  We show that optical and electrical measurements on pentacene single crystals
can be used to extract the density of states in the HOMO-LUMO bandgap. It is
found that these highly purified crystals possess band tails broader than those
typically observed in inorganic amorphous solids. Results on field effect
transistors (FETs) fabricated from similar crystals are also compared. The FET
data imply that the gap state density is much larger within 5-10 nm of the gate
dielectric. These results are discussed in terms of both crystal phase domains
and structural disorder mechanisms.
