Innovation for optical, electrical and economic improvement of thin-film PV technology

By Joop van Deelen & Marco Barink, TNO

Innovation in the field of thin-film cells, in addition to economy of scale and the manufacturing learning curve, is an important element in keeping the price of this technology competitive. Most papers on these cells focus on their technology; however, the economic potential of the technology is also important. Of even greater significance, a realistic estimation of the potential, along with the associated costs, of advanced technology, is part of the equation for profitability. Two examples of technology – metallic grids and texturing – are given in this paper; the designs are discussed, and a brief economic analysis is presented for various scenarios of the technologies. Although the profitability of these technologies can be considerable, it is shown that one should be wary of basing decisions purely on potential and on ideal scenarios, and how the cost of a technology can turn a great prospect into a trade-off.

First Solar goes back to the future

By Mark Osborne, senior news editor, Photovoltaics International

Leading CdTe thin-film module producer First Solar is shifting it business emphasis back to module sales after becoming a leading PV project developer as part of a mid-term business plan that takes advantage of its restored cost-per-watt advantage and two new module products that will be introduced in the coming years that are intended to further its competitive position. We analyze the key metrics behind the transition, such as R&D expenditure, module conversion efficiencies and production capabilities and cost reductions.

Solar Frontier in talks over CIS thin-film production plant in Saudi Arabia

By Mark Osborne, Senior News Editor

CIS thin-film module manufacturer Solar Frontier has signed a memorandum of understanding with Saudi Aramco and the Saudi Arabian National Industrial Cluster Development Program (NICDP) on the feasibility of establishing a thin-film module production plant in Saudi Arabia.

Significant progress in CIGS thin-film solar cell technology reported at IWCIGSTech7

By Rutger Schlatmann & Hans Werner Schock, Helmholtz-Zentrum Berlin für Materialien und Energie, Michael Powalla, Centre for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW)

An efficient exchange of knowledge is essential to move the technology forward. In June 2016, the IW-CIGSTech workshop was organized for the seventh consecutive year in a row. This time, the event took place as a parallel event to the EU-PVSEC/Intersolar Europe in Munich. In the workshop, representatives from industry and academia gathered to discuss the latest developments in the fast-developing field of CIGS (Cu(In,Ga)(Se,S)2) based solar cells. As a result of last year’s workshop, a joint, community-wide effort resulted in the broadly acknowledged “White Paper for CIGS thin film solar cell technology”. In this article, we provide a brief impression of the progress and challenges reported in this year’s workshop.

Damp-heat-induced degradation of layers in CIGS solar cells

By Mirjam Theelen, TNO/Solliance

Investors require a guarantee of a minimum lifetime for PV installations. It is tempting to provide such a guarantee for a longer lifetime simply by specifying test conditions that are more and more severe. In this paper it is argued that, with a more detailed understanding of the basic mechanisms determining cell material behaviour under specific exposure conditions, not only can the inherent lifetime of solar cells and modules be improved, but also the predictive value and effectiveness of lifetime testing. An overview of the literature contributions regarding the influence of damp-heat exposure of the layers in Cu(In,Ga)Se2 (CIGS) solar cells is presented.

Predicting moisture-induced degradation of flexible PV modules in the field

By Kedar Hardikar, Todd Krajewski & Kris Toivola, MiaSolé

A critical failure mechanism for PV modules is the degradation in performance as a result of exposure to temperature and humidity. In the case of flexible PV modules, moisture-induced damage becomes a greater concern, since the moisture resistance of barriers and polymer packaging is expected to be lower than that for conventional glass–glass PV products. The work presented here is aimed at establishing, through the use of accelerated testing, the field lifetime of flexible PV modules with regard to moisture-induced degradation.

Fabrication of high-power CIGS modules by two-stage processing, and analysis of the manufacturing cost

By Kyung Nam Kim, Green School, Korea University, Seoul; Yoonmook Kang, Green School, Korea University, Seoul; Jeong Min Lee, Wonik IPS, Gyeonggi-Do, South Korea; Dong Seop Kim, Wonik IPS, Gyeonggi-Do, South Korea

Of the various copper indium gallium diselenide (CIGS)-formation processes, a so-called ‘two-stage process’, consisting of sputtering and selenization, has been successfully applied in large-scale production thanks to its stable process scheme and high-fidelity production equipment. A CIGS module with a power of 231W, corresponding to a total area-based efficiency of 16% for 902mm × 1,602mm, was demonstrated when this twostage process was employed in a pilot production line at Samsung (although all the technology concerning CIGS production has now been transferred to Wonik IPS, whose main business is to provide production equipment for the semiconductor and display industry). The high-power module suggests significant potential for CIGS modules to compete with multicrystalline Si modules in terms of both cost and performance. This paper addresses the important process technologies for achieving high efficiency on large-area substrates, and presents a cost analysis using the data obtained from the operation of the pilot production line. As a result of the synergistic effect of low material cost and high efficiency of the two-stage process, the CIGS manufacturing cost is expected to be reduced to US$0.34/W.

Closing the gap with silicon-waferbased technologies: Alkali postdeposition treatment improves the efficiency of Cu(In,Ga)Se2 solar cells

By Oliver Kiowski, Centre for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW), Stuttgart, Germany; Theresa M. Friedlmeier, Centre for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW), Stuttgart, Germany; Roland Würz, Centre for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW), Stuttgart, Germany; Philip Jackson, Centre for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW), Stuttgart, Germany; Dimitrios Hariskos, Centre for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW), Stuttgart, Germany

With the introduction of the alkali post-deposition treatment (PDT) for the absorber layer in Cu(In,Ga)Se2 (CIGS)-based solar cells, new efficiency records approaching 22% have become feasible. After gallium incorporation, sodium doping and the three-stage process, this is the next milestone on the CIGS roadmap. In this paper the current understanding of how PDT alters the CIGS surface and affects device parameters is illustrated. A comparative study of cell device parameters from ZSW and the evolution of efficiencies from other institutes and companies with and without PDT is presented.

Routes to increasing efficiency and reducing the cost of thin-film solar panels

By Joop van Deelen, TNO, Eindhoven, The Netherlands; Niels van Loon, TNO, Eindhoven, The Netherlands; Marco Barink, TNO, Eindhoven, The Netherlands; Marieke Burghoorn, TNO, Eindhoven, The Netherlands; Zeger Vroon, TNO, Eindhoven, The Netherlands; Zuyd Hogeschool, Heerlen, The Netherlands; Pascal Buskens, TNO, Eindhoven, The Netherlands; DWI – Leibniz Institute for Interactive Materials, Aachen, Germany

Most development work in the laboratory is dedicated to efficiency enhancements at the cell level; improvements in efficiency can lead to higher cost-competitiveness of PV. However, the cost of panel manufacturing is an important aspect as well. For CIGS panels the deposition of the active layer is an important part of the cost, and decreasing the layer thickness can reduce costs. Moreover, cost of ownership calculations can determine how much benefit can be expected from thinner absorber layers from a cost perspective; clearly, a thinner absorber will result in reduced absorption. To avoid losses, modelling can be used to predict the efficiency and viable light management strategies. Other efficiency-enhancing technology is related to the fact that most thin-film solar panels are monolithically interconnected. The area loss involved in this type of interconnection, and the trade-off between conductivity and transmittance of the front contact, impose limits on the maximum efficiency. The impact of improving both of these aspects is demonstrated in this paper. A viable way to improve the front contact is by supplementing the front contact with a metallic pattern. The benefit and the impact of different configurations and dimensions of the cell and metallic pattern are presented.

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.