Exciton Diffusion Length in P3HT:PCBM BHJ Cells

[Physicist3]

Hi,

I understand that an exciton must reach the D/A interface within the Exciton diffusion length to prevent recombination, however, what are the factors during device fabrication that can affect the Exciton diffusion length of a P3HT:PCBM BHJ OSC?

 

[DrDu]

You don't really expect an answer on that question, throwing in just some abbreviations without context, do you?

 

[TeethWhitener]

You don't really expect an answer on that question, throwing in just some abbreviations without context, do you?

Some translations:
D/A: donor/acceptor
P3HT: a thiophene polymer, the electron donor in question
PCBM: a fullerene-based electron acceptor material
BHJ OSC: bulk heterojunction organic solar cell

Now that that's out of the way, it seems like you're (referring to Physicist3) conflating a few terms here. The "exciton diffusion length," as measured by experiment, is a "typical" diffusion length of an exciton in pure P3HT. The distance that each individual exciton diffuses will follow a probability distribution, so that one can't predict ahead of time how far one particular exciton will diffuse before quenching. Bulk heterojunction cells typically capture the "long tail" of this distribution. For P3HT, the typical diffusion length is around 3-5 nm, but some excitons diffuse further. Since a P3HT:PCBM cell usually phase separates with domains on the order of 10-15 nm, some of the excitons generated in a P3HT domain are able to diffuse to a PCBM domain, where electrons can be collected by an electrode to do meaningful work.

To answer your question, then, I've never seen a study where the typical exciton diffusion length in P3HT is dependent on BHJ device fabrication. It's true that the measured diffusion length in pure P3HT varies a lot from paper to paper (I've seen anywhere from 1 nm to 10 nm), but as far as I'm aware, the main limit to efficiency in BHJ solar cells is the length scale of the phase separated donor/acceptor pairs.

This paper:

http://pubs.rsc.org/en/content/articlehtml/2011/nr/c0nr01002b

suggests that part of the success observed in P3HT:PCBM BHJ's may be attributed to an anomalously large charge transfer radius between the two phases, which is kind of an interesting idea. Basically, instead of the whole exciton (electron-hole pair) diffusing from its origin to the donor/acceptor interface, the exciton delocalizes over a large distance; i.e., the electron-hole separation increases. Still, however, I'm not sure the fabrication procedure would affect the size of the charge transfer radius. I'm inclined to think it's more an intrinsic property of the material.

 

[TeethWhitener]

Follow up: http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.114.026402 landed in my inbox this morning, and it got me thinking about your question again. (Note: the paper itself is only of tangential relevance.) I got to thinking about the mechanism by which excitons diffuse in P3HT.

In general, a photon creates an exciton on a single P3HT chain, and then (presumably), that exciton undergoes some sort of hopping via energy transfer from one chain to the next until it either recombines or the excited electron is absorbed by the acceptor PCBM. The catch is that this hopping is STRONGLY dependent on the geometry of the system. I don't know for certain, but my gut tells me that in a system with two P3HT chains that are the same length and oriented for maximum orbital overlap, an exciton created on one chain will be much more likely to hop to the next chain than if the next chain were of a different length and oriented differently. In other words, I would expect that the exciton hopping probability increases with the degree of crystallinity of the material.

So to more fully answer your question, if I had to guess, I'd say that the exciton diffusion length depends on P3HT's crystallinity. Hang on a sec, this sounds familiar...

Yep, check out this paper. Looks like a group in Korea showed somewhat of a dependence of exciton diffusion length on crystallinity in P3HT (though the error bars are a little large). So in theory, if you were to prepare the BHJ phase-separated material in a way that retained high crystallinity of the P3HT phase, you might see an enhancement in exciton diffusion length. Sounds like a good master's thesis project.