Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft;
op gezag van de Rector Magnificus prof. ir. K. C. A. M. Luyben; voorzitter van het College voor Promoties
in het openbaar te verdedigen op woensdag 29 oktober 2014 om 10:00 uur
door
Pavel BABÁL
Inžinier, Slovenská Technická Univerzita geboren te Bratislava, Slowakije
Dit proefschrift is goedgekeurd door de promotor: Prof. dr. M. Zeman
Copromotor:
Dr. ir. A. H. M. Smets
Samenstelling promotiecommissie:
Rector Magnificus, voorzitter
Prof. dr. M. Zeman, TechnischeUniversiteit Delft, promotor Dr. ir. A. H. M. Smets, Technische Universiteit Delft, copromotor Prof. dr. J.A. La Poutre, Technische Universiteit Delft
Prof. dr. L.D.A. Siebbeles, Technische Universiteit Delft
Prof. dr. E. Vlieg, Radboud University Nijmegen
Dr. W. Soppe, Energie Centrum Nederland
Dr. A. Gordijn, Forschungszentrum Jülich GmbH
Prof. dr. E. Charbon, Technische Universiteit Delft, reservelid
Copyright © 2014 P. Babál All rights reserved.
No part of this material may be reproduced, stored in a retrieval system, nor transmitted in any form or by any means without the prior written permission of the copyright owner.
ISBN 978-94-6203-682-6 Cover photo credits: Marek Babál
1 INTRODUCTION 1
1.1 SOLAR CELLS TYPES 2
1.2 THIN-FILM SOLAR CELLS AND THEIR ADVANTAGES 4
1.3 REFLECTING LAYERS IN THIN-FILM SILICON SOLAR CELLS 7 1.3.1 Desired Intermediate Reflector Properties & Simulations 7
1.3.2 Zinc oxide as a reflecting layer 8
1.3.3 Silicon oxide as a reflecting layer 9
1.3.4 Advanced concepts of reflecting layers 10
1.4 THE OBJECTIVE OF THIS THESIS 12
2 PROCESSING AND CHARACTERIZATION OF THIN-FILM SILICON
LAYERS AND SOLAR CELLS 15
2.1 SOLAR CELL PROCESSING 15
2.1.1 Plasma Enhanced Chemical Vapor Deposition (PECVD) 15 2.1.2 Magnetron Sputtering 17 2.1.3 Thermal and electron beam evaporation 18 2.1.4 Reactive Ion Etching (RIE) 19 2.2 SOLAR CELL CHARACTERIZATION 20 2.2.1 External Parameters 20 2.2.2 External Quantum Efficiency (EQE) 21
2.3 SINGLE LAYER CHARACTERIZATION 23
2.3.1 Raman spectroscopy 23
2.3.2 Fourier transform infrared spectroscopy 24
3 SILICON OXIDE NANOSTRUCTURE AND MATERIAL CHARACTERIZATION 27
3.1 INTRODUCTION 27 3.2 EXPERIMENTAL DETAILS 30 3.3 MATERIAL DEVELOPMENT 31 3.3.1 Single layer properties 31 3.4 NANOSTRUCTURE ANALYSIS 39 3.4.1 TEM analysis 39 3.4.2 Raman spectroscopy analysis 45 3.4.3 Fourier transform infrared spectroscopy analysis 48 3.4.4 X-ray photoelectron spectroscopy comparison 62 3.5 DISCUSSION AND CONCLUSION 67
4.2 RESULTS 74
4.2.1 N-doped nc-SiOx:H as a back reflector 74
4.2.2 P-doped nc-SiOx:H as a p-layer 79
4.3 CONCLUSION 80
5 INTERMEDIATE REFLECTORS IN TANDEM SOLAR CELLS 83
5.1 BRAGG STACKS AS INTERMEDIATE REFLECTORS 84 5.1.1 Choice of materials and ASA simulations 84 5.1.2 Bragg reflector on glass 88 5.1.3 Cell integration 93 5.2 CONTROL OF INTERFACE TEXTURING 97 5.2.1 Influence of front TCO texture 97 5.2.2 Etched zinc oxide as textured intermediate reflector 101 5.2.3 Mechanical polishing of the intermediate reflector 106 5.3 CONCLUSIONS 110 6 CONCLUSIONS 113 6.1 NANOSTRUCTURE 113 6.1.1 Crystalline phase 114 6.1.2 Amorphous phase 115 6.2 CELL INTEGRATION 116 6.3 INTERMEDIATE REFLECTOR CONCEPTS 116 6.3.1 Distributed Bragg Reflectors 116 6.3.2 Intermediate reflector texturing 117 6.4 RECOMMENDATIONS 117 BIBLIOGRAPHY 119 SUMMARY 127 SAMENVATTING 129 LIST OF PUBLICATIONS 133 ACKNOWLEDGEMENTS 135
The enormous dependency on fossil fuels as a source of energy [1] is the cause of many global problems. Their uneven distribution around the world has been a source of political and economic problems, limiting development and causing (armed) conflicts [2]. In addition, the abundance of fossil fuels is limited and even with new deposits being discovered they will eventually run out. Therefore, it is highly important to replace these sources of energy with other (practically) unlimited, widely available, inexpensive, and safe alternatives.
One of these alternatives is solar energy converted directly to electricity via photovoltaic (PV) cells (more commonly known as solar cells). An amount of 1.13 x 1018 kWh of solar radiation reaches the earth’s surface every year, where 19.6 x 1012 kWh per year is the global electric power consumption (in 2010) [3]. Therefore, even with increasing global electricity consumption, if only a fraction of this immense amount of solar energy could be converted, the electrical energy problems that humanity faces now and will face in the future could be solved in a sustainable way. What’s more, 1.3 billion people, mostly from the poorest regions of the world, have no access to grid connected electric energy [4],[5]. Battery systems charged by solar cells are the most promising and cost effective solution for providing off-grid electric energy. They can provide these
remote locations with electricity at a reasonable price without having to wait for the construction of expensive grid infrastructure. This will provide the local populations new opportunities for development and education [6].
Solar cells are seen as one of the most promising alternative, renewable, and eco-friendly sources of energy and are by far the most newly installed type of electricity generation in recent years (Fig. 1.1). Figure 1.1 shows that two times more new PV capacity was installed in 2011 than new wind or gas capacity. This high PV capacity of new installations was partially due to subsidized feed-in tariffs. However, prices of PV systems are dropping and coming near to or - in most locations – are equaling grid prices (grid parity) [8]. For PV technology to become successful on a large scale, the materials used in solar panels must satisfy strict requirements. These materials need to be abundant (Fig. 1.2), cheap, and non-toxic. Their processing and recycling should be cheap as well.
1.1 Solar cells types
To date, there are various PV technologies on the market and many are being researched and developed. Each type of PV technology has its specific advantages and disadvantages, such as low material costs, low production costs and high solar to electrical power conversion efficiency.
To date, the most widespread PV technology is based on bulk wafer-based crystalline silicon (c-Si) cells. They dominate the market with an 84% share in 2012 [7] and are predicted to dominate the market in the
Figure 1.1: Different technologies for the production of electricity according to their global installed capacity in 2011 [7].
1.1 SOLAR CELLS TYPES
Figure 1.2: The abundance of elements in the upper earth’s crust plotted against their atomic number [9].
decade to come (Fig. 1.3). Commercial crystalline silicon modules have efficiencies between 16-20%. Taking into account their production costs, the levelized cost of the generated energy (in this example in the United States; non-dispatchable) is 144.3 $/MWh while conventional sources of electric energy such as coal or natural gas are substantially cheaper, with 100,1 and 67,1 $/MWh respectively [10]. In addition, the conversion efficiency of such laboratory scale c-Si single-junction solar cells is approaching its theoretical limit. Often the Shockley Queisser limit is used with a value of ~30%, however, it only takes into account radiative recombination which is not the dominant charge carrier recombination mechanism for a indirect bandgap material as silicon [11]. By taking into account Auger recombination as well, the theoretical limit drops to 29,4% [12]. Current state of the art crystalline solar cells are close to this limit, with 25,6% and 25% for HIT and PERL cells respectively [13], [14].
Thin-film (TF) PV technology, compared to bulk silicon solar cells, use much less material with a thickness of on average just a few micrometers. The consumption of material in TF solar cells is therefore much lower than in 200 micrometer-thick c-Si wafers and TF cells in general use cheaper processing methods. However, TFs still face many challenges, where the most important one is raising the conversion efficiency. The module efficiency of TF technology on the current market is lower than c-Si modules, being between 10-13%. Yet still their annual production capacity is predicted to grow in the coming years (Fig. 1.3). Some TF PV technologies produce more energy from each module per invested amount (described as the price per Watt peak), however, other costs need to be taken into account as well.
Next to module costs, the costs of a PV system contain non-modular costs, often called the balance of system costs (BOS). These costs include, for example, the structural frame, inverter, installation, etc., and currently account for ~68% of the entire PV system cost [15]. The cost of a PV system per watt decreases with increasing installed capacity as shown in the learning curve in Figure 1.4. However, the non-modular costs decrease at a slower rate than the module costs per installed power. From about 2 GW of installed production capacity the limiting factors for price reduction become the non-modular costs. Therefore, due to the lower efficiency of TF PV modules, they need more area to produce this energy compared with c-Si modules and that raises their non-modular costs. The fact that the costs of the whole PV system usually scale with the area of the system and not with the installed electrical power further underlines the necessity for higher conversion efficiencies of TF solar cells.
1.2
Thin-film solar cells and their advantages
There are a variety of TF PV technologies based on different semiconductors, including germanium, III-V semiconductors, cadmium telluride (CdTe), copper indium gallium selenide (CIGS) and organic materials. III-V semiconductor solar cells are expensive cells with high conversion efficiencies from 25-44% for lab-scale cells [17]. CdTe and CIGS laboratory scale cells have conversion efficiencies between 15-18%. CdTe and CIGS solar modules also have reasonably high efficiencies (11-13%) but their cost heavily depends on the cost of the elements that
1.2 THIN-FILM SOLAR CELLS AND THEIR ADVANTAGES
constitute them [18], [19]. Especially Indium and Telluride are scarce materials in the earth’s crust (Fig. 1.2), greatly limiting their potential for production up-scaling. Neither of these materials meets the desired prerequisites needed for successful large scale PV implementation, being cheap, abundant, and non-toxic materials. An additional shortcoming of CdTe and CIGS is the significant loss in efficiency when laboratory-sized cells are up-scaled to modules (~35%).
One way of increasing TF cell efficiency is by stacking cells on top of each other, making a multijunction cell (Fig. 1.5). In case of two stacked cells, they are called tandem cells (Fig. 1.5a, b). Tandem cells take advantage of better utilization of bandgap energy and spectrum using two different bandgap materials in the same solar cell. Gallium arsenide based multijunction devices are the best demonstrators of this concept, being the most efficient solar cells to date (efficiencies of over 40% under solar concentration and laboratory conditions [20]).
TF Silicon PV technology uses the concept of multijunctions as well. The most common combination of materials constituting a tandem cell are a-Si:H and nc-Si:H. It combines the advantages of the low-cost TF deposition technology with higher spectrum utilization than in single junction TF Si cells. Due to spectral mismatch between the solar spectrum and the absorber material bandgap, the bandgap of one PV cell is not enough to capture all incoming photons. Thermalization and non-absorption constitute the greatest losses in solar cells. Tandem cells are an easy way to absorb a greater portion of the solar spectrum with the
Figure 1.4: The learning curve of PV modules and systems comparing their price per watt with cumulative installations [16].
added advantage of a high output voltage. Currently, laboratory scale single junction and a-Si:H/nc-Si:H tandem cells have a stable efficiency of 10,7% and 12,3% respectively (Fig. 1.5a, b). To-date the most state of the art TF silicon devices are triple junction cells achieving an initial efficiency of 16,3% and stable efficiency of 13,4 % at laboratory scale (Fig. 1.5e, c) [21], [22].
From the TF materials, TF Si is the only material that meets all the prerequisites of a PV material. It is cheap, abundant and easily processable [23]. However, the efficiency of TF Si modules is still low compared with other PV technologies, being around 10%. The usual thickness of TF Si double-junction solar cells studied in this thesis is around 3 μm. This thickness is a compromise of deposition rate and as large as possible light absorbance of the layers. In addition, larger thicknesses reduce the charge carrier collection efficiency because of the relatively poor charge transport in these materials. Therefore, to enhance light absorption, light trapping methods are introduced into the cell. These methods include, for example, the application of anti-reflective coatings, textured interfaces, and back reflectors. Besides their lower costs, other advantages of TF Si cells include the possibility to be flexible, lighter weight, and ease of integration into other products. A relatively small difference between laboratory-scale cell and module efficiency exists, showing the mature status of the technology.
Figure 1.5: State-of-the-art laboratory scale multijunction TF silicon solar cells and their stable efficiencies.
a b c d e
nc-Si:H / a-Si:Ge:H / a-Si:H n-i-p / n-i-p / n-i-p a-Si:Ge:H /
a-Si:Ge:H / a-Si:H n-i-p / n-i-p / n-i-p nc-Si:H / nc-Si:H /
a-Si:H n-i-p / n-i-p / n-i-p a-Si:H / nc-Si:H
p-i-n / p-i-n a-Si:H
1.3 REFLECTING LAYERS IN THIN-FILM SILICON SOLAR CELLS
Hydrogenated amorphous Si (a-Si:H) solar cells are drift based devices, in comparison to c-Si which are diffusion based devices. To successfully extract charge carriers from the cell, an intrinsic silicon layer has to be sandwiched between a doped p- and n-layer which create an electric field in the intrinsic layer. This intrinsic absorber layer is defect rich and its electric properties are, to a certain extent, metastable. The conversion efficiency of an a-Si:H solar cell decreases after exposure to light. This effect is called the Staebler-Wronski effect (SWE) and was first described in 1977 [24]. The efficiency reaches a steady state after about 1000 hours exposure to illumination. The typical relative degradation of the cell efficiency upon light soaking is between 10% to 25% for a-Si:H cells [25]. Nc-Si:H cells practically do not suffer from light induced degradation. As the generation of light induced metastable defects is a bulk process, the SWE is less profound for thinner layers.
1.3
Reflecting layers in thin-film silicon solar cells
Due to the usage of thinner and more stable TF a-Si:H absorber layers, it is necessary to include reflecting layers at the back of the junctions to enhance the optical path of light through the absorber layers. A method of light trapping in multijunction cells, other than using a back reflector, is incorporating between each junction an intermediate reflector (IR). The basic function of any IR in a multijunction cell is the back reflection of incident photons with energies higher than the bandgap of the absorber layer above it with the aim of increasing spectral utilization. The IR is an additional tool that helps in matching the currents of the individual junctions. Note that the tandem cell is a series-connected device in which JSC is determined by the lowest current coming from a junction. To date, a-Si:H/nc-Si:H tandem cells have been studied for almost two decades. Optimization of these cells brought intense research in the field of IRs. During that time, many interesting IR materials, concepts, simulations, and results were achieved which will be shortly summarized.
1.3.1 Desired Intermediate Reflector Properties & Simulations
The development of device grade nc-Si:H [26], [27] led to a new type of TF Si double junction, the a-Si:H/nc-Si:H tandem. A logical next step was the integration of IRs into tandems [28]. A well-functioning IR needs to meet certain requirements. These requirements include a high reflectance of low-wavelength photons (<600 nm, in case of a-Si:H/ nc-Si:H tandems) and a high transparency for high-wavelength photons (>600 nm). This can be achieved by properly tuning the thickness of low
refractive index film (for high index contrast with the silicon absorber layers) and high band gap materials. Optical analysis of IRs with a small refractive index (n < 2.0) revealed possibilities for significant improvements of the top cell short-circuit current density (JSC) (>25%) or thickness reductions of the top cell absorber layer (>50%) [29]. The material should also have minimal parasitic absorption.
If the IR material is p- or n-doped it can serve as the p- or n-layer in the active part of the junction. Another important quality of IR materials is high transversal conductivity, as the tandem cell is a series connected 2-terminal device. As-low-as-possible activation energy of doped layers is desired to reduce device voltage loss as the IR forms the tunnel recombination junction (TRJ) in multijunction cells. The effects of IRs inside tandem cells have been extensively simulated, confirming the necessity of IRs to obtain tandem cells with high performance [30] [31][32]. Simulations proved to help in the optimization process of IR parameters for cell application.
As mentioned earlier, the two junctions of a tandem cell absorb different wavelengths of light. This light needs to be scattered for optimum absorption. Optimal light scattering for the top- and bottom junction requires different textured surfaces for the top- and bottom junctions respectively. In case of subsequent nc-Si:H layer deposition, the nanocrystals in an IR can serve as a seed layer providing high quality nc-Si:H material growth.
Combining all these requirements, an IR can have a theoretical quadruple functionality, serving as the reflecting, doped, texturing, and seed layer. Only a limited number of materials exhibit most or all of these properties. In addition, the material of the IR should be cheap and abundant. In this section, the most commonly used materials will be discussed.
1.3.2 Zinc oxide as a reflecting layer
Aluminum-doped zinc oxide (ZnO:Al) and boron-doped zinc oxide (ZnO:B) are commonly used as a front TCO. These TCOs have several favorable properties: high electron mobility and therefore high conductivity, wide bandgap (3.3 eV), and low refractive index (n~2 at 600 nm) which results in high reflectance at an interface with silicon layers (nSi~4). ZnO:Al is commonly deposited by sputtering or a low pressure chemical vapor deposition (LPCVD). Disadvantages of ZnO for use as an IR include its limited doping ability. The material is also deposited by different methods than the other layers in the TF Si cell, complicating the flow chart of cell processing.
1.3 REFLECTING LAYERS IN THIN-FILM SILICON SOLAR CELLS
ZnO was the first material considered as an IR in an a-Si:H/nc-Si:H tandem cell [28]. The thickness of the IR was between 30-80 nm, providing decent reflective properties. Since then, it has been a popular material for use as an IR [33]. At EPFL in Neuchatel, the research was broadened to applying a 1.5 μm thick asymmetric IR made of ZnO:Al in a tandem cell [34]. The asymmetric IR (with a random textured surface) proved to be effective, enhancing the JSC of the top cell, without the need of increasing its absorber layer thickness. The scattering effects at the asymmetric IR/Si interface provide the optimum light in-coupling into the top a-Si:H solar cell thanks to the 300 nm lateral feature size. This allows a thinner a-Si:H junction to be deposited, which boosts the stabilized tandem cells efficiency, due to reduced light induced degradation of the thinner a-Si:H absorber layer. The initial efficiency of this laboratory-scale cell was reported at 11.2% and stable at 9.8% [34].
1.3.3 Silicon oxide as a reflecting layer
The first successful integration of silicon oxide (SiOX) into thin-film silicon solar cells was reported by the Fuji company [35]. Yamamoto et al. was the first to apply SiOX as an IR [36]. The beneficial properties of this material sparked great interest and led to significant research efforts in SiOX-based reflecting layers from many groups around the world [37]–[39]. In the past few years, silicon oxide became widely used in thin-film silicon PV and has been applied in the most state of the art solar cells with the best performance to date [21], [40], [41]. Silicon oxide has also been applied in hetero- junction solar cells based on monocrystalline Si wafers [42].
Similar as ZnO used as an IR, silicon oxide improves the top cell current by reflecting back light because of a smaller refractive index compared to silicon. Doped silicon oxide has greater optical transparency than the standard a-Si:H doped layers due to its higher band gap. It serves as a better TRJ in tandem cells because of a high transversal conductivity and lower activation energy compared with doped a-Si:H layers. In single junction cells it also improves electron collection [43]. Silicon oxide has several advantages over ZnO. It has a lower lateral conductivity, preventing potential activation of shunt paths between the two sub-cells when used as an IR. Silicon oxide can be deposited in the same chamber as the rest of the silicon-based layers of the cell and it can be doped to a desired level in-situ. It can serve as a seed layer with its tunable nanocrystalline structure, hence it has been termed as hydrogenated nanocrystalline silicon oxide (nc-SiOX:H). However, surface texturing possibilities of nc-SiOX:H are limited.
The properties of nc-SiOX:H with respect to plasma enhanced chemical vapor deposition (PECVD) conditions have been extensively studied [43]–[45]. In general it can be concluded that:
1. Adding more CO2 into the gas mix is accompanied by more oxygen incorporation leading to a lower refractive index and higher band gap while the conductivity drops.
2. The material can be doped by incorporating doping gases into the gas mix, for instance PH3 or B2H6. N-doping in general is considered more efficient than p-doping since the presence of boron in the plasma is detrimental for silicon crystal formation.
3. A minimal pressure and H2 flow is needed to achieve a crystalline phase in the material.
Developing p-doped nc-SiOX:H is a more complicated task compared to n-doped nc-SiOX:H as the presence of boron in the plasma influences film growth, crystallinity, and properties more profoundly than phosphorus (see section 3.3). After the successful application of nc-SiOX:H as an n-layer, it was used as a p-layer, as well showing reduced parasitic absorption in reference to a conventional p-doped silicon carbide layer [46], [47]. The material showed improved refractive index matching between the TCO and absorber layer, thus improved antireflective properties [48], [49]. The integration of a nc-SiOX:H p-layer has been shown to increase the open-circuit voltage (VOC) [50]. From the processing point of view, the application of p-doped nc-SiOX:H simplifies the p-layer in comparison to the use of silicon carbide; only one layer is necessary as there is no need for a buffer layer with a higher band gap that prevents electrons in the i-layer to diffuse into the p-layer [51]. 1.3.4 Advanced concepts of reflecting layers
To improve the absorption enhancement in tandem solar cells even further, many advanced concepts are being explored. In case of single junction cells, the best back reflectance was achieved with a combination of nc-SiOX:H, ZnO:Al, and silver back reflector [52]. This configuration of two (or more) optically active materials stacked on top of each other can be considered as a photonic crystal or Bragg reflector. Photonic crystals in general are extremely appealing for incorporation into solar cells as reflective layers [53] and have been studied in the past [32], [54]–[56]. Bragg reflectors are categorized as 1D photonic crystals. High reflectance can be achieved with two materials with different
1.3 REFLECTING LAYERS IN THIN-FILM SILICON SOLAR CELLS
refractive indexes forming a stack (Fig. 1.6). The bigger the refractive index contrast, the greater is the reflection. The characteristic parameter for the Bragg-stack of a homogeneous film is the optical film thickness hopt = n1 · d1 = n2 · d2 of both layers. By tuning the thickness of each layer and the number of periods that are repeated, reflectance up to 100% in a narrow wavelength range (also called the photonic band gap) can be achieved. To broaden this range, modulated photonic crystals can be applied [57]. A Bragg stack with a photonic band gap that spectrally matches the absorption edge of a-Si:H can enhance the overall absorption in the top cell up to a factor of 1.22 when compared to a solar cell without an IR [56]. Most of the previous work on Bragg stacks serving as IRs was simulation work. Therefore the practical application is of great interest and will be described in detail in section 5.1.
The two junctions constituting the a-Si:H/nc-Si:H tandem have different band gaps and therefore absorb light of different wavelengths. To increase absorption, light is scattered at textured interfaces when entering the solar cell. The texture is adopted by all layers deposited on top of it. As this texture may induce scattering beneficial for the top cell absorption, it may seem flat for light reaching the bottom cell. Due to limited scattering in the bottom cell the JSC does not increase. Therefore, after the top cell deposition, it would be beneficial for the interface texture to change. Obermeyer et al. simulated a double structure diffractive interlayer for the p-i-n configuration [58]. They concluded that for good scattering in a 200 nm thick top cell the period size of surface features should be 300-400 nm and 1200-1600 nm for the bottom cell. With these ideal feature periods, a current gain of up to 20% is predicted. It is beneficial to increase the texture period size of the bottom cell not just for gaining more JSC, but as well for better nc-Si:H material quality resulting in higherVOC and fill factor (FF ). Defective regions are known to form above the sharp valleys of the small texture features [59], [60].
Figure 1.6: A 1D photonic crystal sandwiched between the top a-Si:H and bottom nc-Si:H cell of a tandem.
Increasing the feature size or flattening the surface, as was shown by Boccard [61] can prevent these defective regions from forming, thus improving VOC and FF.
In case of an n-i-p configuration, the native texture of ZnO was used to scatter light in the top cell, as shown by Soderstrom et al. [34]. At first, the deposition substrate had large features beneficial for bottom cell scattering. After the deposition of the bottom cell, when growing the ZnO IR, these initial features were canceled out and taken over by the native texture of the ZnO from an LPCVD process. This native texture has roughly 300 nm periods, beneficial for top cell scattering. This work proved how beneficial it is to re-scatter light between the two junctions. As this was done in an n-i-p configuration, in section 5.2 an asymmetric IR will be shown in a p-i-n cell.
1.4
The objective of this thesis
Different materials can be used as reflective layers. In this thesis, the main focus is on doped nc-SiOX:H which has a combination of good optical properties (low refractive index, high bandgap) and electrical properties (high conductivity, low activation energy). TF Si solar cells are mostly made by PECVD from silane and hydrogen gas. Nc-SiOX:H layers can be deposited in the same chambers as the rest of the solar cell, making it a versatile material to work with. Nc-SiOX:H layers can be characterized for various optical, electrical, and nanostructural properties. Details of the nc-SiOX:H layer and solar cell processing and measurements will be explained in Chapter 2.
It has been established that nc-SiOX:H works well as doped reflective layers in TF Si solar cells. However, deeper insight between the optimized optical and electrical properties of nc-SiOX:H and the nanostructure is unknown. In Chapter 3, the nanostructural nature of nc-SiOX:H is studied in detail to reveal the relation between the optical and electrical properties and its nanostructure. It is examined in detail whether both the amorphous and crystalline phases play an important role in the material. The following questions about the amorphous phase will be answered: is the amorphous tissue homogeneous or heterogeneous? What is the composition and is the quality of the amorphous tissue related to the optimized optical and electrical properties? The following questions about the crystalline phase will be answered: what (minimum) fraction of crystalline material is optimal? What is the optimum size of the crystal grains? Finally, how universal are the processing and nanostructure in relation to optical and electrical properties? All the above mentioned topics are covered in Chapter 3.
1.4 THE OBJECTIVE OF THIS THESIS
The developed nc-SiOX:H layers have device grade optical and electrical properties, but whether they can improve device efficiency in reference to solar cells without nc-SiOX:H will be answered in Chapter 4. The performance of single junction a-Si:H cells with the integrated device grade materials will be shown. N-doped nc-SiOX:H layers (Fig. 1.5, yellow layer) are applied as the n-layer and back reflector with Ag. P-doped nc-SiOX:H (Fig. 1.5, red layer) are applied as the p-layer, serving as the window layer. The n-doped nc-SiOX:H improves the spectral response over the entire spectrum. What is causing this unexpected effect will be answered in Chapter 4.
The n- and p-doped nc-SiOX:H meet in the middle of a tandem cell. They are primarily doped layers but by adjusting their thickness and refractive index they serve as a functional IR. Can higher tandem cell efficiency be achieved by increasing the number of IR layers, making Bragg reflectors? In Chapter 5 this question will be answered as well as how can Bragg reflectors be efficiently designed and what rules apply to their design? How critical is the thickness and refractive index of each layer for the desired optical performance? Can simulation software design an effective Bragg reflector?
The second part of Chapter 5 will be about interface texturing which brings up the following questions: Which materials can easily be textured on a cell without damaging it? Is it possible to separate the textures between the two junctions of a tandem cell to achieve optimal Asahi-like
Figure 1.7: Schematic overview of topics covered in this thesis and their depicted positions in a tandem cell.
Chapter 3 Chapter 4 Chapter 5
Intermediate reflector development and tandem incorportion P- and n-doped silicon
oxide development and nanostructure
study
Single junction cell results with p- and n-doped silicon oxide
scattering in the top cell and optimal scattering and defect-free growth for the bottom cell? Can this be achieved with wet-etching steps? Can this be achieved with a combination of polishing/wet etching? All these issues will be covered in Chapter 5.
To test the newly-developed silicon oxide material and the IR concepts, complete thin-film silicon solar cells need to be deposited and characterized. This chapter will describe all the processing methods and equipment necessary to make the silicon oxide layers and TF Si solar cells as discussed in this thesis. The applied contact configuration and measurement of a solar cell will be described as well.
2.1
Solar cell processing
2.1.1 Plasma Enhanced Chemical Vapor Deposition (PECVD)
Thin-film layers can be grown by Chemical Vapor Deposition (CVD). Precursor gasses react and decompose on a substrate (usually glass) surface producing the deposited layer. Normally, gas temperatures above 600°C are needed to break the chemical bonds of the precursor gasses. Using a plasma, the temperature to dissociate precursor gases can be much lower as the energetic free electrons and ions provide the necessary energy to break the chemical bonds.
To ignite a plasma a rapidly alternating electric field is applied between two electrodes. Atoms and/or molecules are ionized in the
plasma. Highly energetic electrons collide with gas molecules initiating the chemical reaction. This process is called Plasma Enhanced Chemical Vapor Deposition (PECVD). When the plasma is generated between two electrodes with an alternating bias, PECVD is referred to as a capacitively coupled plasma. Ionization events produce new electrons and ions to maintain the plasma. For example, for a-Si:H layer growth, the electrons in the plasma have energies typically ranging between 0 and 30 eV. The power provided by the induced alternating electric field may not always be sufficient to free electrons to start up the plasma. Often a spark needs to be used to provide highly energetic electrons to ignite the process chain reaction.
The SiH4 molecule can dissociate into SixHy radicals and ions. The dissociated ions and radicals can be deposited on the grounded substrate electrode. Figure 2.1 gives a scheme of a typical PECVD setup. Inside the chamber, the substrate is placed between two parallel capacitively coupled electrodes. The generator is set to radio frequency (RF: 13.56 MHz) for the growth of a-Si:H, silicon oxide and silicon carbide layers, while for the nc-Si:H (intrinsic) layers a very high frequency (VHF: 40.68 MHz) is used. A matching box with two capacitors is used to regulate the reflected power. The heater sets the substrate temperature to around 200°C. A constant low pressure of a few mbars is maintained by a pump system. The in-flow of precursor gasses are controlled by the gas system consisting of mass flow controllers. SiH4 is the main gas used to deposit all layers in combination with either CO2, CH4, H2, PH3, and B2H6. The main advantages of RF-PECVD compared to other deposition techniques are:
• Deposition at low substrate temperature (typically around 200°C) • Variety of available substrates: from glass to flexible foils
• Large deposition area possible • Effective doping of layers
• Cost-effective on industrial scale 2.1.1.1 Amigo cluster tool
The samples for this thesis were prepared in a six-chamber cluster tool from Elettrorava called the AMIGO. Each chamber is designated for the deposition of different types of silicon alloys. There is one chamber for p-doped silicon alloys, one for n-doped silicon alloys, one for intrinsic a-Si:H, one for intrinsic nc-Si:H, one chamber for special silicon alloys
2.1 SOLAR CELL PROCESSING
like silicon carbides and nitrides, and a sputtering chamber for ZnO:Al. The deposition gases in each chamber are injected through a showerhead electrode. All of the chambers operate at RF biasing frequency except the chamber dedicated to intrinsic nc-Si:H which operates at VHF: 40,68 MHz. To preserve the necessary vacuum conditions, all six chambers are connected through a transport chamber. This transport chamber is equipped with a robot arm which takes samples from a load lock. The load lock chamber has a capacity to handle 5 substrates.
2.1.2 Magnetron Sputtering
Sputtering is a Physical Vapor Deposition (PVD) technique. A target is bombarded by ions which sputter species (atoms, molecules, nanoparticles) of material. These species deposit on a substrate. This technique is used to deposit aluminum doped zinc oxide (ZnO:Al) that serves either as the front Transparent Conductive Oxide (TCO) contact in a solar cell or as the IR. A RF generator applies an alternating electric field between the material target and substrate. As a result, a fraction of an inert gas, in this case argon, is ionized. These positive argon ions are accelerated towards the target and sputter species from the target. These species are deposited onto the substrate. To avoid any drastic increase of temperature of the target due to the ion bombardment, the target is cooled with water.
Figure 2.2: Schematic diagram of a sputtering process.
In order to increase the ion-current density, a magnetic field is applied perpendicular to the electric field and the electrons are confined near the surface of the electrode. The combination of sputtering and focusing of electrons using a magnetic field is thus referred to as magnetron sputtering. A scheme of the general sputtering technique is shown in Figure 2.2. The sixth chamber of the AMIGO cluster tool is equipped with this deposition technique.
2.1.3 Thermal and electron beam evaporation
Thermal and electron beam evaporation are physical vapor deposition (PVD) methods used to deposit thin layers of material, mainly metals used as BRs and contacts in both single layers and complete devices. In these techniques a (metal) target is heated up until it begins to evaporate. Evaporated metal then reaches a cold substrate where it deposits. This process is performed under a vacuum environment to minimize the contamination with other substances. In this work, thin films of silver (Ag) that act as back reflectors are deposited using this method. On top of the Ag, chromium (Cr) and aluminum (Al) are deposited. Ag is covered to prevent it from oxidizing in air. Cr is separating Ag and Al because in the annealing step these two metals can diffuse into each other.
Another approach is the thermal evaporation technique used to deposit Ag. In this PVD approach, high electrical current is passed through a metallic boat that contains the metal, warming it up to its evaporation
2.1 SOLAR CELL PROCESSING
Figure 2.3: Schematic drawing of the thermal evaporation technique (a) and the electron beam evaporation technique (b).
temperature (Fig. 2.3a). The Cr and Al are evaporated using the electron beam evaporation technique which heats the evaporant located in a crucible by using a magnetically focused beam of high energy electrons (Fig. 2.3b). The advantage of this method is that the temperature is not limited by the melting point of the filament, allowing the evaporation materials with high evaporation temperatures. The evaporants are present in a crucible which is cooled by water during evaporation. In this work all metal evaporation was done using the Provac PRO500S. It is a single chamber high-vacuum system equipped with a 4-pocket 10 kV electron gun and a boat for thermal evaporation. Pumping cycles and deposition recipes are full programmable. Desired contact sizes are created by masking.
2.1.4 Reactive Ion Etching (RIE)
Reactive ion etching (RIE) is used to selectively remove thin-film layers. In this thesis it was used mainly for removing (etching) deposited nc-SiOX:H from around the metal contact to achieve a better defined size of the contact area and reduce additional current collection.
RIE is a technique very similar to PECVD, except gases with large molecular weights are used. These gases etch atoms away from the layer surface instead of depositing on it. These gases are ionized between two parallel plates by RF bias. Because of the greater impulse moment of the ions, they are not deposited, but sputter atoms from the thin film. The setup used is a single chamber Alcatel system with load lock. Etching gases CF4 and CHF3 in a He carrier gas were used at a pressure of 0,5 mbar. A plasma was ignited with these gases the same way as in PECVD (Fig. 2.1).
2.2
Solar cell characterization
Solar cells were processed by PECVD deposition of all silicon alloys on a TCO substrate, metal contact deposition via evaporation, and eventual RIE for better contact area definition. The solar cells are then measured to determine their external parameters and spectral response.
2.2.1 External Parameters
The external parameters are a set of measured characteristics used to describe the electrical behaviour of a solar cell. The main external parameters are: The short circuit current density (JSC), the open circuit voltage (VOC), the fill factor (FF ) and the efficiency (η). These parameters are measured at 25°C at an irradiation intensity of 1000 W/ m2 of the AM 1.5 spectrum. In addition, the series resistance (R
S) and the parallel (shunt) resistance (RP) can be determined using a J-V curve.
A typical J-V curve that relates these variables is shown in Figure 2.4. The intersections with the J=0 and V=0 axis of the curve are marked by the VOC and JSC. There is a certain current (Jm) and voltage (Vm) where the output power is maximum. The shape of the J-V curve greatly affects the resulting maximum power output. The FF is used to characterize the“rectangleness”of the J-V curve defined by the ratio between the maximum power and the product of JSC and VOC. The equation is as follows:
(2.1)
The efficiency of a solar cell can be defined in terms of the external parameters as:
(2.2)
where Pin is the power density of the incident solar radiation.
All the external parameters are obtained using the PASAN Flash solar simulator, which provides the standard AM 1.5 solar spectrum and has an automatic probing stage. The J-V characteristics of solar cells exposed to light pulses of 4 ms are measured. Solar cells reported in this work were deposited on 2,5 x 5 cm substrates using two different contact configurations. The first configuration accommodates about fourteen
2.2 SOLAR CELL CHARACTERIZATION
individual cells of 4x4 mm. The second configuration has four cells of 1x1 cm. The measured FF and VOC using the PASAN are considered trustworthy. In contrast, the JSC is not reliable due to the imprecise determination of the cell area and discrepancies of the light source in reference to the AM 1.5 spectrum. The JSC was taken from the EQE measurement as this technique is independent on the light source or contact area.
2.2.2 External Quantum Efficiency (EQE)
The spectral response or the external quantum efficiency (EQE) of a solar cell is defined as the percentage of photons incident on the device’s front surface that produce charge carriers which are collected at its terminals. In the PVMD group, the setup used is formed by a light source, a monochromator and a current amplifier (Fig. 2.5). The
Figure 2.4: J-V curve.
solar cell is exposed to monochromatic light and the photo-generated current is measured. The current is integrated over the wavelength and multiplied by the AM 1.5 spectrum, giving the JSC. The JSC is measured under short-circuit conditions (no bias voltage). In addition, in some experiments, EQE was measured using a reverse bias of -1V to ensure that most of the light excited charge carries are collected. In this manner, comparing EQE at no bias and reverse bias gives an indication of the charge carrier recombination in a solar cell and shows its potential of optical performance.
When measuring the EQE of a tandem cell (Fig. 2.6), both series connected junctions need to be producing light excited electron-hole pairs in order to collect current. In the standard EQE configuration it is not possible to extract the spectral response of each junction. For this purpose, bias light is incident onto the cell. To measure the top cell, infrared bias light is used as it passes through the top cell and is solely absorbed in the bottom cell. To measure the bottom cell, blue light is used, which is solely absorbed in the top cell. This way the biased cell is conductive and the photocurrent of the non-biased cell can be measured. Since the bias light intensity is constant, only the charge carriers in the non-biased junction, generated by absorption of light transmitted through the monochromator, are detected.
Figure 2.6: An example of a spectral response measurement of a tandem cell measured with the EQE setup.
2.3 SINGLE LAYER CHARACTERIZATION
Figure 2.7: The Raman spectrum between 150-750 cm-1 with the black and red Gaussians showing
the amorphous and crystalline phonon modes respectively. The ratio of the area of these Gaussians constitutes the I521/I480 ratio as a measure of crystallinity.
2.3
Single layer characterization
2.3.1 Raman spectroscopy
Raman spectroscopy is a fundamental method, by which the structure and composition of materials is investigated. The crystalline volume fraction of the nc-Si:H based thin-films can be determined by Raman spectroscopy. Upon interaction with valence electrons, most photons scatter elastically. This spectroscopy technique is based on the small fraction that scatters inelastically. The inelastically scattered photons have a frequency that is slightly shifted compared to the initial frequency before scattering. This shift in energy is called the Raman Effect (shift). As probing light a monochromatic light source is used (usually a laser). The recorded Raman shift spectrum reflects the phonon density of states. Within the detected spectral range the transverse-optic modes of crystalline and amorphous silicon are visible at 521 and 480 cm-1 respectively (Fig. 2.7).
The Raman spectra of the samples were measured with a Renishaw InVia, grating 1800 lines/mm Raman microscope with a 180° back scattering geometry. It can measure Raman shifts from 250 to 3000 cm-1. The phonon modes for determining the crystallinity of silicon samples can be found in a scan between 150 to 750 cm-1 (Fig. 2.7). The microscope is equipped with a 25 mW Argon laser operating at 514
nm and a Helium-Neon laser with a wavelength of 633 nm operating at the same power and spot size (Fig. 2.8). The intensity of the lasers can be regulated from 0,001 to 100%, where 5% was most commonly used. This beam is focused on the sample via mirrors and lenses. The incident laser beam is scattered by the sample and is then redirected back into a spectrophotometer. The elastic scattered light is filtered out by a notch filter. The remaining inelastically scattered light is diffracted into a spectrum and collected by a spectrometer (Fig. 2.8). There it is converted into electric signal which is later processed by a computer to determine the Raman shift spectrum. The resulting spectrum is fitted with Gaussians from which the crystalline content in nc-SiOX:H is estimated via the I521/I480 ratio.
2.3.2 Fourier transform infrared spectroscopy
Fourier Transform Infrared Spectroscopy (FTIR) is the most widely used infrared spectroscopic method. Various oxygen-related bonding configurations in nc-SiOX:H can be distinguished with this method. Vibration modes of non-symmetric bonding configurations are detected in the infra-red absorption spectra. The absorption intensity related to a certain mode is a measure for the number of associated bonds present in the material. The measurement method uses a Michelson interferometer and a beam splitter. The source beam is split into two beams by the beam splitter (Fig. 2.9). One split beam is reflected off a stationary mirror and the other from a moving mirror. The two beams interfere and pass through the sample into the detector. The absorption spectrum is obtained by Fourier-transforming the complex interference of these two superimposed beams. Multiple measurements are taken to reduce the
2.3 SINGLE LAYER CHARACTERIZATION
Figure 2.9: Configuration of a FTIR setup and the components involved.
noise in the measurement [62].
In this technique, light having a black body radiative spectrum from an incandescent lamp is directed onto a film deposited on a c-Si wafer (prime wafer, 500 μm). The c-Si wafer is transparent for wavenumber range of interest (500–2200 cm-1). Therefore, glass is not used as a substrate since it has a high absorption coefficient in this range. Part of the radiation is transmitted and some is absorbed by the film. The transmittance spectrum of the bare substrate and the substrate with a deposited film is measured. The resulting transmittance spectrum of only the film is obtained by dividing the spectrum of the substrate with film by the spectrum of the bare substrate. The transmittance is measured as a function of the position of the moving mirror (see Fig. 2.9) in the interferometer. This measured interferogram is converted into a spectrum as a function of frequency using a Fourier Transform. The resulting spectrum shows the absorption peaks of the deposited film versus the wavenumber (Fig. 2.10).
The absorption peaks are linked to certain vibration modes (bending, wagging, and stretching). FTIR can provide information on the chemical bonding and indirectly provide information on the quality of a film and its components. It can help in developing and optimizing a material with certain desired properties [62]. An overview of the stretching modes
examined in this thesis is shown in Table 2.1.
The setup used to perform FTIR measurements is a Thermo Electron Corp model Nicolet 5700. The infrared light source is an Ever-Glo lamp (9600 – 20 cm-1). A KBr beam splitter (7400 – 350 cm-1) and DTGS detector (6400 – 200 cm-1) are used. The sample compartment contains two holders mounted on a shuttle which automatically switches between the reference wafer and the wafer with the deposited film during the measurement.
Figure 2.10: Sample FTIR absorption spectrum. Table 2.1: Assignments of stretching modes.
Wavenumber [cm-1] Assignment 1050 (Si-O-Si)SiX stretching (single oxygen) 1106 Interstitial oxygen (O2i) 1135 (Si-O-Si)OX stretching (oxygen rich)
2100 Si-H stretching on void surfaces
2140 Si-HX stretching /
OSi-H stretching
2180 O2Si-H stretching
3.1 Introduction
In current state-of-the-art multijunction TF Si solar cells, silicon oxide layers form a vital part of the devices. These bi-functional layers are used as doped layers and (intermediate) reflector layers for light management [21], [22], [28], [41], [45], [52], [63]. The low mobility of holes in the intrinsic layer requires the holes to be collected at the front side of the cell where most carriers are generated due to light absorption. For that reason, the p-layer is at the front side of the cell. To couple as much light as possible into the absorber layer, the p-layer needs to have minimal reflection and parasitic absorption. This can be achieved with silicon oxide which is a low refractive index (n) material. Its refractive index has values between that of a-Si:H and the TCO to achieve a welcome refractive index grading (na-Si:H > nSiOx > nTCO). On the contrary, the reflection of the n-layer serving as the BR needs to be high. This can be as well achieved via a low n silicon oxide layer. Due to a larger refractive index mismatch between nc-SiOX:H and the silicon layers, nc-SiOX:H layers reflect light in the desired spectral range of 500-700 nm back into the top cell. Directing more light into the intrinsic a-Si:H absorber layer is an important optimization tool to achieve current
matching with the other cell(s) in the multijunction. In addition, it allows the i-layer to be kept reasonably thin in order to minimize the impact of light-induced degradation [24]. Apart from these strict optical criteria, the silicon oxide layers need to be sufficiently conductive to transport charges to the TCO and metal contacts.
Silicon oxide is a material which is cheap, abundant, and easy to process. It helps to reduce parasitic absorption losses in doped layers because of its higher bandgap in reference to silicon layers. As mentioned in section 1.3.3, silicon oxide can be multifunctional as it serves as a reflective and doped layer. In addition the p-doped silicon oxide can serve as a seed layer for the growth of an intrinsic nanocrystalline absorber layer in a p-i-n configuration. To what extent is the presence of crystalline grains in silicon oxide necessary and what grain properties are required to achieve good optical and electrical properties? Similarly, what is the importance of the amorphous tissue and its role in the functioning of the material to have good optical and electrical properties?
As vitally important as nc-SiOX:H is, not much is known about its nanostructure. In this thesis, four possible cases of heterogeneous nanostructure are considered that could potentially represent the real nanostructure, as depicted in Figure 3.1. This heterogeneous material is currently interpreted to be a matrix with a crystalline phase made of crystalline silicon (c-Si) grains and an amorphous phase of a-Si:H and amorphous silicon oxide (a-SiOX:H) tissue (Fig. 3.1(d)). Can doped amorphous silicon oxide work just as well (Fig. 3.1(a))? Is the presence of crystalline Si grains (Fig. 3.1(b)) or amorphous silicon (Fig. 3.1(c)) necessary? There are reports of the nc-SiOX:H material possessing purely silicon filaments aligned in the layer growth direction being responsible for good transversal conductivity [47], [64]. These filaments are surrounded by isolating amorphous silicon oxide tissue having very low refractive index values. Films with such nanostructure show anisotropy in the conductive properties, where the lateral conductivity is low and the transversal conductivity high. When integrated as part of an IR in a tandem cell, the lower lateral conductivity has the potential to quench possible interconnections between shunts in the top and bottom cell. Therefore, the doped nc-SiOX:H has an advantage over zinc oxide (ZnO) which facilitates this shunting of tandem cells due to its high lateral conductivity.
In this chapter, the dependence of conductivity on the nanostructure is thoroughly studied. Silicon filament-like structures have been observed in the p-type material studied here as well. Using TEM and Raman spectroscopy, a small amount of the crystalline silicon phase was detected. The question arises about what is the role of the crystalline
3.1 INTRODUCTION
grains in the silicon filaments. Samples under varying conditions were deposited observing that not all nc-SiOX:H material has the same crystalline properties. High values of lateral conductivity were achieved in samples with varying crystallinity, showing isotropic behavior. In addition, the conductivity of nc-SiOX:H still differs between samples with similar crystallinity and crystalline grain size. Therefore, the (lateral) conductivity of nc-SiOX:H is of great interest as a parameter to study the nanostructure of nc-SiOX:H. The conductivity depends on the presence of the crystalline phase as well as the properties of a-Si:H tissue in the filaments and the properties of a-SiOX:H tissue.
This chapter presents a thorough study of the nanostructure of nc-SiOX:H using different measurement methods. First, the relation between measured optical and electrical single layer parameters and the deposition parameters will be studied (section 3.3). To improve the insights into the nanostructure of the nc-SiOX:H, the properties of the crystalline silicon grains were studied in detail by TEM imaging (section 3.4.1) and Raman Spectroscopy (section 3.4.2). The elemental composition and bonding configurations of the a-SiOX:H tissue and the quality of the a-Si:H tissue were studied using FTIR (section 3.4.3) and XPS (section 3.4.4) measurements. Layers deposited at various deposition parameters were integrated into solar cells. The performance of these cells is analyzed in the next chapter.
Figure 3.1: Planar depiction of four cases of silicon oxide nanostructure: (a): pure amorphous silicon oxide, (b) crystalline silicon directly embedded in amorphous silicon oxide, (c) regions of amorphous silicon embedded in amorphous silicon oxide, (d) crystalline silicon surrounded by amorphous silicon embedded in amorphous silicon oxide.
3.2
Experimental details
The nc-SiOX:H based films were deposited on Corning Eagle XG glass using the AMIGO RF-PECVD cluster tool (section 2.1.1). Samples were deposited in series in which the RF power, pressure, temperature, and gas flows of B2H6 (for p-doped material), PH3 (for n-doped material) H2, and CO2 were varied (Tables 3.1, 3.2). For each series, only one parameter is varied while the others are kept constant at values used to process the device grade material. The deposition time was kept constant. Consequently the film thickness varies from sample to sample but is mostly around 250 and 120 nm for n- and p-doped nc-SiOX:H, respectively. The best solar cell performance was obtained with device grade nc-SiOX:H materials processed under the conditions listed in Tables 3.1 and 3.2. These conditions include a temperature of 180°C, electrode gap of 14 mm, and bias frequency set at 13.56 MHz as well. For electrical characterization purposes aluminum contacts were evaporated (section 2.1.3) onto the films. Next, they were annealed for 30 minutes at 130°C to obtain better electrical contact between the film and metal interface.
Table 3.1: Variation of deposition parameters of p-doped nc-SiOX:H. The device grade material
deposition conditions are shown in red.
Table 3.2: Variation of deposition parameters of n-doped nc-SiOX:H. The device grade material
deposition conditions are shown in red.
Parameter Variation & best material
Power 0.08 – 0.29 - 0.5 W/cm2 Pressure 1.4 – 2.2 - 3 mbar Temperature 120 – 180 - 240°C CO2:SiH4 1.17 – 1.75 - 3.5 SiH4:B2H6 250 – 400 – 700 H2:SiH4 0 - 213 – 250
Parameter Variation & best material
Power 0.06 – 0.07 - 0.14 W/cm2
Pressure 0.5 – 1.25 - 2.75 mbar CO2:SiH4 1 – 1.6 - 2.2 SiH4:PH3 25 – 42 - 125
3.3 MATERIAL DEVELOPMENT
The single nc-SiOX:H layers were characterized by reflection-transmission (RT; page 34), conductivity measurements (σ; page 35) in a lateral contact configuration, activation energy (Ea; page 35), Raman spectroscopy (section 2.3.1), and X-ray photoelectron spectroscopy (XPS; page 60). Simultaneously with the films on glass, films were deposited on c-Si wafers for FTIR measurements (section 2.3.2). Both planar view and cross sectional images of the device grade materials were made using TEM imaging (page 43).
3.3
Material development
3.3.1 Single layer properties
First, the observed dependence of the optical and electrical properties of nc-SiOX:H on the processing parameters is shortly summarized. The requirements for an ideal reflector layer based on nc-SiOX:H are low values of activation energy (Ea) and refractive index (n) combined with a high value of band gap and transverse conductivity (σt). Note again, that the measured conductivity is in the lateral direction (page 35). In Figure 3.2 and 3.3, these measured parameters are plotted against the main deposition parameters of n-doped and p-doped nc-SiOX:H, respectively. For each deposition an optimum level of conductivity was found. The activation energy showed lowest values for conditions with lower power, pressure, and PH3:SiH4 ratio settings. For the CO2:SiH4 ratio series the lowest Ea was found between ratios of 1-1.6. In single junction cells, the Ea is not a critical parameter affecting cell performance, but Ea becomes important in tandem cells when nc-SiOX:H is applied as an IR to form tunnel recombination junction for minimum loss in VOC.
The optical parameters as bandgap and refractive index are developing as expected with each deposition parameter except the PH3:SiH4 ratio. In that case it has to be noted that the explored parameter range was small (Fig. 3.2). Table 3.3 shows a summary of the dependence of the material properties of nc-SiOX:H as a function of the chosen deposition parameters. Figures 3.2 and 3.3 show that the optical and electrical parameters desirable for an IR are in competition. The device grade material was therefore found as a compromise between the optical and electrical requirements (Table 3.2). This tradeoff between optical and electrical properties was also observed by others [42], [44], [64], [66].
Figure 3.2: The measured optical and electrical properties vs. the deposition parameters of n-doped nc-SiOX:H.
3.3 MATERIAL DEVELOPMENT
Figure 3.3: The measured optical and electrical properties vs. the deposition parameters of p-doped nc-SiOX:H.
Reflection-Transmission Spectroscopy (RT)
Reflection-transmission spectroscopy (RT) is a technique used to obtain the reflectance, transmittance, and absorption of thin films as a function of wavelength. From the measured R(λ) and T(λ), the refractive index (n), and extinction coefficient (k) can be obtained. In addition, these values are fitted to a predefined optical model of a glass substrate with a thin a-Si:H film on top to determine the film thickness and Tauc band gap (Eg) of a-Si:H.
The spectral measurement ranges from 1.17 – 2.95 eV (375 – 1060 nm). The measurement technique consists of the following steps: A 50W halogen lamp shines light on a film deposited on Corning 7059 glass. The reflected and transmitted spectra are detected with separate silicon photodiodes. Absorption is determined form the relationship A(λ) = 1–T(λ)–R(λ). Finally, Scout software via a predefined model performs the calculations of the thickness and optical constants of the film on glass by fitting the measured spectra [91].
Table 3.3: Summary of the dependence of trends in material properties of n-doped nc-SiOX:H;
↑=increasing, ↓=decreasing, s=strong, w=weak.
Table 3.4: Summary of the dependence of trends in material properties of p-doped nc-SiOX:H;
↑=increasing, ↓=decreasing, s=strong, w=weak.
Parameter σl n600 Ea Eg Dep. Rate
Power ↑ ↓s ↓ ↑ ↑ ↑ Pressure ↑ none ↑s none ↓w ↓w CO2 flow ↑ ↓s ↓w none ↑ ↑ PH3 flow ↑ ↓ none ↑ none none
Parameter σl n600 Ea Eg Dep. Rate
Power ↑ ↓s ↓w ↑s none ↑s Pressure ↑ ↑ ↑ ↓ ↓ ↑s CO2 flow ↑ ↓s ↓s ↑s ↑w ↓ B2H6 flow↑ none ↑ ↓ ↓ ↑ SiH4 flow ↑ ↓w ↑s ↑w ↓s ↑s H2 flow ↑ ↑ none ↓ ↑ ↓s Temperature ↑ ↑ ↑ ↑s ↓ ↓s
3.3 MATERIAL DEVELOPMENT
Figure 3.a: A band diagram depicting the p-i-n device with the Ea and band gap shown in the
doped layers. For intrinsic layers the Ea is half the value of the band gap.
Dark Lateral Conductivity and Activation Energy
Conductivity (σ) is the ability of a material to pass electric charge. The activation energy (Ea) is a measure of the energy difference between the Fermi level and the conduction band edge for electron transport (valence band for hole transport) [23] (Fig 3.a).
In calculating Ea, the sample has coplanar aluminum contacts with well defined dimensions and gap evaporated on its surface. Electrical conductivity is obtained by:
(3.1)
where R is the resistance of the layer, d the distance between the electrodes, L the length of the electrodes and t the film thickness (Fig. 3.b). R is determined from Ohm’s law following R = V/ I. Therefore a voltage (1–100 V, depending on whether the sample is doped or intrinsic) is applied onto the electrodes for each measurement. The experimental setup measures the variation of the electrical conductivity of the material at different set temperatures. First the sample is annealed for 30 min. at 130°C. In complete darkness, the
temperature is then ramped down from 130 to 60°C in steps of 5°C, taking a current measurement at each step. The activation energy is then obtained by fitting the current-temperature relationship to:
(3.2)
Table 3.b: A sample with parallel contacts for conductivity measurements. The dimensions for calculating conductivity are indicated.
where σ is the measured electrical conductivity, σO is a constant, kb is the Boltzmann constant and T is the absolute temperature. The equation can be rearranged to extract the Ea:
(3.3)
A linear relation between the temperature and the natural logarithm of the electrical conductivity, where Ea is the slope, is evident.
This device grade n-doped nc-SiOX:H material performs the best in a solar cell when applied as an IR or in combination with silver as a back reflector (BR). It is worth to note that n for this material (measured at 600 nm) is among the higher values obtained (n=2.6-3). This comes from the trade-off between optical and electrical properties. The welcome drop in Ea and rise in conductivity is accompanied by a fall in the bandgap (Fig. 3.4). The same trends were noticed for the p-doped nc-SiOX:H as well (Fig. 3.5). The conductivity of the n-doped material
3.3 MATERIAL DEVELOPMENT
Figure 3.4: Measured material properties of n-doped silicon oxide: decreasing of activation energy (Ea) and bandgap (Eg) and increasing of conductivity with increasing refractive index (n). The welcome lower refractive index (higher bandgap) is accompanied with lower conductivity.
Figure 3.5: Measured material properties of p-doped silicon oxide: decreasing of activation energy (Ea) and bandgap (Eg) and increasing of lateral conductivity with increasing refractive index (n) at 600 nm; same trends as in case of the n-doped silicon oxide.
