Potential-induced degradation of thin- film modules: Prediction of outdoor behaviour

By Thomas Weber, Project Manager, PI-Berlin; Juliane Berghold, Head of the PV Technology and R&D Services Business Unit, PI-Berlin

The current standards (IEC 61646 and IEC 61730-2, and IEC 62804 draft for c-Si only) are clearly insufficient to guarantee satisfactory long-term stability and energy yield for thin-film modules, given that reports from the field, as well as from laboratory test results (beyond IEC testing), in some cases show significant degradation of IEC-certified modules. Accordingly, thin-film modules can also exhibit degradation effects, such as TCO corrosion and power degradation, because of potential-induced degradation (PID). This paper presents the results obtained for thin-film modules subjected to bias and damp-heat (BDH) conditions in both indoor and outdoor tests. In order to assess module lifetimes for different thin-film technologies with respect to PID, indoor- and outdoor-determined leakage currents are compared and analysed, taking into account weather data and results from accelerated ageing tests. Finally, on the basis of simulations and investigations for different installation locations, module lifetimes are estimated and discussed.

CIGS thin-film solar cells – Breakthroughs for further efficiency improvements

By Stephan Buecheler, Laboratory for Thin Films and Photovoltaics, Empa ; Fabian Pianezzi, Laboratory for Thin Films and Photovoltaics, Empa ; Patrick Reinhard, Laboratory for Thin Films and Photovoltaics, Empa ; Enrico Avancini, Laboratory for Thin Films and Photovoltaics, Empa ; Lukas Kranz, Researcher, Laboratory for Thin Films and Photovoltaics, Empa ; Fan Fu, Laboratory for Thin Films and Photovoltaics, Empa ; Ayodhya N. Tiwari, Head of Laboratory for Thin Films and Photovoltaics, Empa

During the past two years remarkable performance improvements have been reported for polycrystalline Cu(In,Ga)Se2 (CIGS), CdTe and perovskite thin-film solar cells. In this paper the key breakthroughs in CIGS thin-film technology are reviewed and the scope for further performance improvements by analysing the stillremaining electrical and optical losses in record-efficiency CIGS solar cells is discussed. On the basis of this analysis it is believed that conversion efficiencies up to 25% are achievable with CIGS solar cells in the mid term. Furthermore, the potential for the concept of polycrystalline multi-junction solar cells to push efficiencies even further, towards 30%, is discussed. Finally, a short review of the CIGS market and an outlook from an industrial perspective are presented.

Competitiveness of CIGS technology in the light of recent PV developments - Part II: Cost-reduction potential in CIGS production

By Ilka Luck, Founding Partner, PICON Solar GmbH

A detailed analysis of state-of-the-art CIGS technology has resulted in a direct cost of ownership (CoO) of €0.44/Wp for this PV module type. However, the reduction in production costs, although impressive, is not sufficient for CIGS to become competitive with today’s c-Si technology. In order to answer the question as to whether CIGS will ever be able to challenge c-Si, the cost-reduction potential of CIGS is investigated. The impact of savings is evaluated in respect of the material segment, production equipment, energy and labour, production yield, device efficiency and absorber thickness. A total cost-reduction potential of around €0.21/ Wp is identified, which would be enough to put CIGS back into the game (the direct CoO will continue to be dominated by material and equipment depreciation, adding up to 68%). These cost reductions, however, cannot be realized immediately: within the next two years, €0.03/Wp is expected to be feasible, while it will take two to four years for the next €0.107/Wp. For the final €0.073/Wp, a time frame of at least five years is predicted, with corresponding costs for the technology developments. Provided that someone is willing to spend the necessary amount of time and money, the second part of the answer regarding CIGS’ competitiveness will depend on how c-Si evolves within this time period.

Competitiveness of CIGS technology in the light of recent PV developments – Part 1: The state of the art in CIGS production

By Ilka Luck

For some years CIGS was seen as the great white hope of the PV industry, until c-Si revealed its true competitiveness in mass production. Most companies dedicated to the commercialization of CIGS, many of which were VC financed, did not survive this development. Nonetheless, the industry has recently seen new corporate entrants with impressive plans for the roll-out of CIGS. The motives for these strategic actions are of interest, so a cost-of-ownership calculation was performed for a state-of-the-art CIGS production: the result is that current production cost for a CIGS module is €0.44/Wp, with material and depreciation being the main cost drivers. Although significant progress has been made in the last few years, this is still higher than the production costs for standard c-Si modules. However, the costs for CIGS coating materials, which correspond to the wafer in a c-Si module, are significantly lower than those for a wafer. Could this be a motive for the actions that have been witnessed in the CIGS industry? The next task would be to evaluate the further costreduction potential of CIGS and the likelihood of its realization.

Degradation studies of aluminiumdoped zinc oxide

By Mirjam Theelen, Thin Film Technology, TNO Solliance, Photovoltaic Materials and Devices, Delft University of Technology; Zeger Vroon, Thin Film Technology, TNO Solliance; Nicolas Barreau, Assistant Professor, Institut des Matériaux Jean Rouxel (IMN); Miro Zeman, Professor, Photovoltaic Materials and Devices, Delft University of Technology

This paper describes the degradation of sputtered aluminium-doped zinc oxide (ZnO:Al) layers which were exposed to damp heat (85°C/85% relative humidity). The ZnO:Al samples were characterized by electrical, compositional and optical measurements before, during and after damp heat exposure. Hall measurements showed that the carrier concentration stayed constant, while the mobility decreased and the overall resistivity thus increased. This mobility decrease can be explained by the enhancement of the potential barriers at the grain boundaries because of the occurrence of additional electron-trapping sites. X-ray diffraction (XRD) and optical measurements demonstrated that the crystal structure and transmission in the range 300 –1100nm did not change, thereby confirming that the bulk structure stayed constant. Depth profiling showed that the increase of the potential barriers was caused by the diffusion of H2O/OH- through the grain boundaries, leading to adsorption of these species or to the formation of Zn(OH)2 or similar species. Depth profiling also revealed the presence of carbon, chloride and sulphide in the top layer, which indicates the possible presence of Zn5(CO3)2(OH)6, Zn5(OH)8Cl2•H2O and Zn4SO4(OH)6•nH2O. Furthermore, white spots appeared on the ZnO:Al surface during damp heat exposure. The spots contained elements, such as silicon and calcium, which might have migrated from the glass and which reacted with species from the environment, including oxygen, carbon and chlorine.

A methodology for testing, characterization and prediction of edge seal performance in PV modules

By Ajay Saproo, Director of Reliability and Process Integration, MiaSolé; Dan Vitkavage, MiaSolé; Todd Krajewski, Process Development Manager for Flexible Modules, MiaSolé; Kedar Hardikar, Staff Scientist, MiaSolé

A critical failure mechanism of PV modules is the degradation in performance as a result of exposure to temperature and humidity during a typical product lifetime of over 25 years. The time to failure of a PV module attributable to moisture ingress under given field conditions involves multiple factors, including encapsulant and edge seal moisture barrier performance as well as the degradation rate of particular solar cells when exposed to moisture. The aim of the work presented here is to establish a conservative estimate of field lifetime by examining the time to breakthrough of moisture across the edge seal. Establishing a lifetime model for the edge seal independent of the characteristics of the encapsulant and solar cells facilitates the design optimization of the cells and encapsulant. For the accelerated testing of edge seal materials in standard temperature- and humidity-controlled chambers, a novel test configuration is proposed that is amenable to varying dimensions of the edge seal and is decoupled from encapsulated components. A theoretical framework that accounts for the presence of desiccants is developed for analyzing the moisture ingress performance of the edge seal. Also developed is an approach to analyzing test data from accelerated testing which incorporates temperature dependence of the material properties of the edge seal. The proposed equations and functional forms have been validated by demonstrating fits to experimental test data. These functional forms and equations allow the prediction of edge seal performance in field conditions characterized by historical meteorological data. In the specific case of the edge seal used in certain MiaSolé glass–glass modules, this work has confirmed that the edge seal can prevent moisture ingress well beyond the intended service lifetime in the most aggressive climate conditions evaluated.

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.