Scientific Method —

Boosting the efficiency of buckyball photovoltaics

Organic photovoltaic devices have some big potential advantages: they could be …

Right now, the efficiencies of mass-produced silicon photovoltaic panels are closing in on 20 percent, and various thin-film technologies aren't that far behind. Given those gains, organic photovoltaic systems, where efficiencies linger well below 10 percent, would seem to be an afterthought. But solar materials based on organic polymers have a few advantages, in that they're not rigid and opaque like silicon and don't rely on the (sometimes toxic) metals of thin film technologies—there's lots of carbon around, and it's cheap. So there's a healthy amount of effort going into increasing the efficiencies of the organics in the hope that they can become competitive for some applications.

Some good news on that front was published in Nature Materials over the weekend. By adding a thin layer of an organic ferroelectric material to a buckyball-based photovoltaic device, researchers have tripled its efficiency. And, in the process, they've created a photovoltaic device that can be reprogrammed to push current in the opposite direction.

The authors of the paper provide a long list of reasons that organic photovoltaic devices aren't as efficient as other options. Some of these are pretty simple: they don't absorb light very well, and the charges within them aren't as mobile as they are in other materials. But the biggest problem, accounting for over half the inefficiency, is that the process induced by light rapidly reverses itself.

When light strikes a photovoltaic device, it pries an electron loose and pushes it into a higher energy state, which allows it to move away from the site where it originated. This leaves behind an area of positive charge, or hole. Both the electron and the hole can migrate to nearby electrodes, which is how we manage to extract a current from the devices. But, since they have opposite charges, they're just as happy to find each other and recombine, wasting the energy deposited by the photon. It's this waste that helps kill the efficiency of organic photovoltaics.

One of the easiest ways to prevent that from happening is to provide an external electric field, which will send the electrons and positively charged holes in opposite directions. Unfortunately, to create a significant field inside the photovoltaic material, this external field has to be fairly large, and maintaining it is challenging. So, the authors of the new paper put a miniaturized electric field inside the photovoltaic device.

This is where the ferroelectric material comes in. These have a very useful property: once polarized by an eternal electric field, they hang on to that polarization, creating a local electric field in the same orientation. A sheet of ferroelectric that was polarized such that it ensured a positively charged electric field extended into the photovoltaic polymer would attract electrons, grabbing them before they could recombine with the holes they had left behind. Better still, organic ferroelectrics are available; the authors use the catchily named Vinylidene fluoride-trifluoroethylene. They simply coated one of the electrodes with it, and then polarized it with an external electric field.

In this work, they combined ferroelectrics with an organic photovoltaic material based on buckyballs, the spherical version of a carbon nanotube. On its own, the combination of buckyballs and metal electrodes had an efficiency of 1.5 percent. But, in samples where the ferroelectric polymer was used to coat an electrode and polarize it, efficiency shot up to 4.9 percent. That's a pretty substantial gain.

When both electrodes were coated with a film of the Vinylidene fluoride-trifluoroethylene, it made the behavior of the solar cell programmable. Polarize it one way, and electrons would be gathered at one electrode; flip the polarization, and the opposite occurred. This also suggest that, should efficiency drop with time, the polarization could simply be refreshed with another dose of an external electric field—not obviously useful, but kind of interesting.

The authors also looked at the structure of the ferroelectric coating, and found that it alternated between very thin areas and what they termed "nanomesas," which are tiny, flat-topped ridges. Using an atomic force microscope, they showed that current only flowed to the electrode efficiently in the thin areas between the nanomesas. So, they think that these thick regions help attract the charges, which then exit the system through the thin ones.

Five percent efficiency isn't exactly going to set the world on fire, but there is a lot of room for improvement. The approach should work with any organic photovoltaic material, so it can be adapted to anything that works better than buckyballs. And it might be possible to optimize the deposition process so that there's a more efficient arrangement of nanomesas, one that carefully balances attracting charges with allowing them through to the electrodes. If it works out well enough, we just might eventually see some flexible, translucent solar materials reaching the market.

Nature Materials, 2011. DOI: 10.1038/NMAT2951  (About DOIs).

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