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Now in disordered organics, the band picture is thrown out the window, from what I can tell (due to lack of symmetry). But don't HOMO/LUMO levels basically take the place of conduction/valence bands in molecules? In a organic system (a lot of molecules with no order), then, I am correct to believe that the HOMO and LUMO levels broaden into a Guassian density of states? However, the LUMO 'band' does not act like a conduction band in that the states are still localized. From my reading, it appears that the Fermi Level is in the LUMO 'band'. My question is, how can the LUMO 'band' have filled states since a single molecule of course has no electrons in the LUMO. What am I missing?

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Yes, your analogy does work. HOMO/LUMO play the same role for organic semiconductors as the valence band and conduction band play for inorganic semiconductors. In fact, there is a whole discipline of organic electronics, see e.g.:

For example, plastic solar panels may usefully become organic in the near future. I guess that it's not a good idea to use organic semiconductors for the state-of-the-art miniature microprocessors because the molecules are too still too large, irregular, and "dirty".

Otherwise, I don't quite understand what you're missing. Everything you write is true: the LUMO band is unoccupied in the ground state - but it has available states that may be occupied when the voltage makes it energetically preferred. It's exactly the same thing as the conduction band for other semiconductors. In both cases, these states may be shared by the whole material, so they may become delocalized. I recommend you to search Google Scholar for delocalized LUMO. All the best, LUMO

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Thanks. I think I figured out my problem. I thought I was reading that electrons existed in the LUMO states in the ground state equilibrium (which would no longer make it unoccupied). However the conditions were such that there is a voltage across the sample and charge is injected from an electrode (as you suggested was the case). This is what is putting electrons into the LUMO states and thereby making an overall non-zero net charge in the semiconductor. – BeauGeste Feb 16 '11 at 19:47

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