silicon heterojunction solar cells
silicon heterojunction solar cells
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 maandag 23 februari 2015 om 10:00 uur
door
Dong ZHANG
Master of Science, Ulm University in Germany geboren te Rongcheng, China
Copromotor:
Dr. R. A. C. M. M. van Swaaij
Samenstelling promotiecommissie:
Rector Magnificus voorzitter
Prof. dr. M. Zeman Technische Universiteit Delft
Dr. R. A. C. M. M. van Swaaij Technische Universiteit Delft
Dr. A. W. Weeber Energieonderzoek Centrum Nederland
Prof. ir. L. van der Sluis Technische Universiteit Delft Prof. dr. ir. R. Dekker Technische Universiteit Delft Prof. dr. R. E. I. Schropp Technische Universiteit Eindhoven Dr. S. De Wolf École polytechnique fédérale de Lausanne
This work was carried out with an EOS subsidy through Energy research Centre of the Netherlands.
D. Zhang
Surface passivation and optical design of silicon heterojunction solar cells Ph.D. thesis, Delft University of Technology, with summary in Dutch, 2015
Published and distributed by CPI, Wöhrmann PrintService, Zutphen, the Netherlands.
ISBN: 978-94-6203-802-8
Copyright© 2015 D. Zhang
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means without the prior written permission of the copyright owner.
1 Introduction 1
1.1 Solar energy . . . 1
1.2 Photovoltaics . . . 2
1.3 Motivation for developing SHJ solar cells . . . 4
1.4 Challenges for making high-efficiency SHJ solar cells . . . 9
1.5 Aim and outline of this thesis . . . 14
1.6 Contribution to the research field . . . 16
2 Experimental 19 2.1 Fabrication process . . . 19
2.2 Characterization techniques . . . 26
2.3 Optical simulation . . . 39
3 Influence of ITO deposition on silicon heterojunction solar cells 43 3.1 Introduction . . . 43
3.2 Experimental details . . . 45
3.3 Results and discussion . . . 47
3.4 Conclusion . . . 53
4 Influence of intrinsic a-Si:H on silicon heterojunction solar cells 55 4.1 Introduction . . . 55
4.2 Experimental details . . . 56
4.3 Results and discussion . . . 56
4.4 Conclusion . . . 63
5 Optical analysis and double-layer antireflection design 67 5.1 Introduction . . . 67
5.2 Experimental details . . . 69
5.3 Results and discussion . . . 70
5.4 Conclusion . . . 84
6 Optical enhancement of SHJ solar cells with hydrogenated
amorphous silicon carbide emitter 85
6.1 Introduction . . . 85
6.2 Experimental details . . . 86
6.3 Results and discussion . . . 88
6.4 Conclusion . . . 91
7 Conclusion and outlook 97 7.1 Conclusion . . . 97 7.2 Outlook . . . 99 Samenvatting 117 Summary 121 Appendix 125 List of publications 127 Acknowledgements 129 Curriculum vitae 133
1
Introduction
1.1 Solar energy
Following the economic growth and the constant increase of the worldwide population, more and more energy is demanded to sustain a desirable modern life. In the Energy Outlook 2030 released by BP p.l.c. in 2013, the global population is predicted to reach 8.3 billion in 2030 [1]. The energy consumption will increase faster than the population growth: by 36% from 2010 to 2030 and double by 2050 [2]. Now the primary energy source is still based on fossil fuels, like gas, oil and coal. However, this energy resource is not only depleting but also controversial from the viewpoint of climate change. It was reported that between 2010 and 2060, the combustion of fossil fuels will result in 496 gigatonnes of cumulative CO2 emission. Consequently the global mean temperature is expected to rise by about 1.3◦C [3]. Fig. 1.1 shows the shrinkage of the Arctic ice area and the increase of the sea level as the evidence of global warming. The sea level rise may be followed by other effects like submergence of coastal regions and saltwater intrusion to groundwater [4].
In 1954 the first civilian nuclear power plant was built in Obninsk, Russia, revealing the magnificent power of nuclear energy. However, the catastrophic Chernobyl disaster in 1986 and the Fukushima Daiichi nuclear disaster in 2011 reminded the world of its potential danger. Renewable energy, including solar energy, hydro, wind energy, biofuel, biomass and geothermal energy, is
(a) (b)
Figure 1.1:(a) Dwindling Arctic sea ice and (b) Global average absolute sea level change
from 1880 to 2011 [5]
considered to be the ecologically compatible energy resource. Therefore, more and more attention has been paid to renewable energy over the last decade. In 2013, renewable energy already had about a 25% share of the power generation. This share is expected to exceed 30% by 2030 [1]. Among all renewable energy resources, solar energy has its unshakable position due to its many advantages. Solar energy is the most abundant energy resource on earth: almost all energy on earth is from the sun. The earth actually receives 5 × 1021 kJ/year from the sun, which is 15000 times more than the world annual energy demand [6].The sun has lived for 4.6 × 109 years and is supposed to last for another 5 × 109 years. Therefore, solar energy can be considered to be an inexhaustible source. Rapid development of relevant technologies is certainly critical for wide application of solar energy.
1.2 Photovoltaics
An effective way to use solar energy is conversion to electrical energy, which can be easily transformed into any other form of energy and is also very easy to transmit, control, and monitor. The method to generate electrical power by converting solar radiation to direct electricity is called photovoltaics (PV). The
device realizing photovoltaic energy conversion is the solar cell. The history of solar cells goes back to 1839 when is the photovoltaic effect (i.e. the development of an electric potential across a layer of material upon illumination) was demonstrated by A. E. Becquerel at the age of 19 [7]. By 1883 the first solar cell, which was selenium based, was made by C. Fritts, although its efficiency is only around 1% [8]. Benefiting from silicon electronics technologies and application in aerospace industry, crystalline silicon (c-Si) solar cells have developed step by step since the first silicon solar cell with a "grown-in" junction was made in 1941 [9, 10]. There are many milestones of technologies including improvement of Si material quality [11], diffusion-formed junction [12], shallow junction [13], the back surface field (BSF) [14], anisotropic surface texturing [15], and oxide passivation [16], increasing the performance of c-Si solar cells towards higher and higher efficiency. In 1999 Zhao et al. [17, 18] presented their "passivated emitter rear locally diffused" (PERL) silicon solar cell with an efficiency of 25%, which had been the best-performing single-junction c-Si solar cell for the past 15 years. With the interdigitated back contact technology, recently SunPower also demonstrated c-Si solar cells with the efficiency 25% [18, 19].
In order to reduce the material cost of the c-Si solar cell, thin-film technology has gained considerable attention. Thin film solar cells normally incorporate several hundred nanometers or at most several micrometers thick material, consequently reducing the material consumption by three orders of magnitude. Besides, when such a thin film is deposited on a flexible substrate, a flexible solar cell can be made. The materials used for thin-film PV include amorphous or nanocrystalline silicon and their alloys [20], copper indium gallium selenide (CIGS) [21], CdTe [22], and organic materials [23]. More details about amorphous silicon (a-Si) and a-Si PV will be discussed since it is relevant to this thesis.
Films of a-Si were first prepared by sputtering or thermal evaporation. It is easy to understand that the material at that time had no useful semiconductor characteristics, since it was very defective, preventing doping and photoconductivity. Hydrogenated amorphous silicon prepared by a radio-frequent (RF) discharge from silane (SiH4) was reported for the first time in 1965 [24], although it still had an extremely high resistivity. At that time the reactor consisted of a coil outside a quartz chamber but the mechanism essentially was not different from the parallel electrodes commonly used nowadays. After several years, development of a-Si with good semiconductor properties [25, 26] and especially the capability to dope this material [27] drew attention to this material. In 1974 Lewis et al. [28] reported the importance of
hydrogen in amorphous germanium. Therefore, a-Si deposited from glow discharged SiH4 was well recognized as hydrogenated amorphous silicon (a-Si:H). In 1976 the a-Si:H solar cell was already developed at RCA laboratories. Only 3 years later Sanyo started to market a-Si:H solar cells, although they were just used to power calculators. After decades of development, the efficiency of single-junction a-Si:H solar cells has reached a stabilized efficiency of 10.1% [29]. Using a triple-junction solar-cell structure incorporating both a-Si:H and hydrogenated nanocrystalline silicon (nc-Si:H), a stabilized efficiency as high as 13.4% has been realized [30]. However, most of the thin-film PV technologies still have the common issue of material stability and comparatively low efficiency.
In order to make solar cells with both high efficiency and low cost, many novel concepts have emerged or have been introduced to PV, e.g. plasmonics [31], intermediate band [32], quantum dots [33], and hot carriers [34]. However, most concepts are still being researched and far away from industrial application.
1.3 Motivation for developing SHJ solar cells
In general a solar cell contains an absorber layer in which photons of incident radiation are absorbed, thereby generating electron-hole pairs. In order to separate the electrons and holes from each other, so-called semi-permeable membranes are attached to both sides of the absorber [35]. The important requirement for the semi-permeable membranes is that they selectively allow only one type of charge carrier to pass through. In order to minimize the injection of holes from the absorber into the n-type semiconductor an energy barrier should be introduced in the valence band, ∆EV, at the interface between the n-type semiconductor and the absorber (Fig. 1.2). Ideally, this can be achieved by choosing an n-type semiconductor that has a wider bandgap (Eg) than that of the absorber and the energy difference between the bandgaps is fully located in the valence band of the two materials. Similarly, the injection of electrons from the absorber into the p-type semiconductor can be suppressed by use of a p-type semiconductor with a wider bandgap than that of the absorber, with the band offset fully within the conduction band, ∆EC. The requirement of having the band offset in the conduction band means that the electron affinity, Xe, of the p-type semiconductor is smaller than of the absorber. The additional advantage of applying membrane materials with large band gaps is to allow a larger fraction of photons in the solar spectrum to be transmitted through the membranes to the absorber. Fig. 1.2 shows a schematic band diagram of an
illuminated ideal solar cell structure with an absorber and semi-permeable membranes. When the absorber and doped layers are based on the same material, for example c-Si, we denote the junction as a homojunction. When the absorber and membrane materials have different semiconductor properties, such as different energy bandgaps, we describe the junction as a heterojunction.
Figure 1.2:Schematic band diagram of an idealized heterojunction solar cell structure at
the open-circuit condition.
Since c-Si has a band gap of 1.12 eV and a-Si:H of about 1.7 eV, a-Si:H and c-Si can form a heterojunction. By combining advanced a-Si:H and c-Si technologies, silicon heterojunction (SHJ) solar cells were named artificial constructed junction - heterojunction with intrinsic thin-film (ACJ-HIT) solar cells by Sanyo in 1992 [36], showing a high performance. Fig. 1.3 shows the schematic structure of the commonly-used SHJ solar cells. In practical SHJ solar cells, the band offsets between different materials are located in both the conduction and valence band. This can result in the formation of transport barriers between the absorber and the membrane for the carriers. Fig. 1.4 shows the band diagram of SHJ solar cell based on n-type c-Si, showing the transport barrier and carrier transport modes. A transport barrier is formed for holes at the interface between the two materials. The holes can drift through narrow ’spike’ barriers by tunneling, trap-assisted tunneling and/or thermionic emission [37].
More advantages of the SHJ solar cell can be shown by comparing it to the PERL cell [17], which had been the best-performing single-junction c-Si silicon
c-Si absorber intrinsic a-Si:H p-type a-Si:H emitter
intrinsic a-Si:H n-type a-Si:H BSF front ITO
back ITO metal
EC EV EC EV n-type c-Si Χe p-type a-Si:H EF n-type a-Si:H a b c
Figure 1.4:Schematic band diagram of a practical a-Si:H/c-Si heterojunction. Carrier
transport through the energy barrier at the interface by means of a) tunneling, b) trap-assisted tunneling and c) thermionic emission is highlighted by the purple circle.
solar cell before 2014. The fabrication of the PERL solar cell starts with the texturing of a high-quality wafer to form an "inverted pyramid" structure in order to reduce surface reflection and to increase internal reflection on the rear side [38]. Then silicon oxide is thermally grown on both sides of the wafer to passivate surface defects. Small openings in the silicon oxide are made to provide access for the metallic contacts to connect to the silicon regions that have been heavily doped. In order to reduce the reflection further, the surface of the PERL cell is coated with MgF2/ZnS double antireflective coating. When making a SHJ solar cell first an n-type c-Si wafer is randomly textured to provide effective light trapping. Intrinsic a-Si:H is then deposited on both sides of the wafer for passivation, followed by p-type a-Si:H deposition on one side to form the emitter and n-type a-Si:H deposition on the other side for the BSF. Two layers of transparent conductive oxide (TCO) are required to enhance carrier transport to the contacts because the doped a-Si:H layers are thin and resistive. The cell is finalized by screen printing of metal grids. As we can see, fabrication of the PERL c-Si solar cell involves complicated and demanding processing steps. Optical lithography is needed for several steps, e.g. surface texturing for inverted pyramids, local oxidation and local dopant diffusion. High-temperature processing such as thermal oxidation at 1000◦C is required. By comparison, the fabrication of SHJ solar cells is much simpler. The wafer surface is randomly textured. The emitter, the passivating layers, and the BSF can be all deposited by plasma enhanced chemical vapor deposition (PECVD) at a temperature below 250 ◦C. Therefore, wafer bowing is suppressed due to the low processing temperature of the SHJ solar cell. This enables the use of thinner wafers, which results in reduction of the material cost and increase of the open-circuit voltage (Voc). Low temperature processing and excellent surface passivation of wafers make the use of low-quality wafers feasible, which contributes to further reduction of the material cost. The processing time to fabricate SHJ solar cells is also shorter than that to fabricate homojunction cells [39], benefiting the industrial production yield. Furthermore, the performances of SHJ solar cells surpassed the PERL cell as announced by Panasonic in April, 2014 [40]. In Table 1.1 the external parameters of the PERL cell and the best-performing SHJ solar cell are compared. These parameters are the short-circuit current density (Jsc), Voc, and fill factor (FF). The heterojunction band structure and good surface passivation of a-Si:H in SHJ solar cells limits the saturation current density, leading to a higher Vocthan the PERL cell. However, the incorporation of a-Si:H layers in SHJ solar cells causes optical parasitic absorption losses, which result in a slightly lower Jsc. It should be noted that, in addition to all the advantages introduced previously the record efficiency of the SHJ solar cell was obtained on
a much larger solar cell area than the PERL cell.
PERL c-Si silicon solar cell HIT®solar cell
Efficiency 24.7% 25.6%
Voc(mV) 706 740
Jsc(mA/cm2) 42.7 41.8
FF 0.828 0.827
Area (cm2) 4 143.7
Table 1.1:Comparison of external parameters between the PERL c-Si solar cell made by
UNSW [17, 18] and the SHJ solar cells made by Panasonic [40].
Although the SHJ solar cell comprises both a-Si:H and c-Si materials, it does not exhibit a strong performance degradation under light exposure, as is the case for thin-film a-Si:H solar cells, or a strong temperature-dependence of the performance, as is the case for wafer-based c-Si solar cells. Maruyama et al. [41] reported that there is no light-induced degradation after 5 hours of high-intensity illumination and a smaller drop in performance with increasing temperature in comparison with conventional c-Si solar cells. The absence of light-induced degradation is due to the fact that the a-Si:H layers in SHJ solar cells are very thin (only several nanometers) and so provide a negligible contribution to the overall power generation [42]. Further, it has been observed that a solar cell with improved surface passivation and a correspondingly higher Voc, exhibits an improved temperature-dependence [41].
1.4 Challenges for making high-efficiency SHJ solar
cells
In order to make high-efficiency SHJ solar cells the electrical and optical losses in the device have to be understood and reduced as much as possible. Those losses are illustrated in Fig. 1.5 and include (i) optical losses that limit the Jsc, (ii) recombination losses that mainly influence the Voc, and (iii) resistance losses affecting the FF. Several solutions have been considered to reduce these losses. In order to reduce the optical losses surface-texturing has been applied to wafers to provide efficient light trapping, the optical properties of TCO and a-Si:H layers have been optimized to reduce their parasitic absorption, and the aspect ratio of the grid electrodes has been increased to minimize the shaded area. For the reduction of recombination losses cleaning of wafer surfaces prior to a-Si:H
deposition is very important, as this removes recombination centers from the surface. The interface defect-state density can be reduced by saturating the dangling bonds on the wafer surface with hydrogen termination and using high-quality a-Si:H deposition. The resistance losses can be suppressed by decreasing the series resistance of the device. In this respect highly conductive TCO, good ohmic contacts at the contact interfaces, and a-Si:H thickness control are very important. In the following sections some of these critical issues are discussed in more detail.
1.4.1 Wafer surface texturing and post-texturization treatment
Wafer texturing is a common light-trapping method used in c-Si based solar cells. For single crystalline silicon wafers, a "random pyramid" texture is widely used. This texture is typically formed by treating a<100> c-Si wafer with an etching solution. The etching solution normally contains an alkaline chemical, e.g. NaOH, KOH or tetramethylammonium hydroxide (TMAH) as etchant, water as a solvent, and isopropanol as a wetting agent. The <111> and <100> crystalline orientations are etched at different rates, resulting in a random pyramidal surface structure in which the<111> crystalline plane is exposed. In this way, incident light that is reflected on one facet of a pyramid can impinge on a facet of another pyramid, giving the reflected light a second chance to enter the wafer. Besides, the light refracted on the surface will increase its optical path in the wafer, enhancing the light absorption. In spite of the optical benefit, the textured surface requires proper post-texturization treatment for good surface passivation. The post-texturization treatment includes surface cleaning and morphology control. Surface cleaning, which will be discussed in more detail in section 2.1.1, is aimed at removing organic and metallic contaminants, particles and native oxide on the wafer surface. Morphology control is important since it is closely related to the anti-reflective effect [43] and a-Si:H growth [44]. For instance, it is reported that chemical polishing after texturing can suppress epitaxial growth during a-Si:H deposition by reducing the number of pyramid valleys [44]. However, chemical polishing can cause pyramid rounding, enhancing the reflection of the wafer [43]. Therefore, a trade-off has to be found between passivation and anti-reflection by means of morphology control.
1.4.2 Wafer surface passivation
Previously, we mentioned that epitaxial growth during a-Si:H deposition, which is detrimental for surface passivation, can be influenced by the morphology of the wafer surface. In addition to the surface morphology, the deposition conditions of Si:H are very important for suppressing epitaxial growth. The deposition of a-Si:H layers should be performed at elevated temperatures to minimize the defect density in the material. However, epitaxial growth may occur when a-Si:H is deposited onto a c-Si wafer at over 140◦C [45]. This deteriorates the performance of the SHJ solar cell predominantly by reduction of the Voc. Ion bombardment induced by RF power during PECVD can help to prevent this epitaxial growth, but in turn it can result in surface damage if the power is too high. Therefore, there are trade-offs for both temperature and RF power. Epitaxial growth can be
avoided by optimizing the RF power and growth temperature [46].
An alternative method to avoid epitaxial growth is to replace the a-Si:H passivating layer with a silicon alloy or other passivating material. For instance, Fujiwara et al. and Sritharathikhun et al. applied a-SiO:H to form the heterojunction with c-Si [47, 48]. Silicon oxide has good passivating properties and epitaxial growth can be effectively suppressed because the coordination number of oxygen is only two. Development of other materials as passivating layers e.g. a-SiC:H [49] in SHJ solar cells also have been reported. Nevertheless, so far no other passivating material can perform better in SHJ solar cells than a-Si:H.
However, avoiding epitaxial growth of a-Si:H is only a prerequisite to have a good surface passivation. The microstructure of a-Si:H, e.g. the ratio of Si-H2 bonds to Si-H bonds, the hydrogen content, and the bulk defect density all have a significant impact on the quality of the surface passivation [50, 51]. Post-deposition treatments like post-annealing can also influence the surface passivation by hydrogen diffusion and local microstructure reconfiguration [50, 52].
1.4.3 Controlling thickness of a-Si:H based layers
The thickness of a-Si:H is very important for achieving high-efficiency SHJ solar cells for several reasons. On one hand, the intrinsic a-Si:H passivating layer has to be thick enough to provide high-quality c-Si surface passivation [53], and the thickness of the p-type a-Si:H emitter needs to be sufficiently large, in order to avoid being depleted since it is sandwiched between two n-type materials, i.e., TCO and n-type c-Si [37]. On the other hand, the a-Si:H layer thickness should be as thin as possible in order to minimize optical and electrical losses of the solar cell. The optical loss originates from the fact that the a-Si:H has very high absorption coefficients at wavelengths of shorter than 700 nm and it is reported no light absorbed by p-type a-Si:H emitter and only 30% of the light absorbed by intrinsic a-Si:H contribute to the current [54]. Besides, the high resistivity of a-Si:H also limits its thickness which can be used in the solar cell in order to have a high FF [36]. In-situ ellipsometry is a very useful way to control the thickness of the layers during the deposition [55].
1.4.4 Parasitic absorption losses in a-Si:H and ITO layers
As previously mentioned, a-Si:H can cause parasitic absorption losses in the wavelength region shorter than 700 nm where the photon energy is larger than
the bandgap energy of a-Si:H. For this reason both doped and intrinsic a-Si:H layers should be as thin as possible, provided no depletion of the emitter will occur and good c-Si surface passivation is assured. Except the thickness control of a-Si:H, the use of wide-gap silicon alloys, e.g. a-SiC:H and a-SiO:H, to replace a-Si:H as emitter material seems to be a promising approach to reduce the parasitic absorption of the emitter. This issue will be discussed in Chapter 6 when our SHJ solar cell with the p-type a-SiC:H emitter is presented. Besides a-Si:H, tin-doped indium oxide (ITO), which is a commonly used TCO material in SHJ solar cells for carrier transport and antireflection, also causes parasitic absorption losses in the ultraviolet and infrared regions of the solar spectrum. Since ITO has a wide bandgap of about 3.75 eV, high-energy photons in the ultraviolet region are consequently absorbed. In addition, the free carriers in ITO can also absorb infrared light. It is not straightforward to change the bandgap of a material and therefore researchers have been trying to limit the free charge carrier density of ITO in order to make it more transparent in the infrared region of the solar spectrum. In order to assure its high conductivity as well as high transparency, ITO with high mobility and low free charge carrier density has been developed by investigating deposition conditions and post-deposition treatment [56, 57]. Recently hydrogen-doped indium oxide (IO:H) [58] was demonstrated as a very promising alternative to ITO for application in SHJ solar cells. Due to the parasitic absorption losses resulting from a-Si:H and ITO, the highest Jsc achieved in the SHJ solar cell is still lower than that in the homojunction silicon solar cell as shown in Table 1.1.
1.4.5 Metallic grids
The industrial method of making grid electrodes is to use a screen-printing process. Applied resin-bonded silver paste typically exhibits the shape of a grid line with a spreading area, as shown in Fig. 1.6(a). The shaded area should be minimized in order to maximize transmission of light into the solar cell. If the cross-sectional area of the grid electrode is kept constant (in order to maintain a high conductance) the shaded area can be reduced by minimizing the spreading area and increasing the aspect ratio of the grid electrode, i.e. the width (w) should be reduced and correspondingly the height (h) should be increased to keep the area of the cross-section constant. In order to accomplish these two approaches, the screen-printing process parameters and the properties of the paste, such as its viscosity and rheology, need to be optimized. Fig. 1.6(b) shows the optimum shape of the grid electrode [59].
disadvantages when it is applied to SHJ solar cells. Due to the low thermal budget, the line resistivity and contact resistance of low-temperature screen-printed silver are limited. In order to sustain low series resistance of the contact, multiple printing runs are required, consequently leading to an increase of the line width [60]. Since copper has similar resistivity to the silver (1.7 µΩ·cm for silver, 1.6 µΩ·cm for copper) and its price is 100 times cheaper, copper is a promising alternative for the grid electrode. The screen-printable copper paste has been demonstrated in Trina c-Si solar cells to replace the conventional silver paste for making the busbars without any drop of cell efficiency [61]. Copper can also be deposited by electroplating, which can make more effective use of material. In 2012, Kaneka [62] demonstrated the successful application of copper electroplating to SHJ solar cells for metal grids, achieving a solar-cell efficiency of 23.5% on an area of 225 cm2. This accomplishment will definitely make the SHJ solar cell more competitive to other technologies in the future PV industry.
(a) (b)
Figure 1.6:Schematic diagrams of (a) a conventional grid electrode with a spreading
area and (b) an ideal grid electrode without a spreading area and a high aspect ratio
1.5 Aim and outline of this thesis
Although the SHJ solar cell has been developed for more than 20 years and a very high efficiency of 25.6% has been achieved in 2014, the material science and device physics for making high-efficiency SHJ solar cells are still not completely understood. Therefore, in this thesis, various aspects of the SHJ solar cells have been studied including ITO sputtering conditions, post-annealing of the device, the intrinsic a-Si:H passivating layers, optical design, and application of the wide-gap emitter.
with an introduction about mechanisms of involved equipment. Fabrication of SHJ solar cells starts with cleaning the c-Si wafers. Subsequently plasma enhanced chemical vapor deposition (PECVD), sputtering, and evaporation are used to deposit a-Si:H, ITO, and metal contact, respectively. The a-Si:H/c-Si interface is characterized by measuring the minority-carrier lifetime. Electrical properties of a-Si:H based layers were obtained by dark conductivity and activation energy measurements. For optical characterization of layers reflectance and transmittance measurements were carried out, as well as spectroscopic ellipsometry. Solar-cell parameters were obtained by quantum efficiency and J-V measurements. In addition, the optical models are introduced that are used for optical simulation of SHJ solar cells.
ITO is used as an electrical contact and current transport layer, as well as an antireflective coating in the SHJ solar cell. Since ITO has to be sputtered on the a-Si:H layer, which is only about 10 nm thick, it is very important to investigate how the ITO deposition process itself influences the properties of the thin a-Si:H layer underneath, especially the passivation offered by a-Si:H. In chapter 3, we investigate how the ITO sputtering process parameters influence the passivation quality, in particular the power density and deposition temperature. Additionally, we demonstrate how post-annealing recovers the passivation quality degraded by sputtering.
One of the main characteristics of the SHJ structure is the use of intrinsic a-Si:H to passivate the c-Si surface of the SHJ solar cell. Apart from its influence on the passivation, the impacts of the intrinsic a-Si:H layer thickness on carrier transport and parasitic absorption are also significant. In chapter 4, we present a comprehensive study of the influence of the intrinsic a-Si:H layer thickness on the optical and electrical performance of SHJ solar cells. Intrinsic a-Si:H layers in the emitter and BSF are studied separately since at each junction a different doped a-Si:H layer is applied on top of the intrinsic a-Si:H layer.
For solar cells, anti-reflection (AR) measures are very important for reducing reflection losses and increasing the light absorption. In SHJ solar cells, parasitic absorption losses in the ITO and the a-Si:H emitter can reduce the benefits gained from the AR coating. In chapter 5 we present an accurate optical analysis of SHJ solar cells in the wavelength range from 300 nm to 1200 nm. Based on our optical analysis, we show that for the AR design the Jsc can be maximized by optical simulations alone, provided that the c-Si wafer absorbance is maximized since this absorbance dominates the Jsc. We show that this procedure is more accurate than simply minimizing the reflectance due to the influence of parasitic absorption caused by ITO and a-Si:H. In this contribution, we illustrate this approach in the design of a double-layer AR coating of both planar and textured SHJ solar cells.
In order to validate our model, solar cells with the optimized AR coating are fabricated.
In SHJ solar cells based on n-type c-Si wafers, p-type a-Si:H, which is normally used as the emitter material, has a relatively high absorption coefficient at wavelengths shorter than 700 nm and the photogenerated carriers in this emitter cannot be collected. In chapter 6, p-type a-SiC:H is presented as alternative wide-gap emitter material and we compare the electrical and optical properties of this material to the properties of our p-type a-Si:H. With the aid of our ASA software, optical analysis of the SHJ solar cells utilizing the two different emitters is carried out by comparing optical simulation to experimental characterization results. The solar-cell performance of devices with both emitters is presented and discussed. We demonstrate that p-type a-SiC:H has the capability to replace p-type a-Si:H as a better emitter material in SHJ solar cells.
1.6 Contribution to the research field
In order to investigate the methodology of making high-efficiency SHJ solar cells, research has been carried out on materials making up these solar cells and on device structures. The contributions of this thesis to the research field of SHJ solar cells are as follows:
• The clear influence of the ITO sputtering conditions and post-annealing on the passivation of a-Si:H has been demonstrated. For ITO sputtering a power density as low as possible is suggested in order to reduce the degradation of the passivation quality. The use of elevated temperatures during ITO sputtering, which results in simultaneous annealing, can partially recover the degradation of the passivation quality caused by ITO sputtering itself. However, the elevated temperature used in ITO sputtering can cause passivation degradation when it crosses a threshold value. Post-annealing is very effective in recovering the passivation degradation resulting from ITO sputtering at the room temperature.
• The comprehensive impacts of intrinsic a-Si:H layer thickness on optical and electrical performance of SHJ solar cells are shown. On both the emitter and BSF sides, a thinner intrinsic a-Si:H layer leads to higher FF. The Voc is very sensitive to the intrinsic a-Si:H layer thickness on the emitter side, whereas the intrinsic a-Si:H layer thickness on the BSF side can be even zero to gain FF. In the thickness range we investigated,
parasitic light absorption losses from intrinsic a-Si:H layers on both the emitter and BSF sides are not obvious.
• A double-layer AR coating of SiOx in combination with ITO has been designed for application in SHJ solar cells. We showed that the optimization of the optical performance should be based on maximizing the absorption in the c-Si, rather than based on minimizing the solar-cell reflection. In this way the influence of parasitic absorption in the ITO and intrinsic a-Si:H layer is minimized. Using this coating, a very high current density of 40.5 mA/cm2was achieved.
• P-type a-SiC:H is successfully implemented to SHJ solar cells as a wide-gap emitter, showing better optical and comparable electrical performance than the p-type a-Si:H emitter. The optical enhancement caused by the p-type a-SiC:H emitter originates from the reduction of not only the parasitic absorption loss but also the reflection loss. Using the p-type a-SiC:H emitter, the Jsc gain of about 1 mA/cm2 can be realized. An efficiency of 20.7% is achieved in our SHJ solar cell with the p-type a-SiC:H emitter.
2
Experimental
2.1 Fabrication process
In this section, the process of making silicon heterojunction (SHJ) solar cells will be presented together with an introduction about the operation mechanisms of equipment used for making the solar cells. The fabrication of the SHJ silicon solar cell starts with the wafer cleaning in order to reduce the amorphous/crystalline silicon interface defects originating from the contaminants and native oxide on the wafer surface. After wafer cleaning, the plasma enhanced chemical vapor deposition (PECVD) is used to deposit intrinsic and doped hydrogenated amorphous silicon (a-Si:H) for making the passivating layer, the emitter and the back surface field (BSF). Then the indium tin oxide (ITO) is deposited by plasma sputtering as the antireflective coating and the contact. Finally, the front and back metal contact are evaporated by thermal and e-beam evaporation.
2.1.1 Wafer cleaning
The fabrication of the heterojunction silicon solar cell starts with a Float Zone (FZ) n-type crystalline silicon (c-Si) wafer (Topsil, Denmark) with a resistivity of 25 Ω·cm. The reason for choosing FZ wafers is the ultrahigh purity leading to a very high bulk lifetime, which can reach values as high as 35 ms [63]. High
quality bulk material will make the influence of bulk recombination negligible when we investigate the surface recombination.
There are generally two ways of wafer cleaning: dry and wet cleaning. Dry cleaning has been suggested to fit the mass production better [64]. Plasma cleaning using CF4/O2 or H2 [65, 66] has been investigated but so far no passviation result obtained from dry cleaning as good as from wet cleaning has been demonstrated. The main problem is the increased interface defect density caused by the ion bombardment in the plasma process. Therefore, wet cleaning is still the dominant and best-performing cleaning technology for making SHJ solar cells. In this thesis, the wet cleaning is used for the fabrication process. There are two goals of the wafer cleaning process for SHJ solar cells. The first is to remove all contaminants on the wafer surface. Secondly, the defective native oxide should be completely removed and the surface should be terminated with hydrogen, which can protect the wafer surface from oxidization during the transport before the a-Si:H deposition. An effective cleaning process is a prerequisite to obtain good interfaces between c-Si and intrinsic a-Si:H [67]. The contaminants can lead to recombination centers at the interface between a-Si:H and c-Si:H, which is part of the p-n junction. Many institutes working on SHJ solar cells carry out this wafer cleaning procedure in various ways, but the basic mechanisms have been well studied. Possible contaminants on the silicon wafer could be organic contaminants, particles and metal elements [68] and the removal of these contaminants can be realized in the following ways:
• Organic contaminants can be effectively removed by strong oxidants like UV/O3, O2plasma, H2O2, concentrated HNO3and H2SO4[68, 69]. • Megasonic energy can be applied to remove particles with less damage on
the silicon surface than when using ultrasonic energy. By using an oxidant together with weak alkalic chemical like e.g. NH3·H2O and H2O2in RCA Standard Clean 1(SC 1), a repeated formation and removal of oxide can occur on the wafer surface, which can clean particles from the surface. • Regarding metallic contaminants, aqueous ammonia can form complex
with several metal elements and strong acid can also be used in order to remove the contaminants.
After the contaminants are removed, the native silicon oxide can be easily etched again by the HF solution and simultaneously the wafer is passivated by the hydrogen.
In this thesis, most of the results have been obtained using planar wafers. For the planar wafer the cleaning procedures are described in the appendix. Pure HNO3 acid as a strong oxidant is used to remove organic contaminants. Immersion in 69.5% HNO3 as the second cleaning step is aiming at cleaning metal contaminants. Before the native silicon oxide is stripped, the wafer needs to be rinsed completely in order not to bring HNO3to the HF solution. After the HF dip, the wafer is rinsed again in the de-ionized (DI) water, dried with nitrogen and then ready to be loaded into the deposition chamber.
In order to optimize the cleaning process in our facility a series of wafers has been subjected to the cleaning procedure described in the appendix. On both sides of these cleaned wafers a 30-nm thick a-Si:H layer has been deposited using the same deposition conditions as for solar cells and then the effective minority carrier lifetime (τeff) was measured with the Sinton lifetime tester (this tester will be discussed in more detail in section 2.2.1). This lifetime is an indication of the effectiveness of the cleaning process. Fig. 2.1 shows that the optimum HF dip time is in between 60 s and 90 s. For an optimal cleaning the control over the HF dip is important: this etching time should be long enough to remove all the native oxide; however, if this time is too long the immersion of the wafer in the HF solution can increase the surface microroughness [70], resulting in the increase of the surface defect density. This increase in microroughness results from a slow oxidation of the silicon, followed by fast oxide removal on the surface [71]. Besides, the last drying step is found to be very important. Since a piece of glass wafer contacts one side of the wafer for protection during a-Si:H deposition on the other side, the moisture will make the silicon wafer stick to the glass wafer in the vacuum. Eventually, after optimization of the cleaning process, a high τeffwas achieved as shown in Fig. 2.2
In order to reduce the reflection of the solar cell and to induce light scattering in the c-Si absorber, textured wafers are needed. In this case, a (100) c-Si wafer is textured in a mixture of 5 wt% TMAH and 2 wt% isopropanol for 15 min at a temperature of 80 ◦C. Then the textured wafer will go through the cleaning procedure described in the appendix.
2.1.2 Plasma enhanced chemical vapor deposition (PECVD)
The a-Si:H material is deposited by radio frequency (RF) PECVD. The gas precursors are mainly SiH4 as the source of the silicon, B2H6 as the source of acceptors, PH3as the source of donors, and CH4 as the source of carbon. After the RF generator is switched on, the gas precursors are electrically discharged, and a mixture of ions, radicals, molecules, atoms, and electrons form in the
0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 0 2 4 6 8 1 0 1 2 M i n o r i t y c a r r i e r d e n s i t y : 1 0 1 5 c m - 3 H F c o n c e n t r a t i o n : 0 . 5 5 % E ff e c ti v e m in o ri ty c a rr ie r lif e ti m e ( m s ) H F d i p T i m e ( s )
Figure 2.1:The effective minority carrier lifetime of the wafers as a function of the HF
dip time. The wafers, which are (111) oriented, have 30 nm thick a-Si:H layers deposited on both two sides.
1 E 1 4 1 E 1 5 1 E 1 6 0 2 4 6 8 1 0 1 2 1 4 E ff e c ti v e m in o ri ty c a rr ie r lif e ti m e ( m s ) M i n o r i t y c a r r i e r d e n s i t y ( c m - 3)
Figure 2.2:The effective minority carrier lifetime of the wafers as a function of the
minority carrier density. The wafers, which are (111) oriented, have 30 nm thick a-Si:H
layers deposited on both two sides. The τeffat the minority carrier density of 1015cm-3is
plasma, starting the deposition by complex physical and chemical reactions in the plasma and on the sample surface.
In order to have a good interface between c-Si and a-Si:H, the wafer needs to be fixed rapidly on a stainless steel sample holder right after it is cleaned and dried, and then transferred to the vacuum deposition chamber of the RF-PECVD setup in order to avoid re-oxidation. In Fig. 2.3(a) a sketch of our PECVD system is shown. The system has four different chambers for deposition of different materials to prevent cross-contamination. A robot arm in the transport chamber is programmed to move in three dimensions for picking up and loading the samples. Fig. 2.3(b) shows a schematic structure of one deposition chamber with a loaded sample inside. As a part of the sample holder, there is a back metallic plate which is used to confine the plasma between the sample and the electrode on the bottom. In order to avoid the direct contact between metal and the wafer, a clean glass wafer is inserted between them. The properties of a-Si:H is closely related to its deposition conditions including the flow rate of the gas precursors, the pressure in the chamber and the sample temperature [20].
Chamber 1 Chamber 2 Chamber 3 Chamber 4 Loadlock Window Robot arm Transport chamber (a) Heater Pump matching network Sample holder Sample RF generator Gas precursor Plasma Quartz wafer (b)
Figure 2.3:Sketches of (a) the PECVD system with multiple deposition chambers and
(b) one deposition chamber with a loaded sample
2.1.3 Sputtering
Different from PECVD, sputtering is a physical process. An inert gas, e.g. Ar, is used and Ar ions in the plasma, which are accelerated in the plasma sheath, impinge on the material target. In this case the material is ITO (In2O3:SnO2 =
9:1). ITO atoms or clusters that come out from the ITO target due to the ion bombardment, will diffuse to and deposit onto the substrate.
After the intrinsic, n-type, and p-type a-Si:H layers have been deposited by RF-PECVD, ITO is deposited by RF sputtering. The sample in the sputtering chamber is directly heated by a halogen bulb. ITO is deposited through a shadow mask to define the size of the solar cell. Fig. 2.4 shows the schematic structure of our sputtering system. This sputtering system consists of two vacuum chambers: a loadlock and the deposition chamber.
Heater Pump matching network Sample holder Sample RF generator Ar Glass Plasma ITO target Shutter Loadlock
Figure 2.4:Schematic structure of the sputtering machine.
2.1.4 Evaporation
Evaporation is a deposition method by which the melt material evaporates from a source material and condenses on a substrate. Mainly depending on the melting points of the material, thermal evaporation or e-beam evaporation can be used as demonstrated in Fig. 2.5(a). Thermal evaporation is realized basically by applying high voltage to a resistor, which is a tungsten boat in this case. The glowing tungsten boat melts the material with a relatively low melting point, e.g. Ag which has a melting point of about 960 ◦C, and the evaporation starts. Regarding e-beam evaporation, the electrons are generated normally by thermionic emission and accelerated to attain a high kinetic energy. The electron beam is redirected by a magnetic field and impinges on the evaporation material. Upon striking the material, the kinetic energy is converted to other forms of energy among which
the thermal energy melts the material and triggers the evaporation. Electron-beam evaporation is originally designed for evaporating material with very high melting points, e.g. Cr with a melting point of 3400◦C. Although Al has a lower melting point than Ag, Al is also deposited by e-beam evaporation. The reason is that the melt Al can alloy with the tungsten boat which is used as the sample holder for thermal evaporation.
After the ITO has been deposited, the metal needs to be deposited on the front and back of the solar cell. For the metal contact of the SHJ solar cell, a stack of Ag (100 nm)/Cr (30 nm) /Al (1900 nm) is used. The Al is used as the main lateral conductive part of the metal contact due to the material budget and its resistance to oxidation. The reason for using Ag is that the contact resistance between ITO and Ag is smaller than between ITO and Al. When the solar cell is annealed at high temperature, Al can react with oxygen atoms in ITO and form oxide [72, 73], increasing the contact resistance as shown in Fig. 2.5(b). The thin Cr layer is used between Ag and Al to prevent the two metals mixing during annealing. For the metal deposition, a shadow mask is used to define the deposition area. Fig. 2.6 shows a photograph of a finished SHJ solar cell. There are two different contact sizes: 1 cm2and 0.16 cm2. Pump Sample E-beam Evaporation Thermal Evaporation VoltageDsupply TungstenDboat MeltDmaterial WaterDcooledDholder IngotD HeatedDregionD MagneticDfieldD DeflectedDelectronDbeamD Filament Accelerating electrode (a) 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 0 1 0 2 0 3 0 4 0 5 0 C o n ta c t re s is ta n c e ( Ω * c m 2) A n n e a l i n g t e m p e r a t u r e (oC ) A l / I T O A g / I T O (b)
Figure 2.5:(a) Schematic structure of our evaporation system including electron beam
and thermal evaporation.(b) Contact resistance of ITO and Al as well as Ag as a function of temperature. The contact resistance is measured by transmission line measurement.
a-Si:H ITO
Metal
Figure 2.6:Picture of a SHJ solar cell fabricated in the facility of the PVMD group at
Delft University of Technology.
2.2 Characterization techniques
For this thesis, different characterization techniques have been used to characterize the electrical and optical properties of the solar-cell materials and devices. In this section, the characterization methods will be introduced and their measurement principles will be explained. To investigate the surface passivation of the c-Si, measurement of the minority carrier lifetime and Suns-Voc is used. Dark conductivity and activation energy are measured as important semiconductor material parameters. To obtain optical properties of the materials, reflectance and transmittance measurements and spectroscopic ellipsometry are also carried out. Device characterization mainly includes the J-V measurement, external and internal quantum efficiency measurement.
2.2.1 Minority carrier lifetime and Suns-Voc measurement
The minority carrier lifetime describes the characteristic time of the minority carrier density after excitation to decay back to the thermal equilibrium density. It is often interpreted as the average time that free electrons or holes can survive before recombination. Commonly this time is measured to characterize the quality of the bulk silicon material and the surface passivation since it is closely related to the defect density of the bulk or the surface. The measured minority carrier lifetime is called the effective minority carrier lifetime (τeff) defined in Eq. 2.1
1 τeff = 1 τrad + 1 τAug + 1 τSRH + 1 τs = 1 τb + 1 τs , (2.1)
where τrad, τAug, τSRH, τs are the carrier lifetime resulting from the radiative, Auger, Shockley-Read-Hall and surface recombination, respectively, and τbis the bulk lifetime. Since in our experiments high-quality FZ wafers are used, τb-1 is negligible compared to τs-1. Therefore, τeff approximates τs.
In this work the minority carrier lifetime is related to the surface recombination velocity. This quantity describes the recombination rate at the surface of the crystalline silicon wafers or the interfaces with other materials. The relationship between the minority carrier lifetime and the surface recombination velocity can be understood by considering the diffusion and recombination of the electron density (n) in a homogeneous sample with two interfaces. This situation can be represented by the diffusion equation [74]
∂n(x, t) ∂t =D ∂2n(x, t) ∂x2 − n(x, t) τb (2.2) In order to solve n(x,t) for this situation we need the boundary conditions
D∂n(x, t) ∂t |x=−w/2=S1n(− w 2, t), −D∂n(x, t) ∂t |x=w/2=S2n( w 2, t), (2.3)
where t is the time, x is the dimension of the wafer with the coordinate in the center of the wafer as shown in Fig. 2.7, D is minority carrier diffusion coefficient, S1and S2are the surface recombination velocities on two sides of the wafer, and W is the wafer thickness. Sproul et al. [75] showed that when S1and S2are both very high (i.e., S1W/D > 100 and S2W/D > 100 ), τsis equal to D-1(W/π)2, meaning the τsis not limited by the surface recombination velocity but by the diffusion of minority carriers towards the surface. If S1= S2= S, the τscan be expressed by
τs W 2S + 1 D( W π)2. (2.4)
If S is low (i.e., SW/D < 0.25), D-1(W/π)2is negligible compared to W/2S and τ eff≈ τs≈ W/2S. Therefore, the surface recombination velocity can be simply calculated from the measured τeffwhen the preconditions discussed above are fulfilled.
The methodology to measure the τeff can be understood from the continuity equation [76]. Since there is no electrical field and, in case of homogeneous
−𝑤2 0 𝑤2
𝑆2
𝑆1
𝑥 𝑦
Figure 2.7:The coordinate and wafer position.
generation, no diffusion, the continuity equation can be simplified to the following equation:
d∆n
dt =G − U, (2.5)
where the∆n is the excess carrier density, G is the carrier generation rate, U is the recombination rate, and d∆n/dt represents the rate of the carrier density change. The τeffcan then be deduced by the equation:
τeff = ∆n G −d∆n
dt
. (2.6)
In practice, τeff is measured using a Sinton WCT-120 lifetime tester [77, 78] whose schematic is shown in Fig 2.8. The sample is placed on a stage in which an inductive coil is mounted underneath to measure the conductance by an RF bridge circuit. The light intensity of the flash lamp is monitored by a photodiode near the sample. The tester can operate in two modes for the lifetime measurement, which are transient and quasi-steady state (QSS) mode. Depending on the range of the minority carrier lifetime, one of the modes should be chosen. When the minority carrier lifetime is longer than 100 µs, it is more accurate to use the transient mode. In the transient mode, G is equal to zero, which is realized by very short-time flash. Then, Eq. 2.6 is simplified to Eq. 2.7
τeff = ∆n
−d∆n dt
, (2.7)
where∆n can be calculated from the measured photoconductance and d∆n/dt is the slope of the tangent on the curve of the time-resolved∆n decay. In the QSS mode, the time of the flash is much longer than the carrier lifetime. Due to the fact that G d∆n/dt in which G is calculated from the light intensity detected by the photodiode, Eq. 2.6 is simplified to
τeff = ∆n
G . (2.8)
The Sinton lifetime measurement can also give the implied-Voc, which is a prediction for the Voc of the final device, by Eq. 2.9
implied−Voc = kBT
q ln( np
n2i ), (2.9)
where kB is the Boltzmann constant, q is the elementary charge, T is the temperature, n and p are the free electron and hole concentrations, and ni is the intrinsic carrier concentration.
Sample RF generator Phase shifter Attenuator Oscilloscope Computer Power supply
The Sinton Suns-Voc setup [79] measures the voltage of the solar cell in open-circuit condition at different light intensities. The light intensity ranges from a few suns to 0.01 sun. In case of the same spectrum of the lamp, the Vocat one sun of the Suns-Voc measurement should be equal to the Voc of the J-V characteristics. The mechanism of the measurement can be described by Eq. 2.10.
J = J0 exp q(V − JR s) mkBT − 1 ! − suns · Jsc+ (V − JRs) Rshunt , (2.10)
where Jsc is the short-circuit current density inputted on basis of estimation or as an additional measurement, suns represents the light intensity that is equal to the photogenerated current density divided by the Jsc, J0 is the dark saturation current density, Rs is the series resistance, Rshunt is the shunt resistance and m is the ideality factor [80]. Since this measurement is carried out at open-circuit condition, the external current density J equals 0. Besides, exp(qVoc/mkT) is much larger than 1. Therefore, Eq. 2.11 can be deduced from Eq. 2.10.
suns · Jsc= J0exp( qVoc mkBT ) + V Rshunt . (2.11)
With a given Jsc, a plot of Voc versus suns can be obtained. Since this measurement excludes the series resistance of the device, the Voc can be measured before metallization and the information about the shunt and the ideality factor can be obtained. Based on Eq. 2.8 and 2.9, the effective carrier lifetime can also be deduced.
2.2.2 Dark conductivity and activation energy measurement
By measuring the conductance of the semiconductor layer (Gc), its cross-section area (A) and its length (L) in darkness, the dark conductivity of the semiconductor (σd) can be calculated as stated in Eq. 2.12:
σd=Gact· L
A. (2.12)
Taking n-type a-Si:H as an example, the σdis determined by the mobility (µn) and electron density (n) as shown in Eq. 2.13:
σd =qµnn. (2.13)
The n can be expressed by Eq. 2.14: n= NCexp(
EF− EC
Here NCis the effective density of the conduction band states, EFis the Fermi energy, and EC is the minimum attainable conduction-band energy. For n-type a-Si:H, ECminus EFapproximates the activation energy (Eact). Eq. 2.13 and 2.14 can be combined to the following equation:
σd =qµnNCexp(− Eact kBT
). (2.15)
Therefore, by measuring the σd at different temperature, the Eact can be deduced. The sample for this measurement is shown in Fig. 2.9. The test layer, mostly a-Si:H, is deposited on the glass. Then a pair of Al stripes is deposited on the a-Si:H as contacts. The sample is mounted on a temperature-controlled stage. A constant voltage is applied between two metal contacts and the current is measured at different temperatures. As a result, the conductance (Gc(T)) at different temperatures can be calculated. Since the A is the layer thickness times the contact length and the L is the distance between two contacts, the σd(T) can be calculated from Eq. 2.12. Then the Eactcan be deduced from Eq. 2.15.
I I
Glass a-Si:H Measured part of a-Si:H Aluminium contact
Figure 2.9:Schematic sample lay-out for the dark conductivity and activation
measurement.
2.2.3 External and internal quantum efficiency measurement
The external quantum efficiency (EQE) is the fraction of incident photons that result in collected electron-hole pairs at each wavelength. From the EQE measurement, we can see quantitatively how efficiently the solar cell can convert
photons to current at each wavelength. Both optical losses, e.g. reflection and parasitic absorption losses, and electrical losses, e.g. recombination losses, influence the EQE spectrum. By combining EQE and our advanced optical simulation, more detailed analysis about those loss mechanisms can be carried out. This analysis approach is used in Chapter 5 and 6. By integrating the EQE spectrum measured at short-circuit condition with respect to the AM 1.5 spectrum, the Jscof the solar cell can be calculated, using Eq. 2.16:
Jsc=q Z 1200
300
EQE(λ)φ(λ)dλ, (2.16) where φ(λ)is the photon flux density of the AM 1.5 spectrum at each wavelength. For the investigation of the SHJ solar cell, we have chosen the wavelength range from 300 nm to 1200 nm of the solar spectrum. The reason is that for wavelengths shorter than 300 nm, the spectral power density in the AM 1.5 spectrum is almost zero, while photons with wavelengths longer than 1200 nm are hardly absorbed by the c-Si. Based on the EQE and reflectance measurements of the solar cell, the internal quantum efficiency (IQE), which excludes the reflection loss, can be calculated, using Eq. 2.17.
IQE(λ) = EQE(λ)
1 − Rm(λ)
, (2.17)
where Rm(λ) is the measured reflectance of the solar cell. Our EQE measurement setup has been developed in house and is shown in Fig. 2.10. The halogen lamp emits a spectrum with a broad wavelength range. The chopper generates an AC signal, which is amplified by the lock-in amplifier. The monochromator makes the measurement wavelength-selective. The apertures and lens are adjusted to change the focus of the light beam. The EQE of the sample is actually calculated based on a calibration diode with a known EQE (EQEref). Before measuring the sample, the calibration diode is measured first and the lock-in amplifier outputs the voltage at each wavelength (Vref (λ)). Then the sample is measured and the output of V (λ) is used to calculate the EQE of the sample by the following equation:
EQE(λ) = V(λ)
Vref(λ)· EQEref(λ), (2.18)
2.2.4 J-V measurement
The J-V measurement is simply realized by measuring the current of the solar cell at a range of applied voltage. Fig. 2.11 shows a typical J-V curve of a SHJ solar
Halogen lamp Monochramator Chopper Aperture Aperture Lens Solar cell Lock-in amplifier Detection electronics
Figure 2.10:Schematic structure of the EQE measurement setup.
cell. From the J-V measurement four important external parameters are deduced: the open-circuit voltage, Voc, the short-circuit current density, Jsc, the fill factor, FF, and the energy conversion efficiency, η. Besides, other parameters like the maximum power (Pmp), ideality factors, J0, Rsand Rshuntcan also be obtained by additional data processing and simulation.
The J-V characteristics are measured using an Oriel solar simulator to obtain external parameters including Voc, Jsc and FF of the solar cell. For this measurement, the solar cell is illuminated by the Xenon arc lamp, which is adjusted to give a light intensity of 1000 W/cm2 in the wavelength range of 300 nm to 1200 nm. Since multiple solar cells are made on a wafer as shown previously in Fig. 2.6, a shadow mask made of black paper is used during the J-V measurement to define the illumination area which is designed to be the same as the ITO area. Since the Jsc is very sensitive to alignment of the shadow mask and metal grids, the Jscmeasured in the J-V measurement is always calibrated by the EQE measurement for accurate optical analysis. All the reported Voc and FF are the averaged values of three best SHJ solar cells made at the same processing conditions. Note that in the layout of the solar cell we process there are two different sizes of 0.16 cm2 and 1 cm2. We found that the exposure area and the solar cell area have a significant influence on the J-V characterization as illustrated in Fig. 2.11. The Vocof the 0.16 cm2solar cell is about 15 mV smaller than of the 1-cm2 solar cell when the exposure area is equal to the size of the solar cell or the ITO area. This can be explained by the following equation:
Voc = mkBT q · ln( JL· AL J0· A0 − 1), (2.19)
Figure 2.11: J-V curve (black) of a SHJ solar cell under illumination and the power output (red).
where the m is the ideality factor, JL is the photogenerated current density, J0is the dark saturation current density, AL is the illumination area and A0 is the area for the dark saturation current. The ALis dominated by the illuminated area and the ITO area, but the A0is larger than the ALsince it is also largely influenced by the emitter area. Therefore, the measured Voc is smaller than the actual value. Since the perimeter of the 0.16 cm2solar is more comparable to its area than the 1 cm2 solar cell, the reduction of Voc is more obvious. By increasing the exposure area as shown in Fig. 2.12, the Voc of the solar cells with two different sizes saturates at around 690 mV, which is in agreement with our Suns-Voc measurement result. Therefore, we conclude there are two conditions to obtain the actual Voc in the J-V measurement: (i) the solar cell should be large enough that the perimeter of the solar cell is negligible compared to its area; (ii) the exposure area should be larger than the collection area of the solar cell, indicated by the phenomena that FF stops decreasing. The FF decrease can be explained by Fig. 2.13. When the exposure area is larger than the solar cell, some carriers are collected laterally from the highly resistive emitter layer, resulting in the increase of series resistance.
0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 6 5 0 6 6 0 6 7 0 6 8 0 6 9 0 7 0 0 F F ( % ) Voc (m V ) E x p o s u r e a r e a ( c m 2) Vo c F F 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 6 8 7 0 7 2 7 4 7 6 7 8 (a) 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 1 . 2 1 . 4 6 5 0 6 6 0 6 7 0 6 8 0 6 9 0 7 0 0 F F ( % ) Voc ( m V ) E x p o s u r e a r e a ( c m 2) Vo c F F 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 1 . 2 1 . 4 6 8 7 0 7 2 7 4 7 6 7 8 (b)
Figure 2.12: Voc and FF as a function of the exposure area defined by the mask opening
during the J-V measurement for (a) a 0.16 cm2large solar cell and (b) a 1 cm2large solar
cell. Note that the metallic shading is 10%. The red dash line indicates the situation in which the exposure area is equal to the size of the solar cell.
+ ITOp/i + c-Si i/n Mask Light Metal
Figure 2.13: J-V characteristic of the solar cell is measured when the exposure area is
2.2.5 Reflectance and transmittance measurement
Reflectance and transmittance (R/T) measurements of films are used to characterize the optical properties of the material. Based on those measurements, complex refractive indices of the materials can be obtained by fitting the R/T measurements with the SCOUT software [81]. R/T measurements are carried out, using the integrating sphere of the PerkinElmer Lambda 950 UV/VIS spectrometer. Fig. 2.14 shows a schematic representation of the integrating sphere with the sample under reflectance and transmittance measurements. The inside wall of the integrating sphere is coated with spectralon, a polytetrafluoroethylene-based material, which is highly scattering and highly reflective in a very broad wavelength range. Due to multiple reflections, the electromagnetic field is homogeneous in the integrating sphere so that it is sufficient to measure the total intensity of reflected light (Fig. 2.14(a)) and transmitted light (Fig. 2.14(b)) at one point of the sphere. There are two detectors to measure the light intensity in different wavelength ranges. One is a photomultiplier tube for wavelengths below 860.6 nm and the other is the PbS photodetector for wavelengths above 860.6 nm. Before the reflectance measurement of the sample, a piece of spectralon material has to be placed at the transmission port to obtain its reflectance Rref with the sample beam on. The reflectance of the sample is actually calculated by the equation:
R= (Rmeas− Rdark)· Rref
Rref− Rdark
, (2.20)
where Rmeas is the reflectance measured from the sample and Rdark is the reflectance measured from the spectralon material with the sample beam off. Similarly, the T can be measured.
2.2.6 Spectroscopic ellipsometry
Spectroscopic ellipsometry (SE) is now a commonly used method for in-situ or ex-situ thin film characterization because of its many advantages. SE is a fast and non-destructive measurement with very high thickness resolution. Many physical properties of the thin film, like e.g. optical constants, layer thickness, band structure, carrier mobility, carrier concentration, can be obtained by the SE measurement. However, SE is an indirect measurement since it only detects the polarization change of the incident polarized light beam reflected on the thin film as shown in Fig. 2.15. The parameters measured directly from the SE areΨ and ∆ are defined by the equation:
Samp le Reference beam Sample beam Transmission port (a) Samp le Reference beam Sample beam (b)
Figure 2.14:Schematic top view of the integrating sphere for (a) the reflectance
measurement and (b) the transmittance measurement. The detector is on the bottom of the integrating sphere.
tan(Ψ)ei∆ =ρ(n0, n1, n2, d, φ) = rp rs
, (2.21)
where ρ is the ratio of the reflectance of the p-polarized light (rp) divided by the reflectance of the s-polarized light (rs) upon reflection. n0, n1, n2are the complex refractive indices of air, the thin film, and the substrate as shown in Fig. 2.16, and dis the film thickness. The magnitude of the ratio is tan(Ψ), and ∆ is the phase difference. The direct output from the SE measurement will be a plot of Ψ and ∆ as a function of photon energy. Absolute values ofΨ and ∆ contain no directly useful physical information about the materials. A fitting procedure with an optical model is needed to deduce the important information like layer thickness and optical constants. In the process of this modeling, the number of layers must be selected and the optical functions of each layer must be determined. If the optical functions of the layer or the substrate are well known, then tabulated data can be used for the optical functions of materials, but for materials with unknown optical functions, a parameterizable model must be employed. The spectroscopic ellipsometer used in this thesis is from J.A. Woollam Co.
Light source
Polarizer Rotating analyzer
Detector
Sample
F0
Figure 2.15:Sketch of the SE measurement.
Air Thin film Substrate d N0 N1 N2 F0 F1 F2
2.3 Optical simulation
In this thesis, the optical simulation is used for optical analysis of the SHJ solar cell. By optical analysis, the reflectance (R) of the device, the absorbance (A) of each layer, and the transmittance (T) can be evaluated. It is very important for investigating reflection and parasitic absorption losses of the device and for instance designing an antireflective coating. Concerning SHJ solar cells on a planar surface, thin-film optics is applied since the cell consists of several layers with thicknesses similar to the wavelength of visible light. On a textured surface geometrical optics is needed to interpret the light scattering on the facets of pyramids, since the pyramids have a size of several micrometers. In addition, thin-film optics is also applied for the thin layers on top of the facets of the pyramids.
2.3.1 Optics on planar surfaces
A SHJ solar cell on a planar surface can be considered as a layer stack of different materials. Fig. 2.17 shows a one-layer system with two interfaces, from which a calculation for a multilayer system can be expanded. The layer, which is medium 1, is sandwiched by two semi-infinite media, labeled medium 0 and medium 2. The reflection, refraction, and transmission at each interface are illustrated. Depending on the layer thickness two different numerical methods are applied. When the layer thickness is larger than the coherence length of the incident light (e.g. a 280-µm thick Si wafer) R, T and A can be calculated by the following equations: R=|r|2, T =|N2 N0 ||t|2, A=1 − R − T , (2.22)
where r and t are the effective Fresnel amplitude coefficients for reflection and transmission, and N0and N2are the complex refractive indices of medium 0 and medium 2. The r, t and δ are defined by the following equations:
r =r1+ t1t10r2e−2iδ1 1 − r2r10e−2iδ1 , t= t1t2e −iδ1 1 − r2r10e−iδ1 , δ1= 2π λ d1N1, (2.23)
where δ1is the complex phase shift and d1is the layer thickness of medium 1. When the layer thickness is smaller than the coherence length of the incident light (e.g. ITO, and intrinsic and doped a-Si:H layers), R, T and A can be calculated by the following equations:
R=|r1|2+ |t1|2|r2|2|t10|2e−2iα1d1 1 − |r2|2|r10|2e−2iα1d1 , T =|N2 N0| |t1|2|t2|2e−2iα1d1 1 − |r2|2|r10|2e−2iα1d1 , A=1 − R − T . (2.24) I Interface 2 t1 t1t2 t1r2t1' Interface 1 t1r2 t1r2r1' t1r2r1't2 t1r2r1'r2 t1r2r1'r2t1' A R r1 Medium 0 Medium 1 Medium 2
Figure 2.17:Light propagation when the incident light impinges on a single layer with
