Hans-Heinrich Hörhold

Efficient Titanium Oxide/Conjugated Polymer Photovoltaics for Solar Energy Conversion

[19] Since the mixing ratio of the anions in the solution from which the crystals were grown was revealed to be almost identical to the stoichiometry determined by EPMA of the obtained crystal, the mixing ratio of the anions in the solution was adopted in the chemical formula of the crystal. The results of EPMA of k-(BETS)2FexGa1‐xBr1.0Cl3.0 are: for x = 0.50, Fe/Ga/Br/Cl = 0.51:0.49:1.13:2.85 (0.50:0.50:1.00:3.00); for x = 0.40, Fe/Ga/Br/Cl = 0.40:0.60:0.96:2.87 (0.40:0.60:1.00:3.00); for x = 0.30, Fe/Ga/Br/Cl = 0.32:0.68:1.11:3.07 (0.30:0.70:1.00:3.00); for x = 0.20, Fe/Ga/Br/Cl = 0.24:0.76:1.11:3.16 (0.20:0.80:1.00:3.00); for x = 0.10, Fe/Ga/Br/Cl = 0.12:0.88:1.04:3.22 (0.10:0.90:1.00:3.00). [20] Recent susceptibility measurements revealed that the angle between the easy axis of the antiferromagnetic structure and the c axis is about 35 in k-(BETS)2FeCl4 (E. Ojima, T. Sasaki, private communication). While the easy axis of k-(BETS)2FeBr0.6Cl3.4 is approximately parallel to the b* axis.

CHARGE CARRIER TRANSPORT IN CONJUGATED POLYMERS

The charge carrier mobility in a ladder-type poly(paraphenylene) (MeLPPP) and a phenylamino-substituted poly(phenylenevinylene) (PPV) derivative (PAPPV) has been studied in detail by using the time-of-flight (TOF) method. In most cases, the TOF signals feature characteristics of nondispersive charge transport in terms of the disorder formalism, although the transition from nondispersive to the dispersive transport regime was observed in PAPPV. Hole mobilities in the range of 10−4 to 7×10−3 cm2/Vs were obtained, depending on electric field and temperature. The influence of disorder on charge transport of the investigated conjugated polymers is discussed.

Enhanced brightness in organic light-emitting diodes using a carbon nanotube composite as an electron-transport layer

We have studied the effects of using a composite fabricated from carbon nanotubes and a host polymer, poly(m-phenylene-vinylene-co-2,5-dioctyloxy-p-phenylene-vinylene) (PmPV), as an electron-transport layer in organic light-emitting diodes. Double layer devices using this composite as an electron-transport layer, triple layer devices with a composite electron-transport layer, and poly(9-vinylcarbazole) as a hole-transport layer, as well as poly(2,5-dimethoxy-1,4phenylene-vinylene-2-methoxy- 5(2′-ethylhexyloxy)-1,4-phenylene-vinylene (M3EH-PPV) single layer devices were prepared. Current–voltage–luminance and electroluminescent spectral measurements were performed using six different nanotube powder to polymer mass ratios (0%, 2%, 4%, 8%, 16%, and 32%) for all device structures studied. dc transport and photoluminescence behavior of the polymer-nanotube composite were also investigated. Although a potential barrier is introduced at the M3EH-PPV/composite interface, a significant increase in efficiency was ...

Photovoltaic properties and exciplex emission of polyphenylenevinylene-based blend solar cells

By studying the photoluminescence emission and photovoltaic properties of blends of polyphenylenevinylene (PPV)-based electron donating and accepting polymers, the authors observed a strict anticorrelation between relative exciplex emission in the solid state and photovoltaic efficiency of corresponding blend devices. Comparative studies on defined bilayer geometries showed that the overall shape of the current-voltage characteristics under illumination is independent of device geometry and layer thickness. Consequently, they conclude that the photocurrent in PPV-based blends is mainly determined by the efficiency to form free carriers rather than by free carrier recombination.

Novel light emitting and photoconducting polyarylenevinylene derivatives containing phenylene arylamine and phenylene oxide units in the main chain

Abstract A novel type of high molecular and film forming PAV-derivative, namely alternating PAV/PPV copolymers 3–4 , has been prepared by polycondensation using the principle of HORNER reaction (PO-activated olefination). In these polymers backbone conjugation is interrupted by arylamino or ether groups to result in segmented poly(arylene vinylene)s. They consist of well-defined distyrylbenzene (DSB) blocks. Thin films exhibit both photoluminescence and electroluminescence, emitting blue ( 3b ), green ( 3a ), and green yellow ( 4b ) light. It has been found that band gap and oxidation potential are strongly dependent on the nature of Y that joins the DSB units. Thus, model compounds M 3 , M 4 containing only one DSB unit reflect well the characteristic fluorescence and redox behaviour of the respective polymers. The N-containing copolymers 3a , 4a due to their low oxidation potential (0,6 – 0,8 V vs. Ag/AgCl) are of particular interest as photoconducting and electroluminescent materials.

Near-infrared sensitivity enhancement of photorefractive polymer composites by pre-illumination

Among the various applications for reversible holographic storage media, a particularly interesting one is time-gated holographic imaging (TGHI). This technique could provide a noninvasive medical diagnosis tool, related to optical coherence tomography. In this technique, biological samples are illuminated within their transparency window with near-infrared light, and information about subsurface features is obtained by a detection method that distinguishes between reflected photons originating from a certain depth and those scattered from various depths. Such an application requires reversible holographic storage media with very high sensitivity in the near-infrared. Photorefractive materials, in particular certain amorphous organic systems, are in principle promising candidate media, but their sensitivity has so far been too low, mainly owing to their long response times in the near-infrared. Here we introduce an organic photorefractive material—a composite based on the poly(arylene vinylene) copolymer TPD-PPV—that exhibits favourable near-infrared characteristics. We show that pre-illumination of this material at a shorter wavelength before holographic recording improves the response time by a factor of 40. This process was found to be reversible. We demonstrate multiple holographic recording with this technique at video rate under practical conditions.

Investigation of poly(arylene vinylene)s, 41 : Synthesis of soluble dialkoxy-substituted poly(phenylene alkenylidene)s by applying the Horner-reaction for condensation polymerization

Soluble poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-1,2-ethenylene] (MEH-PPV) and its alternating copolymer poly[2,5-dimethoxy-1,4-phenylene-1,2-ethenylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-1,2-ethenylene] (M3EH-PPV) were successfully synthesized by condensation polymerization of substituted terephthalaldehydes with substituted xylylene bisphosphonates via the Horner reaction. This polycondensation method was also applied to the synthesis of the analogous poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-1,3-butadiene-1,4-diyl] (MEH-PPB), which contains a 1,3-butadiene instead of a vinylene unit. The optical data of MEH-PPV and M3EH-PPV (solid film, abs. λ max 484 nm, em. λ max 589 nm) are consistent with those previously reported for MEH-PPV from the dehydrohalogenation route. In comparison to MEH-PPV, the longer alkenylene sequence of MEH-PPB exhibits a slight red shift in its absorption and fluorescence spectra (abs. λ max 502 nm, em. λ max 604 nm). All three polyphenylene alkenylidenes (MEH-PPV, M3EH-PPV, and MEH-PPB) are completely soluble in common solvents. They are well-defined conjugated polymers of high molecular weight (M w > 20000) and possess all-trans structure. Optical quality solid films are easily prepared from these polymers by solution processing. Blends with phenyl-substituted PPV derivatives can also be prepared in this way. This combination of properties is highly desirable for many light-emitting, nonlinear optical, and photoelectrical applications.

The overshoot effect in transient electroluminescence from organic bilayer light emitting diodes: Experiment and theory

Transient electroluminescence (EL) from organic bilayer light emitting diodes addressed by a rectangular voltage pulse often features an overshoot when the voltage is switched off. Experimental results are presented for a variety of hole transporting layers in contact with an oxadiazole layer simultaneously acting both as a blockade for holes and as an electron transport layer. The overshoot occurs in spin coated yet not in vapor deposited samples. A model is developed to rationalize charge recombination under the premise (i) of an interfacial layer of finite thickness between hole and electron transport layers in which both transport molecules coexist and (ii) of interfacial energy barriers impeding both hole and electron passage. It predicts the occurrence of an EL overshoot due to the recombination of stored electrons and holes under the action of their mutual space charge field when the external voltage is switched off. The temporal pattern of the predicted transient EL signal is in good agreement wit...

Hole transport in polyphenylenevinylene-ether under bulk photoexcitation and sensitized injection

Employing the time-of-flight (TOF) technique, the hole mobility in films of polyphenylenevinylene-ether has been measured as a function of electric field and temperature. Charge carriers were generated upon either photoexcitation into the S1←S0 transition of the polymer or via sensitized injection from a thin rhodamine 6G layer. The results will be interpreted in terms of the concept of hopping among polymer segments featuring a Gaussian distribution of energies, its variance being 91 meV. While TOF signals generated via sensitized injection are similar to those in molecularly doped polymers bearing out a transition from nondispersive to dispersive transport, intrinsically generated TOF signal features a cusp at higher temperature and almost no dispersion. The latter phenomenon turns out to be a consequence of charge generation at the tail of the density distribution of hopping states.

Poly(p-phenylenevinylene) derivatives: new promising materials for nonlinear all-optical waveguide switching

Several new derivatives of poly(p-phenylenevinylene) (PPV) are investigated regarding their linear and nonlinear optical material and waveguide properties, including their nonlinear photonic bandgap properties that are induced by photoablated periodic Bragg gratings. The new materials were prepared by means of the polycondensation route, which yields polymers with excellent solubilities and film-forming properties. Comparative data suggest that the new polycondensation-type MEH-PPV (completely soluble, strictly linear and fully conjugated), in particular, is the most promising polymer under investigation to fulfill the requirements for all-optical switching in planar waveguide photonic bandgap structures. UV-photobleaching techniques and photoablation in the UV, VIS, and near-infrared ranges at different pulse durations are investigated. Homogeneous submicrometer gratings that serve as Bragg reflectors have been fabricated in MEH-PPV thin films by application of these methods. The great potential of this type of materials for nonlinear all-optical switching applications that arises from their unique optical properties and their patterning behavior is discussed in detail. Numerical simulations of a switching device based on gap-soliton formation in a nonlinear periodic waveguide structure with the newly obtained material data have been carried out. We show that one can expect photonic bandgap all-optical switching in MEH-PPV planar waveguides. Device performance considering different grating parameters is discussed.