PVI Issue

Photovoltaics International Volume 21

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There have been encouraging signs in recent months of changing fortunes for PV equipment suppliers after a difficult period of consolidation. Shipment figures, actual and forecast, have in many instances seen an upswing, as booming markets in Japan, China and the US continue to drive demand, even as some European markets continue to dwindle. It’s probably too early to call the beginnings of a new PV technology buy cycle, but it seems more a case of ‘when’ rather than ‘if ’ now, and analysts have pointed to mid-2014 as the likely point when supply and demand will be in some kind of equilibrium. Clearly the implication of this is that if demand continues to rise beyond this point, supply will have to keep up, so manufacturers will have to invest in new capacity.

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In this issue...

PVI Paper
Photovoltaics International Papers, Power Generation
Beyond lowering energy costs and demand charges, Superstorm Sandy demonstrated the frailty of centralized power generation. Building owners/operators throughout the Northeast in the USA are evaluating distributed power generation options for supporting building-critical loads during future grid outages. Those options (many of which also incorporate commercial-scale grid storage solutions) include on-site diesel generators, micro-turbines, fuel cells and solar PV systems. As electrical vehicle (EV) charging is added to the mix, the grid requirements and demand costs will further increase. This article will discuss specific value streams for integrating energy storage with PV for commercial buildings, and technologies - specifically, advanced power converters – that will enable those benefits to be achieved.
PVI Paper
Photovoltaics International Papers, PV Modules
As part of the European FP7 R&D project ‘Cu-PV’, the compatibility of copper-electroplated metal wrapthrough (MWT) cells with conductive adhesives has been investigated. The objectives of this project include to reduce, by the use of copper plating, the amount of silver utilized in cell manufacturing, and to demonstrate the compatibility of high-power n-type back-contact module technology with copper-plated cells. The overall goal is to reduce the impact on the environment of cell and module manufacture. MWT module technology as developed by ECN uses conductive adhesive to make the interconnection between cells and a conductive backsheet foil. These adhesives have been proved to result in very reliable modules in the case of cells with fired silver metallization. To determine the compatibility of conductive adhesive with copper-plated cells, component tests were performed, followed by the manufacture of modules with copperplated cells and conductive adhesive interconnections. Climate chamber testing of these modules showed that the adhesive is compatible with the copper-plated cells. The next steps include further optimization of the plating process and additional testing at the module level.
PVI Paper
Materials, Photovoltaics International Papers
Electrically conductive adhesives (ECAs) are an alternative interconnection technology especially suited to high-efficiency cell concepts with new contact structures. This paper describes the basic principles of this emerging interconnection technology and discusses the different material types on the market. Mechanical and electrical characterization methods for conductive adhesives are also presented. Results are included from peel tests, volume and contact resistivity measurements, metallographic investigations, dynamic mechanical analysis and differential scanning calorimetry. Finally, a novel simulation approach for the cure kinetics of ECAs and arbitrary temperature profiles is briefly described and demonstrated by an example of an epoxy adhesive cure.
PVI Paper
Materials, Photovoltaics International Papers
Future high efficiency silicon solar cells are expected to be based on n-type monocrystalline wafers. Cell and module photovoltaic conversion efficiency increases are required to contribute to lower cost per watt peak and to reduce balance of systems cost. Past barriers to adoption of n-type silicon cells by a broad base of cell and module suppliers include the higher cost to manufacture a p-type emitter junction and the higher cost of the n-type mono silicon crystal. Technologies to reduce the cost of manufacturing the p-type emitter by diffusion or implantation of boron are being developed in the industry. To reduce the cost and improve further the quality of n-type mono silicon crystal, SunEdison has developed a continuous Czochralski (CCZ) crystal pulling process, based on the technology of Solaicx, acquired in 2010. This CCZ technique allows production of a crystal with much greater resistivity uniformity, with a lower incorporation rate of lifetimereducing metals impurities, and allows crystal oxygen to be selected independent of production batch size. CCZ is expected to reduce n-type crystal cost below that of current p-type mono crystal.
PVI Paper
Cell Processing, Photovoltaics International Papers
This paper presents the status of imec’s work on the use of copper for the main conductor as an alternative to screen-printed silver front contacts in solar cells. This work is motivated not only by the limitations that Ag screen-printed contacts have regarding solar cell efficiency (high contact shading, limited line conductivity, and poor contact resistance to moderately doped emitters), but also by the PV industry's desire to reduce Ag usage for reasons of cost. Despite the potential advantages of Ni/Cu contacts, their commercialization has been limited because of increased process complexity and doubts over the €/Wp advantage and long-term reliability. These three factors all depend on the specific process and toolset and are discussed in this paper. A relatively simple process sequence is described that uses industrial pilot-line tools and consists of: 1) defining the front-contact pattern by ps-UV laser ablation; 2) self-aligned plating of the contacts using Ni/Cu/Ag; and, finally, 3) sintering in N2 for nickel silicidation. The process sequence is applied to 15.6 x 15.6cm2 p-type CZ-Si PERC (passivated emitter and rear cell) solar cells with 120Ω/sq. homogeneous emitters; average cell efficiencies of 20.5% are achieved over more than 100 cells. Cost analysis results are then discussed, indicating that this Ni/Cu process sequence has a lower cost/piece than equivalent screen-printed PERC cells while also providing ~0.5% abs. higher cell efficiency. Thermal-cycling and damp-heat reliability data that meet extended (1.5 x) IEC 61215 criteria for singlecell laminates and small modules are reported. The improved efficiency potential of applying this metallization sequence to rear-junction n-type PERT (passivated emitter and rear totally diffused) cells is discussed and preliminary results are given.
PVI Paper
Fab & Facilities, Photovoltaics International Papers
For more than a decade, the growth in PV markets surpassed expectations. Then, in 2012, the European market declined for the first time compared with the previous year. As policymakers’ support for PV hesitates over the costs to society of this technology, it is timely to take an overview of the social costs and benefits, also referred to as the ‘external costs’, of PV electricity. In this article, these costs are put into perspective visà- vis those associated with conventional electricity-generating technologies. The external costs of electricity can be broken down into: 1) the environmental and health costs; 2) the costs of subsidies and energy security; and 3) the costs for grid expansion and reliability. Included in these costs are the increased insurance, health, social and environmental costs associated with damages to health, infrastructure and environment, as well as tax payments that subsidize producers of electricity or fuels, their markets and the electricity infrastructure. A life cycle assessment (LCA) of the environmental impact is used in the quantification of the associated environmental and health costs. Because the environmental footprint of PV electricity is highly dependent on the electricity mix used in PV module fabrication, the environmental indicators are calculated for PV electricity manufactured using different electricity mixes, and compared with those for the European electricity mix (UCTE), and electricity generated by burning 100% coal or 100% natural gas. In 2012$, coal electricity requires 19–29¢/kWh above the market price, compared with 1–1.6¢/kWh for PV manufactured with 100% coal electricity. The sum of the subsidies, avoided fossil-fuel imports and energy security, and the economic stimulation associated with PV electricity deployment, amounts to net external benefits. Integrating high penetrations of renewables, with the same reliability as we have today, appears to be fully feasible and within the cost horizons of the current activities of system operators.
PVI Paper
Photovoltaics International Papers, Thin Film
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.
PVI Paper
Photovoltaics International Papers, PV Modules
The aim of this paper is to shed some light on what difference the quality of a PV product makes to the customer and how much effort is required to deliver it. From the customer’s point of view, the quality of a PV product is key to a worthwhile investment, since the value of a PV system is defined by its cost compared with its yield over the entire lifetime, or the levelized cost of electricity (LCOE). But while many manufacturers make more or less the same promises, in this paper a closer look is taken at what is really involved in living up to those promises. If quality is understood to be a fundamental attitude that is reflected in every single process along the entire value chain, only then will this eventually lead to high-quality products and services. The paper discusses in detail the principles, methods, tests and processes required to secure a superior quality brand.
PVI Paper
Cell Processing, Photovoltaics International Papers
Passivated emitter and rear cells (PERC) are considered to be a next generation of industrial solar cells, and several companies have already started pilot production. The much-reduced rear-surface recombination in PERC cells requires improvements to the front side, for example the emitter, in order to further increase the conversion efficiency in the future. This paper presents an evaluation of the emitter technologies of three industrially applicable PERC cell concepts: 1) with an ion-implanted emitter, 2) with a chemically polished rear surface, and 3) with a selective emitter formed by gas phase etch-back (GEB). The results are compared with a reference high-efficiency POCl3-diffused PERC cell. The three industrial PERC concepts utilize lean industrially applicable process flows which reduce the phosphorus concentration at the wafer surface. Accordingly, when compared with the POCl3-diffused emitter, the ion-implanted and GEB emitters obtain significantly lower emitter saturation current densities of 40 to 60fA/cm2 for emitter sheet resistances of 90 to 130Ω/sq. When applied to large-area PERC cells with screen-printed metal contacts, the ion-implanted and GEB emitter cells demonstrate up to 10mV higher open-circuit voltages than the POCl3-diffused reference PERC cell, and achieve conversion efficiencies of 20.0 and 20.3%, respectively. The next steps in further increasing the efficiency are outlined.
PVI Paper
Cell Processing, Photovoltaics International Papers
According to the ITRPV (International Roadmap for PV), a large fraction of future solar cells will be n-type and rear-contact cells with the highest efficiencies and fabricated using low-cost processes. As the standard p-type silicon solar cell in mass production is completely optimized and has therefore reached its cost limit, it is currently very difficult for new solar cell concepts to be cost effective from the outset when introduced into production. Consequently, in the current market situation, the introduction of new solar cell concepts to the market is not straightforward. The only way to achieve this is to use the fully adapted standard processes employed in today's manufacturing lines and only upgrade them with a few industrially approved process steps – such as laser ablation and boron diffusion – in order to implement low-cost device structures with stable efficiencies well above 20%. This paper gives an overview of n-type cell concepts already present on the market and of promising technologies ready for pilot production; the latter were summarized and discussed at the 3rd nPV workshop in April 2013 in Chambéry, France. The consequences for module manufacturing, as well as for measurement techniques and for requirements in respect of new standardization for cell and module characterization, will also be discussed..

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