The Case of the Complexed Charge Carrier: The Mysterious Efficiency of Organic Solar Cells

There’s nothing humans love more than a mystery, which explains our ongoing fascination with whodunits, a genre tracing its roots back to the works of Edgar Allan Poe, and best known in its current incarnation of CSI: Everywhere.  Curiosity may well kill the cat (or at least lead to an attempt on the life of our hero(ine) in the nail-biting climax of the plot), but it’s also the engine of discovery.  Scientific research is, if you choose to look at it in a certain light, the systematic investigation and solving of mysteries.  Whodunit may be an argument for private eyes, philosophers, and priests, but howdunit is the question for scientists.

One thus-far unsolved mystery is the case of the organic solar cell, or more specifically, the question of how such devices manage to display power conversion efficiencies much larger than those predicted by theoretical calculations. It’s nice to have a positive mystery to tackle – something working better than it should is, after all, something of a rarity (at least it was in my research experience)– and unlocking the answer to this question is a goal that Martijn Kemerink and his colleagues at Eindhoven University of Technology have set themselves. Conceptually, the team explain, such high conversion efficiency values shouldn’t be possible at heterojunction interfaces; to generate power, the cells need free-flowing electrons and holes that result from the dissociation of charge-transfer complexes.  In order to flow freely, the charge carriers need to overcome the binding energy holding them together as electron-hole pairs, and it’s their inability to achieve this escape that should, in theory, limit their efficiency.  Experimentally, however, the carriers aren’t conforming to this expectation, and the team at Eindhoven have come up with a model that may explain why.

To form the initial charge transfer complex, one of the carriers, given energy by the band offset between the semiconductor layers, is hopping across the interface, forming its complex after moving.  As relaxation occurs in the disorder-broadened density of states, the carriers can dissociate and flow freely, making their contribution to the power conversion. To return to our howdunit, our intrepid detectives have posited that the culprit initially escapes by jumping the garden fence, then slips away in the ensuing confusion, making a clean getaway. Whether this is the true version of events remains to be seen – more work is needed to test the modeled behavior against experimental reality – but understanding these processes is a vital step towards creating economically viable plastic solar cells, with all their promise for cheap, large-scale access to renewable energy.  A mystery, I think we’d all agree, that’s well worth solving.

DOI: 10.1002/adfm.201200249

Beetle-mimetic Velcro?

Most people go through a phase in life where they are fascinated by insects (usually when you’re young and insatiably curious; my cousin’s six-year-old boy was delighted to number an insect vacuum among his gifts from Santa). While some of us then go on to develop phobias and even downright loathing, a few people retain their sense of wonder at the splendors of the natural world, and scientists who study the form and function of biomaterials have produced some incredible synthetic analogues of things that come naturally to our fellow life-forms. Superhydrophobic and self-cleaning surfaces, gecko-foot adhesive patches, mussel-glue, and photonic crystals inspired by weevils are just some of the things we’ve featured on Materials Views, and now we can report on a Velcro-like interlock mechanism inspired by beetles.

In work featured on the latest cover of Advanced Materials, Professor Kahp-Yang Su and his team at Seoul National University have created a synthetic reproduction of the mechanism used by beetles to keep their wings tucked away when not in use. Along the edges of its carapace, a beetle has thousands of tiny, densely packed hairs that slide together and prohibit any sideways slip when the carapace closes over the folding wings. In actual fact, the mechanism is more closely related to the children’s toy Stickle Bricks than to that of Velcro, as it does not use hooks and loops to generate adhesion. Professor Suh’s team mimicked this structure using dense arrays of polymer fibers, where Van der Waal’s interactions between the fibers provide the adhesive force. Simple and elegant, and a very pretty cover picture to highlight an excellent piece of research.

DOI: 10.1002/adma.201103022





Hemostatic Materials: Anti-hemorrhagic Dressings for Wounds

I’ve been playing a lot of Mass Effect over the holidays. It’s fun, but, as with most games, I’m epically bad at it, and the only thing that keeps me in the missions for any length of time is the convenient convention of health recovery. Skulk behind a wall or crate for a few moments, and you miraculously revitalize. Most RPGs have this convention, justified in this instance by the plot existence of your battle suit, which monitors your condition and distributes treatment as required to get you back up to full speed.

The idea of such powered armor has been hanging around in sci-fi for a good half-century, most notably in Robert A. Heinlein’s novel Starship Troopers, but in recent times has started to make the transition towards science fact through programs such as the US Army’s Future Soldier Initiative. While the complete suit may still be some distance away, some of the concepts and components are the goals of active research programs, as recent work published by Paula Hammond and her team at MIT demonstrates.

The report in Advanced Materials details development of a hemostatic material for use as field dressings in emergency wound treatment. Current practices for staunching bleeding wounds are generally based on compression (for example, tourniquets) but this can be difficult to control or even achieve in the field, depending on where on the body a wound is received. Hemostatic alternatives, which work by enhancing the clotting process) have previously been produced, but suffer from a variety of disadvantages such as expense, impractical application in field conditions, or adverse immune response. Professor Hammond’s team have come up with an alternative that uses a layer-by-layer assembly technique to build films of thrombin (a clotting agent) and tannic acid directly onto absorbent gelatin sponges. Hydrogen bonding between the tannic acid and the thrombin allows the construction of the films without the need for any other ingredients, maximizing the load of the clotting agent, and all the materials used in the study are already approved for clinical use, so could potentially be deployed as a viable treatment system in a short time. The thrombin load, once applied to a wound, was effective in promoting rapid clotting within one minute: a control test using uncoated, regular cotton gauze bandages and compression failed to stop bleeding from a wound over a twelve-minute period, showing this research’s very real potential to save lives in emergency situations and operating theaters.


Image Credit: arrighi / CC BY 2.0

Shrilk: A Chimera of Chitosan and Fibroin for Biodegradable Plastics

Current plastics technology is overwhelmingly reliant on feedstocks from fossil fuels, and, in many cases, are still non-biodegradable substances (although recycling is able to deal with an ever-increasing range of materials).  With declining reserves of resources and the correlated rise in interest in sustainable materials, research towards viable alternatives for plastics in packaging, consumer products and even medical supplies is continuing apace.

Materials scientists have once again turned to the lessons of nature in addressing this issue.  Javier Fernandez and Donald Ingber of Harvard University took chitosan (commonly found in the shells and carapaces of mollusks and insects) and fibroin (silk), and deposited them sequentially to develop a layered composite material that they christened “shrilk”: a chimera of the shrimp and silkmoths that provided the raw materials.  The dry material demonstrates strength and toughness similar to aluminum alloys.  When water is added, strength is sacrificed but the elasticity improves.  The films can be deposited on patterned molds to impart a specific patterning to the surface, and being able to tune or switch the elasticity allows simple engineering of 3D structures.  The materials are biocompatible (and thus feasible for biomedical applications: the authors envision a tissue-engineering scaffold in the paper), biodegradable, and biosourced, presenting a truly sustainable and environmentally friendly wrapper for your next plastics purchase.

Original paper: 10.1002/adma.201104051

Luminescent Solar Concentrators – the Search for Photovoltaic Windows

With an ever-increasing number of column inches being dedicated to renewable energy as the great climate change debate rages, the search for viable, scalable solutions to humanity’s dependence on fossil fuels continues.  Often, currently available solutions suffer from bad press regarding aesthetics, efficiency, or just plain bad luck:

The aesthetic argument always plays strongly to the crowd, and so one avenue of research into renewable energy is looking at ways in which energy tech can be incorporated into existing structures and materials, taking advantage of the fact that there are things we need to build anyway.  Windows, for example, present a large surface area for trapping light – imagine if every window in the Burj al Khalifa could be photovoltaic – all 26,000 plus of them.  One technology that could achieve this is the luminescent solar concentrator (LSC), the topic of a review published recently in Advanced Energy Materials. LSCs are simple devices that use a plastic or glass sheet as a waveguide.  The waveguide has lumophore molecules associated with it, either embedded in the material, coated on the top or bottom surface, or entrapped as a liquid layer between two plates.  When light hits the glass, it is absorbed by the lumophores and re-emitted at a longer wavelength.  Some of the light is then trapped in the waveguide by total internal reflection.  At the end of the waveguide, if a photovoltaic cell is attached, the light can be converted to electricity.

Michael Debije and Paul Verbunt take a detailed look in their review at the past thirty years of LSC development, noting that extensive commercialization has not yet taken place, largely hindered by the modest conversion efficiencies when compared to silicon solar cell outputs.  However, with more and more types of lumophore becoming available (a particularly interesting approach is the use of quantum dots, which can be tailored simply by adjusting their size), the options are becoming ever-more promising, and it might turn out that maximum efficiency may not be the only guiding light in LSC design.  Aesthetics may also have a part to play – what color of window would you prefer?  And how about creating energy by painting a little sunshine of your own?

Original paper: 10.1002/aenm.201100554