Transient photoconductivity in a discotic hexagonal plastic crystal

Relationship between structure and electroluminescence of oligo(p-phenylenevinylene)s

The preparation of LEDs with poly(p-phenylenevinylene) (PPV) as emitting material is well established, However, due to the presence of a distribution of conjugated chain lengths in the polymer, systematic investigations of the electroluminescence with polymeric materials are difficult, as far as the optical emission is concerned. We are studying the relationship between structural variation of substituted oligo(p-phenylenevinylene)s and their electroluminescent behaviour using a series of distyrylbenzenes with a variety of substituents in order to investigate their influence on the electroluminescence (EL). Furthermore, we synthesized a homologous series of monodisperse oligo(2,5-dipropoxy-…

Photoconductivity in the columnar phases of a glassy discotic twin

Discotic Liquid Crystals - A New Class of Fast Photoconductors

We showed for the first time that discotic liquid crystals are well suited for a new class of fast photoconducting materials. Due to their spontaneous orientation and their dynamical fluctuations in the mesophase, they show exceptionally high mobilities of 1·10−3 cm2/Vs, about two to three orders of magnitude higher than those obtained for conventional amorphous polymers. Further on, the Gaussian transport (for holes in the mesophase) is remarkable, which is characterized by the existence of a conduction band and the absence of trapping states. In contrast, the charge carrier transport in amorphous photoconductors is generally trap-dominated which limits technical properties, leading to low…

Fast photoconduction in the highly ordered columnar phase of a discotic liquid crystal

THE search for organic materials suitable for electronic applica-tions dates back to the early 1950s. But the only organic systems known so far to show electronic charge-carrier mobilities comparable to the amorphous inorganic semiconductors that are the main-stay of the microelectronics industry are zone-refined organic single crystals1–4. Single crystals are difficult and costly to process, however, and are not suitable for device applications. Here we show that a highly ordered columnar (stacked) phase of disk-like organic molecules can exhibit high mobilities for photoinduced charge carriers, of the order of 0.1 cm2 V-1 s-1—higher than for any organic material other than single-crystal …

The mobility of charge carriers in all four phases of the columnar discotic material hexakis(hexylthio)triphenylene: Combined TOF and PR-TRMC results

Electronic Properties of Discotic LC-Materials

Abstract We have made extensive studies of the charge-carrier mobilities in the discotic hexagonal mesophases of triphenylene-based discotic liquid crystals. Using the time-of-flight technique, transient photocurrents were measured yielding charge-carrier mobilities for various electric fields and temperatures. Starting from promising results obtained with the monomeric discotic liquid crystalline model compound hexapentyloxytriphenylene (H5T), we synthesized a “discotic twin” with two triphenylene units linked together by a suitable spacer. Additionally, we synthesized a discotic liquid crystalline oligomer consisting of four triphenylene units which are bound to a flexible cyclosiloxane r…

Photoconductivity in Discotic Liquid Crystals: A New Class of High-Mobility Materials

Abstract Using a time-of-flight technique, different transport mechanisms, deep trapping, multiple shallow trapping and Gaussian transport, can be observed in the different temperature and phase regions of the liquidcrystalline (LC) photoconductor hexapentyloxytriphenylene (HPT). Transient photocurrents and carrier mobilities for various temperatures, electric fields, and sample histories were examined. The ideal intrinsic Gaussian transport, observed for holes in the mesophase, puts HPT into a new class of highmobility materials with both hole mobilities on the order of 1.10−3cm2/Vs and a steplike current decay. These features result from the fact that there is obviously neither a position…

Light-emitting diodes based on phenylenevinylene oligomers with defined chain lengths

Abstract We present characteristic electroluminescence data on single-layer devices based on three different oligo phenylenevinylene compounds with defined chain lengths. Light-emitting diodes (LEDs) were prepared by subsequent vacuum evaporation of the oligomer film and the A1 top electrode onto an indium—tin oxide (ITO) substrate. The LEDs show light emission in the spectral range of yellow to orange depending on the conjugation length of the compound used. We report photoluminescence and optical absorption data of the thin sublimed films, together with the wavelength dependence of the electroluminescence and with the current—voltage characteristics of the LED devices. The redox behavior …