Friday, February 19, 2010

Is "organic semiconductors" a misnomer?

A wide range of organic molecular materials such as pentacene and polythiophene are attracting considerable interest because of the prospect of "plastic electronics". They can be used to fabricate devices such as light emitting diodes, photovoltaic cells, and field effect transistors, which are traditionally made with inorganic semiconductor materials such as silicon and gallium arsenide. Consequently, these organic materials are often referred to as organic semiconductors. This may seem reasonable because:
  • they have a conductivity that is activated in temperature
  • there is an energy gap of several eV to the lowest optically excited state
  • they can be used to make "semi-conductor type" devices
On the other hand, they have properties that are significantly different from inorganic semiconductor materials. These all relate to the fact that electronic states tend to be localised on single molecules whereas in inorganic semiconductors one can have states which are delocalised over many atoms.
  • they do not have well-defined conduction and valence bands (e.g., their mobility is almost always much less than that required for band transport) [just because you can calculate something does not mean it exists!]
  • they have a mobility that is thermally activated
  • energy gaps associated with optical absorption and conduction are significantly different
  • electronic correlations significantly modify the ordering of electronic states (e.g. there is a large gap between singlet and triplet excited states)
Because there is no band transport one cannot define a scattering time and one should not talk about "band bending" near an interface.

I think that referring to these materials as organic semiconductors has led a lot of confusion and debatable reasoning in the literature. I think "organic electronic materials" or "organic photonic materials" is much more appropriate.


  1. Ross, there are organic semiconductors that do show evidence of band-like conduction. Specifically, look at the work of Norbert Karl on anthracene single crystals back in the '80s, where they found time-of-flight mobility numbers in the range of 35 cm^2/Vs, with mobility increasing with decreasing T. More recently, look at the work done on rubrene single crystals by various groups.

    FWIW, the standard definition of semiconductor, so far as I know, refers only to the presence of a moderate (say 0.5-3 eV) gap between a band of filled states and a band of empty states.

  2. Moreover, one can certainly talk about band bending near interfaces. There is still some local electronic structure reasonably described by bands of (possibly localized) single-particle electronic states. In equilibrium across an interface the chemical potential for the electrons is uniform, meaning that the local energy levels can shift relative to the chemical potential due to charge transfer. Those energetics must self-consistently account for the charge transfer via the Poisson equation/Gauss' law.

    There are many complications that do show up in these materials (e.g., the polaronic nature of the charge carriers; the competition between polaron formation via charge transfer and the formation of interfacial dipoles at interfaces), but the framework of semiconductor physics remains useful in thinking about these systems....

  3. Doug,

    Thanks for your thoughtful comments. I appreciate them because you are actually making devices from them and have an interest in the basic underlying physics of the materials.

    I agree that there are some clean single crystal materials which exhibit evidence of band transport and so I am o.k. with referring to them as semiconductors.

    However, the bulk of organic materials used in photovoltaics and LEDs do not. In those materials I am claiming there is no band of states (all of the states are localised on single molecules) and so do not justify by the standard definition being called semiconductors.

    I agree there is local electronic structure near interfaces and the chemical potential varies spatially. But again to me that is different to claim there is a band whose top or bottom is varying with space.

    I agree there are many insights gained from comparisons with inorganic semiconductors. However, some of my provocative point is to emphasize just how different many of these materials are. Acknowledging the similarities AND differences will leader to greater insight.

  4. I think that we may be having a semantic disagreement here. When I say "band", I mean a distribution of electronic states over an energy range (in which there are so many states so closely spaced that there is essentially a continuum of states), bounded above and below by energy ranges where there are no electronic states. In these terms, the electronic states (let's ignore interactions for a moment, and say "single-particle states", though many-body states are fine, too) can be extended or localized - it doesn't matter.

    Please correct me if I'm wrong, but I think you're using "band" to refer only to extended, Bloch-like states that can be labeled by a wavevector parameter, k....

  5. Just to be clear about your points, when you say that "energy gaps associated with optical absorption and conduction are significantly different," is the reason that there are generally large reorganization energies in organic molecules, so the optical absorption occurs at the frozen (Franck-Condon) energetic structure, but transport has an activation energy smaller than the optical gap, because it takes advantage of nuclear motion?

  6. Ross, by what you're calling the "standard defn of a semiconductor" wouldn't you have to say amorphous silicon is not a semiconductor?

  7. McGinness' original mobility gap model for conduction in polyacetylene derivatives ("melanins" ) came right out of Mott's model for conduction in amorphous inorganic semiconductors such as the chalcogenide glasses. If the latter compounds are semiconductors, then so are conductive organic polymers.