Improvements in CdTe module reliability and long-term degradation through advances in construction and device innovation

By Imran Kahn, Integration Manager of Device Technology, First Solar; Lou Trippel, PV Module Product Line Director, First Solar; Nicholas Strevel, Technical Sales Manager, First Solar; Chad Kotarba, Engineer, First Solar

Recent advances in cadmium telluride (CdTe) research and development have improved the long-term power output degradation and extended reliability test performance of First Solar’s thin-film CdTe PV modules. This paper reviews the characterization results of the new First Solar cell structure with improved back-contact design that better manages the fundamental power-output degradation mechanism. First Solar's proprietary ‘Black’ series module construction significantly enhances the long-term durability and extended test performance of the modules. The accelerated lab-testing methods, field testing and associated analyses are discussed. These advances in the solar cell performance, coupled with upgraded module materials, further substantiate the long-term power-generating capability of First Solar's CdTe PV modules in harsh operating conditions.

CIGS manufacturing: Promises and reality

By David Jimenez, President, Wright Williams & Kelly, Inc.

Economic issues are the driving forces behind PV adoption. Even technological advances are measured against their impacts on cost per watt, levelized cost of energy (LCOE), and total cost of ownership for energy (TCOe™). This sixth paper in a series covering business analysis for PV processes looks at two approaches to manufacturing thin-film copper-indium-gallium-diselenide (CIGS) PV – sputtering and co-evaporation – and their potential areas for cost improvement.

Current topics in CIGS solar cell R&D - Part 2: Buffer layers and metastabilities in CIGS

By Niklas Papathanasiou, Head of CIGS Solar Cell Development, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH/PVcomB

This is the second part of a review article series about current topics in R&D concerning Cu(In,Ga)(Se,S)2 – or CIGS – solar cells. In the first part, which appeared in the previous edition of Photovoltaics International, the focus was on CIGS absorber layer formation. This second part will discuss another essential part of CIGS solar cells – the buffer layer – in conjunction with metastabilities in these types of cell.

Current topics in CIGS solar cell R&D: Overcoming hurdles in mass production

By Niklas Papathanasiou, Head of CIGS Solar Cell Development, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH/PVcomB

Since the demonstration of the first CuInSe2 solar cell in 1974 by scientists at Bell Laboratories, a lot of effort has been put into the development of cost-effective processes for highly efficient Cu(In,Ga)(Se,S)2 – or CIGS – solar cell devices. In 2012 these efforts led to the first gigawatt CIGS solar module production facility operated by Solar Frontier, a company that has a long history in R&D and originates from ARCO Solar, who developed the first commercial CIGS solar modules at the beginning of the 1990s. However, several start-up companies employing CIGS technology are presently struggling in the currently harsh market environment. Even though world-record laboratory solar cells now demonstrate 20.3% efficiency using a three-stage co-evaporation process, and full-size modules achieve 14.6% employing a similar method, efforts in research and development are more important than ever in order to increase cell efficiency, to bridge the gap between cell and module efficiencies, and to develop cost-effective and robust manufacturing processes. This paper gives an overview of current research topics under investigation by research institutes and industry, with a main focus on CIGS absorber formation. Along with other research results published by groups all over the world, this paper covers recent research results obtained at the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) and briefly mentions the work of the Photovoltaic Competence Center Berlin (PVcomB), a joint initiative of the Technical University of Berlin (TU Berlin) and HZB.

Baseline meets innovation: Technology transfer for high-efficiency thin-film Si and CIGS modules at PVcomB

By Björn Rau, Technology Manager / Deputy Director, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH/PVcomB; Felice Friedrich, Head of Analytics Group, Technical University Berlin/PVcomB; Niklas Papathanasiou, Head of CIGS Solar Cell Development, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH/PVcomB; Christof Schultz, Engineer, HTW Laser Research Group, University of Applied Sciences Berlin (HTW)/PVcomB; Bernd Stannowski, Head of TF Si R&D Group, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH/PVcomB; Bernd Szyszka, Professor, Technical University Berlin/PVcomB; Rutger Schlatmann, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH/PVcomB, Director, University of Applied Sciences Berlin (HTW)/PVcomB

Thin-film PV modules are one of the most sustainable options for the generation of electricity, with low material consumption and short energy-payback times. Both of these factors are essential for paving the way towards a terawatt PV market. However, the cost-competitive production of PV modules has become extremely difficult, and module producers are facing huge challenges. A rapid technology transfer from research to industry is therefore required in order to introduce innovations for lower production costs and higher conversion efficiencies. At the Competence Centre Thin-Film- and Nanotechnology for Photovoltaics Berlin (PVcomB), founded by the Helmholtz-Zentrum Berlin (HZB) and the Technical University Berlin, two R&D lines for 30 x 30cm2 modules based on thin-film silicon and copper indium gallium (di)selenide (CIGS) respectively are operated. Robust baseline processes on a high efficiency level, combined with advanced process and device analytics, have been established as a basis for the introduction and development of further innovative technology steps, and their transfer to industry.

Laser structuring of thin films for flexible CIGS solar cells

By Gediminas Račiukaitis, Department of Laser Technologies, Center for Physical Sciences and Technology (CPST); Simonas Grubinskas, Department of Laser Technologies, Center for Physical Sciences and Technology (CPST); Paulius Gečys, Research Fellow, Department of Laser Technologies, Center for Physical Sciences and Technology (CPST); Klaus Zimmer, Senior Scientist and Group Leader, Leibniz Institute of Surface Modification; Martin Ehrhardt, Leibniz Institute of Surface Modification; Anja Wehrmann, Leibniz Institute of Surface Modification; Alexander Braun, CTO, Solarion AG

Thin-film solar cells (TFSCs) still hold unlocked potential for achieving both high efficiency and low manufacturing costs. The formation of integrated interconnects is a useful way of maintaining high efficiency in small-scale solar cells by their connection in series to form a module. Laser scribing is widely used for scribing a-Si- and CdTe-based TFSCs to form interconnects. The optical properties of the ternary copper-indium-gallium (di)selenide (CIGS) compound are well suited to the solar spectrum, with the potential to achieve a high photoelectrical efficiency. However, since it is a thermally sensitive material, new approaches for the laser-scribing process are required, to eliminate any remaining heating effects. For flexible CIGS solar cells on non-transparent substrates (metal foils or polymer), the scribing process faces additional challenges. This is one reason why ultrashort laser pulses yield better results in terms of scribing quality and selectivity. The modelling of laser energy coupling and an extensive characterization of laser scribes allow approaches to be developed for laser scribing of CIGS solar cells on flexible polymer substrates. The measured high efficiency of the resulting high-speed laser-scribed, integrated CIGS mini-modules proved the capability of this approach.

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.