All That Matters

This week I am attending the Materials Research Society Fall meeting in Boston, where there is a big focus on energy. Catalysis, fuel cells, batteries, solar cells, solar fuel, you name it. And I had a discussion with some researchers from the inorganic solar cell community, who asked me what is with the organic solar cells? There is a lot of university research in this area they said, but at industrial trade shows in comparison you don’t see as many start-ups working on organic solar. Eight19 is an exception to this that comes to mind.

And as we’ve discussed, the problem is basically efficiency. There have been a lot of advances in inorganics recently, with single films now easily reaching efficiencies above 20%. A thin film GaAs solar cell this year achieved a record efficiency of 28.2%! These highly efficient cells are only about 1 micrometre thick(!), which means they are also quite flexible and bendable. And what’s more, fabrication is also very cheap. To make a thin-film solar cell doesn’t even waste an expensive wafer any more, there are techniques to remove the devices from the substrate and to reuse the wafer for the fabrication of the next cell.

In contrast, organic solar cells are much less efficient, less than half what those record breakers achieve – whether it is dye-sensitized cells or polymer-based ones. In the official, verified solar cell efficiency tables (reference below), GaAs as said achieves 28.2%, silicon thin films 19.1%, silicon crystals 25%, CIGS (of Solyndra fame) 19.6%. On the other hand, dye-sensitized solar cells achieve 10.9% and organic polymers 8.3%. And if you’re wondering, the absolute record is held by the more expensive so-called inorganic multijunction cells at 43.5%, but for concentrated light, not normal light.

But such huge differences in efficiency are known. Typically, the argument made in favour of organic solar cells is cost. But is that so? As explained, the latest generation of inorganic thin-film cells are very cheap to make as well. Moreover, one of the most expensive parts of solar cells are the panels that hold the cells, as well as installation. Assuming that these costs are half of the costs of solar modules (a not unreasonable approximation), fabricating organic solar cells that even would be only 10% to 20% the cost of inorganic ones will cut the cost per panel by 40% to 45%. Yet, with efficiencies of less than half of the inorganic ones, you need twice the amount of panels, so it won’t come cheaper. [...]

Understanding the properties of something chaotic such as a bowl of spaghetti may seem a daunting task. But that’s what Garry Rumbles from the National Renewable Energy Laboratory in the USA, Natalie Stingelin from Imperial College London in the UK, and coworkers are trying to do. With success. They study polymers – long spaghetti-like molecules made of repeating atomic subunits – and have now uncovered how the microstructure of these polymers controls the behaviour of optically generated electrical charges in such a tangled molecular web, with important implications for the design of electronic devices.

The physical properties of polymers depend a lot on the length of the molecules as a whole, the atomic make-up of their structural units and the physical interactions between the individual strings. That’s why polymers come in so many forms, from hard plastics to stretchable synthetic rubbers. And what Stingelin and Rumbles now show is that also their electronic properties depend not only the chemical make-up of the polymers, but also the details of their structure and their molecular weight. This has dramatic consequences for the search of new polymers for various optical and electronic applications, says Stingelin. “Are there otherwise wonderful polymers out there that were cast aside because their creators tested the wrong molecular weight? We think it’s quite possible.”

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The crisis at the Fukushima Daiichi nuclear plants is a real tragedy. Tens of thousands of people have been evacuated around the plants, many of which continue to live in shelters with little comfort and privacy. And even worse, there are more than 27,000 people that are either dead or declared missing as a consequence of the earthquake and the tsunami.

The stream of media reporting on the status of the Fukushima plants is continuing, although ironically we are now in a situation where although the continuing release of radiation into the plant’s immediate environment is accumulating to radiation levels that are worryingly high, the broader interest on the issue outside of Japan appears to have ebbed away. And that despite the fact that these problems will be with us for months, if not years.

What is still going strong in the media, however, is the debate on the future of nuclear energy. Some see the accident as a sign that we should stop all nuclear power plants – immediately – whereas others such as George Monbiot see the fact that the implications of this accident so far seem geographically limited as a sign to support nuclear power. Unfortunately, this pro/contra nuclear is where the debate stops, and there appears little movement on either side.

It’s about our energy future

What I am missing in this entire debate is the vision for our energy future. That’s because a sustainable energy supply is a complex issue, where broad brush strokes such as pro or contra nuclear unfortunately don’t help. Take the German government’s decision to shut down seven of its oldest nuclear reactors: unlike the shutdown of nuclear reactors in Japan this hasn’t led to power cuts in Germany. So where does the missing energy come from? This power is bought on the international market. So who can offer spare capacities of around seven gigawatts power or more? My guess is that most likely it’s nuclear energy from elsewhere….

But short-term politics and Fukushima-related knee-jerk reactions aside, how do we envision our energy future? [...]

The past year has been a great year for science with major advances in several areas. Too many exciting results to mention here. Instead, to reflect about the past year I have chosen a representative paper for each month of the year that I hope can serve as an example of the great science going on in a number of research fields. Of course, this is a highly subjective and personal collection, and indeed there might be others worth mentioning. But the aim was also to provide a balanced overview of the year that covers a variety of topics.

Of course, if you have an exciting paper to add, please feel free to use the comments section below to let us know!

Anyway, enough said, here are some of my highlights from the past year:

Simulations of electronic excitations in an iron-based superconductor. Image by Oak Ridge National Laboratory via flickr.

JANUARY – iron-based superconductors

Since they were discovered in 2008, iron-based superconductors, the pnictides, have been one of the hottest topics in condensed matter physics. Part of their appeal stems from the fact that they are based on iron, which is a magnetic element. Normally, magnets and superconductivity exclude each other.

The iron-based compounds have a similar crystal structure as the so-called cuprates, which are the materials with the highest superconducting temperatures known. The mechanism for these high-temperature superconductors is unknown, and studying the iron-based superconductors may also be relevant to the understanding of the cuprates.

This paper published in Science shows for the first time that the electrons in the iron-based superconductors show a periodic arrangement that is different to the periodicity of the atoms in the crystal. Similar observations have been made in the cuprates, and their understanding is considered important to the mechanism of high-temperature superconductivity.

Chuang, T., Allan, M., Lee, J., Xie, Y., Ni, N., Bud’ko, S., Boebinger, G., Canfield, P., & Davis, J. (2010). Nematic Electronic Structure in the “Parent” State of the Iron-Based Superconductor Ca(Fe1-xCox)2As2 Science, 327 (5962), 181-184 DOI: 10.1126/science.1181083

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Rechargeable batteries. In comparison to lithium-ion batteries these are an older but more cheaper technology generation, made from metal hydrates. Image by comedy_nose via flickr.

The title of this blog post is a bit tongue in cheek, but the situation isn’t that far from the truth when it comes to rechargeable batteries such as lithium-ion batteries. Ever since lithium-ion batteries were first commercialized in 1991 by Sony, based on work by John Goodenough and others, they have been highly successful in the market. Open your mobile phone and read the battery label, almost certainly it is a lithium battery. The lithium stores electrical charges in the battery’s anode. During discharge of the battery the lithium moves to the cathode, where the charge is released. Lithium-ion batteries are also used in electric cars, in laptops, for electric power tools and so on. The market is huge.

On the other hand, if you use these rechargeable batteries, their real-world problems are pretty clear. Storage capacity could be better, particularly for electrical cars. Then, these batteries should be rechargeable more often without degrading, and last but not least the charge cycle should be reasonably fast.

The success of lithium iron phosphate

The bottleneck in the storage capacity of lithium-ion batteries is how much lithium the electrodes can take up. In particular the cathodes are a problem, their capacity is smaller than that of the graphite anodes used. One of the best cathode materials, proposed by Goodenough early on, is lithium iron phosphate (LiFePO₄). Unfortunately, lithium iron phosphate as studied by Goodenough didn’t work well, it didn’t conduct electrical current! In 2002, Yet-Ming Chiang and colleagues from MIT then published a paper where they fabricated lithium iron phosphate that is made conducting through the addition of other metals. Furthermore, Chiang also discovered that if nanoparticles are used instead of bulk to make the cathode, the surface area of the electrodes is increased and hence their efficiency goes up.

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This morning I read an article by the Scientific American editor David Biello on an important topic: the importance of rare earth elements for our economy, and the power of those few countries that export them on a larger scale. (disclaimer: Scientific American is part of Nature Publishing Group, my employer)

David hits an important point there. But to my mind, the problem is far more critical and fundamental than this single, focussed example suggests, and we need to act on it soon.

Salt production at Salar de Uyuni. This salt flat harbours 50% of the world's lithium reserves. Image by Ricampelo via Wikimedia Commons.

The issue is that rare earth elements such as neodymium are essential to green energy and our economy. Neodymium is part of Nd₂Fe₁₄B, a powerful permanent magnet that is used for electromotors, read heads of hard disk drives, etc. Each wind turbine apparently uses 300 kg of neodymium, each Toyota Prius about 1 kg. At present, China produces 97% of all neodymium.

And this is the problem. China has implemented export controls for its rare earth elements resources. In a recent diplomatic spat with Japan, they temporarily restricted the export of rare earth elements to Japan. But the Chinese should not take all the blame for a little realpolitik. Heard of the 1973 oil crisis?

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