We show that optical and electrical measurements on pentacene single crystals can be used to extract the density of states in the highest occupied molecular orbital–lowest unoccupied molecular orbital band gap. 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 fabricated from similar crystals imply that the gap state density is much larger within 5–10 nm of the gate dielectric. Thus, organic thin-film transistors for such applications as flexible displays might be significantly improved by reducing these defects.

• Received 4 December 2003

DOI:https://doi.org/10.1103/PhysRevLett.93.086802