Photovoltaics International Papers

Solar-grade silicon (SoG-Si) based on metallurgical refining processes, often called upgraded metallurgical-grade silicon (UMG-Si), is expected to play an important role in achieving the solar industry’s necessary cost targets per Wp in order to compete with other energy sources. The broad term ‘UMG-Si’ currently embraces types of silicon feedstock that differ quite substantially in product quality and performance. This paper presents a summary of the work carried out by Elkem on low-cost production of silicon feedstock via a flexible, recycling metallurgical processing route with the lowest carbon footprint on the market. Results are given that qualify Elkem Solar Silicon® (ESS™) as a SoG-Si, with comparable efficiencies to polysilicon (poly-Si) from the traditional Siemens process. The latest results on the performance of modules based on ESS are reported. An indication of the stability of older modules based on SoG-Si feedstock from Elkem is also considered. On the basis of the results, there is no reason to expect modules based on ESS to differ from other commercial modules based on poly-Si. ESS is therefore shown to be a viable alternative to conventional poly-Si, but with the additional benefit of lowering specific energy use and cost per Wp.

The selective emitter (SE) concept features two different doping levels at the front surface of the cell. Both doping profiles are tailored individually to best suit their specific purposes, thus achieving both low contact resistance of the emitter electrode and low recombination in the emitter and at the Si/SiNx:H interface. This paper details the experience gained since the first tools for generating an SE structure were installed two years ago. The approach taken is discussed and a presentation given of the physical concept and properties of SE technology, along with the different aspects that have to be considered when integrating SE into an otherwise unchanged production facility.

Low-temperature thermal stresses in a manufactured photovoltaic module (PVM) based on crystalline silicon (Si), before the PVM is fastened into a metal frame, are assessed on the basis of a simple, analytical (mathematical), easy-to-use and physically meaningful predictive stress model. The PVM considered comprises the front glass, ethylene vinyl acetate (EVA) encapsulant (with silicon cells embedded into it) and a laminate backsheet. The stresses addressed include normal stresses that act in the cross sections of the constituent materials and determine their short- and long-term reliability, as well as the interfacial (shearing and peeling) stresses that affect the assembly’s ability to withstand delaminations. The interfacial stresses also determine the cohesive strength of the encapsulant material. The calculated data, based on the developed model, indicate that the induced stresses can be rather high, especially the peeling stress at the encapsulant-glass interface, so that the structural integrity of the module might be compromised, unless the appropriate design-for-reliability (DfR) measures, including stress prediction and accelerated stress testing, are taken. The authors are convinced that reliability assurance of a photovoltaic (PV) product cannot be delayed until it is manufactured – such an assurance should be considered and secured, first of all, at the design stage.

China has become the largest manufacturing base for crystalline silicon modules in the world, and is becoming increasingly reliant on a domestic supply base. This article discusses the emergence of local supply chains and the strategic responses of global suppliers to this domestic competition. It proceeds to review a set of conclusions from four case studies of formulated material supply within China that can apply to supply chain participants in the PV industry, concluding with some strategic considerations for suppliers on the cusp of entering the Chinese market.

India is a unique market. As part of an India-specific strategy on the part of the players, solar capital costs in India have significantly fallen in relation to the global average. This paper describes the trend for lower cost modules and services to be offered by module suppliers and EPC companies in order to capture the greatest share of the Indian market. In this context, more importance is being given to gaining a greater market share than earning a higher return.

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.

What does an industry need for sustainable, long-term success? A market, customers and suppliers, and – most certainly – excellent products that can be sold. When looking at various different mature industries there is one thing they all have in common – they have industry-initiated roadmaps! With SEMI’s experience in the semiconductor industry over the last 40 years, the example of the International Technology Roadmap for Semiconductors (ITRS) has proved that pre-competitive industry collaboration among the supply chain and among competitors leads to a reduction in costs, a better time to market and an increased efficiency. Moreover, it helps all players to benefit from jointly solved manufacturing challenges.

Although the different roadmaps for PV vary somewhat from each other, the bottom line always remains the same: exponential growth is predicted over the next 5 –10 years. The latest cell technologies meet the demand for grid parity even in central Europe and PV will therefore continue to be the most popular source of renewable energy. In consequence, the whole PV industry has developed from a niche product towards mass production. Every player along the entire value chain is now faced with the need to stay profitable while meeting the ever-increasing demands of the market. Implementing suitable automation can improve competitiveness and thus pave the way to becoming or remaining successful in this turbulent market.

Recently, PV demand forecasting has seen greater contributions from countries that had previously been lumped together in the rest-of-world (RoW) bucket – a category previously reserved for the collective PV demand from countries or regions outside of major (FiT-stimulated) European PV markets. Research has shown that PV adoption outside Europe will not simply increase overall PV demand levels, but will assist in smoothing out erratic demand cyclicality. At first glance, the increased gigawattage of demand being added from the RoW grouping provides an essential component in driving long-term industry growth scenarios. Non-European PV demand is forecast to increase from approximately 30% to 60% of global PV demand between 2011 and 2016. However, more tangible benefits of having an increased number of countries feeding into the global demand mix extend beyond just the significant ‘growth’ potential this situation offers to the PV supply chain. Of these various benefits, perhaps the one that will provide the greatest level of comfort to the PV supply chain will be a collective ‘smoothing’ effect in quarterly demand swings. This should have a positive effect on factory shipment schedules and hopefully provide an end to some of the boom-and-bust cycles that have negatively impacted the fortunes of the PV supply chain during 2010 and 2011.

Flexible copper-indium-gallium-(di)selenide (CIGS) absorbers offer a wide range of possible applications in rigid as well as flexible and lightweight solar module designs. The main advantage of CIGS in comparison to the well-known flexible module technology based on amorphous silicon is its currently higher efficiency and the promising optimization potential of its efficiency in the future. Because of low cell thicknesses of less than 40µm and the general sensitivity of CIGS to moisture, it is a challenge to develop suitable interconnection and encapsulation technologies that promote long-term reliability of solar modules. Selected aspects of our work in this area will be discussed in this paper.