Martin Pfeiffer

Heliatek (Germany)

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Low-voltage organic light-emitting diodes by doped amorphous hole transport layers

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE (2002)

Jan Blochwitz Martin Pfeiffer Xin Zhou Ansgar Werner Michael Hofmann

Organic light emitting diodes generally suffer from higher operating voltages compared to inorganic ones. This limits their application in passive or active driven displays based on OLED-technology. As was previously shown by our group, p-type doping of the hole injection and transport layer of an organic light emitting diode (OLED) by co-evaporation of a matrix and an acceptor molecule leads to lower operating voltages of the device. In OLEDs using doped transport layers, the use of a proper buffer layer between the doped layer and the light emission layer is essential to yield a high current efficiency and a low operating voltage at the same time. In order to further enhance the device efficiency, we apply here the doping concept to OLEDs with a light emission layer which is doped with a fluorescent dye. This approach proves that doping of the transport layer is able to improve the optoelectronic properties of already highly efficient OLEDs. The doped hole injection and hole transport layer is a Starburst layer p-type-doped with tetrafluoro-tetracyano-quinodimethane (F₄-TCNQ). As blocking layer, a diamine (TPD) is used. The emitter layer consists of quinacridone (QAD) doped aluminum-tris-(8-hydroxy-quinolate) (Alq₃). Holes are injected from untreated ITO, electrons via a lithium-fluoride (LiF)/aluminum cathode combination. For this OLED layer sequence, we achieved a luminance of 100cd/m² in forward direction at the lowest operating voltage reported for completely non-polymeric OLEDs (3.2-3.4V) with a current efficiency of around 10cd/A.

10.1117/12.457498

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Citations (6)

n‐Type Doping of Organic Thin Films Using Cationic Dyes

Advanced Functional Materials (2004)

Anett Werner F. Li Kentaro Harada Martin Pfeiffer Torsten Fritz

Abstract We present an approach to stable n‐type doping of organic matrices using organic dopants. In order to circumvent stability limitations inherent to strong organic donors, we produce the donor from a stable precursor compound in situ. As an example, the cationic dye pyronin B chloride is studied as a dopant in a 1,4,5,8‐naphthalene tetracarboxylic dianhydride (NTCDA) matrix. Conductivities of up to 1.9 × 10 –4 S cm –1 are obtained for doped NTCDA, two orders of magnitude higher than the conductivity of NTCDA doped with bis(ethylenedithio)‐tetrathiafulvalene as investigated previously, and four orders of magnitude higher than nominally undoped NTCDA films. Field‐effect measurements are used to prove n‐type conduction and to study the doping effect further. The findings are interpreted using a model of transport in disordered solids using a recently published model. Combined FTIR, UV‐vis, and mass spectroscopy investigations suggest the formation of leuco pyronin B during sublimation of pyronin B chloride.

Cationic polymerization

Tetrathiafulvalene

Sublimation

10.1002/adfm.200305053

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Citations (98)

Kuṛux Index

BRILL eBooks (1972)

Martin Pfeiffer

10.1163/9789004643932_016

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Citations (0)

Electrical characteristics of Au-PbPc-Au thin film sandwich devices on exposure to O₂and Cl₂

International Journal of Electronics (1994)

R. A. Collins A. K. Abass Martin Pfeiffer

The semiconducting characteristics of Au-PbPc-Au thin film sandwich devices were studied as a function of exposure to oxygen (in the form of air) and chlorine. Both O2 and Cl2 were found to generate acceptor levels within the PbPc bandgap. Both ohmic and space charge-limited conduction were observed. The experimental data were analysed within the framework of the model of Schmidlin and Roberts. Devices exposed to air for extended periods showed Schottky behaviour. The width of the Schottky barrier was estimated as60nm, the barrier height 15eV and the rectifying ratio at ±1 V was 200. The Schottky barrier disappeared after vacuum annealing at 423 K.

Ohmic contact

Acceptor

Metal–semiconductor junction

Air gap (plumbing)

10.1080/00207219408925978

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Citations (3)

From ant-catcher to author: editors’ advice to young myrmecologists

Asian Myrmecology (2013)

John R. Fellowes Martin Pfeiffer

Advice (programming)

Source

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Citations (0)

Fermi level determination in organic thin films by the Kelvin probe method

Journal of Applied Physics (1996)

Martin Pfeiffer Karl Leo N. Karl

We show that the Kelvin probe method (KPM) is well suited for in situ measurements of the Fermi level of organic vapor-deposited dye layers. The method works well even for high resistivity materials, is nondestructive, and does not need any top contacts. First results for zinc-phthalocyanine (ZnPc) and perfluorinated ZnPc (ZnPc–F16) are presented. The KPM results show that a 2.5% admixture of ZnPc–F16 to a ZnPc matrix leads to a 0.2 eV lowering of the Fermi level of ZnPc, i.e., ZnPc–F16 has acceptorlike properties relative to ZnPc.

Organic semiconductor

10.1063/1.363757

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Citations (72)

Impact of rain-forest logging on helminth assemblages in small mammals (Muridae, Tupaiidae) from Borneo

Journal of Tropical Ecology (2007)

Konstans Wells Lesley R. Smales Elisabeth K. V. Kalko Martin Pfeiffer

Parasites are ubiquitous in wild animals, with host-specific life histories considered as major determinants of prevalence and parasite assemblage patterns. It is predicted that habitat differences in logged rain forests influence population performances of small mammals and consequently may change the infection patterns of local animal populations with regard to endo- and ectoparasites. We investigated patterns of helminth species assemblages (Nematoda, Platyhelminthes) in two rat species ( Leopoldamys sabanus , Niviventer cremoriventer ) and two tree shrew species ( Tupaia tana , T. longipes ) in three logged and three unlogged rain forests in Borneo by examining 337 faecal samples with non-invasive faecal egg count (FEC). Nematode eggs prevailed in 95% of all samples with up to five (mean 1.9 ± 1.1) morphotypes. Whereas members of Strongylida were most prevalent in L. sabanus , T. tana and T. longipes , Spirurida dominated in N. cremoriventer that revealed at the same time the lowest average nematode prevalence and FEC. Cestode eggs were only found in L. sabanus and T. tana . Composition and abundance patterns of the parasitic helminth assemblages were influenced by logging. As hypothesized, species richness of nematode morphotypes and mean number of infections per host of T. longipes were larger in logged than in unlogged forest. In contrast, L. sabanus was more heavily infected with cestodes in unlogged than in logged forest and also revealed larger egg counts for strongylids and spirurids in unlogged forest. Our results suggest that forest degradation and altered environmental conditions influence helminth diversity and infection patterns of small mammals with contrasting trends among host species. The inconsistent logging-induced changes in helminth assemblages from different hosts indicate that specific sets of habitat-host-parasite interactions are uniquely influenced by the effects of logging. Consequently, predictions on changes of parasite diversity and prevalence with regard to habitat disturbance need to be based on the individual life histories of the hosts (and the parasites).

10.1017/s0266467406003804

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Citations (52)

Leuco Crystal Violet as a Dopant for n-Doping of Organic Thin Films of Fullerene C₆₀

The Journal of Physical Chemistry B (2004)

Fenghong Li Ansgar Werner Martin Pfeiffer Karl Leo Xianjie Liu

The triphenylmethane dye crystal violet (CV) and its leuco base, leucocrystal violet (LCV), are investigated as dopants for n-type doping of fullerene C60. Conductivities up to 8 × 10-3 S/cm at 30 °C are achieved when C60 is doped with CV. Mass spectroscopy and optical spectroscopy (UV/VIS/NIR absorption and Fourier transform infrared transmission) confirm that the leuco base LCV is formed during sublimation of the cationic CV dye. When the commercially available LCV was directly used as a dopant in C60, a maximum conductivity of 1.3 × 10-2 S/cm was obtained at 30 °C. We found that in both cases, the leuco base became reoxidized to the cationic form by electron transfer to electron-accepting matrixes, leading to the doping effect. The donor properties of LCV in a charge-transfer complex with 7,7,8,8-tetracyanoquinodimethane (TCNQ) were confirmed by UV/VIS/NIR absorption and Fourier transform infrared (FTIR) spectroscopy. C60 anions were observed in the FTIR or NIR absorption spectra of the mixed films of C60 and LCV. Photoinduced charge transfer between LCV and C60 provides free electrons, which increase the n-type conductivity. The electron transfer becomes irreversible by hydride abstraction.

Tetracyanoquinodimethane

Crystal violet

10.1021/jp0478615

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Citations (120)

Very-low-operating-voltage organic light-emitting diodes using a p-doped amorphous hole injection layer

Applied Physics Letters (2001)

Xin Zhou Martin Pfeiffer Jan Blochwitz Anett Werner A. Nollau

We demonstrate the use of a p-doped amorphous starburst amine, 4, 4′, 4″-tris(N, N-diphenyl- amino)triphenylamine (TDATA), doped with a very strong acceptor, tetrafluoro- tetracyano-quinodimethane by controlled coevaporation as an excellent hole injection material for organic light-emitting diodes (OLEDs). Multilayered OLEDs consisting of double hole transport layers of p-doped TDATA and triphenyl-diamine, and an emitting layer of pure 8-tris-hydroxyquinoline aluminum exhibit a very low operating voltage (3.4 V) for obtaining 100 cd/m2 even for a comparatively large (110 nm) total hole transport layer thickness.

Triphenylamine

Acceptor

10.1063/1.1343849

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Citations (310)

Efficient Vacuum‐Deposited Organic Solar Cells Based on a New Low‐Bandgap Oligothiophene and Fullerene C₆₀

Advanced Materials (2006)

Kerstin Schulze Christian Uhrich R. Schüppel Karl Leo Martin Pfeiffer

Organic solar cells based on a new oligothiophene derivative (see figure) and fullerene C60 exhibit power efficiencies of up to 3.4 %. α,α′-Bis(2,2-dicyanovinyl)-quinquethiophene (DCV5T) features a reduced optical gap with high absorption coefficients between 450 and 650 nm. Dicyanovinyl substituents on the oligothiophene rings lower the bandgap and increase the ionization energy of these oligomers, enabling the formation of photoactive heterojunctions with C60 and yielding open-circuit photovoltages as high as 1 V.

10.1002/adma.200600658

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Citations (324)