Luminescence characterizations and parameter drifts of CIGS solar cells

By Thomas Ott, Research Assistant - RECIS Project, University of Applied Sciences Ulm; Thomas Walter, University of Applied Sciences Ulm; Dimitrios Hariskos, Centre for Solar Energy and Hydrogen Research (ZSW); Oliver Kiowski, Centre for Solar Energy and Hydrogen Research (ZSW); Raymund Schäffler, Scientific Associate, Manz CIGS Technology GmbH

Lifetime guarantees of more than 20 years are a target for the long-term stability of solar modules. An important point for the future of CIGS solar cells is to understand the impact of metastable behaviour on long-term stability. Accelerated ageing under open-circuit conditions leads to a drop in open-circuit voltage (Voc). A decrease in the net doping density is responsible for the drop in Voc and consequently the drop in the photoluminescence (PL). In the initial state the electroluminescence (EL) ideality factor exhibits a value close to unity, as expected from theory. After the dark anneal an increase in the EL ideality factor is observed, and an EL measurement at constant voltage shows a decrease in EL: both these behaviours are due to a pile-up of negative charges at the heterointerface. The application of a positive bias or an illumination during the endurance test leads to an optimization of stability. This paper shows that PL and EL can distinguish between bulk and interface properties and are well suited for the detection of degradation mechanisms.

Calyxo’s advanced CdTe module designed for hot climates

By Michael Bauer, CTO, Calyxo; Frank Becker, Head of Development, Calyxo; Hubert-Joachim Frenck, Head of Production, Calyxo; Jochen Fritsche, IP Management and Product Developmen, Calyxo; Kenneth Kormanyos, President of Calyxo USA, Inc.the US-based R&D Group, Calyxo

This paper presents Calyxo’s recent advances in product design that have resulted in independently confirmed peak aperture-area efficiencies of 13.4% for modules and 16.2% for cells. Some insight is given into a suitable product design for achieving the highest reliability possible, even in hot climates such as Australia, with no signs of degradation during the first three years of deployment in the field. These technical advances and the midterm production-cost target of US$0.50/Wp allow a forecast levelized cost of electricity (LCOE) of under US$0.10/KWh, especially in sunny regions of the world.

Si nanorod-based thin-film solar cells on glass

By Silke Christiansen, Max Planck Institute for the Science of Light (MPL), and Institute of Photonic Technology (IPHT),; Michael Kiometzin, Max Planck Institute for the Science of Light (MPL) and Institute of Photonic Technology (IPHT)

Advances in nanofabrication for enhancing the efficiency of optical devices, such as solar cells and photo-detectors, via nanostructuring have attracted a great deal of interest. A photoconversion strategy employing nanorods (NRs) has emerged as a powerful way of overcoming the limitations of planar wafer-based or thin-film solar cells. But there is also a broad spectrum of challenges to be tackled when it comes to putting into practice cost-effective NR solar cell concepts. ROD-SOL is a 10-partner, ‘nanotechnology for energy’ project with end-users, equipment manufacturers and institutes from six countries forming the consortium. The aim of the project is to provide the photovoltaic market with a highly efficient (> 10%), potentially low-cost, thin-film solar cell concept on glass, based on silicon nanorods. This paper presents the project’s achievements and discusses what the future might hold for nanotech-based solar energy production.

Crystalline silicon thin foils: Where crystalline quality meets thin-film processing

By Frédéric Dross, Research Engineer & Team Leader, IMEC; Kris Baert, Programme Manager of Solar Cells, SOLO Department, IMEC; Jef Poortmans, Program Director, Strategic Programme SOLAR+ and Department of Solar and Organic Technologies, IMEC

Today, crystalline-Si photovoltaics (PV) dominate the market, accounting for more than 85% of market share in 2010. A large scientific community made up of academic as well as industrial stakeholders strives to find solutions to improve device efficiencies and to drive down costs. One of the important cost elements of a module is the c-Si wafer itself. This paper discusses the fabrication of a carpet of c-Si foils on glass, either by layer transfer of an epitaxially-grown layer or by bonding of a very thin wafer, and processing this c-Si thin-foil device into a photovoltaic module. This could constitute an advantageous meet-in-the-middle strategy that benefits not only from c-Si material quality but also from thin-film processing developments.

Critical subsystems for thin-film PV manufacturing equipment

By John West

Sales of critical subsystems used in thin-film PV manufacturing equipment are expected to reach $324M in 2011, and the outlook is for this figure to grow by 3.74% in 2012 to $336M. This expectation is going against the trend for the industry as a whole, which is predicted to decline next year as revenues from cell and module manufacturing weaken. The reason for this countermovement is the opportunities available to manufacturers who are willing to invest in the latest thin-film PV equipment to drive down costs and force unprofitable competitors out of business. While the same opportunities exist for crystalline silicon manufacturing, the number of well-resourced companies signalling their intention to invest in thin-film technologies should ensure a positive year for suppliers of equipment and critical subsystems to this segment of the industry.

Plasma-enhanced chemical vapour deposition of ZnO for photovoltaic TCO application

By Jenny Schmidt; Alexander Michaelis; Isabel Kinski

In terms of material properties, plasma-enhanced chemical vapour deposition (PECVD) of ZnO has advantages over sputtering techniques, due to the variety of available precursors, and the different dopants for achieving certain levels of n-type and, controversially discussed, p-type transparent conductive oxides (TCOs) on various substrate materials. This paper considers the deposition of boron-doped zinc oxide for n-type TCO-application on substrates of dimensions up to 50×50cm2 and at a temperature range of 50 to 450°C using a PECVD reactor with a plasma frequency of 13.56MHz. The materials’ characteristics such as transparency, carrier concentration and structural properties are discussed as a function of the deposition parameters. The deposition temperature strongly affects the crystallographic and morphological appearance of the deposited thin films, which was investigated using field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD) methods. The electronic band structure-dependent characteristics were studied using ultraviolet-visible (UV-vis) spectroscopy and Hall measurements. Secondary ion mass spectrometry (SIMS) measurements complete the characterization methods for qualitatively verifying the incorporation of dopants and impurities. Results are reported for columnar-grown boron-doped ZnO with optical transparency greater than 80% in the visible range and a maximum carrier concentration of 1020cm-3.

Structure and stability of a-Si/μc-Si tandem solar cells deposited on LPCVD-grown ZnO:B and sputtered ZnO:Al

By Brad Tinkham; Clement David; Andreas Neumann; Daniel Sixtensson; Thierry Girardeau; Fabien Paumier

Because of its attractive electronic and optical properties, zinc oxide (ZnO) has found widespread use as a front and back electrode in commercial solar cells. ZnO can be deposited on glass using a variety of different methods, of which vacuum-based techniques are the most commonly used in industrial applications. Aluminium-doped sputtered ZnO:Al (AZO) has been studied intensively for use as a front contact in a-Si/μc-Si tandem cells. The implementation of AZO in series production has been hindered by reproducibility issues stemming from the combination of deposition and subsequent etching steps that are necessary to tune the ‘haze’ of the layers for optimal light scattering. Boron-doped ZnO:B (BZO), deposited by low-pressure chemical vapour deposition (LPCVD), has become a cost-effective option for module manufacturers, since the desired layer morphology can be produced as grown without the need of post-growth chemical etching. This paper addresses the different aspects of using AZO and BZO layers as front contacts for a-Si/μc-Si tandem modules fabricated in series production. The properties of the underlying ZnO layers put restrictions on the layer properties and process parameters that are used in the deposition of a-Si and μc-Si.

Polarized light metrology for thin-film photovoltaics: Manufacturing-scale processes

By Robert W. Collins, Center for Photovoltaics Innovation & Commercialization and Department of Physics & Astronomy, University of Toledo; Nikolas J. Podraza, Center for Photovoltaics Innovation & Commercialization and Department of Physics & Astronomy, University of Toledo; Lila R. Dahal, Center for Photovoltaics Innovation & Commercialization and Department of Physics & Astronomy, University of Toledo; Kenneth R. Kormanyos, President & Senior Research Fellow , Calyxo USA; Sylvain Marsillac, Department of Electrical & Computer Engineering, Old Dominion University

In situ, real-time and off-line polarization spectroscopies have been applied in studies of large-area spatial uniformity of the components of multilayer stacks in hydrogenated silicon (Si:H) and cadmium telluride (CdTe) thin-film photovoltaic (PV) technologies. Such reflection spectroscopies involve first the measurement of spectra in the reflected-to-incident polarization state ratio of the light wave (or the ellipsometry angles of the reflecting multilayer stack), and then the analysis of these spectra to determine the thicknesses and properties of component layers of the stack. In addition, expanded capabilities result from measurement/analysis of the irradiance ratio and the degree of polarization of the reflected beam, simultaneously with the polarization state ratio, particularly for rough surfaces with in-plane roughness scales of the order of the optical wavelength or greater that scatter and depolarize the light beam. This paper provides examples of 1) real-time monitoring of texture etching of the transparent conducting oxide ZnO:Al; 2) real-time monitoring and off-line mapping of roll-to-roll deposited hydrogenated amorphous silicon (a-Si:H); and 3) large-area mapping of coated glass panels used in low-cost CdTe PV technology. For a-Si:H and CdTe thin-film PV technologies, the focus is on the characterization of the window layers, which are p-type protocrystalline Si:H and n-type cadmium sulphide (CdS), respectively. Analysis of the thickness, phase and structure of the window layer material over the area of the PV panel is critical in order to design processes for uniformity of high performance. Descriptions are given of future directions in novel instrumentation development that will enable mapping for uniformity evaluation at the high speeds required for on-line analysis.

Reactive magnetron sputtering of ZnO:Al

By Bernd Szyszka, Head of the Large Area Coating Department, Fraunhofer IST; Volker Sittinger, Senior Scientist, Large Area Coating Department, Fraunhofer IST; Wilma Dewald, Junior Scientist, Magnetron Sputtering Group, Fraunhofer IST; Florian Ruske, Senior Scientist, Institute for Silicon Photovoltaics, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH

Transparent conductive oxides (TCOs), such as aluminium-doped zinc oxide (ZnO:Al), play an important role in thin-film photovoltaics. As a material for front contacts, ZnO:Al is standard in industrial-scale production, especially in the field of Cu(In,Ga)Se2 solar cells. Over the last few years, there has been a strong push to use ZnO:Al films on glass as substrates for amorphous or amorphous/microcrystalline silicon solar cells, and these films have now been introduced as an alternative to the typically used fluorine-doped tin oxide (SnO2:F) films in production. Sputtering coaters for large area deposition of ZnO:Al are widely available, and ZnO:Al films are produced in these coaters by sputtering of ceramic targets. This technology offers high process stability and is therefore favoured over reactive sputtering of metallic targets. With respect to cost and quality, however, the reactive process is an interesting alternative. In this paper we will give an overview of the process of reactive sputtering of ZnO:Al and discuss the most important insights.

Polarized light metrology for thin-film photovoltaics: research and development scale processes

By Robert W. Collins, Center for Photovoltaics Innovation & Commercialization and Department of Physics & Astronomy, University of Toledo; Jian Li, National Renewable Energy Laboratory, and Department of Electrical Engineering & Computer Science, University of Michigan; Michelle N. Sestak, Center for Photovoltaics Innovation & Commercialization and Department of Physics & Astronomy, University of Toledo; Sylvain Marsillac, Associate Professor, Department of Electrical & Computer Engineering, Old Dominion University

Optical probes based on polarized light spectroscopy, including spectroscopic ellipsometry (SE) and polarimetry, have been applied in research and process development for the three major thin-film photovoltaics technologies, including thin-film hydrogenated silicon (Si:H), cadmium telluride (CdTe), and copper indium-gallium diselenide (CuIn1−xGaxSe2). Real-time SE during materials fabrication has provided insights into the nucleation, coalescence, and structural evolution of these thin films. These insights have led, in turn, to guiding principles for PV performance optimization, as well as future directions for real-time process control. The optical properties deduced simultaneously with the layer thicknesses using real-time SE have been applied to characterize the phase composition of materials (amorphous versus crystalline), the mean free path and grain size, and the relative free carrier concentration. As a result, analytical formulae for the optical properties of PV materials have been developed with free parameters that are linked to basic materials properties. This paper shows how the formulae and associated parameter-property relationships can serve as a database for analyzing complete PV stacks, with future prospects for mapping layer thicknesses and basic materials properties in on-line monitoring applications for large-area PV plates and modules.

CdTe thin-film modules: basic developments, optimizing performance and considerations in design

By Frank Becker, Ddepartment Head of the Metallization Group, Calyxo GmbH; Hubert -Joachim Frenck, Head of Production, Calyxo GmbH

A growing number of thin-film photovoltaic module producers are either trying to keep up with the current cost leader or aiming to differentiate on product design. Calyxo is dedicated to both keeping the pace in the US$0.50/Wp race and introducing new product generations, therefore delivering more value to the customer. We have tried to improve the methodology and approaches for knowledge building in the individual process steps, by learning the relevant interactions between them, as well as ramping volume and lowering manufacturing cost in the first production line. Developing and building the deposition equipment suited to the high process temperatures of approximately 1000°C at atmospheric pressure took some time, but the technology itself now enables Calyxo to benefit from significant cost savings both on capital investment and operational cost – compared to some well-known vacuum deposition methods. Besides the continuous decrease in manufacturing costs, even early on in building the manufacturing capacity, the ability to design the product itself according to the needs of the customers proved itself to be a decisive factor in ensuring competitiveness. This paper aims to give an insight into some of the basic design features of a new product generation and how the so-called new CX3 product will generate more watts by improved performance: delivering better customer value by decreased voltage to save on BOS costs and ensuring further increased field durability through an optimized package design.

The thin-film PV equipment market

By John West, Managing Director, VLSI Research

As recently as a couple of years ago, solar panels based on thin-film manufacturing technology were being promoted as the low-cost alternative to crystalline silicon. Not only was it cheaper, but thin film also had a convincing roadmap which guaranteed this cost advantage for the foreseeable future. That was 2008, when persistently high polysilicon prices seemed inevitable as demand for solar electricity boomed. We now know that assumption to be false, and although we all knew polysilicon prices would fall eventually, no one predicted the speed and magnitude with which they crashed: in the space of several months, prices reached the point where any advantage associated with the lower materials costs of thin-film manufacturing were completely blown away.

Light trapping in nanotextured thinfilm silicon solar cells

By Rahul Dewan, Electronic Devices and Nanophotonics Laboratory, Jacobs University Bremen; Dietmar Knipp, Assistant Professor of Electrical Engineering, Jacobs University Bremen

Conversion efficiencies of thin-film silicon solar cells can be increased by nanotexturing of the cells. This nanotexturing step allows for a larger fraction of the incoming light to scatter and diffract, so that both the total absorption of light in the solar cell and the short circuit current is enhanced. In this study, we investigate the optics of thin-film silicon solar cells by numerically simulating Maxwell’s equations by a finite-difference time-domain algorithm. Starting with periodically textured solar cells, the influence of the texture period and height on the quantum efficiency and short circuit current were investigated. With this understanding of the optimized surface texture for periodically textured solar cells, the possibility of interpreting the optics of randomly textured solar cells will be discussed.

Despite multiple challenges, the maturing thin-film PV sector looks set to increase market share

By Shyam Mehta, Senior Analyst, GTM Research

The three most viable thin-film photovoltaic technologies – cadmium telluride (CdTe), copper-indium gallium (di)selenide (CIGS), and amorphous silicon (a-Si) – continue to mature and grow technologically and in market stature. But apart from the dominance shown by CdTe leader First Solar, the rest of the TFPV manufacturers have had a fairly difficult time making significant commercial inroads as the price of mainstream crystalline-silicon modules plummeted over the past couple of years. Other factors delaying the long-predicted age of thin film include bankability challenges and difficulties in reducing production and system costs. Yet entrants in all three thin-film categories have reason for optimism, as they push toward a competitive market position. This paper provides an overview of the current status of the thin-film PV sector and its players, offering insights into why certain companies might emerge successfully in the years ahead.

Controlling surface texture of sputtered ZnO:Al using different acidic singleor multi-step etches for applications in thin-film silicon solar cells

By Jorj I. Owen, IEK5-Photovoltaik, Forschungszentrum Jülich GmbH; Jürgen Hüpkes, IEK5-Photovoltaik, Forschungszentrum Jülich GmbH; Hongbing Zhu, IEK5-Photovoltaik, Forschungszentrum Jülich GmbH; Eerke Bunte, IEK5-Photovoltaik, Forschungszentrum Jülich GmbH

Magnetron-sputtered ZnO:Al is often used as a front contact in thin film silicon solar cells due to its transparent conductive oxide (TCO) properties that allow texturization by chemical etch processes to introduce light trapping. The transparency, conductivity, and surface texture after etching depend strongly on the sputtering conditions. Consequently, the typical preparation method is to find the right balance in TCO properties and light scattering, leading to a very narrow sputtering parameter window. It is preferable to separate the electro optical optimization from that of texturization to allow for a larger process window and improve ZnO:Al film properties further. This paper presents some methods of controlling the surface features using various mixtures of two step etching processes in aqueous solutions of HF and HCl. Results include methods for controlling the density of craters, texturizing compact ZnO:Al films, and fabricating novel modulated surfaces with more than one characteristic feature size. The two step etch process enables the creation of good surface textures even on high rate material that, via state of the art HCl etching, tend to lead to poor solar cell performance.