advances in photovoltaics conference report
tariffs, leading to a large uptake of domestic-based PV modules. These are typically 3 kWp systems that provide homeowners with around half their required power usage. In Sydney about 8 percent of homes have these fitted.
Green pointed out that large arrays are also being deployed around the globe, with up-take relatively high in the US and Germany. These are set to compete with wholesale conventional power generation some time soon. Goals for PV uptake vary greatly from country to country, given variable climates, but the overall figure for Europe is a 12 percent PV power contribution by 2020. In the UK, the aim is to deploy 2 GW by the end of this decade.
The cost required to make PV continues to fall. Green says that there’s been a pretty consistent 20 percent ‘learning rate’ since the late 1970s – in other words, a doubling in production has led to about a 20 percent drop in cost. This trend is expected to continue. Falling production costs have enabled PV to now get very close to grid parity in some parts of the world, but there’s still some way to go before this technology hits wholesale price. A proper evaluation of the value provided by PV must not just consider generation costs, but also power demand cycles: Green illustrated this point with a plot of daily power demand in Germany, and the potential contribution from different technologies. Here PV can flatten out the peaks in demand in the middle of the day – it is having significant impact on this already.
In Japan there has been significant investment in hydro- storage technology to make use of the nuclear output, which has to be continuous. A similar approach could be applied to handling the PV input to the grid, flattening out the daily PV input cycle.
Green also discussed PV technology, beginning by offering his take on the competition between first-generation silicon-wafer cells and second-generation rivals based on thin films. He argued that those who had written-off silicon-wafer devices for economic reasons have ignored the huge potential for manufacturing cost reductions – much of this development has, until now, been in Asia, and may have been missed.
Green explained that the increased competition in the market place has spurred silicon wafer manufacturers to develop new, cheaper products based upon ‘quasi- monocrystalline’ silicon ingots, which are being produced in larger and larger sizes. A spin-off benefit of the size increase is the ability to produce a higher proportion of high-quality, oriented grain growth in the centre of the ingot, using a centrally placed seed crystal combined with directional solidification. There’s scope for further advances here, plus a strong driver to do it, since the silicon wafer is about half the total cell cost. To highlight the tremendous advances made by the
silicon PV industry, Green showed the gains in achievable efficiency and the extent of cost reduction since the emergence of the silicon p-n junction ‘black cell’ in 1974. Improvements to date have come from refinements to passivation and rear contacts, along with developments associated with surface texturing. In the near term, one of the biggest opportunities to trim costs is to find alternatives for the role of silver.
Light trapping technology came up several times during the day – in Green’s talk he explained that optimised rear-surface scattering can increase the optical thickness of a cell to about 40 or 50 times its geometric thickness. An efficiency of 25 percent could result without the need to resort to any new technologies.
Green also described some of the advantages associated with moving to thin film technology: Less materials usage, large manufacturing capabilities with integrated module fabrication, and improvements to aesthetics and ruggedness. According to him, chalcogenides – and, further ahead, dye-sensitised and organic cells – offer great potential, as well as capabilities for flexible devices. He showed delegates a plot of efficiency versus cost per unit area, which revealed reductions in cost for thin film technology, offset by some drop in efficiency. However, it’s still early days.
In Green’s opinion, there are many different technologies that can be used to form third-generation cells, and he believes that it’s not clear which of these is the frontrunner. He pointed out that one contender is stacked cells with different bandgaps that can access the whole solar spectrum – predictions are for 49 percent efficiency in three-cell stacks, going up to 58 percent in a six-cell stack. Efforts within Green’s group include research into stacked silicon-based technology, with bandgap manipulation using quantum confinement in silicon dots having diameters of just 2-5 nm. Another
November/December 2011
www.compoundsemiconductor.net 47
CdTe-solar cell manufacturer Abound Solar won a 1.5 MW contract in Polpenazze Del Garda, Italy
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