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Heinz Von Seggern - One of the best experts on this subject based on the ideXlab platform.

  • trap controlled Hole Transport in small molecule organic semiconductors
    Applied Physics Letters, 2007
    Co-Authors: Arne Fleissner, Hanna Schmid, Christian Melzer, Heinz Von Seggern
    Abstract:

    The influence of trap concentration on Hole Transport is investigated by an optical time-of-flight method for the amorphous small molecule organic semiconductor N,N′-bis(1-naphtyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamin (α-NPD) doped with neutral Hole traps by codeposition of 4,4′,4″-tris-[N-(1-naphtyl)-N-(phenylamino)]-triphenylamine (1-NaphDATA). α-NPD doped with 120ppm 1-NaphDATA exhibits nondispersive Hole Transport like undoped α-NPD, but trap-controlled with reduced mobility. The trap depth derived from the mobility decrease coincides with the ionization potential difference of α-NPD and 1-NaphDATA. The transition to dispersive Transport for increasing trap concentration to 1160ppm is explained by an energetic relaxation of optically generated charge carriers within a density of states broadened by traps.

  • transition from non dispersive to dispersive Hole Transport in a small molecule organic semiconductor controlled by molecular doping
    MRS Proceedings, 2006
    Co-Authors: Arne Fleissner, Hanna Schmid, Christian Melzer, Roland Schmechel, Heinz Von Seggern
    Abstract:

    The influence of charge carrier traps on charge carrier Transport is studied in a small molecule organic semiconductor model system by means of an optical time-of-flight method. The model system consists of the Hole Transport material N,N'-di(1-naphtyl)-N,N'-diphenylbenzidine (α-NPD, sometimes denoted as α-NPB) either undoped or doped with various concentrations of the small molecule 4,4',4”-tris-[N-(1-naphtyl)-N-(phenylamino)]-triphenylamine (1-NaphDATA), which is known to create Hole traps in α-NPD. In case of undoped α-NPD, non-dispersive Hole Transport is observed and the Hole mobility is determined as 6·10 −4 cm 2 /Vs in the examined electric field range, being in good agreement with published data. Depending on the intensity of the laser light employed for optical charge carrier generation, current transients both in the space-charge regime and in the small signal case are obtained. In the small signal case the current transients do not exhibit the expected flat current plateau before the characteristic kink that marks the transit time, but feature a cusp instead. A tentative mechanism for its formation is proposed. The influence of the trap concentration on charge carrier Transport is studied by introducing 1-NaphDATA as a molecular dopant. It is demonstrated that the Hole Transport in α-NPD can be controlled by varying the doping concentration of 1-NaphDATA. Increasing the trap concentration, a transition from non-dispersive Transport in undoped α-NPD to non-dispersive but trap-controlled Transport with reduced mobility and further to dispersive Transport is observed.

  • trap engineering in organic Hole Transport materials
    Journal of Applied Physics, 2001
    Co-Authors: N Von Malm, Jurgen Steiger, Roland Schmechel, Heinz Von Seggern
    Abstract:

    Chemical impurities with known highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital energies were incorporated in organic Hole Transport materials. The effect of these dopants on quantity and depth of trap levels, Transport properties, and luminescence of organic light emitting devices was examined. This was achieved by investigating current–voltage characteristics, luminance–voltage characteristics, and utilizing the method of thermally stimulated current for trap level detection. It was found that 4,4′,4″-tris-[N-(1-naphthyl)-N-(phenylamino)]triphenylamine (1-NaphDATA) doped into N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (α-NPD) generates a trap level whose activation energy corresponds to the HOMO energy difference between dopant and matrix molecules. Therefore, the detected electronic states can be assigned to Hole traps. The influence of those traps on the charge Transport will be reported. For doping α-NPD into 1-NaphDATA no new trap levels could be detected.

Anders Hagfeldt - One of the best experts on this subject based on the ideXlab platform.

Norio Nagayama - One of the best experts on this subject based on the ideXlab platform.

  • anisotropic surface Hole Transport property of triphenylamine derivative single crystal prepared by solution method
    Applied Surface Science, 2016
    Co-Authors: Minoru Umeda, Mitsuhiko Katagiri, Sayoko Shironita, Norio Nagayama
    Abstract:

    Abstract This paper reports the anisotropic Hole Transport at the triphenylamine-derivative single crystal surface prepared by a solution method. Triphenylamine derivatives are commonly used in a Hole-Transport material for organic photoconductors of laser-beam printers, in which the materials are used as an amorphous form. For developing organic photovoltaics using the photoconductor’s technology, preparation of a single crystal seems to be a specific way by realizing the high mobility of an organic semiconductor. In this study, a single crystal of 4-(2,2-diphenylethenyl)-N,N-bis(4-methylphenyl)-benzenamine (TPA) was prepared and its anisotropic Hole-Transport property measured. First, the Hole-Transport property of the TPA was investigated based on its chemical structure and electrochemical redox characteristics. Next, a large-scale single crystal formation at a high rate was developed by employing a solution method based on its solubility and supersolubility curves. The grown TPA was found to be a single crystal based on the polarization micrograph observation and crystallographic analysis. For the TPA single crystal, an anisotropic surface conduction was found, which was well explained by its molecular stack structure. The measured current in the long-axis direction is one order of magnitude greater than that of amorphous TPA.

Karl Leo - One of the best experts on this subject based on the ideXlab platform.

  • Hole Transport material variation in fully vacuum deposited perovskite solar cells
    APL Materials, 2014
    Co-Authors: Lauren E Polander, Paul Pahner, Martin Schwarze, Matthias Saalfrank, Christian Koerner, Karl Leo
    Abstract:

    This work addresses the effect of energy level alignment between the Hole-Transporting material and the active layer in vacuum deposited, planar-heterojunction CH3NH3PbIx−3Clx perovskite solar cells. Through a series of Hole-Transport materials, with conductivity values set using controlled p-doping of the layer, we correlate their ionization potentials with the open-circuit voltage of the device. With ionization potentials beyond 5.3 eV, a substantial decrease in both current density and voltage is observed, which highlights the delicate energetic balance between driving force for Hole-extraction and maximizing the photovoltage. In contrast, when an optimal ionization potential match is found, the open-circuit voltage can be maximized, leading to power conversion efficiencies of up to 10.9%. These values are obtained with Hole-Transport materials that differ from the commonly used Spiro-MeO-TAD and correspond to a 40% performance increase versus this reference.

  • low voltage organic light emitting diodes featuring doped phthalocyanine as Hole Transport material
    Applied Physics Letters, 1998
    Co-Authors: Jan Blochwitz, Martin Pfeiffer, Torsten Fritz, Karl Leo
    Abstract:

    We show that doping of the Transport layers can strikingly improve the properties of organic light emitting diodes (OLEDs). The electroluminescence onset voltage of diodes containing an vanadyl–phthalocyanine (VOPc) Hole Transport layer intentionally doped with tetrafluorotetracyano-quinodimethan (F4-TCNQ) is reduced by up to an order of magnitude compared to OLED with undoped VOPc. The improved properties of our devices can be explained by the improved conductivity and better injection for a doped Transport layer.

Lars Kloo - One of the best experts on this subject based on the ideXlab platform.