To investigate the impact of impurities on properties of single crystals of rubrene (C<sub>48</sub>H<sub>24</sub>), we consider structural,
charge transport and optical properties. We show that transport properties are strongly influenced by the presence of a
bandgap acceptor state, commonly occurring in crystals of rubrene. The bandgap state is likely caused by incorporation
of an oxygen-related impurity. We demonstrate that the impurity presence can be detected using optical spectroscopy.
The impurity model explains commonly-observed variations of rubrene properties. The acceptor-like characteristics of
the impurity suggests a possible approach for modification of the charge transport in molecular crystals.
The field-effect mobility in crystalline organic FETs is known to be an order of magnitude larger than the mobility observed in thin-film OFETs. We have shown for state-of-the-art crystalline pentacene that the density of gap states, and hence the field-effect mobility, is still limited by residual impurities and disorder in the material. We use photoconductivity and space-charge-limited current (SCLC) in pentacene single crystals to extract the density of states in the HOMO-LUMO bandgap. We find that purified crystals still possess band tails broader than those typically observed in inorganic amorphous solids. Results on field effect transistors (FETs) fabricated from similar crystals imply that the gap state density is much larger within 5-10 nm of the gate dielectric. We also have observed a defect generation phenomenon that is new to organic semiconductors. We use SCLC to study a defect in pentacene single crystals that can be created by bias stress and persists at room temperature for an hour in the dark but only seconds with 420nm illumination. The defect gives rise to a hole trap at E<sub>v</sub> + 0.38eV and causes metastable transport effects at room temperature. Creation and decay rates of the hole trap have a 0.67eV activation energy with a small (10<sup>8</sup> sec<sup>-1</sup>) prefactor, suggesting that atomic motion plays a key role in the generation and quenching process. Clearly, such defect reactions could be a factor in the stability of pentacene OFETs.