Surface, Emitter and Bulk
`Recombination in Silicon and
`Development of Silicon Nitride
`Passivated Solar Cells
`
`Mark John Kerr
`
`June 2002
`
`A thesis submitted for the degree of Doctor of
`Philosophy of The Australian National University
`
`HANWHA 1043
`
`

`

`

`

`Declaration
`
`I certify that this thesis does not incorporate, without acknowledgement, any material
`previously submitted for a degree or diploma in any university, and that, to the best of my
`knowledge, it does not contain any material previously published or written by another person
`except where due reference is made in the text. The work in this thesis is my own, except for the
`contributions made by others as described in the Acknowledgements.
`
`Mark Kerr
`
`

`

`

`

`Acknowledgments
`
`I would like to express indebted gratitude to my supervisor Dr Andres Cuevas. He has
`always been extremely generous with his time, knowledge and ideas and allowed me great
`freedom in this research. His enthusiastic approach to research, his endless excitement for solar
`cells and his effervescent personality have made this experience all the more enjoyable and I am
`greatly appreciative. I would also like to thank Prof. Andrew Blakers for forming part of my
`supervisory panel, for helpful discussions and for establishing the PV labs at the ANU, they are
`a tremendous place to work and study. I am also grateful to Dr Matt Stocks and Dr Rob Elliman
`for their advice as members of my supervisory panel.
`
`I would like to thank Dr Daniel Macdonald for many hours of useful banter, as a source
`and a sounding board for many ideas, and for his in-depth knowledge on gettering
`multicrystalline silicon. I am also extremely grateful to Chris Samundsett for his assistance in
`the labs, for showing me the ropes and for his endless help with various samples, even the ones
`that got dropped on the floor. Equally important, I am grateful for his keen sense of wit and
`balanced perspective on life.
`
`I am very appreciative of Dr Jan Schmidt for introducing me to the world of PECVD SiN
`and for a wealth of useful discussions and contributions. I would also like to thank Dr Ron
`Sinton for developing such wonderful tools for studying recombination and for all the helpful
`advice he provided, especially in the art of lifetime measurements and for the work in Chapter 2
`on the generalised QssVoc technique.
`
`I would also like to thank the many people who have collaborated with me and
`contributed to various parts of this research, in particular to Prof. C. Jagadish and Dr H. Tan of
`EME for use of the PECVD reactor. Dr Jan Bultman of ECN organised the stripping Hall
`measurements of emitter profiles and I am grateful to Dr Pietro Altermatt of UNSW for
`discussion and data on Auger recombination. Many thanks to Dr D. Ruby of Sandia National
`Laboratories, Dr K. Emery of NREL and Dr Stefan Glunz of the Fraunhofer ISE for quantum
`efficiency data and for I-V measurements of solar cells under standard conditions. Dr D. Ruby
`also performed the texturing of mulitcrystalline cells by RIE. Thanks to Dr Stefan Glunz and
`Stefan Rein of the Fraunhofer ISE for the solar cell used in Figure 2.18. Dr Patrick Campbell of
`UNSW performed the montecarlo simulations of photon recycling in silicon and Prof. Martin
`Green provided useful discussion on modeling the limiting efficiency of silicon solar cells.
`Francesca Ferrazza kindly supplied the Eurosolare multicrystalline silicon material and the
`assistance of Dr Saul Winderbaum and Tony Leo of BP Solar is gratefully acknowledged for
`fabricating large area multicrystalline silicon solar cells.
`
`

`

`

`

`Abstract
`
`Recombination within the bulk and at the surfaces of crystalline silicon has been
`investigated in this thesis. Special attention has been paid to the surface passivation achievable
`with plasma enhanced chemical vapour deposited (PECVD) silicon nitride (SiN) films due to
`their potential for widespread use in silicon solar cells. The passivation obtained with thermally
`grown silicon oxide (SiO2) layers has also been extensively investigated for comparison.
`
`Injection-level dependent lifetime measurements have been used throughout this thesis to
`quantify the different recombination rates in silicon. New techniques for interpreting the
`effective lifetime in terms of device characteristics have been introduced, based on the physical
`concept of a net photogeneration rate. The converse relationships for determining the effective
`lifetime from measurements of the open-circuit voltage (Voc) under arbitrary illumination have
`also been introduced, thus establishing the equivalency of the photoconductance and voltage
`techniques, both quasi-static and transient, by allowing similar possibilities for all of them.
`
`The rate of intrinsic recombination in silicon is of fundamental importance. It has been
`investigated as a function of injection level for both n-type and p-type silicon, for dopant
`densities up to ~5x1016cm-3. Record high effective lifetimes, up to 32ms for high resistivity
`silicon, have been measured. Importantly, the wafers where commercially sourced and had
`undergone significant high temperature processing. A new, general parameterisation has been
`proposed for the rate of band-to-band Auger recombination in crystalline silicon, which
`accurately fits the experimental lifetime data for arbitrary injection level and arbitrary dopant
`density. The limiting efficiency of crystalline silicon solar cells has been re-evaluated using this
`new parameterisation, with the effects of photon recycling included.
`
`Surface recombination processes in silicon solar cells are becoming progressively more
`important as industry drives towards thinner substrates and higher cell efficiencies. The surface
`recombination properties of well-passivating SiN films on p-type and n-type silicon have been
`comprehensively studied, with Seff values as low as 1cm/s being unambiguously determined.
`The well-passivating SiN films optimised in this thesis are unique in that they are stoichiometric
`in composition, rather than being silicon rich, a property which is attributed to the use of dilute
`silane as a process gas. A simple physical model, based on recombination at the Si/SiN interface
`being determined by a high fixed charge density within the SiN film (even under illumination),
`has been proposed to explain the injection-level dependent Seff for a variety of differently doped
`wafers. The passivation obtained with the optimised SiN films has been compared to that
`obtained with high temperature thermal oxides (FGA and alnealed) and the limits imposed by
`surface recombination on the efficiency of SiN passivated solar cells investigated. It is shown
`that the optimised SiN films show little absorption of UV photons from the solar spectrum and
`can be easily patterned by photolithography and wet chemical etching.
`
`

`

`The recombination properties of n+ and p+ emitters passivated with optimised SiN films
`and thermal SiO2 have been extensively studied over a large range of emitter sheet resistances.
`Both planar and random pyramid textured surfaces were studied for n+ emitters, where the
`optimised SiN films were again found to be stoichiometric in composition. The optimised SiN
`films provided good passivation of the heavily doped n+-Si/SiN interface, with the surface
`recombination velocity increasing from 1400cm/s to 25000cm/s as the surface concentration of
`electrically active phosphorus atoms increased from 7.5x1018cm-3 to 1.8x1020cm-3. The
`optimised SiN films also provided reasonable passivation of industrial n+ emitters formed in a
`belt-line furnace. It was found that the surface recombination properties of SiN passivated p+
`emitters was poor and was worst for sheet resistances of ~150(cid:58)/(cid:133). The hypothesis that
`recombination at the Si/SiN interface is determined by a high fixed charge density within the
`SiN films was extended to explain this dependence on sheet resistance. The efficiency potential
`of SiN passivated n+p cells has been investigated, with a sheet resistance of 80-100(cid:58)/(cid:133) and a
`base resistivity of 1-2(cid:58)cm found to be optimal. Open-circuit voltages of 670-680mV and
`efficiencies up to ~20% and ~23% appear possible for SiN passivated planar and textured cells
`respectively. The recombination properties measured for emitters passivated with SiO2, both n+
`and p+, were consistent with other studies and found to be superior to those obtained with SiN
`passivation.
`
`Stoichiometric SiN films were used to passivate the front and rear surfaces of various
`solar cell structures. Simplified PERC cells fabricated on 0.3(cid:58)cm p-type silicon, with either a
`planar or random pyramid textured front surface, produced high Voc’s of 665-670mV and
`conversion efficiencies up to 19.7%, which are amongst the highest obtained for SiN passivated
`solar cells. Bifacial solar cells fabricated on planar, high resistivity n-type substrates (20(cid:58)cm)
`demonstrated Voc’s up to 675mV, the highest ever reported for an all-SiN passivated cell, and
`excellent bifaciality factors. Planar PERC cells fabricated on gettered 0.2(cid:58)cm multicrystalline
`silicon have also demonstrated very high Voc’s of 655-659mV and conversion efficiencies up to
`17.3% using a single layer anti-reflection coating. Short-wavelength internal quantum efficiency
`measurements confirmed the excellent passivation achieved with the optimised stoichiometric
`SiN films on n+ emitters, while long-wavelength measurements show that there is a loss of
`short-circuit current at the rear surface of SiN passivated p-type cells. The latter loss is
`attributed to parasitic shunting, which arises from an inversion layer at the rear surface due to
`the high fixed charge (positive) density in the SiN layers. It has been demonstrated that that a
`simple way to reduce the impact of the parasitic shunt is to etch away some of the silicon from
`the rear contact dots. An alternative is to have locally diffused p+ regions under the rear
`contacts, and a novel method to form a rear structure consisting of a local Al-BSF with SiN
`passivation elsewhere, without using photolithography, has been demonstrated.
`
`

`

`Table of Contents
`
`CHAPTER 1: INTRODUCTION............................................................................................. 1
`
`1.1 MARKET OVERVIEW............................................................................................................ 1
`1.2 THESIS MOTIVATION........................................................................................................... 5
`1.3 THESIS OUTLINE.................................................................................................................. 6
`
`CHAPTER 2: QUANTIFYING AND MEASURING RECOMBINATION IN
`CRYSTALLINE SILICON........................................................................................................ 9
`
`2.1 RECOMBINATION MECHANISMS........................................................................................ 10
`2.1.1 Radiative Recombination ........................................................................................... 11
`2.1.2 Auger Recombination................................................................................................. 12
`2.1.3 Bulk Recombination Through Defects ....................................................................... 14
`2.1.4 Surface Recombination Through Defects................................................................... 16
`2.1.5 Emitter Recombination............................................................................................... 18
`2.2 THE EFFECTIVE LIFETIME ................................................................................................. 20
`2.3 MEASURING THE EFFECTIVE LIFETIME............................................................................. 22
`2.3.1 Introduction ................................................................................................................ 22
`2.3.2 Photoconductance Based Techniques......................................................................... 24
`2.3.3 Calibration Functions for Determining the Photoconductance................................... 25
`2.4 DETERMINING DEVICE CHARACTERISTICS FROM THE EFFECTIVE LIFETIME ................... 28
`2.4.1 Determination of Implied Voc vs. Light Intensity Curves........................................... 28
`2.4.2 Determination of Photovoltaic I-V Curves................................................................. 30
`2.5 GENERALISATION OF OPEN-CIRCUIT VOLTAGE MEASUREMENTS ................................... 32
`2.5.1 Introduction ................................................................................................................ 33
`2.5.2 Experimental Illustration of the General Analysis ..................................................... 34
`2.5.3 Determination of the Lifetime from Voc ..................................................................... 37
`2.5.4 Determination of the Photovoltaic I-V Curve from Voc ............................................. 38
`2.5.5 Determination of the Lifetime from the Jsc-Voc Curve................................................ 40
`2.6 DISCUSSION ....................................................................................................................... 42
`2.7 CHAPTER SUMMARY ......................................................................................................... 47
`
`CHAPTER 3: LIFETIME AND EFFICIENCY LIMITS OF CRYSTALLINE SILICON
`SOLAR CELLS......................................................................................................................... 49
`
`3.1 INTRODUCTION.................................................................................................................. 49
`
`

`

`3.2 LIMITS FOR THE EFFECTIVE LIFETIME ...............................................................................50
`3.2.1. Previous Work............................................................................................................50
`3.2.2 Experimental Details...................................................................................................51
`3.2.3 Results and Discussion................................................................................................53
`3.2.4 Conclusions.................................................................................................................55
`3.3 COULOMB-ENHANCED AUGER RECOMBINATION..............................................................56
`3.3.1 Introduction.................................................................................................................56
`3.3.2 Existing Analytical Models.........................................................................................57
`3.3.2.1 Low Injection Models...........................................................................................57
`3.3.2.2 High Injection Models..........................................................................................58
`3.3.2.3 Complete Models..................................................................................................59
`3.3.3 A New Auger Parameterisation ..................................................................................60
`3.3.3.1 The Low Injection Auger Lifetime.......................................................................60
`3.3.3.2 The High Injection Auger Lifetime ......................................................................62
`3.3.3.3 A General Parameterisation..................................................................................64
`3.3.3.4 Validation of the New Parameterisation...............................................................64
`3.3.3.5 Discussion.............................................................................................................66
`3.3.4 Estimate of the SRH Lifetime in Silicon.....................................................................71
`3.3.5 Auger Recombination in Multicrystalline Silicon ......................................................72
`3.3.6 Conclusions.................................................................................................................74
`3.4 LIMITING EFFICIENCY OF SILICON SOLAR CELLS..............................................................74
`3.4.1 Solar Cell I-V Characteristics .....................................................................................75
`3.4.2 Effect of Coulomb-Enhanced Auger Recombination .................................................77
`3.4.3 Effect of Dopant Density and Dopant Type................................................................79
`3.4.4 Effect of Emitter and Surface Recombination ............................................................81
`3.4.5 Effect of Additional Bulk Recombination ..................................................................84
`3.4.6 Comparison with Experimental Devices.....................................................................84
`3.4.7 Conclusions.................................................................................................................86
`3.5 CHAPTER SUMMARY..........................................................................................................87
`
`CHAPTER 4: SURFACE RECOMBINATION OF p-TYPE AND n-TYPE SILICON
`PASSIVATED WITH PECVD SILICON NITRIDE.............................................................89
`
`4.1 INTRODUCTION ..................................................................................................................89
`4.2 OPTIMISATION OF THE PECVD DEPOSITION PARAMETERS ..............................................91
`4.2.1 Process Variables and Experimental Details...............................................................91
`4.2.2 Results and Discussion................................................................................................93
`4.2.2.1 Effect of Gas Flow Ratio and Rate.......................................................................93
`
`

`

`4.2.2.2 Effect of Deposition Temperature, Process Pressure and Process Power............ 94
`4.2.2.3 Effect of Deposition Time.................................................................................... 97
`4.2.2.4 Optimal Parameters for 0.3(cid:58)cm p-type Silicon................................................... 98
`4.2.3 Comparison with Previous Work ............................................................................. 100
`4.2.3.1 Effect of Deposition Temperature...................................................................... 100
`4.2.3.2 Effect of Other Process Variables and Correlation with the Refractive Index .. 100
`4.2.3.3 Effect of SiN Film Thickness............................................................................. 102
`4.2.4 Conclusions .............................................................................................................. 103
`4.3 SURFACE RECOMBINATION PROPERTIES......................................................................... 104
`4.3.1 Review of Previous Work......................................................................................... 104
`4.3.1.1 Low-Frequency Direct PECVD SiN Films........................................................ 104
`4.3.1.2 Remote and High-Frequency Direct PECVD SiN Films................................... 105
`4.3.2 Experimental Details ................................................................................................ 107
`4.3.3 Results ...................................................................................................................... 109
`4.3.4 Discussion................................................................................................................. 113
`4.3.4.1 Comparison with Literature Data for Seff((cid:39)n).................................................... 113
`4.3.4.2 Physical Mechanisms for Recombination at the Si/SiN Interface ..................... 114
`4.3.5 Comparison of Stoichiometric SiN and Thermal SiO2............................................. 118
`4.3.6 Limiting Efficiency of SiN Passivated Solar Cells................................................... 121
`4.3.7 Conclusions .............................................................................................................. 123
`4.4 CHARACTERISATION AND THERMAL STABILITY OF THE OPTIMAL SIN FILMS............... 125
`4.4.1 Absorption Coefficient ............................................................................................. 125
`4.4.2 Etch Rates................................................................................................................. 126
`4.4.3 Thermal Stability ...................................................................................................... 128
`4.5 CHAPTER SUMMARY ....................................................................................................... 130
`
`CHAPTER 5: PASSIVATING DIFFUSED EMITTERS .................................................. 133
`
`5.1 INTRODUCTION................................................................................................................ 133
`5.2 n+ EMITTERS.................................................................................................................... 135
`5.2.1 Previous Work .......................................................................................................... 135
`5.2.2 Experimental Details ................................................................................................ 136
`5.2.3 PECVD SiN Deposition Parameters......................................................................... 137
`5.2.3.1 Effect of Deposition Temperature and Gas Flow Ratio..................................... 137
`5.2.3.2 Effect of Process Power and Process Pressure................................................... 137
`5.2.3.3 Comparison with Previous Work....................................................................... 138
`5.2.4 Passivation Results and Discussion.......................................................................... 139
`5.2.4.1 JoE measurements for planar SiN passivated samples ........................................ 139
`
`

`

`5.2.4.2 JoE measurements for planar thin oxide passivated samples...............................141
`5.2.4.3 JoE measurements for textured samples ..............................................................141
`5.2.4.4 JoE measurements for planar unpassivated samples............................................143
`5.2.4.5 Electrically active phosphorus profiles...............................................................144
`5.2.4.6 Band gap narrowing (BGN) parameters for PC1D.............................................145
`5.2.4.7 Extracted Sp for passivated emitters ...................................................................146
`5.2.5 Industrial Phosphorus Emitters .................................................................................148
`5.2.5.1 Experimental Details ..........................................................................................149
`5.2.5.2 Results and Discussion .......................................................................................149
`5.2.6 Conclusions...............................................................................................................151
`5.3 p+ EMITTERS.....................................................................................................................152
`5.3.1 Introduction...............................................................................................................152
`5.3.2 Previous Work...........................................................................................................153
`5.3.3 Experimental Details.................................................................................................155
`5.3.4 Results and Discussion..............................................................................................155
`5.3.4.1 Jop+ measurements for planar oxide passivated samples.....................................155
`5.3.4.2 Jop+ measurements for planar SiN passivated samples .......................................156
`5.3.4.3 Degradation of the bulk lifetime during boron diffusion....................................160
`5.3.5 Conclusions...............................................................................................................160
`5.4 EFFICIENCY POTENTIAL OF SIN PASSIVATED n+p CELLS................................................161
`5.5 CHAPTER SUMMARY........................................................................................................166
`
`CHAPTER 6: SOLAR CELL STRUCTURES PASSIVATED WITH
`STOICHIOMETRIC PECVD SILICON NITRIDE............................................................169
`
`6.1 INTRODUCTION ................................................................................................................169
`6.2 PREVIOUS WORK .............................................................................................................170
`6.2.1 p-type Float-Zone Silicon Solar Cells.......................................................................170
`6.2.1.1 Front and Rear SiN Passivated Cells..................................................................170
`6.2.1.2 Rear SiN Passivated Cells ..................................................................................171
`6.2.1.3 Cells Incorporating SiN at the Front Surface .....................................................172
`6.2.2 p-type Multicrystalline Silicon Solar Cells ...............................................................172
`6.2.3 n-type Silicon Solar Cells..........................................................................................173
`6.3 DEVELOPMENT OF SIMPLIFIED PERC CELLS WITH SIN PASSIVATION ...........................174
`6.3.1 Cell Design................................................................................................................174
`6.3.2 Cell Fabrication.........................................................................................................174
`6.3.3 Cell Results and Discussion......................................................................................176
`6.3.3.1 Planar Float-Zone Silicon Solar Cells ................................................................176
`
`

`

`6.3.3.2 Planar SiN Passivated Multicrystalline Silicon Solar Cells............................... 179
`6.3.3.3 Random-Pyramid Textured Float-Zone Silicon Solar Cells .............................. 181
`6.3.3.4 Alternative Rear Contact Schemes .................................................................... 184
`6.3.3.5 Local BSF Type Rear Contact Without Photolithography ................................ 186
`6.3.3.6 Conclusions........................................................................................................ 187
`6.4 n-TYPE CELLS WITH SIN PASSIVATION........................................................................... 188
`6.5 SOLAR CELLS WITH AN INDUSTRIAL BELT-LINE EMITTER DIFFUSION........................... 192
`6.5.1 Large Area Planar Float-Zone Silicon Cells............................................................. 192
`6.5.2 Large Area Screen-Printed Multicrystalline Silicon Cells ....................................... 194
`6.5.3 Large Area Fire-Through Multicrystalline Silicon Cells.......................................... 195
`6.5.4 Conclusions .............................................................................................................. 198
`6.6 CHAPTER SUMMARY ....................................................................................................... 199
`
`CHAPTER 7: SUMMARY AND FURTHER WORK ....................................................... 201
`
`7.1 MEASURING RECOMBINATION IN CRYSTALLINE SILICON .............................................. 202
`7.2 LIFETIME AND EFFICIENCY LIMITS OF SILICON SOLAR CELLS ....................................... 203
`7.3 PASSIVATION OF n-TYPE AND p-TYPE SILICON WITH PECVD SILICON NITRIDE............ 204
`7.4 PASSIVATION OF DIFFUSED SURFACES ........................................................................... 205
`7.5 SOLAR CELL STRUCTURES PASSIVATED WITH PECVD SIN........................................... 207
`
`LIST OF PUBLICATIONS ................................................................................................... 211
`
`BIBLIOGRAPHY................................................................................................................... 215
`
`

`

`

`

`CHAPTER 1
`
`Introduction
`
`S
`
`olar cells convert sunlight directly into electricity using the photovoltaic effect. They are a
`promising technology for satisfying current and future energy demands in a sustainable and
`environmentally friendly way. The first commercial use of solar cells was in space applications
`for powering satellites in the late 1950’s. Today, the terrestrial market for solar cells greatly
`exceeds that for space applications, with a variety of end uses including grid connected systems,
`consumer products and for remote area power supply. This rapidly expanding market calls for
`advanced technologies and devices capable of yielding a higher performance at lower cost.
`
`1.1 Market Overview
`
`The market for solar cells has benefited significantly from government based subsidy
`programs over recent years. Figure 1.1 shows the annual worldwide shipments of photovoltaic
`(PV) modules over the last 25 years [1, 2]. Worldwide production at the end of 2000 was 288
`MW of which 287.3 MW was for terrestrial applications. In the period from 1977-1983 the
`market was operating from a very small base and grew at a very high rate. The following decade
`was characterised by a relatively constant growth rate of around 12%pa. In the last four years,
`
`

`

`2
`
`
`
`Chapter 1: Introduction
`
`34% pa growth rate
`
`12% pa growth rate
`
`300
`
`250
`
`200
`
`150
`
`100
`
`50
`
`PV Modules Shipped (MW/Yr)
`
`0
`1975
`
`1980
`
`1985
`1990
`Year
`Figure 1.1: Worldwide shipments of PV modules over the last 25 years.
`
`1995
`
`2000
`
`the market for PV modules has undergone tremendous growth at an annual rate of 34%. The
`resulting average growth rate since 1983 is then 16.6%pa.
`
`Based on the above growth rates, projections of the future market size for PV modules
`are given in Figure 1.2 for the period up to 2010. A growth rate of 25%pa is included, as it has
`been used historically, although some experts are now considering it to be a conservative
`estimate for growth over the next two decades [3]. It can been seen that the landmark
`achievement of a market size exceeding 1GW/Yr is expected to occur during the period 2005-
`2008, if not sooner [4]. Indeed, market growth is expected to be at the higher rates over at least
`the next few years on the basis of capacity expansions already announced [5, 6].
`
`A major determinant of increased market growth will be reduced costs. The cost of
`industrial solar cell modules is presently US$3.5-6/Wp [7], resulting in an energy cost of
`US$0.4-0.6/kWh depending of the available solar insolation for a grid-connected system [8].
`The ability to achieve continued cost reductions depends largely on three interrelated factors:
`(cid:120) Mass manufacturing of solar cells in large facilities resulting in economies of scale - The
`European study of Bruton et al. [9], based essentially on established technologies with some
`conservative extrapolations, has concluded that module costs could be reduced to around
`1Euro/Wp ((cid:124)US$1/Wp) for a range of technologies. Materials would be approximately two-
`thirds of the total cost/Wp for a large plant.
`(cid:120) Improved cell efficiency – A large proportion of the costs of installed PV systems are area
`dependent. Therefore if cell efficiency can be increased without increasing the manufacturing
`cost, significant reductions in energy cost can be achieved. A high efficiency approach is all the
`more important to make the best used of the high material cost. High efficiency approaches now
`
`

`

`Chapter 1: Introduction
`
`3
`
`34% pa
`
`25% pa
`
`16.6% pa
`
`6000
`
`5000
`
`4000
`
`3000
`
`2000
`
`1000
`
`Projected PV Market Size (MW/Yr)
`
`0
`2000
`
`2002
`
`2004
`2006
`Year
`Figure 1.2: Projected market size for PV modules using a variety of growth rates for the
`period up to 2010.
`
`2008
`
`2010
`
`at the industrial scale include the laser grooved buried grid (LGBG) cells produced by BPSolar
`under the Saturn(cid:165) name, HIT(cid:165) cells from Sanyo and OECO cells from ASE.
`(cid:120) Reduced material costs – Green has argued that as a cell technology matures, the constituent
`materials dominate the costs [10]. This is particularly true for silicon wafer based technolo