advances in photovoltaics conference report
different wavelengths highlighting different effects. Irvine summarised current work for the team, including enhancing absorption with light capture through scattering, optimisation of back reflectors and the addition of super-absorber material to replace CdTe. Here iron pyrite is undergoing trials: Nominally FeS2
, but it is
notoriously difficult to control stoichiometry. Future work will also look at combining organics and inorganics in hybrid devices to extend the wavelength range.
Nano-structuring silicon
A long-standing heavyweight within the solar industry is Sharp. Matthias Kauer represented this company, providing an overview of research into PV, which is a key technology for the company and one that has been contributing a growing fraction of product sales since 1959.
According to Kauer, solar cell products and PV sales are particularly important in Europe, using both silicon and thin-film technologies. He says that Sharp places an emphasis on local production for local consumption, and can live up to this mantra thanks to manufacturing at sites such as the Sharp plant in Wrexham, UK, which ships 500 MW of PV cells based on crystalline silicon per annum. Further ventures are planned in Europe, such as the setting up of a silicon thin-film cell manufacturing facility in Italy. In Sakai, Japan, thin-film solar cell fabrication is going on alongside a major LCD facility where there are economies of scale, and overlap in the use of glass supplies.
Looking forward, Kauer said that cost reduction rates indicate that solar technology should hit grid parity in about five-to-ten years. He believes that once this happens, PV will expand into new markets, such as electric vehicles and ‘town’ power generation.
Sharp’s R&D takes place in Oxford, UK, with a range of areas and products. Activities include the development of a proof of concept of a novel, nanostructured thin-film silicon solar cell. This features a very high aspect ratio light trapping ‘moth-eye’ surface as a substrate for subsequent film growth – specifically 1.5 µm pillars with a 0.5 µm spacing.
Another interest being pursued by Sharp is that of multi- junction solar cells based on a stack of lattice-matched III-V materials. These are currently being fabricated by MBE. Kauer pointed out that there’s a ‘missing’ material with a bandgap around 1 eV that needs to fit into the stack: Ge/X/GaAs/GaInP. He said that the solution to X might be InGaAsN.
Sam Stranks from Oxford University, UK, described the development of a PV composite ‘blend’. This was based on single-walled carbon-nanotubes, typically 1 µm long and 1 nm in diameter, wrapped in a single layer of the polymer P3HT and dispersed in a matrix. Stranks admitted that efficiencies are currently low, but pointed out that there are plenty of possibilities for optimisation
Delegates were also briefed on efforts concerning cost and efficiency gains uncovered by fabrication studies at Loughborough University, UK, including optimisation of groove shape for buried contacts by laser ablation (for concentrator PV devices). This is achieved by tuning laser power, stage speed, energy, pulse rate and duration. Characterisation is via coherence correlation interferometry, a fast, large-area method.
The requirement for a fast, cheap, in-line method for surface scattering studies was illustrated by work at the Laboratoire de Physique des Interfaces et Couches Minces, Palaiseau. They are using Mueller polarimetry for monitoring surface texture optimisation to control light management in devices.
Finally, Ralph Gottschalg from Loughborough University gave an entertaining and extremely informative talk on the problems of testing cells and modules in order to make true comparisons and gain trustworthy data. Earlier in the day delegates had been made aware of the need for robust testing regimes and effective comparators. Gottschalg picked up the story, warning delegates that the situation is a minefield! Although researchers may assume that testing by placing two cells alongside each other will give a true comparison, this is not the case for all sorts of reasons – including ground reflection, wind speed, and pigeons! Given all these issues, it might seem that the best way forward is to revert to ‘laboratory conditions’. But there are many downsides with that approach, warned Gottschalg.
The biggest of these is that the lab cannot replicate the conditions in the field. Round-robin results performed in a series of high-quality European laboratories gave frighteningly large variations in results (+/-3 percent), and agreements were worse with varying irradiance. The description of the current attempts to develop international standards – requiring standard conditions, standardised data for module behaviour, agreed methods for modelling energy yield, and standardised reporting – raised many smiles.
As if all this information were not enough for this one- day gathering, there was a lively and varied poster show to take in as well. Presentations illustrated fabrication, structure, characterisation, and modelling of PV cells and devices, as well as some novel applications. The next meeting is planned for early Fall 2012, by which time we can expect significant progress and further exciting developments.
© 2011 Angel Business Communications. Permission required.
November/December 2011
www.compoundsemiconductor.net 49
of the blend that could lead to long-lived charge separation. Potential levers include the density, spacing, size and directionality of the nanotubes. He revealed that he and his co-workers are now using time resolved photoluminescence studies to characterise the structures.
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