Physics and Technology of Amorphous-Crystalline Heterostructure Silicon Solar Cells

Engineering Materials

Wilfried G.J.H.M. van Sark · Lars Korte Francesco Roca (Eds.)

Physics and Technology of Amorphous-Crystalline Heterostructure Silicon Solar Cells

ABC

Dr. Wilfried G.J.H.M. van Sark Utrecht University Copernicus Institute Science Technology and Society Budapestlaan 6 3584 CD Utrecht The Netherlands E-mail: [email protected] Dr. Lars Korte Helmholtz-Zentrum Berlin für Materialien und Energie Inst. Silizium-Photovoltaik Kekuléstraße 5 12489 Berlin Germany E-mail: [email protected]

ISBN 978-3-642-22274-0

Dr. Francesco Roca ENEA - Agenzia Nazionale per le Nuove Tecnologie, l’Energia e lo Sviluppo Economico Sostenibile Unità Tecnologie Portici, Localitá Granatello P. le E. Fermi 80055 Portici Napoli Italy E-mail: [email protected]

e-ISBN 978-3-642-22275-7

DOI 10.1007/978-3-642-22275-7 Engineering Materials

ISSN 1612-1317

Library of Congress Control Number: 2011934499 c 2012 Springer-Verlag Berlin Heidelberg  This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Typeset & Cover Design: Scientific Publishing Services Pvt. Ltd., Chennai, India. Printed on acid-free paper 987654321 springer.com

Preface

The development of hydrogenated amorphous (a-Si:H) / crystalline silicon (c-Si) heterojunction (SHJ) solar cells has recently accelerated tremendously. This is not just triggered by the recent expiration of core patents of Sanyo Electric Company, but most of all due to the high efficiency that has been proven to be achievable in practice (being close to the theoretical limit for c-Si) and the very advanced architectures that can be realized with this technology, such as fully back contacted solar cells with very thin wafers. The low temperature processing and reduction of materials resources is bringing grid parity rapidly within reach, even in countries with little solar irradiation, and this way of processing is highly cost competitive with the ‘classic’ c-Si solar cells with diffusion processed junctions. SHJ photovoltaic technology merges the best of the worlds of both high efficiency crystalline silicon technology and thin film technology. Institutes and companies entering this field have found that high conversion efficiencies can quickly be accomplished based on the nearly complete elimination of surface defect states. A consortium of 12 partners has been working together in the HETSI project (in full: heterojunction solar sells based on a-Si/c-Si), funded by the European Commission in the framework of the 7th Research Framework Programme from 2008 to 2011. In the scope of this project, a workshop was held at Utrecht University in 2010, to present and discuss the status as well as the issues in amorphouscrystalline heterojunction silicon solar cells. At this workshop the idea was born to collect all the present understanding as well as the ongoing innovations in a book, as one of the broad dissemination activities of HETSI. The result is a comprehensive collection of the knowledge available at the most prestigious laboratories in Europe involved in SHJ solar cell research. It is an authoritative review of present-day research topics and future opportunities in this field. It is an invaluable asset to anyone who is involved in this field, but also to the increasing numbers of researchers and industrialists who are entering this rapidly evolving solar photovoltaic technology.

Ruud E.I. Schropp Debye Institute for Nanomaterials Science Section Nanophotonics Faculty of Science Utrecht University

Acknowledgements

The editors would like to thank all the many authors and co-authors that have contributed to this book. It is their knowledge, which gives the book the value it has. We also would like to thank all institutions and individuals, who granted permission to publish figures, supplied data for this book or provided valuable feedback. This book originated from a workshop organized at Utrecht University in February 2010 within the framework of the project HETSI (heterojunction solar cells based on a-Si/c-Si), which ran from February 2008 until February 2011, and was funded by the European Commission in the framework of the 7th Research Framework Programme. Partners in this project were: Institut National de l’Energie Solaire (INES, FR), Centre National de la Recherche Scientifique (CNRS, FR), Energieonderzoek Centrum Nederland (ECN, NL), Utrecht University (UU, NL), Agenzia Nazionale per le Nuove Tecnologie, l'Energia e lo Sviluppo Economico Sostenibile (ENEA, IT), Interuniversity MicroElectronics Centrum (IMEC, BE), Institut de Microtechnologie - Ecole Polytechnique Fédérale de Lausanne (EPFL, CH), Helmholtz-Zentrum Berlin für Materialien und Energie (HZB, DE), SOLON SE (DE), Photowatt SAS (FR), Q-Cells SE (DE), and ALMA Consulting Group SAS (FR). In the workshop many experts presented an overview of the state-ofthe-art in physics and technology of amorphous-crystalline heterostructure silicon solar cells, including a hands-on training session on computer modelling of cells. In this book, the presentations have been converted in comprehensive chapters. To our opinion, thanks to the many contributors that are world-renowned experts in their respective fields, the book as a whole contains a thorough overview of amorphous-crystalline heterostructure silicon solar cells, from the fundamental physical principles to the experimental and modelling details. We hope that it will serve as a reference base for the ever-growing scientific and industrial community in the photovoltaics field. Statements of views, facts and opinions as described in this book are the responsibility of the author(s).

Wilfried van Sark Lars Korte Francesco Roca

There is one forecast of which you can already be sure: someday renewable energy will be the only way for people to satisfy their energy needs. Because of the physical, ecological and (therefore) social limits to nuclear and fossil energy use, ultimately nobody will be able to circumvent renewable energy as the solution, even if it turns out to be everybody’s last remaining choice. The question keeping everyone in suspense, however, is whether we shall succeed in making this radical change of energy platforms happen early enough to spare the world irreversible ecological mutilation and political and economic catastrophe. Hermann Scheer (1944 – 2010), Energy Autonomy: The Economic, Social and Technological Case for Renewable Energy, Earthscan, London, UK, 2007, page 29.

Table of Contents

Chapter 1: Introduction – Physics and Technology of Amorphous-Crystalline Heterostructure Silicon Solar Cells . . . . . . . . . . . . Wilfried van Sark, Lars Korte, and Francesco Roca

1

Chapter 2: Heterojunction Silicon Based Solar Cells . . . . . . . . . . . . . . . . . . Miro Zeman and Dong Zhang

13

Chapter 3: Wet-Chemical Conditioning of Silicon Substrates for a-Si:H/c-Si Heterojunctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heike Angermann and J¨ org Rappich

45

Chapter 4: Electrochemical Passivation and Modification of c-Si Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J¨ org Rappich

95

Chapter 5: Deposition Techniques and Processes Involved in the Growth of Amorphous and Microcrystalline Silicon Thin Films . . . . . . . . Pere Roca i Cabarrocas

131

Chapter 6: Electronic Properties of Ultrathin a-Si:H Layers and the a-Si:H/c-Si Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lars Korte

161

Chapter 7: Intrinsic and Doped a-Si:H/c-Si Interface Passivation . . . . . . . Stefaan De Wolf

223

Chapter 8: Photoluminescence and Electroluminescence from Amorphous Silicon/Crystalline Silicon Heterostructures and Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rudolf Br¨ uggemann Chapter 9: Deposition and Properties of TCOs . . . . . . . . . . . . . . . . . . . . . . Florian Ruske Chapter 10: Contact Formation on a-Si:H/c-Si Heterostructure Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mario Tucci, Luca Serenelli, Simona De Iuliis, Massimo Izzi, Giampiero de Cesare, and Domenico Caputo

261 301

331

XII

Table of Contents

Chapter 11: Electrical Characterization of HIT Type Solar Cells . . . . . . . Jatin K. Rath

377

Chapter 12: Band Lineup Theories and the Determination of Band Offsets from Electrical Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Jean-Paul Kleider

405

Chapter 13: General Principles of Solar Cell Simulation and Introduction to AFORS-HET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rolf Stangl and Caspar Leendertz

445

Chapter 14: Modeling an a-Si:H/c-Si Solar Cell with AFORS-HET . . . . . Caspar Leendertz and Rolf Stangl

459

Chapter 15: Two-Dimensional Simulations of Interdigitated Back Contact Silicon Heterojunctions Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . Djicknoum Diouf, Jean-Paul Kleider, and Christophe Longeaud

483

Chapter 16: Technology and Design of Classical and Heterojunction Back Contacted Silicon Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Niels E. Posthuma, Barry J. O’Sullivan, and Ivan Gordon

521

Chapter 17: a-Si:H/c-Si Heterojunction Solar Cells: A Smart Choice for High Efficiency Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Delfina Mu˜ noz, Thibaut Desrues, and Pierre-Jean Ribeyron

539

Author Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

573

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

575

List of Contributors

Heike Angermann Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institut für Silizium-Photovoltaik, Kekuléstraße 5, D-12489 Berlin, Germany [email protected]

Djicknoum Diouf Laboratoire de Génie Electrique de Paris, CNRS UMR8507, SUPELEC; Univ Paris-Sud, UPMC Univ Paris 06, 11 rue Joliot-Curie, Plateau de Moulon, F-91192 Gif-sur-Yvette Cedex, France [email protected]

Rudolf Brüggemann Institut für Physik, Carl von Ossietzky Universität Oldenburg, 26111 Oldenburg, Germany [email protected]

Ivan Gordon imec, Photovoltaics/Solar Cell Technology, Kapeldreef 75, B-3001 Leuven, Belgium [email protected]

Domenico Caputo Department of Electronic Engineering Rome University “Sapienza”, Via Eudossiana 18, 00139 Rome, Italy [email protected]

Simona De Iuliis ENEA - Research Center Casaccia, Via Anguillarese 301, 00123 Rome, Italy [email protected]

Giampiero de Cesare Department of Electronic Engineering Rome University “Sapienza”, Via Eudossiana 18, 00139 Rome, Italy decesare@ die.uniroma1.it Thibaut Desrues CEA-INES, Savoie Technolac, 50 avenue du lac Léman - BP258, F-73375 Le Bourget du Lac – Cedex, France [email protected]

Massimo Izzi ENEA - Research Center Casaccia, Via Anguillarese 301, 00123 Rome, Italy [email protected] Jean-Paul Kleider Laboratoire de Génie Electrique de Paris, CNRS UMR8507, SUPELEC; Univ. Paris-Sud, UPMC Univ. Paris 06, 11 Rue Joliot-Curie, Plateau de Moulon, F-91192 Gif-sur-Yvette Cedex, France [email protected]

XIV Lars Korte Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institut für Silizium-Photovoltaik, Kekuléstraße 5, D-12489 Berlin, Germany [email protected] Caspar Leendertz Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institut für Silizium-Photovoltaik, Kekuléstraße 5, D-12489 Berlin, Germany [email protected] Christophe Longeaud Laboratoire de Génie Electrique de Paris, CNRS UMR8507, SUPELEC; Univ Paris-Sud, UPMC Univ Paris 06, 11 rue Joliot-Curie, Plateau de Moulon, F-91192 Gif-sur-Yvette Cedex, France [email protected]

List of Contributors Jörg Rappich Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institut für Silizium-Photovoltaik, Kekuléstraße 5, D-12489 Berlin, Germany [email protected] Jatin K. Rath Utrecht University, Debye Institute for Nanomaterials Science, Section Nanophotonics, P.O. Box 80000, 3508 TA Utrecht, The Netherlands [email protected] Pierre-Jean Ribeyron CEA-INES, Savoie Technolac, 50 avenue du lac Léman - BP258, F-73375 Le Bourget du Lac – Cedex, France [email protected]

Delfina Muñoz CEA-INES, Savoie Technolac, 50 avenue du lac Léman - BP258, F-73375 Le Bourget du Lac – Cedex, France [email protected]

Francesco Roca ENEA - Agenzia Nazionale per le Nuove Tecnologie, l'Energia e lo Sviluppo Economico Sostenibile Unità Tecnologie Portici, Localitá Granatello P. le E. Fermi 80055 Portici Napoli Italy [email protected]

Barry O'Sullivan imec, Photovoltaics/Solar Cell Technology, Kapeldreef 75, B-3001 Leuven, Belgium [email protected]

Pere Roca i Cabarrocas Laboratoire de Physique des Interfaces et des Couches Minces, CNRS Ecole Polytechnique, 91128 Palaiseau, France [email protected]

Niels Posthuma imec, Photovoltaics/Solar Cell Technology, Kapeldreef 75, B-3001 Leuven, Belgium [email protected]

Florian Ruske Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institut für Silizium-Photovoltaik, Kekuléstraße 5, D-12489 Berlin, Germany [email protected]

List of Contributors Wilfried G.J.H.M. van Sark Utrecht University, Copernicus Institute, Science, Technology and Society, Budapestlaan 6, 3584 CS Utrecht, The Netherlands [email protected] Ruud E.I. Schropp Utrecht University, Debye Institute for Nanomaterials Science, Section Nanophotonics, P.O. Box 80000, 3508 TA Utrecht, The Netherlands [email protected] Luca Serenelli ENEA - Research Center Casaccia, Via Anguillarese 301, 00123 Rome, Italy [email protected] Rolf Stangl Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institut für Silizium-Photovoltaik, Kekuléstraße 5, D-12489 Berlin, Germany [email protected]

XV Mario Tucci ENEA - Research Center Casaccia, Via Anguillarese 301, 00123 Rome, Italy [email protected] Stefaan De Wolf Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and thin-film electronics laboratory (PVlab), Breguet 2, 2000 Neuchâtel, Switzerland [email protected] Miro Zeman Delft University of Technology, Photovoltaic Materials and Devices group, Mekelweg 4, 2628 CD, Delft, The Netherlands [email protected] Dong Zhang Delft University of Technology, Photovoltaic Materials and Devices group, Mekelweg 4, 2628 CD, Delft, The Netherlands [email protected]

List of Abbreviations, Units, and Signs

1D 2D 3D 4-BrB 4-NB 4-NBDT AC (ac) ACJ-HIT

: : : : : : : :

AD AFORS-HET AFM AIST

: : : :

ALD AM1.5 AM1.5G APCVD APM AR ARC AS a-Si:H a-SiC:H a-SiO:H ATR ATR-FTIR

: : : : : : : : : : : : :

BACG BEHIND

: :

BHD BP BSF CB CBM CDMR CFSYS

: : : : : : :

one dimensional two dimensional three dimensional 4-bromobenzene 4-nitrobenzene 4-nitrobenzene diazonium tetrafluoroborate alternate current artificially constructed junction-heterojunction with intrinsic thin film analog to digital automat for simulation of heterostructures atomic force microscopy National Institute of Advanced Industrial Science and Technology (Japan) atomic layer deposition air mass 1.5 air mass 1.5, global chemical vapour deposition at atmospheric pressure ammonia/hydrogen peroxide mixture anti-reflection anti-reflection coating admittance spectroscopy hydrogenated amorphous silicon hydrogenated amorphous silicon carbide hydrogenated amorphous silicon oxide attenuated total reflection attenuated total reflection Fourier transform infrared (spectroscopy) back amorphous-crystalline silicon heterojunction back enhanced heterostructure with interdigitated contact Brooks-Harring-Dingle band pass back surface field conduction band conduction band maximum capacitance detected magnetic resonance constant final state yield spectroscopy

XVIII

List of Abbreviations, Units, and Signs

CIGS CNRS CPM CPM CS c-Si CV / C-V CVD cw CZ DB DBR DC (dc) DH DIN DIW DOS ECN EDMR EFG EL EMA ENEA

: : : : : : : : : : : : : : : : : : : : : : :

EWT EPFL epi-Si EPR EQ EQE ESR FE FF FPD FSF FSRV FTIR FTIR-SE FZ GB HETSI HF HIT HJ HP HR-TEM

: : : : : : : : : : : : : : : : : : : : : :

copper indium gallium selenide Centre National de la Recherche Scientifique hydrochloric acid/hydrogen peroxide mixture constant photocurrent mode capacitance spectroscopy crystalline silicon capacitance-voltage chemical vapour deposition continuous wave czochralski dangling bond dielectric Bragg reflector direct current dihydride Deutsches Institut für Normung dionised water density of electronic states Energieonderzoek Centrum Nederland electrically detected magnetic resonance edge-defined film-fed-growth electroluminescence effective medium approximation Agenzia Nazionale per le Nuove Tecnologie, l'Energia e lo Sviluppo Economico Sostenibile emitter wrap through Ecole Polytechnique Fédérale de Lausanne epitaxially grown crystalline silicon electronic paramagnetic resonance equilibrium external quantum efficiency electron spin resonance front emitter fill factor flat panel displays front surface field front surface recombination velocity fourier-transform infrared fourier-transform infrared ellipsometry float zone grain boundary heterojunction solar cells based on a-Si c-Si hydrofluoric acid heterojunction with intrinsic thin-layer solar cell heterojunction hot plate high resolution transmission electron microscopy

List of Abbreviations, Units, and Signs

HSM HWCVD HZB IBBC IBC IBC-SiHJ IBC-HJ I/E imec INES IP IPA IPE ISE IQE ISFH ITO IV / I-V IZO KOH LBL LBSF LCD LID LPCVD LSM MH MIGS MIS MOCVD MOS MPL MS MTCE

: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :

MW μc-Si:H μPCD μW-PCD MWT NB NDMR NUV-PES ODMR OECE PC

: : : : : : : : : : :

XIX

high stretching mode hot wire chemical vapour deposition Helmholtz-Zentrum Berlin für Materialien und Energie interdigitated backside buried contact interdigitated back-contact interdigitated back contact silicon heterojunction interdigitated back contact heterojunction iodine/ethanol Interuniversity MicroElectronics Centre Institut National de l’Energie Solaire internal photoemission isopropyl alcohol internal photo emission Institut für Solare Energiesysteme internal quantum efficiency Institut für Solarenergieforschung Hameln tin-doped indium oxide current-voltage indium zinc oxide potassium hydroxide layer by layer local back surface field liquid crystal display light induced degradation low pressure chemical vapour deposition low stretching mode monohydride metal-induced gap states metal insulator semiconductor metal organic chemical vapour deposition metal oxide semiconductor modulated photoluminescence magnetron sputtering multitunneling with successive recombination through carrier capture or reemission into the band microwave hydrogenated microcrystalline silicon microwave photo conductive decay microwave detected photoconductance decay metal wrap through nitrobenzene noise detected magnetic resonance near ultraviolet photoelectron spectroscopy optically detected magnetic resonance oblique evaporation of contact personal computer

XX

List of Abbreviations, Units, and Signs

PC PCD PDS PECVD pEDMR PERL PERT PES PESC PL PLD PMMA pm-Si :H por-Si PRECASH

: : : : : : : : : : : : : : :

PS PV PVD QSSPC RCA RCPCD RE RECASH

: : : : : : : :

RF RT SAF

: : :

SC SCR SDPC SDT SE SE SEM SHJ SlSF

: : : : : : : : :

SOD SPM SPV SR SRH

: : : : :

planar conductance photoconductance decay photothermal deflection spectroscopy plasma enhanced chemical vapour deposition pulsed electrically detected magnetic resonance passivated emitter and rear locally diffused passivated emitter rear totally diffused photoelectron spectroscopy passivated emitter solar cell photoluminescence pulsed laser deposition poly methyl methacrylate polymorphous silicon porous silicon point rear emitter crystalline/amorphous silicon heterojunction photoyield spectroscopy photovoltaics physical vapour deposition quasi-steady-state photoconductance radio corporation of america resonance-coupled photoconductive decay rear emitter rear emitter crystalline/amorphous silicon heterojunction radio frequency room temperature Salpetersäure – Ammoniumfluorid – Flusssäure (etch mixture of nitric acid, 70% HNO3, ammonia fluoride, 40% NH4F, and hydrofluoric acid, 50% HF) semiconductor space charge region spin dependent photoconductivity spin dependent transport spectroscopic ellipsometry selective emitter scanning electron microscopy crystalline silicon heterojunction Schwefelsäure – Salpetersäure - Flusssäure (etch mixture of sulphuric acid, 96% H2SO4, nitric acid, 70% HNO3, and hydrofluoric acid, 50% HF) spin-on dopant sulphuric peroxide mixture surface photovoltage spectral response Shockley-Read-Hall

List of Abbreviations, Units, and Signs

TBAF TCO TDS TE TFT TFT-LCD TH TLM TR TRMC UNSW UPS UU UV-NIR UV-VIS VB VBM VHF VFP VIGS XPS

: : : : : : : : : : : : : : : : : : : : :

tetrabutylamonium hexafluorophosphate transparent conductive oxide thermal desorption spectroscopy texture etch thin film transistor thin film transistor-liquid crystal display trihydride transfer length method transient transient microwave conduction University of New-South Wales ultraviolet photoelectron spectroscopy Utrecht University ultraviolet-near infrared ultraviolet-visible valence band valence band maximum very high frequency voltage filling pulse method virtual induced gap states x-ray photoelectron spectroscopy

XXI

Chapter 1

Introduction – Physics and Technology of Amorphous-Crystalline Heterostructure Silicon Solar Cells Wilfried van Sark1, Lars Korte2, and Francesco Roca3 1

Utrecht University, Copernicus Institute, Science, Technology and Society, Budapestlaan 6, 3584 CD Utrecht, The Netherlands 2 Helmholtz-Zentrum Berlin GmbH, Department Silicon Photovoltaics, Kekuléstraße 5, D-12489 Berlin, Germany 3 ENEA - Agenzia Nazionale per le Nuove Tecnologie, l'Energia e lo Sviluppo Economico Sostenibile - Unità Tecnologie Portici, Localitá Granatello, P. le E. Fermi, 80055 Portici, Napoli, Italy

1.1 General Introduction Although photovoltaic solar energy technology (PV) is not the sole answer to the challenges posed by the ever-growing energy consumption worldwide, this renewable energy option can make an important contribution to the economy of each country. According to the New Policies Scenario of the “World Energy Outlook 2010” published in November 2010 by the International Energy Agency (IEA) [1], it is to be expected that the share of renewable energies in global energy production increases threefold over the period 2008-2035, and that almost one third of global electricity production will come from renewables by 2035, thus catching up with coal. The “Solar Generation 6” report of the European Photovoltaic Industry association published in October 2010 [2] predicts in its Solar Generation Paradigm Shift Scenario that by 2050, PV could generate enough solar electricity to satisfy 21% of the world electricity needs, i.e. a total of up to 6750 TWh of solar PV electricity in 2050, coming from an installed capacity of 4670 GW in 2050. This is to be compared with 40 GW installed in the world at the end of 2010 [3]. After the first solar cell was demonstrated in silicon 55 years ago [4] the cost has declined by a factor of nearly 200, and high-throughput mass-production compatible processes are omnipresent all over the globe. More than 90% of the current production uses first generation PV wafer based crystalline Silicon (c-Si), a technology with the ability to continue to reduce its cost at its historic rate [5,6]. The direct production costs for crystalline silicon modules are expected to be around 1 €€ /Wp in 2013, below 0.75 €€ /Wp in 2020 and lower in the long term, as stated in the Strategic Research Agenda of the European Photovoltaic Technology Platform [7]. W.G.J.H.M. van Sark et al. (Eds.): Physics & Tech. of Amorphous-Crystalline, EM, pp. 1–12. springerlink.com © Springer-Verlag Berlin Heidelberg 2012

2

W. van Sark, L. Korte, and F. Roca

However the challenge of developing photovoltaic technology to a costcompetitive alternative for established fossil-fuel based energy sources remains enormous and new cell concepts based on thin films of various types of organic and inorganic materials are entering the market. Thin film silicon (TFS), cadmium telluride (CdTe), copper indium selenide (CIS) generally are denoted as the second generation of PV technologies and are currently considered a very interesting market alternative to crystalline silicon. Advanced thin film approaches such as dye-sensitized titanium oxide (TiO2) and blends of polythiophene and C60 (P3HT:PCBM) [8] are showing fast progress. World-record solar cell efficiencies are regularly updated, see e.g. [9], and some interesting initiatives related to their industrialization and commercialization have recently been undertaken. For large scale PV deployment in large power plants or in building integrated applications it is a prerequisite that the performance of solar energy systems is enhanced by assuring low cost in production and long term reliability (>25 years). This requires the following issues to be addressed: 1) increase of the efficiency of solar irradiation conversion; 2) decrease of the amount of materials that are used, while these materials should be durable, stable, and abundant on earth; and 3) reduction of the manufacturing and installation cost. The fantastic boom of thin film technology in recent years can suggest further development on the medium to long term due to the application of innovative concepts to conventional materials and developments of new classes of thin film materials stemming from nanotechnologies, photonics, optical metamaterials, plasmonics and new semiconducting organic and inorganic sciences, most of them recognized as next (third) generation approaches. On the other hand the growth of the PV industry is also requesting well proven technology in order to sustain the emerging market; here, crystalline silicon has a long history of ‘pulling rabbits out of the hat’ [5]. Today, the industry has reached a new level of scale that is mobilizing vast new resources, enthusiasm, skills, and energy in order to reduce wafer thickness, enhance efficiency and improve processes related to substrate cleaning, junction realization, surface passivation, contact realization. We see that PV’s historic price reduction is a result from the combined effects of step-by-step evolutionary improvements in a wide variety of areas rather than one or two huge breakthroughs [5,6]. For example, processes such as dry texturing, spray-on phosphorus doping sources or impurity gettering have become standard, while last but not least actions related to increase the factory size and automation further lead to cost reductions (“economies of scale”). In contrast, larger values of the conversion efficiency of PV technology have been reached with the realization of sophisticated crystalline silicon (c-Si) cell structures, involving numerous and very complicated steps. This approach inevitably implies an increase of costs, which is not compatible with industrial production requirements that demand simple, high-throughput and reproducible processes. In order to realize reliable devices characterized by high efficiency and low cost, an approach has been developed on the basis of amorphous/crystalline silicon heterojunction solar cells (SHJ), which combines wafer and thin film technologies. In this area impressive results were achieved by Sanyo Electric with the so called a-Si/c-Si Heterojunction with Intrinsic Thin layer (HIT) solar cell [10,11]. This technology showed excellent surface passivation (open circuit voltage (Voc) values

1 Introduction – Physics and Technology

3

of around 730 mV) and the highest power conversion efficiency to date for a cell size of 100.4 cm2: 23.0% was obtained [11].

1.2 Amorphous Crystalline Heterojunction Solar Cells The design of the silicon hetero-junction solar cell is based on an emitter and back surface field (BSF) that are produced by low temperature growth of ultra-thin layers of amorphous silicon (a-Si:H) on both sides of a thin crystalline silicon wafer-base, less than 200 µm in thickness, where electrons and holes are photogenerated. The low temperature a-Si:H deposition lowers the thermal budget in the production of the cell (see Fig. 1.1), and at the same time will allow for highthroughput production machinery. Taken together, this can lead to a considerable lowering of manufacturing costs thus opening opportunities for the production of GWp/year manufacturing plants to sustain the booming PV market. p/n junction diffusion

screen printing & firing

1000 ARC 800

5’ 0,5’

600

400

200

p/n junction formation by PECVD

Electrical Contacts

10’ screen printing & annealing

TCO

3’

Lower temperature

Process temperature (C°)

30’

10’ 10’

0

Shorter process time Time (min)

Fig. 1.1 Authors’ estimated thermal budget and process time for the conventional c-Si technology (top curve) and SHJ technology (bottom curve).

The idea of making solar cells from silicon heterojunctions is a rather old one: It was first published in 1974 by Walther Fuhs and coworkers from the University of Marburg (Germany) [12]. However, it turned out that to realize the Voc potential > 700 mV inherent to the heterojunction concept, it is mandatory to include additional, very thin (of the order of 10 nm) undoped – so called intrinsic – a-Si:H buffer layers between the wafer and the doped (emitter or BSF) a-Si:H layers. Briefly, the reason is that the defect density in a-Si:H increases strongly with doping, and this leads to an increase in interface defect density at the a-Si:H/c-Si junction, thus to enhanced recombination and a lower Voc. This finding is the essence of a patent filed by Sanyo in 1991, which can be seen as the “core patent”

4

W. van Sark, L. Korte, and F. Roca

Fig. 1.2 Development of the number of both publications and citations related to silicon heterojunction solar cells over time [14].

for the subsequent successful commercialization of their so-called “HIT” concept. This patent has expired in 2010. A more in-depth discussion of the intellectual property aspect can be found in [13]. As a consequence, over the last decade, there have been many encouraging results on developing alternative concepts making use of a-Si:H/c-Si heterojunctions for high efficiency cells, such as omitting the undoped buffer and lowering the doping levels in the emitter and BSF, working on p-type c-Si substrates (the HIT cell is produced on n-type material), or on modifications to the a-Si:H layers like using a-Si:H/µc-Si stacks, a-SiC:H etc. This is reflected in the steadily increasing number of publications and citations related to a-Si:H/c-Si heterojunction solar cells, cf. Fig. 1.21. Still, it appears that among other factors, the expiry of the mentioned “core patent(s)” has contributed significantly to the strongly increased interest in HIT-type cells seen in the last few years. Today, many research groups and industries are pursuing intense R&D to further develop the a-Si:H/c-Si heterojunction technology. One such consortium has received funding from the European Commission in the framework of the 7th Research Framework Programme to develop a knowledge base and optimized device structure based on new insights in the physics and technology of wafer-based silicon heterojunction devices, within the project “Heterojunction Solar Cells based on a-Si c-Si” (HETSI) [15] 2. 1

The database used for this analysis does not contain the proceedings of the European photovoltaic conferences prior to ~ 2008. 2 The partners (acronym, country) are Institut National de l’Energie Solaire (INES, FR), Centre National de la Recherche Scientifique (CNRS, FR), Energieonderzoek Centrum Nederland (ECN, NL), Utrecht University (UU, NL), Agenzia Nazionale per le Nuove Tecnologie, l'Energia e lo Sviluppo Economicamente Sostenibile (ENEA, IT), Interuniversity MicroElectronics Centrum (IMEC, BE), Institut de Microtechnologie - Ecole Polytechnique Fédérale de Lausanne (EPFL, CH), Helmholtz-Zentrum Berlin für Materialien und Energie (HZB, DE), SOLON SE (DE), Photowatt SAS (FR), Q-Cells SE (DE), and ALMA Consulting Group SAS (FR).

1 Introduction – Physics and Technology

5

24 22

cell efficiency [%]

20 18 16 14 12 p/n

10

n/p Sanyo R&D Sanyo Production NREL R&D HZB R&D Europe R&D

8 6

1995

2000

2005

2010

year Fig. 1.3 Development of a-Si:H/c-Si heterojunction cell efficiency vs. time. Both (n)a-Si:H/(p)c-Si and (p)a-Si:H/(n)c-Si cell structures are shown.

The reported cell efficiencies have developed accordingly: Fig. 1.3 gives a (non-exhaustive) overview on the progress over time, where the distinction is made between (n)a-Si:H/(p)c-Si type cells and the “canonical” (p)a-Si:H/(n)c-Si structure as used by Sanyo. There is evidence for the gap in cell efficiencies between the two doping sequences being due to differences in fundamental device physics (carrier mobilities, band offsets), cf. Chapter 6 in this book. Furthermore, it is apparent that the Sanyo HIT cell has a significant lead on the reported cell efficiencies, by ~2% absolute at the time of writing. Nevertheless, others are covering lost ground at a fast pace: The latest reported cell efficiencies from NREL (US) are 18.2% on n-type and, interestingly, 19.3% (Voc of 678 mV) on p-type wafers [16]. In Europe, the highest efficiencies reported so far are 21.0% obtained at Roth & Rau Switzerland in cooperation with EPFL Neuchâtel [17] and up to 19.6 % (20 % on 100 cm²) with a Voc up to 718 mV on industrially relevant surfaces, i.e. large area 148 cm² pseudo-square n-type c-Si industrial wafers [18]. Recently Sanyo reported on opportunities to reach impressive efficiencies over 23% based on the utilization of very thin wafers (