a new approach to solar cells?

This paper has been out for a while, but I've been meaning to write something about it anyway. Most organic solar cells use nice, normal carbon-based compounds as donor and acceptor materials. While some of these have decent charge carrier mobility ("decent" is a relative term!), there is another important factor to consider--the exciton lifetime. This is the amount of time the exciton can last before recombination. Recombination occurs when the electron and the hole annihilate each other. This may or may not involve release of a photon. (In OLEDs, recombination is the desired outcome. Not so for solar cells.)

Photoexcitation will give a singlet exciton. ΔS=0, no spin flips with excitation, right? In most light-atom-only (read: organic) compounds, L-S coupling is observed, and ΔS=0 holds. For heavier atoms, spin-orbit coupling takes over. Consequentially, ΔS=0 doesn't matter so much.[1] This enables intersystem crossing to a triplet state...which is cool, because triplet excitons last a lot longer than singlet excitons.[2] Increasing the exciton lifetime also increases the exciton diffusion length. This means the solar cell can be thicker! If it's thicker, it can absorb more light.

This is the rationale behind using Pt-OEP. A heavy metal, such as platinum, enables spin-orbit coupling. In other words, the excitons used by the solar cells in the aforementioned paper are triplets! Unfortunately, Pt-OEP has a very low charge carrier mobility and doesn't absorb at all the wavelengths emitted by the sun.

Still, the paper's worth reading if you haven't gotten around to it already (it was published in 2005).

[1] The first rule of quantum is: the rules don't always apply. Forbidden transitions still occur.

[2] Never mind that spin-orbit coupling affects this also...


Mike said...

Umm, too much chemist talk ;)

Do you know anything about the "new" Grätzel Cells with TiO2?

Milo said...

So here is a technical question: How stable are these things to potential real world conditions? For instance, if I have an organic based solar cell sitting on top of my 2005 Camry charging the battery, the temperature of the cell could get to what, 35 or 40C? Over time, I would expect that the increased temp combined with atmospheric O2 and constant irradiation would degrade the poor ligand.

Ψ*Ψ said...

Mike: I'm planning to post something on Grätzel cells at some point, um, soon.
Milo: Oh, man. Just don't talk about stability with organic solar cells. It's just depressing.

milkshake said...

I noticed that they are comparing this Pt-porphyrine to phtalocyanines in the article (when discussing the charge mobility).

There is a curious class of boronate-chained dimethylglyoxime complexes - they are super easy to make (one pot from commercial stuff) and they are very chemicaly stable:
Angew. Chem. Int. Ed. Vol 1(6), (1962), p 333-334
DOI: 10.1002/anie.196203333

I wonder how would they fare in this application

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