Energy efficiency workshop: day 1, part 2

The fourth speaker of session 1 was Garry Rumbles, a cool guy with a British accent who defended the bulk heterojunction cell. The main message I got from his talk was that the Band diagram is wrong (and therefore that physicists are dumb!). The reason why the Band diagram is wrong is because it hides a lot of the details that are present in molecules, such as the locations of the triplet states, and of various trap states. Also, it doesn't show the fact that holes and excitons annihilate each other. Maybe this is because of the coulombic attraction, although I'm not sure.

He pointed out that the big breakthrough, credited to Alan Heeger, was that you could make solar cells with C60 as an electron acceptor, and with this solar cell architecture you get very slow recombination rates. In P3HT the exciton diffusion time is 300 to 500 ps, which I guess is pretty short, and so the distance excitons diffuse is less than 10 nm. I knew this already. Then he went on to discuss some stuff that was probably important but which I didn't write down and don't recall. Finally, he showed a plot of how a solar cell degrades--it can last for ~50 days, although it seems to have degraded quite a bit by the end of it. He also commented that the most efficient organic cells that have been recently made were made in companies, not in government-funded research labs or universities. And he talked about intrinsic and extrinsic sources of degradation. Extrinsic ones that he listed were oxygen, water, and UV-presumably, they react with the active material, but the material could be protected. Also, there are effects like Delamination, PSS (not sure what this means now but it's in my notes) and Reducing the metal. Intrinsic effects were oxidizing effects, due to polarons (or related to polarons), reducing species, and intermal conversion (which presumably heats up the system and causes thermal damage).

I guess that last point is interesting because internal conversion is the way in which plants dissipate heat, and is deemed "safe" compared to the alternative of a long-lived excited state chlorophyll. I guess heat damage is an issue in things like computers and optical devices, where we need to have chillers and heat sinks. At the same time, plants are probably a lot more exposed to air than computers or laminated (i.e. plastic-enclosed) solar cells, and so they heat from them probably gets conducted away a lot faster. Or maybe there's just less heat to be released. In any case, I wonder where the boundary is between internal conversion=good and internal conversion=bad. Maybe it's not such a good thing in solar cells.

Then he talked about Time Resolved Microwave Conductivity (TRMC) which is this technique that measures charge transfer rates and recombinations lifetimes, I think. I'm not really clear on this but the technique seems cool, and he harped on the importance of considering the triplet state in these calculations, something that I guess solid state physicists don't do. He also mentioned that P3HT has pretty good Intersystem crossing. There was a question at the end about tirplets, and how the are too low in energy to really charge separate.

Which brings us to lunch. I ended up chatting with this guy who works on singlet fission (into triplets). He emphasized the importance of fission rather than intersystem crossing as the desirable mechanism. I guess I had never differentiated these two mechanisms in my mind, so I suppose I should look into that. And at lunch I met some grad students from the Kapteyn Group, who were all cool.

The next talk was about the hydrogen economy. The main point the guy tried to make is that we should only focus our research funding on things that will scale, in time. He doesn't think Carbon sequestration will every become commercially relevant. and apparently crystalling PV cells pay for themselves in 30 years, thin films in 1-2 years, and wind in a few months. And he thinks wind could surpass nuclear energy by 2020. And apparently hydrogen requires 30% of the stored energy in order to generate the fule. But I was a little bored and started doing other things in my notepad.

Then there was a guy who talked about the redox chemistry of water oxidation. I was not really awake, although surely all that is important. THe take home message there was that doing a thorough kinetic analysis of your system is an important thing to do.

And then there was another guy who talked about making photoelectrochemical cells. His main point was that metal oxides are the class of materials taht are most promising. They are stable ("most rocks are oxides") and can be semiconductors. He's doing combinatorial analysis to look for candidate metal oxides, and he's developed modules that can be given to high school students who can help out with this. I thought that was cool, and something I probably wouldn't have liked, at least in theory, as a high school student.

The final talk was by this wonderful lady--she talked about CO2 reduction, but I was tired and learned nothing. But I do remember, either from her talk or the preceding one, that there are catalysts available for making methanol or hydrocarbons out of CO2( without having super-high energy intermediates) but not ethanol--so , that's is a problem for GM potentially.

The poster session was excellent. I learned about triplet fission, and talked to someone who worked on doing 2D spectroscopy with pulse shaping.