PVI Issue

Photovoltaics International Volume 15

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The 15th edition of Photovoltaics International hopes to bring some optimism to 2012 by tackling the key factors on how to cope with the current situation. Contributions come from MIT on using TCAD as a viable method for modelling metal impurity evolution; Alternative Energy Investing presents a comprehensive look at materials cost; and efficiency improvements are on offer by ECN. REC Solar and Tata BP Solar report on module lifetime and IMS Research gives us a brief rundown of 2012 global market expectations.

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In this issue...

PVI Paper
Photovoltaics International Papers, Power Generation
In trying to introduce its relatively new technology to traditional utility customers, the photovoltaic industry often finds itself in the awkward position of trying to sell a product to a customer who may not want to buy. The up-front capital costs of new solar plants (that deliver power only intermittently) can be less than appealing. Large-scale grid integration will therefore be accelerated by PV technologies that best fit the profile of traditional power sources. In addition to low cost, this includes high capacity factors and the ability to better match demand during daylight hours. Concentrator photovoltaic (CPV) power plants are now being integrated into the grid at megawatt scales. By performing light collection using acrylic, silicone, or glass optics instead of semiconductors, the material cost balance of PV is fundamentally shifted. The world’s most efficient solar cells can then be employed, and maintaining tracking of the sun becomes economically favorable across vast sunny locales worldwide. With AC system efficiencies in excess of 25%, the resulting CPV power plants produce high energy yields throughout the year and deliver the high capacity factors demanded by utility customers. Since semiconductors are a minority component cost, manufacturing capital costs are lower than for any other PV technology, allowing for rapid scale-up and field deployment. This article will describe the state of the art of CPV technology, field performance results, and the outlook for near-term deployments.
PVI Paper
Photovoltaics International Papers, PV Modules
This paper presents a novel glue-membrane integrated backsheet specifically for PV modules, which has been designed and fabricated by utilizing a flow-tangent cast roll-to-roll coating process combined with a plasma technique. Polyethylene terephthalate (PET) is adopted as a substrate and is surface activated and etched by atmospheric plasma. Then a special coating formulation containing reactive fluoropolymers is applied to both sides of the PET, followed by thermal curing, resulting in a glue-membrane integrated coating layer with a polyurethane structure. Finally, a monolayer of silane molecules is grafted onto the surface via plasma-enhanced deposition to provide the surface medium with surface energy, rendering excellent long-term adhesion to ethylene vinyl acetate (EVA). Scanning electron microscope (SEM) images have revealed that plasma etching and activation significantly improves compatibility between the PET and the coating layer, resulting in a tight and strong integration between the two. It has also been confirmed by SEM that the obtained novel backsheet integrates the glue layer and the membrane layer perfectly. There is no clear boundary between the two layers, distinguishing the novel backsheet from the conventional layer-by-layer laminated backsheet. The unique glue-membrane integrated structure has already been demonstrated by many practical applications under harsh environmental conditions to have significant advantages over other backsheets regarding delamination, blistering and discoloration. Furthermore, the novel backsheets showed excellent barrier properties, weatherability (85°C, 85% RH, 1000h), mechanical properties and electrical isolation properties. Because it is a promising photovoltaic material, the novel backsheet has already been widely used in China for PV module encapsulation and has obtained extensive praise from customers.
PVI Paper
Photovoltaics International Papers, Thin Film
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.
PVI Paper
Photovoltaics International Papers, Thin Film
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.
PVI Paper
Materials, Photovoltaics International Papers
Most high-efficiency solar cells are fabricated from monocrystalline Czochralski silicon (Cz-Si) wafers because the material quality is higher than multicrystalline silicon (mc-Si) wafers. However, the material study presented in this paper reveals strong variations in the material quality of commercially available Cz-Si wafers, leading to a loss in solar cell efficiency of 4% absolute. The reason for this is the presence of defects, which appear as dark rings in photoluminescence (PL) images of the finished solar cells. It is shown that these efficiency-limiting defects originate from oxygen precipitation during emitter diffusion. It is demonstrated that an incoming inspection in the as-cut state is difficult, as strong ring structures in as-cut wafers turn out to originate most often from thermal donors. These are dissolved during high-temperature treatments and are therefore harmless, whereas moderate ring structures in the as-cut state may become severe. That is why critical wafers can be identified and sorted out reliably only after emitter diffusion, by using QSSPC-based lifetime measurements or PL imaging. The two-year statistics gathered from the research line at Fraunhofer ISE on the occurrence of ring defects in Cz-Si wafers indicate that ring defects are highly relevant in terms of material yield.
PVI Paper
Photovoltaics International Papers, PV Modules
Non-destructive methods for measuring photovoltaic modules are discussed in this paper, with the aim of comparing different quality-assurance methods for different module technologies (e.g. crystalline and thin-film technologies: a-Si, CdTe, CIS). For a complete quality control of PV modules, a combination of fast and non-destructive methods was investigated. In particular, camera-based measurements, such as electroluminescence (EL) and infrared (IR) technologies, offer excellent possibilities for determining production failures or defects in solar modules, which cannot be detected by means of standard power measurements. These methods are applied effectively in quality control and development support, and EL is already an important characterization tool in industry and research. Most short circuits reduce the voltage in their surrounding area and appear dark in EL images. However, as this failure is not always critical and apparent, short circuits are only precisely identifiable in combination with IR measurements. Therefore, to quickly detect at high resolution the most common defects in a PV module, a combination of EL and IR measurements is advisable.
PVI Paper
Fab & Facilities, Photovoltaics International Papers
As a relative newcomer to the industrial world compared to more mature manufacturing sectors, the PV industry has not yet been subject to consistent environmental regulatory standards internationally. Like all industries that have preceded it, PV manufacturing is seeing its regulatory future evolve as PV producers migrate to different regions of the world. With this global expansion come significantly different levels of regulatory stringency, reflective of local conditions and cultures.
PVI Paper
Cell Processing, Photovoltaics International Papers
This paper presents examples of recent process developments at ECN in silicon solar cells on n-type monocrystalline base material. For all PV manufacturers, the challenge is to increase module efficiencies while maintaining low production cost. An effective way to move to higher and more stable efficiencies, using low-cost industrial-type processing, is n-type solar cell technology. The solar cell considered in this paper is the n-pasha cell – a bifacial solar cell with homogeneous diffusions and screen-printed metallization. The n-pasha cell is currently produced on an industrial scale by Yingli Solar; in 2011 a maximum solar cell conversion efficiency of 19.97% was obtained using this cell concept on 239cm2 n-type Cz at the ECN laboratory. The focus of the paper will be increasing efficiency by optimization of the cell process, in particular the front-side metallization, and by improvements to the rear-surface passivation. These two steps have contributed an increase in efficiency of 0.8%, allowing cell efficiencies of 20% to be reached.
PVI Paper
Cell Processing, Photovoltaics International Papers
Coupled device and process simulation tools, collectively known as technology computer-aided design (TCAD), have been used in the integrated circuit industry for over 30 years. These tools allow researchers to quickly home in on optimized device designs and manufacturing processes with minimal experimental expenditures. The PV industry has been slower to adopt these tools, but is quickly developing competency in using them. This paper introduces a predictive defect engineering paradigm and simulation tool, while demonstrating its effectiveness at increasing the performance and throughput of current industrial processes. The impurity-to-efficiency (I2E) simulator is a coupled process and device simulation tool that links wafer material purity, processing parameters and cell design to device performance. The tool has been validated with experimental data and used successfully with partners in industry. The simulator has also been deployed in a free web-accessible applet, which is available for use by the industrial and academic communities.
PVI Paper
Photovoltaics International Papers, PV Modules
Crystalline silicon solar modules installed in the field are exposed to atmospheric conditions and experience stress, which induces a wear-out phenomenon in various parts of the modules and degrades performance over time. The performance eventually reaches a point where the output power falls below an acceptable level. Thermal cycling (TC) and damp heat (DH) are two important reliability tests for estimating infant failures related to materials and the manufacturing process, as well as providing the information on performance degradation with respect to time. In this study, modules composed of 156mm × 156mm multicrystalline silicon cells were subjected to TC and DH tests. By applying acceleration models, such as the Norris-Landzberg model for TC and the Hallberg-Peck model for DH, the minimum guaranteed life was calculated. The electrical and reliability results were interpreted and explained on the basis of the respective models.
PVI Paper
Materials, Photovoltaics International Papers
The purpose of this paper is to give an overview of the use and potential of diamond wire for the silicon-shaping process in the PV industry. The current market and future prospects for helping to meet the goals of 2020’s roadmap of thinner wafers and reduced $/W are described.
PVI Paper
Market Watch, Photovoltaics International Papers
Predicting what will happen to the global PV market is very nearly an impossible task. Its underlying principles are very similar to the dozens of other electronics markets that IMS Research studies, but the key difference in the PV industry is the very close link to, and ultimate dependence on, government policy. In a few years’ time, the introduction, halting or change (or rumoured change) of a single government’s PV policy will have little effect on the global industry, and the huge swings in demand will be less common and less severe. The reasons for this are clear. First, because of geographic diversification in the industry, a single country will account for a smaller portion of the global total (unlike in 2011, when Germany and Italy accounted for more than half of global demand) and thus individual governments’ policy changes will have a smaller impact. Second, if system prices continue to drop rapidly (and IMS Research believes they will), a growing number of regions will achieve the ‘holy grail’ of grid parity and will thus no longer depend solely on government policy to drive their markets.
PVI Paper
Market Watch, Photovoltaics International Papers
The solar photovoltaics market in the United Kingdom was virtually non-existent until April 2010, when the long-awaited feed-in tariff scheme was implemented. Yet, despite coming late to the game, the UK’s solar industry took off immediately, installing more than 80MW in the first 12 months alone. Now, just two years down the line, the market is placed as the world’s eighth largest. This paper will take a look back at how the UK got to this point as well as considering just how bright the future of this fast-paced market will realistically be.
PVI Paper
Cell Processing, Photovoltaics International Papers
In the photovoltaics industry, contacts to crystalline silicon are typically formed by the firing of screen-printed metal pastes. However, the stability of dielectric surface passivation layers during the high-temperature contact formation has turned out to be a major challenge for some of the best passivating layers, such as intrinsic amorphous silicon. Capping of well-passivating dielectric layers by hydrogen-rich silicon nitride (SiNx), however, has been demonstrated to improve the thermal stability, an effect which can be attributed to the atomic hydrogen (H) diffusing out of the interface during firing, and passivating dangling bonds. This paper presents the results of investigations into the influence of two different dielectric passivation stacks on the firing stability, namely SiNy/SiNx (y < x) and Al2O3/SiNx stacks. Excellent firing stability was demonstrated for both stack systems. Effective surface recombination velocities of < 10cm/s were measured after a conventional co firing process on 1.5Ωcm p-type float-zone silicon wafers for both passivation schemes. On the solar cell level, however, better results were obtained using the Al2O3/SiNx stack, where an efficiency of 19.5% was achieved for a large-area screen-printed solar cell fabricated on conventional Czochralski-grown silicon.
PVI Paper
Fab & Facilities, Photovoltaics International Papers
Several PV module producers have performed a carbon footprint analysis and published a sustainability report as part of their corporate social responsibility policy. Comparison of carbon footprint results is difficult because several international standards and life cycle assessment (LCA) databases are used. No product footprint category rules (PFCR) or product category rules (PCRs) for photovoltaics exist, so LCAs are performed with varying underlying assumptions. Furthermore, a fair comparison can only be made when all environmental footprints of a product are taken into account.
PVI Paper
Materials, Photovoltaics International Papers
The PV industry is undergoing dramatic changes. Like a carnival ride gone dreadfully wrong, exhilaration has been supplanted by dread; joy has been replaced by fear. Just look around you – provided you are able to turn your head to defy the g-forces acting upon you as we bank and turn wildly along. You will see PV companies closing their doors for good. You will see extraordinarily talented people throughout the supply chain, shifting positions everywhere and looking for safe-haven jobs. And you will also see once-leading PV companies burning cash and losing their status as ‘bankable’. Everywhere we turn, we see companies in the supply chain shuttering production as if to balance markets.
PVI Paper
Photovoltaics International Papers, PV Modules
The improved performance and reduced manufacturing costs of photovoltaic (PV) modules that have been achieved in recent years have positioned this technology as an economically attractive renewable electric energy source. In order to verify that this also has a positive impact on energy payback time (EPBT) and carbon footprint, the Energy Research Centre of the Netherlands (ECN) has conducted a life cycle analysis (LCA) for REC Peak Energy-series PV modules produced by Renewable Energy Corporation (REC). The LCA study was based on a full set of actual production data obtained for the first quarter of 2011 from REC’s manufacturing sites. Because REC is an integrated manufacturer, the LCA study includes internal data for the production steps from polysilicon production to module assembly, as well as for all materials and transportation associated with production. ECN used generic figures for installation, operations and recycling together with the REC data to assess the environmental impact indicators. For polysilicon produced in the USA, and for wafers, cells and modules produced in Singapore, an EPBT of 1.2 years was achieved, with a corresponding carbon footprint of 21g CO2-eq/kWh for PV systems located in southern Europe (1700kWh/m2year irradiation). For modules with wafers and cells produced in Norway, the corresponding values were 1.1 years and 18g CO2-eq/kWh. A key contributor in achieving these values is REC’s highly efficient fluidized bed reactor (FBR) process for the production of polysilicon.
PVI Paper
Photovoltaics International Papers, Power Generation
As yet, procedures for long-term tests of photovoltaic modules in outdoor conditions have not been considered by international standardization committees. Although many laboratories perform long-term PV outdoor tests, a commonly agreed and standardized procedure has so far not been adopted. The European Distributed Energy Resources Laboratories’ (DERlab) approach to filling the gap of international standardization has led to the development of a basic protocol that complies with European and international standards, while providing specific common guidelines and procedures for measuring the energy yield of PV modules for at least one year in outdoor conditions. The DERlab procedures for long-term PV module testing are described in this paper, and the range of analyses that can be derived from the data, such as module degradation, are discussed. The paper also presents the DERlab approach to measuring module performance in outdoor conditions, which can be used to complement energy-rating methods suggested in international standards. DERlab has created consistent measuring procedures that allow the direct comparison of the energy yield of solar modules taking into account the site-specific factors of different locations and varying climatic conditions, as well as a maintenance guideline.

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