The Brain, it is Full: CU workshop, day 1 part 1

Today was the first full day of this energy efficiency workshop that I'm attending. There were 8 full talks (~45 minutes each) plus a brief introduction to energy policy in the beginning by a guy who works for the Department of Energy, which I didn't really follow too well other than that it seems like the DOE is funding a lot of weapons research and a lot of fuel.

The first talk was by John Benner, who talked about the economics of solar cells. He said that photovoltaics are in a pretty typical "growing curve" in terms of how markets develop, but there will need to be a lot of breakthroughs in order for solar-generated electricity to become cheap enough to be competitive with fossil fuels. Showed some calculations of projected growth rates. He also mentioned that because people have underestimated growth of the solar cell market in their predictions, there hasn't been enough supply of solar cells to meet the demand. Also, that we might be in a "bubble" wrt solar cells, where the cost of the modules is going down, abut the price is not because people will pay. Later, though, there was a discussion about how much energy it takes to make a solar cell; it turns out that you don't get any net energy out of your installation until you stop building new solar cells.

The next talk was by Arthur Frank, from NREL, who talked about dye sensitized solar cells, and about the properties of titania and sensitizers that were desirable for these cells. It turns out that organic molecules are catching up with the more expensive ruthenium based dyes in terms of efficiency. Also, the surface roughness of the titania determines things like charge transport. There was also a comment at the end about trap states in titania, and he said that no one really knows much about traps. Presumably traps are defects in semiconductor structure--at least that's how I always thought about them--but they may not be bad. The person had asked whether there are different kinds of traps (i.e. deep vs. shallow), and whether it's possibly that one kind slows down diffusion and the other kind encourages exciton recombination. And at the end there was a comment about how some Japanese engineers had tested a solar cell for 3 years and it dropped in efficiency by 5%, but this wasn't due to the dye leaking (some dye-sensitized solar cells are made of liquid electrolytes).

Then we had a break. I ate a bagel and some fruit. Then Brian Gregg, also from NREL, talked about small molecules solar cells, and about doping organic semiconductors to improve their performance. The highest efficiency organic solar cell is 5%--not good enough! He talked a little bit about why the conventional band diagram that people use to describe p-n junction solar cells in silicon is the wrong picture to have; how a state diagram is more accurate. Also, unlike Silicon, you can have an open circuit voltage in excitonic solar cells even if your electrodes have the same work function, because you only have hole transport through the material, and not electron transfer. This has to do with the energies of only one of the HOMO or LUMO lining up with the work function of the electrode. There was also something about how you can use chemical potential gradients to help you with your solar cell in addition to electric field gradients, even though you can't do this in silicon. I didn't quite understand that
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Then he talked a lot about interfaces and the problems with recombination; however, I wasn't sure if he was omitting recombination in bulk because it wasn't important, or because that wasn't what he studied.
He also talked about how In small molecules, the binding energy of the exciton is about 0.25 eV, which is more than the thermal energy available at room temperature (kT). So excitons have to diffuse to an interface and then be split at an interface, in order for them to be useful. But even at the interface the coulombic attraction doesn't go away. There are ways to overcome this, but they end up wasting 0.9 eV, which is a lot. It think this involves some sort of thermal energy, but I didn't understand how it works. He also had a way of overcoming the coulombing attraction energy, but hasn't demonstrated it yet. It involved doping, and he proceeded to talk about the effects of "doping" for the rest of the talk. (Actually, a professor from Maryland who I talked to later and who works on SPM told me that she doesn't think doping is the right term; that really, you're polarizing the molecule differently). But yes, apparently some recent work out of Peter Peumans' group at stanford showed that super-purifying P3HT doesn't actually make a better device--but people don't know exactly what these dopants are.

So Brian Gregg's group has apparently done some "chemical" doping, which increases the dielectric constant of these molecules (the ones he used was PPEEB) and through that maybe increases the mobility (if I understood correctly). And through the Poole-Frenkel equation, which is a super-oversimlification that works, this increases the conductivity.

The next guy was Garry Rumbles, and talked about bulk heterojunction cells, and his talk was cool, so I'll write about that later.