A lot of papers on materials for organic electronics and photonics will discuss transport and optical properties in terms of conduction and valence bands, concepts that are valid and useful for inorganic crystalline semiconductors.
But, I do not think such bands exist for most of these materials. This can be seen from the magnitude of the transport mobility. These notes show a simple self-consistency argument which shows that if band transport is meaningful (i.e., one can talk about electrons with a definite wavevector and which are occasionally scattered) then the mobility must be much larger than about 1 cm^2/Vsec.
I derived this result over a year ago but then discovered this appears to have been well known back in the 70's, and seems to have been forgotten.
For example, the result is clearly stated:
in equation (24) of a 1963 paper by Glarum.
in equation (224) on page 24, of a classic 1971 review of Metallic Oxides by John Goodenough
page 346, of the second edition of Pope and Svenberg's Electronic processes in organic crystals and polymers.