Heiko Thiem

Characterization of thick film poly(triarylamine) semiconductor diodes for direct x-ray detection

Thick film (≥5 μm thick) semiconducting polymer diodes incorporating poly(triarylamine) (PTAA) have been produced and applied as direct x-ray detectors. Experiments determined that a rectifying diode behavior persists when increasing the thickness of the active layer above typical thin film thicknesses (<1 μm), and the electrical conduction mechanism of the diodes has been identified. Direct current and photoconductivity measurements on indium tin oxide/poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)/PTAA/metal diodes confirm that carrier conduction occurs via a Poole–Frenkel mechanism. The energy band structure of diodes (having gold or aluminum top electrodes) has been elucidated and used to explain the resulting electrical characteristics. Theoretical calculations show that, upon irradiation with x-rays, the diode quantum efficiency increases with increasing polymer film thickness. The diodes produced here display characteristics similar to their thin film analogs, meaning that they may be oper...

Electronic Properties and Supramolecular Organization of Terminal Bis(alkylethynyl)-Substituted Benzodithiophenes

Benzodithiophene (BDT) was symmetrically bisubstituted in the terminal positions with five different alkynes C≡C-(C(n)H(2n+1)) with n = 4, 6, 8, 10, 12. The materials were characterized as potential materials for field-effect transistor applications. Electrochemical measurements in solution and photophysical measurements in solution and in the solid state, together with UV photoelectron spectroscopy in air and quantum-chemical calculations, elucidate the nature of the frontier orbitals and of the excited states as well as their deactivation pathways. Structural information on the molecular assembly in the solid state, both at room temperature and at elevated temperatures, is obtained by a combination of DSC, polarized optical microscopy, and 2D-WAXS, which point to the crystallinity of the compounds in all phases and reveal π-stacking arrangements independently of the length of the alkyl side chains.

New fluorene-bithiophene based oligomers for the use in organic field effect transistors

We report the synthesis of oligo(9,9-dioctyl-2,7-diyl)-alt-bithiophenes 4a–e. The molecular weight was tuned by an end-capping reaction with the monofunctional 2-bromo-9,9-di-(n-octyl)-fluorene 2. All oligomers showed nematic mesophases and no sign for crystallisation upon cooling. The clearing temperatures were in the range from 80 to 288°C in oligomers with different molecular weight. A plot of 1/P n versus the clearing temperature resulted in a clearing temperature of 366°C for the ideal polymer. First orientation experiments on rubbed polyimide films gave a maximum orientation of 8.2 to 1 parallel and perpendicular to the rubbing direction.