms of polycrystalline thin films 38 4.1 Introduction 38 4.2 Thin film growth modes 39 4.2.1 Frank - van der Merwe growth 39 4.2.2 Volmer - Weber growth 39 4.2.3 Stranski - Krastanov growth 40 4.3 Film formation processes 41 4.4 Nucleation of thin films 42 Condensation and the driving force for nucleation 4.4.1 45 4.4.2 Contact wetting angle 46 4.4.3 Atomisitic surface processes 49 4.5 Post-nucleation growth phase 50 4.5.1 Island growth and capture zone 51 4.5.2 Step flow processes and facet formation 52 4.5.3 Coalescence of growth islands VI 55 4.5.4 Formation of a complete film 55 4.5.4.1 Channel formation 56 4.5.4.2 Secondary nucleation and complete film formation 56 4.6 Selective review of thin film growth literature 57 4.6.1 Thin film growth experiments — the role of oxygen 59 4.6.2 Studies of cadmium telluride thin film growth 60 4.6.2.1 Epitaxial growth of CdTe 61 4.6.2.2 Growth of CdTe thin films for solar cell applications 63 4.6.2.3 Post-growth annealing of CdTe thin films in solar cells 65 4.7 References for Chapter 4 69 5 Experimental techniques 69 5.1 Introduction 69 5.2 Thin film growth techniques and device processing 69 5.2.1 Close space sublimation 72 5.2.2 Chemical bath deposition 74 5.2.3 Metal organic chemical vapour deposition 75 Transparent conducting oxides and deposition substrates 5.2.4 75 5.2.5 Post growth treatment of CdTe/CdS devices with CdC12 75 5.2.6 Contacting procedures 76 5.3 Physical characterisation of devices and layers 76 5.3.1 Atomic force microscopy 77 5.3.2 Film thickness measurement 77 Scanning electron microscopy secondary electron imaging 5.3.3 78 5.3.4 Energy dispersive X-ray analysis 79 5.3.5 X-ray diffraction devices 81 5.4 Electrical/optical characterisation of CdTe/CdS 81 5.4.1 Current — voltage measurement 81 5.4.2 External quantum efficiency 81 5.4.3 Electron beam induced current 82 5.4.4 Optical beam induced current 83 5.5 References for Chapter 5 VII 6 OBIC and EQE measurement of CdTe/CdS devices 85 6.1 Introduction 85 6.2 Review of OBIC/spatially resolved EQE 85 6.2.1 OBIC systems 86 6.2.2 Key features of EQE as a function of wavelength 87 6.2.3 OBIC studies of thin film solar cells 91 6.3 OBIC/EQE apparatus: system design and testing 93 6.3.1 System design 93 6.3.1.1 General considerations 93 6.3.1.2 Focussing optics 94 6.3.1.3 Sample and lens positioning