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

Adam, Dieter; Schuhmacher, Peter; Simmerer, J.; Häußling, Lukas; Siemensmeyer, K.; Etzbachi, K. H.; Ringsdorf, Helmut; Haarer, D.

doi:10.1038/371141a0

Cited by 1,211 publications

(748 citation statements)

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“…Extremely large hole mobility was observed in the discotic phase. [1][2][3] Discotic liquid crystals have a potential technological importance due to their semiconducting properties combined with their processability. When the discotic liquid crystal is heated to mesophase it self-organizes in one-dimensional discotic columnar structure and this columnar structure is preserved upon cooling to a solid state.…”

Section: Introductionmentioning

confidence: 99%

Optical properties of substituted phthalocyanine rare-earth metal complexes

Yoshino

Lee

Sonoda

et al. 2000

Journal of Applied Physics

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Comparative study of optical properties of alkylthio-group-substituted phthalocyanine rare-earth metal sandwich complexes (͓(C n S͒ 8 Pc͔ 2 M,MϭEu,Lu,Tb) is presented. Photoluminescence and photoconductivity of ͓(C n S͒ 8 Pc͔ 2 M complex is very weak. Two photoluminescence bands were observed at around 400-650 and 720-800 nm in chloroform solution corresponding to the Soret and Q bands in the absorption spectra, respectively. However, the emission from Eu 3ϩ ion ͑as well as Tb 3ϩ ) was not found compared with other Eu complexes because the 5d levels of the Eu 3ϩ ion lie higher than the triplet level of the ligand. The significant enhancement of the photoconductivity of ͓(C 16 S͒ 8 Pc͔ 2 M after C 60 doping is reported. The photoconductivity is positive at the low electric field in the ohmic regime while it becomes negative at the high electric field upon photoexcitation with strongly absorbed light. The negative photoconductivity is attributed to space-charge effects. The mechanism of photoluminescence and photoconductivity are discussed by taking the electronic energy schemes of phthalocyanine ligands and lanthanide ion and C 60 into consideration.

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Section: Introductionmentioning

confidence: 99%

Optical properties of substituted phthalocyanine rare-earth metal complexes

Yoshino

Lee

Sonoda

et al. 2000

Journal of Applied Physics

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“…They exceed those measured in cross-linked networks from nematic mesogens ͑ϳ10 −4 cm 2 V −1 s −1 ͒, 4 those measured in smectic mesogens dispersed in a host network ͑ϳ10 −3 cm 2 V −1 s −1 ͒, 5 and those previously reported for cross linked films by the authors. 6 Although these values are not as large as those for single-molecular crystals, 7 or those in nonreactive liquid crystal smectic mesophases, 8 or indeed columnar mesophases, 9 the formation of a solid, stable, insoluble, and semiconducting layer, will allow easy incorporation of these materials in actual device structures ͑field-effect transistor, organic light-emitting device͒, in much the same manner as the polymeric materials are currently used. The mobility data for the various materials is summarized in Table I.…”

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confidence: 99%

High mobility ambipolar charge transport in a cross-linked reactive mesogen at room temperature

Kreouzis

Baldwin

Shkunov

et al. 2005

Applied Physics Letters

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By photochemically cross linking the molecules of a reactive end group functionalized calamitic liquid crystal ͑bis͑heptylphenyl͒-bithiophene-oxetane͒ in the smectic mesophase, an insoluble rigid molecular network is formed which remains structurally unchanged on cooling to room temperature. The charge transport is investigated by the time-of-flight method and the resulting material shows long-range ͑ϳ10 m͒ ambipolar transport with nondispersive field independent hole mobility, hole = 0.016 cm 2 / V s and dispersive field dependent electron mobility, typically electron = 0.028 cm 2 /V s ͑both at 4 ϫ 10 4 V cm −1 and at 15°C͒. These desirable charge transport properties make this system a promising candidate for organic electronic applications, such as charge transport layers in organic light-emitting diodes and field effect transistors. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.2117632͔Highly ordered thermotropic liquid crystalline mesophases display desirable high mobility charge transport; 1 these usually occur at elevated temperatures, and upon cooling, the formation of a polycrystalline solid has a detrimental effect on the charge transport 1 as the grain boundaries form charge carrier barriers. These barriers have the effect of reducing the carrier range and long-range mobility. It has been proposed that the problem of maintaining the enhanced transport properties present in the mesophase, at temperatures below the crystalline to mesophase transition, can be solved by the use of reactive mesogens ͑RMs͒. These are comprised of liquid crystal molecules with reactive end groups at the ends of the aliphatic chains. They form thermotropic mesophases and can be irreversibly photopolymerized in the presence of an appropriate photoinitiator, forming a stable network. It is expected that as the morphology is frozen-in, the transport properties will be stabilized. 2,3 The formation of an insoluble semiconducting film has the advantage of simplifying the fabrication of multilayer devices using sequential solution-based application methods to add further layers. 3 In addition, RMs can be patterned using optical masks.Initial time-of-flight ͑TOF͒ studies on a nonreactive calamitic liquid crystal bis͑alkylphenyl͒-bithiophene ͑PTTP͒ ͑Fig. 1͑a͔͒ in the smectic phase ͑SmG͒ showed ambipolar transport ͓see Fig. 2͒ with resulting hole mobility hole = 0.044 cm 2 V −1 s −1 and electron mobility electron = 0.07 cm 2 V −1 s −1 ͑these are both dispersive and field dependent, quoted here at 4 ϫ 10 4 V cm −1 ͒, indicating that this particular calamitic core was a promising candidate for the preparation of an oxetane containing reactive mesogen. Being a thermotropic liquid crystal at elevated temperatures, this behavior was, of course, lost at room temperature, yielding featureless trapping decays in the photocurrent transients.The RM used in the cross linking experiment ͑PTTP-oxetane͒ is shown in Fig. 1͑b͒. This was mixed with 0.5 wt % cationic initiator ͑Meerkat͒ and the liquid crystal cell ͑thickness, d =10 m͒ was fil...

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“…6 For example, LC materials with a rod-shaped molecular structure, 7 in which − electron conjugated rigid skeletons are aligned in one direction with a layered structure in the smectic phase. Another category of LC semiconductors 8 features large, two-dimensional, disk-shaped − electron conjugated molecular cores, which are stacked in close molecular alignment in the columnar phase.…”

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confidence: 99%

Time-of-flight analysis of charge mobility in a Cu-phthalocyanine-based discotic liquid crystal semiconductor

Fujikake¹,

Murashige²,

Sugibayashi

et al. 2004

Applied Physics Letters

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We used a time-of-flight method to study the charge carrier mobility properties of a molecular-aligned discotic liquid crystal semiconductor based on Cu-phthalocyanine. The heated isotropic-phase semiconductor material was sandwiched between transparent electrodes coated onto glass substrates without conventional alignment layers. This was then cooled, and a discotic liquid crystal semiconductor cell was obtained, which we used to make mobility measurements. The material had a fixed molecular alignment due to the supercooling of the hexagonal columnar mesophase. It was clarified that the carrier mobility for electrons was as high as it was for holes at room temperature. The maximum value of negative charge mobility reached 2.60×10−3cm2∕Vs, although negative carrier mobility is often much lower than positive carrier mobility in other organic semiconductors, including conventional Cu-phthalocyanine vacuum-deposited films.

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

Cited by 1,211 publications

References 18 publications

Optical properties of substituted phthalocyanine rare-earth metal complexes

Optical properties of substituted phthalocyanine rare-earth metal complexes

High mobility ambipolar charge transport in a cross-linked reactive mesogen at room temperature

Time-of-flight analysis of charge mobility in a Cu-phthalocyanine-based discotic liquid crystal semiconductor

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