Crystal Growth, Structure, Electronic Band Structure and Electrical Properties of a Pentacene Polymorph
Abstract
Pentacene, C_22H_14, is a highly promising materials for application in thin film transistor devices since recently, high mobilities and good semiconducting behavior have been reported. For this reason we have attempted to grow single crystals of pentacene from the vapor phase. Using a vapor phase transport in a stream of carrier gas, millimeter size crystals were produced. A new polymorph of pentacene was found to crystallize in the triclinic space group P(-1) (Nr.2) with a 3% reduced volume per molecule over the previously reported structure. Both of the pentacene polymorphs, one grown by a vapor phase transport and second known from literature, grown from solution, show a molecular crystal with well spaced molecules and without contacts within the sum of the Van der Waals spacing for pairs of carbon atoms. The mobility and bandwidth of a new polymorph of pentacene, are found to be much larger than expectations based on its crystal structure and calculations of its band structure. Band structure calculations using extended Hückel theory (EHT) support the picture of the pentacene lattice with very little dispersion in the bands near the Fermi energy. In spite of this apparent absence of a well defined conduction pathway in the lattice, our optical measurements show a reasonable bandwidth and our transport measurements show a mobility that is high and increasing as the crystal is cooled. It is not clear yet if there exist a third polymorph of pentacene, since a different than both reported polymorphs, (001) d-spacing has been recorded in thin films. The small differences in the molecular packing seem to affect the electrical properties and enhance or reduce the carrier mobility.
- Publication:
-
APS March Meeting Abstracts
- Pub Date:
- March 2000
- Bibcode:
- 2000APS..MARC21003K