ELECTRON-TRANSPORT ACROSS METAL DISCOTIC LIQUID-CRYSTAL INTERFACES/

Electron transport across micron thick films of columnar hexagonal dis cotic liquid crystal phases homeotropically aligned between metal elec trode surfaces has been studied both experimentally and theoretically. These molecules are unique in their combination of charge transport a long individual molecular columns with liquidlike self-organization. T ypical of organic insulators, a high resistance Ohmic regime is eviden t at fields of less than 0.05 MV cm(-1), due to a low concentration of chemical impurities (n<10(9) cm(-3)), and a space-charge injection re gime at higher fields. Breakdown fields are reasonably high: in hexaki s(hexyloxy)triphenylene they reach similar to 5 MV cm(-1) at room temp erature. Our results show that triphenylene-based discotics form an ex cellent class of highly ordered optically transparent insulators. At h igh temperatures and high fields the current is injection controlled a nd exhibits typical tunneling and space charge limited, nonlinear I-V characteristics. Dramatic jumps in injection currents are observed at phase transitions. The change at the crystalline to liquid crystalline phase transition is mainly due to more efficient ''wetting'' of the e lectrode surface in the liquid crystalline phase, whilst at the liquid crystalline to isotropic phase transition it arises from the enhancem ent in the molecular mobility. The concepts of semiconducting gaps, ba nd mobilities, and carrier injection rates are extended to these new m aterials. The experimental observations are interpreted in a framework which takes into account the important role played by liquidlike dyna mics in establishing the microscopic structural order in, what is, oth erwise a highly anisotropic and weakly bonded ''molecular crystal.'' ( C) 1998 American Institute of Physics.