Low Cost Solar Cells
from Fast Grown Silicon Ribbon Materials
Sven Seren
Low Cost Solar Cells
from Fast Grown Silicon Ribbon Materials
Dissertation
zur Erlangung des akademischen Grades des Doktors der Naturwissenschaften
(Dr. rer. nat.)
an der Universität Konstanz Fachbereich Physik
vorgelegt von Sven Seren
Tag der mündlichen Prüfung: 21. Mai 2007 Referent: Prof. Dr. Ernst Bucher Co-Referent: PD Dr. Giso Hahn
Konstanzer Online-Publikations-System (KOPS) URL: http://www.ub.uni-konstanz.de/kops/volltexte/2007/3312/
URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-33124
Introduction 9
Einleitung 12
Part I
1 RGS 15
1.1 Introduction 15
1.2 RGS Production Principle 16
1.2.1 Discontinuous Setup 16
1.2.1.1 Wafer Specifications 17
1.2.1.2 Comparability of RGS wafers 18
1.2.2 Continuous Setup 18
1.3 RGS Crystal Growth 21
1.4 Gettering and Hydrogenation 24
1.4.1 Gettering 24
1.4.2 Hydrogenation 25
1.5 Electrical Characterisation 25
1.5.1 Charge Carrier Mobility 25
1.6 Solar Cell Process Development 27
1.6.1 Planarisation 28
1.6.1.1 Wafer Thickness Variations 29
1.6.1.2 Crack Reduction 30
1.6.2 Defect Etching 31
1.6.3 High Oxygen RGS: Material Quality 34
1.6.3.1 Process Monitoring 35
1.6.4 Low Oxygen RGS: Material Quality 36
1.6.4.1 Extended Phosphorus Gettering 37
1.6.5 Emitter Formation 38
1.6.5.1 Sheet Resistivity 38
1.6.5.2 Orientation 39
1.6.6 Hydrogenation 40
1.6.6.1 Introduction 40
1.6.6.2 Methods 41
1.6.6.3 High Oxygen RGS 42
1.6.6.4 Low Oxygen RGS 48
1.6.7 Shunting in Low Oxygen RGS 53
1.6.7.1 Introduction 53
1.6.7.2 Areal Shunts and Current Collection 54
1.6.7.3 Point Shunts 57
1.6.7.4 Shunt Avoiding Process 59
1.6.8 Texturisation 63
1.6.8.3 Acidic Texture 66
1.7 Thin and Large Area RGS Solar Cells 73
1.7.1 Thin RGS Wafers 73
1.7.2 Large RGS Wafers 75
1.8 Ga doping (drift cell) 77
1.9 n-type RGS 83
1.10 RGS Mini Module 88
1.11 Summary 90
1.12 Outlook 92
Part II
2 Molded Wafer 95
2.1 Introduction 95
2.2 MW Wafer Casting and Material Characteristics 95
2.3 Solar Cell Process Development 99
2.4 As Grown and Annealed MW Material 102
2.5 Texturisation 103
2.6 Hydrogenation 106
2.7 Summary 107
2.8 Outlook 108
Part III
3 Thermography 109
3.1 Introduction 109
3.2 Lock-In Thermography (LIT) 110
3.2.1 Dark Lock-In Thermography (dLIT) 113
3.3 Illuminated Lock-In Thermography (iLIT) 113
3.4 Measurement Modi 119
3.4.1 Illuminated Lock-In Thermography (iLIT) under different Conditions 119
3.5 Summary 121
Summary 123
Zusammenfassung 126
References 129
Publications 137
Acknowledgments 139
Danksagung 140
Introduction
This thesis was written in the winter of 2006 / 2007, which was the warmest winter on record [1]. This phenomenon is not limited to Germany as the global climate shows the same trend. The expert group of the United Nations for climate IPCC1 published within its topical report [2] a forecast for the year 2100. Therein the most probable scenario anticipates a global warming between 1.7 and 4°C. Should the global warming constitute above three degrees, the mainland ice of Greenland would melt completely, resulting most probably in disastrous consequences for the coastal areas of the earth.
A correlation between the global warming and the increase of greenhouse gas concentrations within the earth’s atmosphere cannot be neglected any longer, this holds in particular for carbondioxide and methane. Todays CO2 concentration in the atmosphere is the highest it has been in the past 650.000 years. Solely the instantaneous reduction of greenhous gases can retard this progression, which cannot be stopped anymore [2].
How to accomplish this, in particular when considering the emerging markets?
Politics on a global level can be factored out because from this side no effective decisions are made even on a local level. This holds in particular for the main- emitting nations. Because a reduction of the global energy consumption will not take place, a solution can be found only in a low-emission energy production, as a CO2
storage solution, for instance in the interior of the earth, is not available nor would it be sustainable. Uranium as a base for nuclear fission would wear out rapidly, if a reasonable fraction of the energy production should be shifted from fossil to nuclear energy sources. Novel techniques, such as nuclear fusion could not be utilised up to now despite the enormous effort made so far (JET2, ITER3). Instead of rebuilding the fusion reactor "sun" on earth, the energy provided by the existing sun could be used more effectively by means of light and thermal radiation, by wind and water power, biomass as well as geothermic power.
This work is attributed to the first attempt, the usage of the electromagnetic radiation of the sun by photovoltaic conversion into electric energy. For a large scale energy production solar cells produced from crystalline silicon are the dominating technique at the moment. For their production quartz sand is reduced to silicon, which has to be cleaned in the gas phase and is subsequently deposited as high-purity silicon. For the production of multicrystalline silicon the material obtained with this technique is crystallised to huge silicon blocks using an ingot casting process. The ingots were then cut down to smaller bricks, from which silicon wafers are wire-cut, the base material for solar cells. Since the diameter of the wire used for the wire sawing process approximately equals the wafer thickness, a silicon loss of roughly 50%
occurs. Due to segregation- as well as contamination-based processes not the whole ingot can be wire-sawed to wafers. Areas of the ingot being in direct contact with the crucible during solidification, as well as upper and lower parts of the ingot cannot be used, which enhance the fraction of wasted silicon further above 50%.
A significant enhancement of the solar fraction of the produced global energy amount can only be reached by increasing the competitiveness, i.e. by a price reduction.
Therefore, cost reduction within the production chain of solar systems has to
1 Intergovernmental Panel on Climate Change
2 Joint European Torus
3 International Thermonuclear Experimental Reactor
proceed. A cost reduction as a result of mass production, however, is currently limited by a shortage of available and sufficiently pure silicon. The photovoltaic industry expanded over the last years with annual growth rates of 30-40%, whereas a broadening of production capacities for silicon dropped far behind. Until this bottleneck is overcome and for competitive photovoltaics also beyond it, the only way is to save silicon, best by using a fast producing and thus cost effective technique.
This leads directly to the content of this work, which describes the characterisation and the solar cell processing of silicon wafers, produced very fast and directly from the silicon melt, i.e. without the indirection of block casting and the silicon loss linked to it.
The first part of this work presents the silicon ribbon material RGS (Ribbon Growth on Substrate). Crystallographic investigations as well as the analysis of material characteristics define the potential of the material, which is still in the R&D phase. In particular, attention is laid on the interstitial oxygen content due to its influence on the hydrogen diffusivity which directly affects the potential for material quality improvement.
For the development of a suitable solar cell process, adapted to the material quality, basic experiments are performed concerning the mechanical planarisation of uneven wafer surfaces, the reduction of cracks induced during planarisation as well as the chemical removal of defect-rich surface layers. The analysis of particular processing steps leads to a solar cell process, which avoids local shunts.
A spatially varying dopant concentration in the wafer can be used to enhance short circuit current densities of solar cells as a result of a drift-field. Wafer and cell based experiments are performed to investigate the assumed depth dependent doping concentration due to segregation and the RGS-specific crystallisation conditions.
Doping with phosphorus, however, leads to n-type wafers which are characterised and processed to estimate the potential of this material.
To enhance cell efficiencies, different surface textures are investigated for this material. Further on, it is tested if scaling effects for the processing of larger RGS solar cells occur and which impact the reduction of the wafer thickness has on cell parameters and the silicon usage per output power.
Another silicon ribbon material, MW (Molded Wafer), will be presented in the second part of the work. This material is still in the R&D phase as well. As a result of the production process and combined with an annealing step at high temperatures, the comparably thick MW wafers show a broad oxygen denuded zone located in the upper wafer fraction. This wafer fraction represents the photovoltaically active zone in a solar cell process adapted to the material characteristics. The influence of the annealing step on the material quality, in particular the annealing temperature, is clarified in terms of solar cell parameters and advanced cell analysis.
The last part of this work addresses the Lock-In Thermography, a measurement technique, which allows the imaging of Joule losses in solar cells already after a very short measurement time. This is of high interest particularly for the silicon ribbon materials presented within this work due to a typical inhomogeneous lateral distribution of the material quality.
The Lock-In calculation significantly enhances besides the lateral also the thermal resolution of the measurement setup, which was built up during this work. This enables the resolution of typical temperature differences produced by shunts in the
μK-range. The conventional Lock-In Thermography is advanced by a new measurement technique, the illuminated Lock-In Thermography (iLIT), which for the first time allows the contactless measurement of arbitrary pn-structures. This enables a monitoring of single solar cell processing steps without contamination.
Einleitung
Die vorliegende Arbeit wurde im Winter 2006 / 2007 verfasst. Dieser war der wärmste seit Beginn der Wetteraufzeichnungen [1]. Dieses Phänomen ist nicht auf Deutschland begrenzt, das globale Klima zeigt denselben Trend. Elf der letzten zwölf Jahre waren die wärmsten seit Beginn der Aufzeichnungen. Die Klima- Expertengruppe der Vereinten Nationen IPCC4 hat in ihrem aktuellen Bericht [2] eine Prognose für das Jahr 2100 veröffentlicht. Das wahrscheinlichste Szenario sagt darin eine Erderwärmung im Intervall 1.7 bis 4°C voraus. Sollte die Erwärmung zum Jahr 2100 aber deutlich über drei Grad liegen, würde das Festlandeis Grönlands vollständig abschmelzen, mit wahrscheinlich katastrophalen Folgen für die Küstengebiete der Erde.
Eine Korrelation der Erderwärmung zum Anstieg von Treibhausgasen in der Erdatmosphäre, allen voran Kohlendioxid und Methan, kann mittlerweile nicht mehr geleugnet werden. Die heutige CO2 Konzentration in der Atmosphäre ist die höchste seit 650.000 Jahren. Einzig und allein die sofortige globale Verringerung der Konzentration an Treibhausgasen kann diese Entwicklung bremsen. Aufzuhalten ist sie bereits nicht mehr [2].
Wie aber kann dies global, auch im Hinblick auf die Emerging Markets, bewerkstelligt werden? Politik auf globaler Ebene kann ausgeklammert werden, da von dieser Seite aus bereits lokal, insbesondere von den größten emittierenden Nationen, keine effektiven Entscheidungen getroffen werden. Da nicht mit einer globalen Reduktion des Energieverbrauches zu rechnen ist, kann eine Lösung nur in der emissionsarmen Energieerzeugung zu suchen sein, denn eine CO2 Speicherlösung etwa im Erdinneren steht nicht zur Verfügung und wäre auch nicht nachhaltig. Uran als Brennmaterial für Spaltreaktoren wäre sehr schnell erschöpft, falls ein merklicher Anteil der Energieerzeugung von fossilen auf nukleare Energieträger verlagert würde. Neue Techniken wie Kernfusion konnten bisher trotz großem Aufwand (JET5, ITER6) nicht nutzbar gemacht werden. Anstelle den Fusionsreaktor Sonne auf der Erde nachzubauen, kann die Energie der existierenden Sonne auf der Erde effektiver genutzt werden, als Licht- und Wärmestrahlung, in Form von Wind- und Wasserkraft, Biomasse und Geothermie.
Diese Arbeit ist dem erstgenannten Ansatz zugeordnet, der Nutzung der elektromagnetischen Strahlung der Sonne durch photovoltaische Energieumwandlung in elektrische Energie. Für eine Energiegewinnung auf großer Skala sind Solarzellen aus kristallinem Silizium die momentan dominierende Technik.
Zu deren Herstellung wird Quarzsand zu Silizium reduziert, das anschließend über die Gasphase gereinigt als hochreines Silizium abgeschieden wird. Das so erhaltene Material wird zur Herstellung von multikristallinem Silizium im Blockgussverfahren zu großen Siliziumblöcken erstarrt, die wiederum in kleinere Blöcke zersägt werden.
Aus diesen kleineren Blöcken werden mit einer Drahtsäge Siliziumwafer, das Ausgangsmaterial für Solarzellen, gesägt. Da der Draht zum Sägen eines Wafers aus dem Siliziumblock aber in etwa die selbe Dicke wie der Wafer aufweist, entsteht ein Silizium-Verlust von etwa 50%. Durch Segregations- und Kontaminationseffekte beim Blockgussverfahren kann weiterhin nicht der ganze Block zu Wafern zersägt werden. Bereiche, die mit der Tiegelwand in Kontakt waren sowie obere und untere
4 Intergovernmental Panel on Climate Change
5 Joint European Torus
Blockbegrenzungen können nicht verwendet werden und erhöhen somit den Anteil an verlorenem Silizium weiter auf über 50%.
Eine signifikante Erhöhung des solaren Anteils an der produzierten Gesamtenergie gelingt nur durch Steigerung der Wettbewerbsfähigkeit, also durch Senkung des Preises. Dazu müssen Kosten in der Herstellungskette von Solaranlagen gesenkt werden. Eine Preissenkung als Folge von Massenproduktion ist jedoch momentan durch einen Engpass an verfügbarem, ausreichend reinem Silizium limitiert. Die Photovoltaikindustrie wuchs die letzten Jahre mit Raten von 30-40% pro Jahr, die Produktionskapazitäten für Silizium wurden jedoch nicht in gleichem Maße ausgeweitet. Bis dieser Engpass überwunden sein wird und Photovoltaik auch darüber hinaus wettbewerbsfähig sein kann, muss Silizium eingespart werden, im besten Fall durch eine schnell produzierende und somit kostengünstige Technik.
Dies führt unmittelbar zum Inhalt dieser Arbeit, die die Charakterisierung und die Solarzellen-Prozessierung von Silizium-Wafern beschreibt, die sehr schnell und direkt aus der Schmelze gezogen werden, also ohne den Umweg des Blockgusses und des damit verbundenen Materialverlustes.
Der erste Teil der vorliegenden Arbeit stellt das Folien-Silizium RGS (Ribbon Growth on Substrate) vor. Kristallographische Untersuchungen sowie die Analyse wichtiger Materialparameter zeigen das Potential des sich noch in der Entwicklung befindenden Materials. Besonderes Augenmerk liegt auf dem interstitiellen Sauerstoffanteil, da er die Wasserstoffdiffusivität und damit das Verbesserungspotential der Materialqualität direkt beeinflusst.
Zur Entwicklung von an die Materialqualität angepassten Solarzellenprozessen werden grundlegende Experimente zur mechanischen Planarisierung unebener Waferoberflächen, zur Reduktion dabei verursachter Risse, wie auch zur chemischen Entfernung defektreicher Kristallschichten durchgeführt. Die Analyse einzelner Prozess-Schritte führt auf einen Zellprozess, der materialspezifische lokale Kurzschlüsse verhindert.
Durch eine mit der Position im Wafer nicht konstante Dotierstoff-Konzentration kann ein Drift-Feld ausgebildet werden. Dieses kann zur Erhöhung der Kurzschluss- stromdichte von Solarzellen genutzt werden. Auf Wafer- wie auch Zellbasis wird untersucht, ob mit Gallium dotierte RGS Wafer aufgrund von Segregation und den RGS-spezifischen Kristallisationsbedingungen die vermutete ortsabhängige Dotierung zeigen.
Dotieren mit Phosphor hingegen führt zu n-Typ Wafern, deren Charakterisierung und Prozessierung das Potential dieses Materials abschätzen.
Um Zelleffizienzen zu erhöhen, werden verschiedene Oberflächen-Texturen an diesem Material getestet. Weiterhin wird untersucht, ob Skaleneffekte beim Prozessieren von größeren RGS Solarzellen zu erwarten sind und welche Auswirkungen eine Reduktion der Waferdicke auf Zellparameter und Siliziumverbrauch bezogen auf die abgegebene Leistung hat.
Ein weiteres Folien-Silizium Material, MW (Molded Wafer), wird im zweiten Teil der Arbeit betrachtet. Auch dieses Material befindet sich noch im Entwicklungsstadium.
Aufgrund des Herstellungsprozesses und in Kombination mit einem Ausheil-Schritt bei hohen Temperaturen besitzen die verhältnismäßig dicken Wafer dieses Materials eine ausgedehnte sauerstoffarme Zone im oberen Waferbereich. Dieser Bereich stellt in einem für dieses Material angepassten Siebdruck-Solarzellenprozess die
photovoltaisch aktive Zone dar. Der Einfluss des Ausheil-Schrittes, insbesondere der Temperatur auf die Materialqualität, wird anhand von Zellparametern und erweiterter Analyse verdeutlicht.
Der letzte Teil dieser Arbeit befasst sich mit der Lock-In Thermographie, einer bildgebenden Messmethode, die es bereits nach sehr kurzer Messzeit ermöglicht, Joulesche Verluste in Solarzellen ortsaufgelöst darzustellen. Dies ist insbesondere bei den in dieser Arbeit analysierten multikristallinen Folien-Silizium Materialien von großem Interesse, da hier typischerweise eine inhomogene Verteilung der Materialqualität vorliegt.
Die Lock-In Berechnung erhöht neben der lateralen auch die thermische Auflösung des im Rahmen dieser Arbeit aufgebauten Messsystems erheblich. Dies ermöglicht es, typische Temperaturdifferenzen in Solarzellen, hervorgerufen durch Shunts im μK-Bereich, aufzulösen.
Die konventionelle Lock-In Thermographie wird um eine neue Messmethode, die illuminated Lock-In Thermographie (iLIT) erweitert, die es erstmals erlaubt, nicht nur Solarzellen, sondern beliebige pn-Strukturen kontaktlos zu messen, was eine kontaminationsfreie Überprüfung einzelner Prozessschritte ermöglicht.
1 RGS
1.1 Introduction
Silicon ribbon materials in general are usually classified by the shape of the meniscus which is built up at the liquid solid interface (three different shapes are distinguished in [3]), by the transport direction of the solidified ribbon with respect to the movement of the liquid-solid interface during crystallisation, according to how the crystallisation heat is removed or according to the seeding of the growing silicon crystal [4]. The most convenient consideration is to distinguish silicon ribbon growth technologies by the way in which the crystallisation heat (latent heat of fusion) is removed. Two types are possible:
For type I technology the heat is transported from the liquid-solid interface through the solidified wafer by a temperature gradient to the surrounding via radiation or a heat sink. Hereby the crystal growth speed is controlled by the heat flux through the wafer and can be kept constant. Maximum growth rates of 8 cm/min can be calculated for a 300 μm thick wafer [5]. However, technically feasible growth rates are much lower due to an upper limit of thermal stress in the wafer for a compromise between fast growth rates and low defect (such as dislocations) densities. E.g. for Edge-defined Film-fed Growth (EFG) silicon the temperature gradient close to the liquid solid interface is about 1000°C/cm and limits the growth speed to ~ 2 cm/min [6]. Silicon crystals grown using this technique show an elongated grain structure in the pulling direction with grain lengths in the cm range where faster growing crystal orientations are preferred.
If the crystal is grown on a substrate material, heat transport into the colder substrate material is more effective than the heat dissipation by radiation, enabling a much faster growth rate. Silicon ribbon materials produced by a supporting substrate are referred to as type II materials. A maximum growth rate of 600 cm/min can be calculated for a temperature gradient of 160°C between the melting temperature of Si and the substrate temperature [7]. Opposite to type I silicon ribbons, type II ribbons show a completely different grain structure due to the areal contact of the crystal to the supporting substrate material during growth. Ribbon Growth of Substrate (RGS) is classified as a type II ribbon material.
The RGS technology was developed by Bayer AG throughout the 1980’s (started in 1984 [7]) and 1990’s. In this phase, two laboratory-scale machines were built to demonstrate the RGS production principle and potential. Bayer stopped its silicon solar activities in 2000 and the RGS technology was developed further by a Dutch consortium of ECN and S’Energy (now Sunergy Investco) together with Deutsche Solar AG. In cooperation with other partners namely the University of Konstanz, National Microelectronics Research Centre (NMRC, now Tyndall National Institute, Ireland) and sunways AG the RGS material quality was improved within the EU funded RGSells project by means of material characterisation and solar cell processing. The results were introduced successively into the wafer production process. Thus the wafer quality could be enhanced significantly leading in combination with improved solar cell processing to enhanced efficiencies of RGS solar cells.
RGS wafers are produced exclusively by ECN. Characterisation and solar cell results presented within this work originate from experiments carried out at the University of Konstanz. Results gathered by other institutes are stated explicitly in the following.
1.2 RGS Production Principle
Silicon Melt Substrate
vP Silicon Foil
300 µm Frame
vC
Silicon Melt Substrate
vP Silicon Foil
300 µm Frame
vC
Figure 1: RGS production principle.
The RGS production principle shown in Figure 1 is quite simple. Substrates kept at a temperature below the silicon melting point are moved at high speed underneath a casting frame filled with liquid silicon which defines the size of the wafers and the solidification front. The silicon sheet cools down by heat transport into the substrate and detaches automatically from the (reusable) substrate material due to the different heat expansion coefficients of silicon and the substrate. In contrast to other silicon ribbon materials such as EFG or String Ribbon (SR)7 the crystal growth (vc) and the crystal transport speed (vp) are decoupled which allows a very fast wafer production. The crystal growth speed can be controlled by the heat extraction capacity and the temperature of the used substrate material whereas the wafer thickness is influenced by the length of the casting frame and the pulling velocity of the substrate plates. Thus, in contrast to other wafer casting technologies, the RGS process is much more variable in both, the wafer size and the wafer thickness [8].
1.2.1 Discontinuous Setup
When Bayer AG developed the RGS technology they started to build a first laboratory-scale machine to demonstrate the RGS production principle and potential.
The layout of the first machine (internal notation: Anlage I) is shown in Figure 2.
During processing a sequence of 10 substrates is moved underneath a casting frame filled with liquid silicon in a “single shot”. The silicon is melted in a crucible above the casting frame (not shown Figure 2) and an outlet at the bottom of the crucible has to be opened exactly in the moment the substrates start to move. The substrates are pre-heated before casting with an infrared heat source to minimise the thermal stress of the silicon crystal. During casting a gas shower provides a reactive gas environment to reduce the surface tension of the silicon [9] which results in flat wafer surfaces and less contamination from the surrounding. The subsequent tempering
unit is used to control the temperature gradient between the surrounding and the wafer and thus the thermal stress in the wafer.
Figure 2: Layout Anlage I.
After the “single shot“ wafer casting is finished, the wafers cool down to room temperature and detach from the substrate by the difference in the thermal expansion coefficient. The substrates have to be cleaned and can then be reused.
The crucible with outlet and valve as well as the casting frame have to be freed from the residual silicon before reusing. This point is important for the carbon concentration [Cs] of the produced RGS wafers. In a continuous melting environment as it is given for other ribbon technologies like SR or EFG, a carbide layer is formed at the inner border of the continuously refilled crucible, acting as a diffusion layer for further carbon originating from the carbon based crucible [10]. Thus the carbon concentrations of silicon wafers being processed in such an environment are significantly lower compared to RGS silicon. A further source for carbon is the carbon based casting frame and the substrate material being in direct contact with liquid silicon during crystal growth. The substrate material controls the heat removal properties, the wetting behaviour, the chemical reactivity and the thermal expansion.
These factors influence the crystal growth as well as the separation of the wafer from the substrate. Thus high emphasis was laid on the development of the substrate material as well as the morphology of the substrate surface to reduce sticking of the wafers and to optimize seeding and thus crystal growth.
1.2.1.1 Wafer Specifications
The interaction between crystal defects such as dislocations or impurities and their behaviour during solar cell processing makes it impossible to predict solar cell efficiencies from wafer parameters. Thus the suitability of a wafer must be tested empirically in a suitable solar cell process [11]. Suitable cell processes developed for wafers originating from the laboratory-scale RGS wafer production machine are described below in paragraph 1.6.
The following wafer characteristics were measured at ECN after each production run which usually contained 10 RGS wafers of the size 13x8.6 cm2 or 13x10 cm2 depending on the used substrate dimensions: the base doping concentration via a 4 point probe tester as well as the [Oi] and the [Cs] content by Fourier Transform Infrared Spectroscopy (FTIR). The measured wafer parameters showed a variation in the [Oi] and the [Cs] content as a function of the wafer position in one production run.
Depending on the RGS process conditions, variable oxygen and carbon contents in dependence of the wafer position were found. The oxygen and carbon contents were
in addition measured at Tyndall with Secondary Ion Mass Spectrometry (SIMS) on bevelled wafer samples [12].
Further on, the as grown RGS wafers can show thickness variations of up to one mm on one single wafer. The thickness variation is a result of process instabilities partially due to the acceleration and retardation of the movable carrier on which the substrates are mounted on.
1.2.1.2 Comparability of RGS wafers
Due to the limited output of the lab-type machine the produced RGS wafers were cut into 5x5 cm2 wafers using a laser to enhance the number of processable wafers and thus to obtain better statistics concerning the solar cell parameters.
The variations in the chemical and physical properties of the RGS wafers complicate a comparison of processed solar cells from even the same RGS production run. Thus the most meaningful results were obtained by comparing 5x5 cm2 wafers or respectively solar cells originating from the same large entire wafer. In the following 5x5 cm2 RGS wafers originating from the same entire RGS wafer are referred to as
“neighboring wafers”.
The inhomogeneously distributed material quality necessitates the application of spatially resolved measurement techniques for characterisation of wafer characteristics such as minority carrier lifetime. Thus, all minority carrier lifetime measurements were performed using the µPCD technique (photoconductance decay measured by microwave reflection) by scanning the wafer in lateral direction.
Spatially resolved solar cell characterisation was preferentially performed using the Light Beam Induced Current (LBIC) technique as well as Lock-In Thermography (LIT). Concerning the latter method, a suitable setup was assembled and a new measurement method was established which is part of this work (see part III).
It has to be pointed out that at the time the machine (Anlage I) was built up the goal was just to demonstrate the possibility of the production of flat silicon sheets according to the RGS technique. Thus no well specified materials in terms of impurity concentrations and contamination were used. As the proof of principle could be demonstrated successfully a successive exchange of machine parts with low impurity concentrations resulted in less impurity containing wafers. After the transfer of Anlage I from Bayer AG to ECN further improvements were performed in terms of construction and usage of lower contaminated materials being in direct contact with liquid silicon. For instance a major improvement in the RGS material quality could be achieved by exchanging the SiO2 crucible and the residual environment to coated or carbon based materials. As an important result the interstitial oxygen content [Oi] of the RGS wafers could be lowered significantly, resulting in enhanced solar cell performances (see paragraph 1.6.4).
1.2.2 Continuous Setup
A major advantage of the RGS technology is the feasibility of a continuously operating process as demonstrated in Figure 3 in opposite to the batch type block casting process used for standard mc material. This should result in a homogeneous wafer quality compared to standard mc wafers originating from different heights of one block. The segregation processes due to the higher solubility of most materials in
the liquid silicon phase as well as side or bottom wall contaminations by the crucible and casting frame result in a height dependent wafer quality for ingot based material.
The spread in the wafer quality leads to a broadening of the solar cell efficiency of 5-10%rel. in dependence of the wafer position in one block [13].
The continuous crystal growth process is enabled by continuously by-passing substrate plates which are arranged in a chain like setup (Anlage II). Thus the setup can work in thermal equilibrium by refilling the crucible with silicon granulate instead of melting a particular amount of silicon and cooling the crucible down after the growth process of a limited amount of wafers.
Figure 3: Layout Anlage II.
Within this work no material produced from Anlage II was analysed. Bayer AG built up Anlage II for the same purpose as Anlage I was built, namely for the proof of concept to continuously produce flat silicon sheets using the RGS technique. Thus no main focus was laid on the impurity concentrations of the used materials. Since Anlage I is much more variable due to the open and much more simple design, all improvements concerning the crystal growth process and the used materials were gathered with Anlage I. The results, however, are directly transferable to the continuous process of Anlage II and as well to a new, completely re-engineered RGS machine (Figure 4) working in thermal equilibrium. This new machine is currently built up by ECN and is expected to produce first wafers within the last quarter of 2007.
Figure 4: Sketch of Anlage III (total length about 8 m) with infrastructure installation.
The production principle of Anlage III is comparable to Anlage II (Figure 3). In contrast to the chain like substrate transport mechanism used in Anlage II the substrate plates are transported in a horizontally circular setup enabling the exchange of defective substrate plates during operation by special maintenance ports (Figure 5).
Figure 5: Setup for continuous wafer casting and outfeeding (top view).
The complete setup is located in a vessel and works under an inert gas atmosphere in thermal equilibrium by subsequently refilling the melting crucible using especially designed feeding systems.
The wafer output with this concept of approximately 1 wafer per second is remarkably high and much faster then any other crystalline silicon production technique. The machine is constructed for the production of 156x156 mm2 wafers and has a silicon consumption of 40-60 kg/h.
1.3 RGS Crystal Growth
The substrate material, from a microscopic point of view, is not totally flat and this in combination with the high surface tension of liquid silicon (higher than mercury) leads to randomly distributed seeding points for the crystal growth. As a result the silicon ribbon shows a randomly distributed crystal orientation and a columnar grain structure in a direction perpendicular to the substrate material (Figure 6).
The crystal growth velocity, i.e. the velocity of the liquid-solid interface propagating off from the substrate, is time dependent and shows a square root dependence with a faster initial growth velocity since the liquid silicon is at the moment when crystal growth starts in direct contact with the colder substrate. Then the growth velocity slows down with increasing thickness of the already solidified silicon due to the additional heat transport through the already solidified fraction. The principle of this kind of heat transfer can mathematically be described with the “classical Stephan problem” [14]. This 1-dim. model is a simple case of the class of transient heat transfer problems with a moving boundary condition, which is in this case given by the solidification interface. The model assumes that a liquid at uniform temperature T1, which is higher than the melting temperature Tm, is confined to a half space x > 0.
At time t = 0 the boundary surface at x = 0 is lowered to a temperature T0 below the melting temperature and maintained at this temperature due to the direct contact to the substrate. As a result solidification starts at the surface x = 0 and a solid interface s(t) moves into positive x direction. Under these assumptions the heat conduction equations can be solved and the position of the solid-liquid interface can be described by
t t
s( )=λ αs (1)
with αs the thermal diffusivity of the solid phase and λ the solution of the equation
) 0 (
) 2 2 ( 2)
( 0
4
0 4
2 2
− =
−
− ⋅
⋅ − +
−
T T c
h
a erfc
e T T
T T a b erf
e
m ps a sf
m l
m λ π
λ λ
λ λ
(2)
with b the ratio of the liquid to solid heat conductivity, a the ratio of the liquid and solid heat diffusivity, hsf the solidification heat and cps the specific heat capacity of the solid phase [15].
However, the assumption for this model involves a constant temperature T0 of the substrate material with temperature independent material characteristics such as the specific heat capacity of the solid phase cps as well as a laminar heat flow in the liquid silicon phase which both is not given. According to eqn. (1) a silicon wafer thickness in the range of 0.8 to 1.6 mm for substrate temperatures in the reasonable range of 1200 to 1350°C can be reached in 1 s. This corresponds to growth velocities in the mm/s range and is much higher compared to other silicon crystallisation methods.
Although for a detailed quantitative analysis a more sophisticated model is needed, the experimental data of the RGS growth process is in qualitative agreement with the estimations shown above. The experimental data show that after a growth period of
1 s a typical wafer thickness of 0.3 - 0.4 mm is obtained which is much thinner than expected according to eqn. (1). The reason for the difference is a non-constant but increasing substrate temperature during crystal growth, a finite heat transfer between liquid silicon and the substrate which slows down the crystallisation speed as well as a non-laminar heat transport within the crucible and casting frame [15]. Further on, when considering a 2-dim. model, a solidification velocity in parallel to the substrate has to be taken into account due to the undercooling of the melt at the point of the substrate where solidification starts (nucleation point). The undercooling of the melt in a region around the nucleation point can lead to an unstable morphological solid- liquid interface. As a consequence the interface can break up into a dendritic morphology parallel to the substrate [20].
The fast and non-linear crystal growth results in a thickness dependency of the grain size, the defect density, and due to segregation effects the distribution of dopant and impurity concentrations.
Figure 6: Optical microscope cross view of a polished and etched (Secco etch [60]) RGS wafer.
To visualize the crystal structure, a RGS wafer was cut into pieces of 1.3x1.3 cm2. The pieces were polished on the cross section and then etched by a Secco etch [60].
This etching solution shows faster etching rates at positions where the crystal structure is disturbed, e.g. at grain boundaries, dislocations or precipitates. Thus the anisotropic etching results in a contrast-rich surface which can be analysed by an optical microscope. Figure 6 shows an overview image stitched together from 6 polished and etched wafer pieces. By comparing wafer regions near the surfaces of thicker areas with thinner wafer areas, extended defect-rich crystal structures at the free wafer surface and the wafer substrate side are visible (Figure 6, left side). During crystal growth the crystallisation speed is slowed down (eqn. (1)) with prolonged crystal growth. The conduction of the solidification heat to the substrate is hindered by the fraction already solidified. Thus the crystallisation conditions change resulting in an altered crystal structure at thicker wafer areas.
Investigated in detail (Figure 7), a columnar grain structure is visible with slightly enlarged grain cross sections towards the free wafer surface due to the directional crystal growth starting from the wafer substrate side. The structure shows a defect- rich layer on both, the wafer front surface and substrate side. This layer has a thickness of about 25 μm and has to be removed before solar cell processing (see paragraph 1.6). Further on, pin holes are visible at locations where incorporations were etched by the etching solution leading to holes in the silicon matrix. In the middle of the sample a region can be identified where the columnar grain orientation is disturbed. Minority charge carries show a limited mobility and a higher recombination rate in such crystal areas due to the higher density of grain boundaries, leading to reduced diffusion lengths and thus to a limited solar cell performance.
Figure 7: Detailed view of one sample of Figure 6 (cross section).
An overview of RGS material characteristics is given in Table 1. The grain size of RGS is compared to other ribbon materials (cm range) quite small due to the high amount of nucleation sites provided by the substrate. The comparably high dislocation density is a result of the thermal stress during crystal growth but is expected to improve when a setup in thermal equilibrium is used (see paragraph 1.2.2).
The amount of boron used for doping is reduced compared to standard mc material (0.5 – 1 Ωcm) resulting in a resistivity of 3 Ωcm. In terms of solar cell processing it was found that silicon ribbon materials show for doping concentrations corresponding to 2 – 4 Ωcm an optimum in solar cell parameters [6]. The reason for this effect is yet not fully understood. A possible explanation is that due to the high carbon content of ribbon materials originating from the carbon based or carbon coated shaping die for type I ribbon materials or the used substrate materials for type II ribbons (RGS) recombination active centres with boron are formed [16]. Thus basically 3 Ωcm material was produced and for most experiments performed within this work 3 Ωcm material was used.
Besides detrimental recombination centres involving transition metals or carbon, oxygen and boron (Fe-B) related defects play an important role for RGS and will be discussed in detail in paragraph 1.6.6 and 1.6.7.
grain
size [μm] dislocation density
[cm-2]
thickness
[μm] resistivity
[Ωcm] [C]
[cm-3] [O]
[cm-3] as grown Ldiff
[μm]
100 - 500 105 - 107 100 - 500 3 > 1018 > 3·1017 ~ 10 Table 1: Material characteristics of as grown RGS silicon.
1.4 Gettering and Hydrogenation 1.4.1 Gettering
Impurities reducing the minority charge carrier lifetime can be gettered internally within the bulk of the wafer e.g. at oxygen precipitates or externally near the wafer surface [27]. The latter mechanism allows a removal of impurities from the active region (bulk) of the solar cell and thus enhances the material quality during processing. Therefore the impurity (e.g. metal atom) has to be released from its located site within the silicon matrix by enhancing the energy above its binding energy in the lattice. Further on, to move the impurity to the surface where it can be easily removed, the diffusivity of the particular defect species in silicon has to be enhanced as well. Thus high temperatures are required to provide the thermal energy necessary for both parts of the gettering sequence. On the other hand the thermal energy should not be too high allowing the captured impurities to be released again from the gettering site. Thus the gettering sequence depends on the material and the impurity species.
Gettering within a standard solar cell process takes place during the emitter formation process step by phosphorus diffusion at temperatures in the range of 800 to 900°C for about 20 min. During the P diffusion a highly P doped SiO2 layer is grown on the wafer surface acting as a (infinite) P source for the subsequent diffusion. During the growth of the SiO2 layer Si self-interstitials (Sii) are injected from the Si-SiO2 interface into the bulk. These interstitials can remove impurities (metals) via a so-called kick-out reaction from substitutional lattice sites to interstitial sites [28].
On an interstitial site the mobility of the impurity is enhanced compared to a substitutional lattice site. Due to the enhanced diffusivity the impurity can diffuse easily to the wafer surface where the solubility of the impurity is higher in the P doped SiO2 layer. Thus the impurity is located and can subsequently be removed after the emitter diffusion by phosphorus glass etching in diluted HF. This injection induced phosphorus gettering mechanism provides an effective reduction of metal atoms in the silicon bulk.
Another well known gettering mechanism takes place during the formation of the aluminium back contact of a solar cell. Aluminium is applied either by evaporation or screen-printing of Al paste on the backside of the silicon wafer. The contact formation to the silicon bulk is enabled by the formation of an eutectic at temperatures above the eutectic point of Al and Si at 577°C. At this temperature the solubility of most metals is enhanced by a factor of ~ 104 in the eutecticum compared to pure silicon [10]. Thus a diffusion gradient is formed towards the eutectic on the back surface of the wafer, leading to an accumulation of the impurity species in this photovoltaically inactive region.
A combination of phosphorus and aluminium gettering which takes place simultaneously during an industrial-type screen-printing process is known to be even more effective than each gettering mechanism by itself [29]. The high temperature processing steps such as the emitter diffusion and the back contact formation are passed within short time periods (several minutes). Thus primarily fast diffusing metals such as Cu, Ni, Co, Au, Fe and Cr can be gettered effectively due to their high diffusion constants allowing the penetration of the entire wafer at temperatures of 850°C within minutes. In contrast, slow diffusion metals such as Ti, V, Mo can hardly be gettered for times and temperatures relevant for solar cell processing [10].
1.4.2 Hydrogenation
Besides gettering of impurities which removes the defect from the silicon bulk, remaining recombination active defects can be passivated (deactivated) with hydrogen which results in enhanced minority carrier lifetimes. The deactivation of defects is due to a saturation of unoccupied bonds with hydrogen atoms, so called dangling bonds. The dangling bond gets electrically passivated with hydrogen and recombination is therefore less active or inactive. The incorporation with hydrogen can either be realised by out-diffusion from a hydrogen-rich source (in solar cell processing a hydrogen-rich PECVD silicon nitride layer (SiN:H) acting simultaneously as an antireflective coating and surface / emitter passivation layer) or by a diffusion from the gas phase (hydrogen plasma).
The hydrogen passivation plays a major role for the defect-rich RGS silicon and will be considered in detail in paragraph 1.6.6.
1.5 Electrical Characterisation 1.5.1 Charge Carrier Mobility
Characterisation of as grown RGS material was performed in terms of temperature dependent Hall measurements. Due to the high density of recombination active defects and grain boundaries the mobility of charge carriers is reduced in RGS compared to monocrystalline silicon. It has to be distinguished between the measured mobility of majority charge carriers and the mobility of the minority charge carriers which are of importance concerning the solar cell performance. Assuming that the scattering mechanisms (i.e. scattering at charged or neutral crystal defects, dislocations, grain boundaries or acoustical phonons) are the same for both type of charge carriers at room temperature [17], the measured hall mobility gives an estimation of the reduction in the diffusion length for RGS compared to monocrystalline material. However, the more important point when evaluating the measured mobility is that during the measurement the current flows from one contact (Figure 8, C 1) parallel to the sample surface to the other contact (C 2). However, in a RGS solar cell the current flow is perpendicular to the surface (from the point of generation G in the bulk to the emitter on the surface, see Figure 8).
Figure 8: Current flow (schematically) in a RGS wafer during Hall measurement perpendicularly to grain boundaries (blue) and solar cell operation (green).
The current flow parallel to the surface is hindered by the grain boundaries in contrast to the current flow in a solar cell. Thus, the measured Hall mobility is lowered by the contribution of the grain boundaries.
The minority carrier mobility is reduced for older oxygen-rich RGS material by a factor of 2-3 compared to monocrystalline material. A detailed overview is given in [18] and [58]. The increased scattering reduces the carrier diffusion constant and thus the diffusion length. Hydrogenation enhances the mobility significantly by passivating bulk defects. Thus, hydrogenation plays a major role in improving RGS material quality. Different hydrogenation methods and their influence on material quality are discussed in paragraph 1.6.6.
Figure 9: Temperature dependent Hall mobility (majority charge carriers) of high oxygen RGS material. Compared to a monocrystalline FZ reference sample the mobility of the RGS material is reduced. Prolonged MIRHP hydrogenation (see paragraph 1.6.6.2) enhances the mobility significantly due to a passivation of bulk defects.
1.6 Solar Cell Process Development
The focus in this work was on solar cell processing and characterisation of RGS material. During the complete period tests of material quality have been performed and a standard (simplified) screen-printing firing through SiN8 process developed in the first period was used for most solar cell processes. The aim was to keep the screen-printing process as simple as possible to obtain a reliable process sequence and to keep the processing steps industrially relevant and thus cost effective. Ongoing from an industrial compatible screen-printing process [19] shown in Figure 10, single process steps were adapted or added to deal with the particular RGS wafer characteristics as described in the following paragraphs.
Figure 10: Industrially compatible screen-printing firing through SiN8 process.
Due to permanent variations in the RGS wafer production environment in terms of testing new materials suitable for silicon melting, wafer casting as well as substrate materials, changes in wafer quality could only be monitored using a stable base line process. The change in the oxygen content of the latest RGS wafers provided by ECN for this work led to simplifications within the cell process which could be proven on lifetime as well as solar cell efficiency level. The main processing issues were:
• damage layer removal by mechanical planarisation, acidic or alkaline etching, texturisation,
• emitter formation by POCl3 diffusion or alternative P-source diffusion,
• bulk passivation and gettering by Al-gettering, P-Al co-gettering, H-plasma passivation, passivation from hydrogen-rich PECVD SiN,
• metallisation by screen-printing,
• AR coating and surface passivation by PECVD SiN.
In the following the development of solar cell processes in terms of characterisation of the as grown material and experiments performed for single processing steps are presented. Thereby single processing steps take into account the specific characteristics of older high oxygen contaminated RGS material available in the first
8 The stoichiometric composition of silicon nitride depends on various deposition parameters of the used PECVD reactor. In this work “SiN“ denotes “SixNy“.
half of this work as well as the specific characteristics of improved low oxygen RGS material.
1.6.1 Planarisation
Before material characterisation or solar cell processing the uneven front surface of the as grown RGS wafers had to be levelled using a commercial wafer- dicing saw. In former experiments the wafer surface was levelled by using a single blade technique [22], [58]. The shape of the blade was flat with a width of 150 µm, which allowed a spacing of the individual cuts of 70 µm. In this way the levelling of a single 5x5 cm2 RGS wafer took about 20 minutes. To shorten this time and make the levelling of the surface possible in an industrial-type way, planarisation tools have been developed in the past [19], which have been adapted to the use of RGS wafers.
The tools currently used have a width of 65 mm, which in principle allows the levelling of one 12.5x12.5 cm2 or four 5x5 cm2 RGS wafers in only two cuts (used dicing saw DISCO 341). Depending on the height differences at the wafer surface, successive cuts at different heights have to be performed to avoid wafer breakage. In Figure 11 the levelling tool as mounted on the spindle of the dicing saw (left) and two 5x5 cm2 RGS wafers (before and after levelling) lying on the vacuum chuck (right) can be seen. It turned out that it is essential to have a flat wafer backside without bending to enable the wafer to be sucked to the vacuum chuck and avoid breakage.
In principle a V-textured surface can be obtained when the surface of the tool is not flat but textured.
Figure 11: Structuring tool as mounted on the spindle of a commercial dicing saw for planarisation of the uneven RGS surface (left). On the vacuum chuck two 5x5 cm2 RGS wafers before and after planarisation can be seen (right).
Although planarisation parameters such as revolution speed of the spindle, specifications of the planarisation tool, feeding speed of the vacuum chuck and abrasion depth of a single planarisation step have been optimised, only a yield of approx. 80% can be obtained. Further on, the high mechanical load during planarisation can induce micro-cracks in the RGS material and is time consuming.
However, at the moment there exists no alternative to the planarisation step. In the future, if RGS wafers can be produced with a flatter free surface, the planarisation step can be omitted at all. It is expected that the new RGS machine which currently is built up will produce almost flat wafers due to the operation in thermal equilibrium of
the continuous production process as well as due to optimised substrate plates (see paragraph 1.2.2).
1.6.1.1 Wafer Thickness Variations
The discontinuous RGS production setup described in paragraph 1.2.1 as well as the non-optimised morphology and material composition of the substrate plates used for wafer growth in the first period of this work resulted in RGS wafers with thickness variations in the range of a few hundred microns. To keep the mechanical load as low as possible the planarisation was stopped at a certain height (Figure 12, left, dashed line).
Figure 12: Left: schematic cross section of the resulting wafer surface of a RGS wafer after planarisation at a fixed height of the planarisation tool (dashed line).
Right: picture of a partially planarised 5x5 cm2 RGS wafer.
As a standard wafer (planarisation) thickness 300 μm was chosen for most experiments. This thickness is in the range of standard mc wafers and thus the comparability of RGS solar cells to mc reference cells processed within each experiment is given as well. This turned out to be an important point to monitor the applied solar cell process due to the inhomogeneity of the RGS material quality.
The lower part of Figure 12 shows a schematic side view of a typical RGS wafer after planarisation. The resulting unplanarised thinner part of the wafer differs in crystal structure compared to the planarised thicker areas as Figure 6 suggests. The thinner wafer regions decrease further after defect etching resulting in thin wafer areas with a rough surface. The resulting height difference of the planarised compared to the unplanarised wafer area causes a smearing of the front metallisation fingers (Figure 13, right cell side) during screen-printing on the according area or even an interruption of fingers resulting in a reduction of the finger conductivity (decreased aspect ratio) and thus an enhanced series resistance of the solar cell. Further on, the broadening of the fingers at such areas results in enhanced shadowing and thus a reduced short circuit current density of the RGS solar cell.
Figure 13: Picture of a 5x5 cm2 RGS solar cell. On the right side smearing and thus broadening of front metallisation fingers due to screen-printing on unplanarised rough surface areas with differences in wafer thickness can be seen.
Although a smearing for partially planarised RGS wafers and thus an increase in the series resistance of the front metallisation occurs, the resulting loss in solar cell efficiency is negligible. Compared to fully planarised and thus totally flat RGS solar cells no loss in efficiency could be observed within a group of neighboring RGS wafers, demonstrating that this kind of loss contribution is less detrimental compared to other RGS specific issues, such as shunting.
To provide a cost effective industrially compatible screen-printing firing through SiN process for RGS silicon this is an important point. The time consuming and thus costly planarisation step could be reduced to a partial planarisation for wafer surface structures of current material originating from the discontinuous production setup.
1.6.1.2 Crack Reduction
Due to the high mechanical load during planarisation micro-cracks can be introduced in the RGS material as shown in Figure 14, leading to shunts in processed RGS solar cells. To reduce the generation of cracks during planarisation, beside the optimisation of planarisation parameters as described above, etching experiments prior to the planarisation step were performed.
Figure 14: Scanning Acoustic Microscope (SAM, [23]) scan of a partially planarised 5x5 cm2 RGS wafer. Cracks as a result of the high mechanical load during planarisation are visible (thick arrows). Areas where the wafers remained unplanarised (i.e. very thin areas of the as grown wafer) are marked with thin arrows.
For that purpose RGS wafers were etched prior to the planarisation step (5 μm per side) using an acidic isotropic etching solution (modified CP6 solution: HNO3 [65%], HF [50%], CH3COOH [99.8%] at the ratio of 43:3:7) to remove distorted crystal layers and thus to reduce tensile stress on the surface of the wafers. The reduction of tensile stress was expected to reduce the formation of micro-cracks in combination with the mechanical load during planarisation. The other half of the wafers was processed without the additional etching step prior to the planarisation. Both groups were processed subsequently to solar cells according to the same cell process.
Figure 15: Fill factors of RGS solar cells in dependence of shunt values. One half of the RGS wafers was etched prior to planarisation.
Figure 15 shows the fill factors (FF) of RGS solar cells in dependence of the related shunt values. Alternative shunt paths are expected to be comparable for both groups of solar cells due to the identical processing. Thus a reduction in the shunt value of a solar cell corresponds to the amount or severity of induced micro-cracks.
As a result, the average shunt values of the wafers etched prior to the planarisation are enhanced from 121 Ωcm2 to 147 Ωcm2 which corresponds to a gain of 18%rel. This might be an indication that the as grown wafers benefit from a reduction of tensile stress by means of etching prior the planarisation.
However, the low absolute values and the broad distribution of the shunt values shown in Figure 15 suggest alternative shunting mechanisms lowering the shunt values and thus the FFs of the solar cells. This will be discussed in detail in paragraph 1.6.7.
1.6.2 Defect Etching
After planarisation of the uneven wafer front side a saw-damage removal is necessary prior to the subsequent emitter diffusion. The planarisation step as a mechanical treatment provides a high mechanical load on the wafers resulting in a heavily disturbed crystal area including micro-cracks. On the other wafer side (substrate side) an at least 25 μm thick layer of disturbed crystal structure has to be removed (Figure 7). Thus after planarisation and before solar cell processing 25 μm per side have to be etched off using an acidic isotropic etch (modified CP6 solution,
see paragraph 1.6.1.2). An anisotropic (e.g. alkaline etch) would result in a rough wafer surface due to different etching rates for each grain orientation. RGS as a multicrystalline material consists of randomly orientated grains and thus no defined surface could be obtained by using a non-isotropic etching solution.
The crystal structure shown in Figure 7 suggests that an etching depth of 25 μm per side should be sufficient to remove disturbed crystal structures on the surfaces of the RGS wafers. On the other hand on the wafers substrate side of the RGS wafers the carbon concentration is increased [24] due to the contact of the liquid silicon with the carbon based substrate plate during solidification as shown in Figure 16.
Figure 16: Distribution of the substitutional carbon concentration [Cs] in a RGS wafer.
Local FTIR measurements showed that the carbon concentration is higher at the front and the rear surface of the wafer.
The enhanced carbon concentration is a result of a supersaturation of the silicon melt with carbon and is known to produce carbon precipitates in the RGS wafers [25]
which are detrimental for the solar cell performance. Thus, to check if the applied etching depth of 25 μm per side is sufficient or if RGS solar cells would benefit from extended etching, solar cells were processed from two groups of RGS wafers. One half of the wafers were etched 25 μm per side, the other half 50 μm per side.
Figure 17: Fill factors of RGS solar cells in dependence of shunt values. The etching depth of one half of the RGS wafers was doubled (triangular symbols).
Resulting shunt values of the processed RGS solar cells are shown in Figure 17. The distribution of the shunt values shows no beneficial effect for the group with extended etching depth. The processed solar cells show for both groups nearly the same mean
values of the shunt resistances Rsh (93 Ωcm2 for an etching depth of 25 μm and 90 Ωcm2 for 50 μm).
As a result, an etching depth of 25 μm per side is sufficient to remove the defect-rich surface layers of planarised RGS wafers.
The absolute values of the shunt resistances of the RGS solar cells shown in Figure 17 are low compared to standard industrial multicrystalline solar cells (Rsh > 1000 Ωcm2). RGS material suffers from different shunting phenomena lowering the FFs of the solar cells. However, shunting can be avoided by means of processing as described in paragraph 1.6.7.4.
To investigate the detrimental influence of the enhanced carbon concentration on the substrate side of RGS wafers (Figure 16) on solar cell parameters, an additional experiment was performed. Two groups of RGS wafers were processed to solar cells. One group was etched after planarisation (25 μm per side) as described above.
The other group was not only planarised on the wafer front side but in addition 30 μm were removed on the wafer substrate side using the planarisation tool, followed by etching of 25 μm per side as well. As a result, 55 μm were removed from the wafer substrate side for the second group.
Figure 18: Fill factors of RGS solar cells in dependence of shunt values. The wafer substrate sides of one half of the RGS wafers were grinded prior to etching.
Due to the mechanical removal of the carbon rich layer on the substrate side of the RGS wafers the mean shunt values improved for the group with the grinded and etched substrate side (Figure 18, triangular symbols) from 74 Ωcm2 to 129 Ωcm2 which corresponds to an enhancement of 74%rel..
Again, the absolute shunt values are significantly lower compared to solar cell processed from standard mc material due to a RGS material specific shunting mechanism.
Figure 19: Illuminated Lock-In Thermography (iLIT) measurement of two RGS solar cells processed from neighboring wafers. The corresponding wafers were processed without (left) and with (right) a grinding of the wafer substrate side. Same scaling for both cells.
However, the possibility to improve shunt values by grinding is difficult to realise. The mechanical load during grinding induces micro-cracks limiting the shunt values and thus the FFs of the solar cells again. Figure 19 shows two iLIT [26] measurements of solar cells processed from wafers originating from the same entire RGS wafer. Thus the material quality of the two wafers is comparable (neighboring wafers). The solar cell shown on the left side in Figure 19 was processed without grinding the wafer substrate side and shows areal shunts especially in the upper right area (yellow / red areas) originating from material and process induced defects (see paragraph 1.6.7).
In contrast the solar cell shown on the right side reveals reduced areal shunting but two strong point shunts (lower left cell area) most probably originating from the additional mechanical load during grinding.
1.6.3 High Oxygen RGS: Material Quality
The available RGS material at the beginning of this work consisted of wafers from the discontinuously working single-shot RGS machine at ECN as described in paragraph 1.2.1, with the fabrication process as taken over from Bayer AG. The main characteristics of these wafers were:
• very high interstitial oxygen concentration [Oi], with a special temperature treatment needed before solar cell processing
• carbon contaminations
• high dislocation density
• typically 150 - 200 μm average grain size
The last three items hold for the current RGS material as well.
Due to the high oxygen concentration an annealing step was applied directly after casting to transfer the interstitial oxygen into large oxygen clusters and to prevent New Donor formation during solar cell processing [21], [22].
To establish a baseline at the beginning of the work, this material was characterised using spatially resolved lifetime measurements9 after different processing steps. In this way the influence of gettering as well as hydrogenation steps on minority carrier lifetime could be monitored. In a first experiment a complete run (10 full size RGS wafers of production run no. 63) were measured in the as grown state without surface passivation (Figure 20). They showed poor lifetimes in the 0.1-0.3 µs range, corresponding to diffusion lengths < 20 µm. Wafers 9 and 10 are grown on a different type of substrate and differ from the other wafers in the thermal load because they are located outside the annealing furnace after crystallisation. Therefore, they normally cannot be used for standard solar cell processing. In these wafers, the high amount of interstitial oxygen [Oi] leads to the formation of recombination active New Donors during high temperature steps, that drastically reduce the minority carrier diffusion length Ldiff and thus cell parameters. The reason why they show higher initial lifetimes might be due to the absence of large oxygen precipitates intentionally formed during the 1-hour annealing in wafers 1-8.
1 2
3 4
5
6 7
8 9
10
Wafer 4 Wafer 10
Figure 20: Mapped minority carrier lifetimes of a set of as-crystallised RGS wafers (production run 63) after planarisation. Same scaling for all measurements.
Measurement performed without surface passivation (measured effective lifetimes equal bulk lifetime values for low bulk values).
1.6.3.1 Process Monitoring
The poor as grown material quality as shown in Figure 20 can be improved during processing by means of gettering and defect passivation with hydrogen, enhancing the as grown minority carrier lifetime. Only by improving material quality significantly during processing the use of the defect-rich RGS silicon in a cost
9 µPCD technique, photoconductance decay measured by microwave reflection using a Janus 300 system [47]
effective industrial-type solar cell process can lead to lower Wp costs for photovoltaics.
Wafer 5
1 2 3
4 5 6
7 8 9
Figure 21: Mapped minority carrier lifetimes from a set of RGS 5x5cm2 wafers (run 63) after POCl3-diffusion (P-gettering), oxidation, Al-gettering and hydrogenation.
Surface passivation by iodine / ethanol solution.
From the full size RGS wafers shown in Figure 20 5x5 cm2 wafers have been cut, which underwent processing steps for solar cell process involving POCl3 diffusion (simultaneously P-gettering), thermal oxidation for surface passivation, Al gettering at 800°C for 30 min after evaporation of 2 µm Al on the backside and an optimised hydrogenation step using microwave-induced remote hydrogen plasma (MIRHP) for 24 h at 450°C. Lifetimes have been measured after each of these steps and can be improved by P-gettering (0.3 µs), Al-gettering (0.5 µs) reaching values of around 1 µs after hydrogenation as shown in Figure 21.
For the high [Oi] RGS material gettering is limited by internal gettering processes at oxygen containing precipitates [30]. Thus impurity (metal) atoms are only partially removed during the high temperature steps from the active region of the solar cell.
The remaining fraction leads to an increased recombination of precipitates located at extended defects such as dislocations and / or grain boundaries.
1.6.4 Low Oxygen RGS: Material Quality
In contrast to the improvements in lifetime reached for the high [Oi] RGS (Figure 21) the beneficial effect of Al / P co-gettering and hydrogenation is clearly enhanced for the recent low [Oi] material. The lower oxygen content leads to higher as grown lifetimes and the gain in lifetime due to gettering and hydrogenation is strongly enhanced. Thus the difference in the oxygen concentration demonstrates the detrimental effect of oxygen precipitates in RGS wafers for the minority carrier lifetime.
