Question

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?

Answer 1

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.:

http://en.wikipedia.org/wiki/Organic_electronics
http://en.wikipedia.org/wiki/Organic_semiconductors

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