I have been reading a paper about trap states in P3HT, which is an organic semiconductor. These materials aren't as well defined as inorganic semiconductors (i.e. silicon etc), because they are largely amorphous. This makes them a nightmare to model. The trap states are basically energy levels that exist inside the "band gap" of these materials. In the paper, two traps are found. One is attributed to oxygen exposure, and one is found to be an intrinsic property of the material itself. By this, we really mean that there are some energy levels with low density of states in between the highest occupied molecular orbital (HOMO, analogous to the valence band) and the lowest unoccupied molecular orbital (LUMO, analogous to the conduction band). This got me thinking: imagine trying to model the density of states of an entire polymer chain, that would be crazy!
It makes one really appreciate looking at highly ordered systems like crystals.
Crazy.
ReplyDeleteFinding the density of states of an entire polymer chain. Seems to complicate things when you have two sets of density of states within the one part.
Suppose we could find a nice expression for density of states for one part. Then when we consider a second part, we'd probably want to acknowledge any interacting/shared states between the two.
With three, we'd want to consider any shared states between any two and the three.
Even if this was a (linear)chain with only two neighbours, I'd like to wonder what would happen when the chain wants to loop onto itself...
Appreciation for ordered systems indeed!
Big sidenote:
ReplyDeleteOrdered biological system? Just like a lattice?
Circogonia icosahedra, oddly enough, is in the shape of an icosahedron and several more organisms/systems in simple shapes as well!
In immediate response to Ann, apparently icosahedra are the easiest shapes to form with identical protein subunits. Want to prove it mathematically :)?
ReplyDeleteFor Dave:
If they could model polymer chains, where would all the fun be? ?!?! This was one of the interesting problems of my Summer Research project last year. The compound we were investigating was supposed to be an ambipolar transistor material (depending on gating material band gaps and applied potential &c.) and so electrons were supposed to go down the middle of the molecules while holes were supposed to go down the sides of the molecules. One caveat: the compound had two whopping-great saturated (i.e. for the non-chemists, single bonded only) carbon chains stuck on each side of the interesting bit of the molecule to make it soluble. So, besides other problems with making it form a nice film on a semiconductor substrate, the big problem for its electrical properties was making all the electron-carrying bits line up in the middle and making all the hole-carrying bits join up too. Without these bits lining up, the compound won't have very good electronic properties ( :( ).
So the question is, is there a way to make these arrangements regular? One way might be to apply a field whilst the film is forming on the substrate....
Liquid crystals are an interesting solution because (I think) they can form 'meshes' that avoid the linear lining up requirement of the compound.
You have to love those oxygen-created trap states!
Another solution is to use small molecules, as opposed to polymers. For example, the most common electron accepting is PCBM, a small molecule. There has been some research done looking into all small molecule devices (these were present in a seminar I attended last week). I suppose that there are lots of problems with conductivity, transport etc.
ReplyDeleteSpeaking of making the molecules line up, and be more regular, what if one were to anneal the film in the presence of a field, and then cross link them (photonically probably...). I wonder if anyone has tried this?
Josh, I suspect that if icosohedra are the easiest shape to form (under certain, mathematically simple conditions), we would have proven it in maths...
ReplyDeleteDavid, I assume when you mention small molecule device you mean something where there is a small group of atoms repeated many times to make the device.
Why would a small moelcule device really complicate thing?
Would it be because the whole thing is of comparable size to an atom or something?
Or would it just be hard to manoeuvre?
The fact that you can have energy levels existing in the forbidden band gap itself is interesting-hence the name "trap" states? Since polymer is made up of repeating structures there should be some kind of trick we can use if we want to model it.
ReplyDeleteIn terms of tricks, remember that although the polymers are made up of repeating units, they are not necessarily ordered. That is, the polymer chains can go in crazy directions. Also, the polymer chains aren't in an isolated environment. They are known as trap states, because electrons, or charge carriers "fall" into the trap states.
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