Guenther Leising

Optoelectronic devices made from multilayer and molecularly doped organic layers

We discuss the photovoltaic and electroluminescence properties of three groups of optoelectronic devices with the following organic materials: (A) single layer devices made of para-hexaphenyl (PHP), pristine methyl-substituted laddertype poly-para-phenylene (mLPPP) and N-(2,6- Diisopropylphenyl)-N-octylterrylene-3,4,11,12-tetracarboxdiimide- (DOTer) sandwiched between ITO and Al electrodes, (B) mLPPP blended with TiO-phthalocyanine (TiO-Pc) and an aromatic macromolecule, (C) multilayer devices consisting of differently arranged layers made of mLPPP, PHP and DOTer. The motivation for these experiments is the optimization of either charge transfer or energy transfer from one molecular to its neighbor molecule. In order to obtain high photocurrents for photovoltaic applications it is favorable to use a combination of a polymer with electron donor properties and an organic substance with high electron affinity, which provides efficient charge transfer resolution in the creation of polarons. In particular we investigated the influence of the location of the photoactive region on the shape and magnitude of the photocurrent action spectra by performing experiments under electrical and optical bias for different excitation conditions. The best photovoltaic yields were obtained for hetero-structure devices.

Influence of the molecular geometry on the photoexcitations of highly emissive organic semiconductors

Para-phenylene type molecules are efficient photoluminescence emitters in the ultraviolet-blue-green spectral range. They are used in light emitting diodes (LEDs) and photopumped lasers. Photoexcited para-phenylene type molecules give rise to strong emission from singlet excitons, bleaching of the singlet exciton absorption, induced absorption from triplet excitons and induced absorption from polarons. Since the latter two processes represent absorption of the emitted light of singlet excitons, the presence of polarons and triplet excitons might be a fundamental problem for laser diodes made from para-phenylene type molecules. In our experiments we modify the molecular geometry by the application of hydrostatic pressures up to 80 kbar in a temperature range of 10 to 300 K. In particular we show how triplet and polaron states, which are present in LEDs under operation, react to the induced geometric changes. The spectra of ground state absorption, excited state emission, bleaching of the singlet exciton absorption, induced absorption from triplet excitons and induced absorption from polarons are significantly broadened and shifted in energy. In order to explain the observed behavior we have performed three-dimensional bandstructure calculations within density functional theory for the planar poly(para-phenylene). By varying the intermolecular distances and the length of the polymer repeat unit pressure effects can be simulated.

Organic light-emitting devices based on new heterocyclic compounds

One approach to increase the overall performance of organic light emitting devices is to separate the light-emitting volume from the ones which are assigned to charge injection or transport. We realized such polymer hetero-layer structures by combination of hole transporting materials like polyparaphenylenevinylene (PPV) with new electron transporting materials, i.e. heterocyclic polymers and heterocyclic low molecular compounds, especially phenyl quinoxalines (PQs). The electronic properties of these heterocyclic compounds have been investigated by various methods including ultraviolet photoelectron spectroscopy. PQs show electron affinities near 3.5 eV and ionization potentials below 6 eV. Measurements of thermally stimulated depolarization currents (TSDC) were carried out in order to study both dipolar relaxation and charge transport processes in single layer devices. The TSDC spectra revealed the prominence of both dipolar relaxation and of charge transport processes. The dipolar processes show activation energies between 0.4 eV and 1 eV which are typical values for small relaxing entities like polymer side groups. Current-voltage and current-luminance characteristics were used to study the prepared heterolayer devices. Double layers made of PPV and polyphenylquinoxaline (PPQ) are characterized by low onset voltages near 2 V and high luminous efficiency of more than 0.8 cd/A. The experimental findings show that PPQs are promising materials in the field of organic electroluminescence.

Plasma reflection of polycrystalline YBa2Cu3O7−x

Abstract We propose a new model for the evaluation of optical taken from polycrystalline YBaCuO at room temperature, which assumes two Drude contributions to the dielectric function. To test the model we performed reflection measurements on three samples of different origin. We found excellent agreement for samples of high density showing the great influence of the morphology on the spectra. Our model allows an estimate of the intrinsic anisotropy of the effective mass of the carriers.

Photovoltaic responses in ionically self-assembled nanostructures containing conjugated polymers and fullerenes

We use the technique of ironically self-assembled monolayers (ISAMs) to produce photovoltaic devices of well-controlled thickness and composition. The ISAM nanostructure fabrication method simply involves the alternate dipping of a charged substrate into aqueous cationic and anionic solutions at room temperature. We have employed several approaches to combine the tetrahydrothiophenium precursor of PPV with fullerenes and other organic materials .We apply modulation spectroscopy for the electro-optical characterization of the ISAM-devices. Analyzing the thickness dependence of the recorded photocurrent action spectra allows us to identify the photoactive region within the devices. The modulation frequency dependence of the photocurrent can be assigned to the influence of trapped charges taking part in the photovoltaic process. By utilizing the ability to control both thickness and composition of the organic layer at a nanometer level of precision, the composition and concentration of these defects has ben systematically varied.

Efficient full-color electroluminescence and stimulated emission with polyphenylenes

We demonstrate the fabrication and characterization of highly efficient red-green-blue (RGB) and white light emitting devices based on poly(phenylene) type materials as the hexaphenyl and the methyl substituted laddertype poly(para phenylene) (m-LPPP). The RGB-devices are fabricated with an external color conversion technique based on PHP, whereas the white light emission is generated by an internal excitation energy transfer from the blue m-LPPP component to a red light-emitting polymer in a polymer blend, which is used as the active layer in a light-emitting diode. We present photophysical properties, like spectral line-shape site selectivity of photoluminescence (PL), and electroluminescence of bulk poly(para-phenylenevinylene) PPV films and isolated PPV chains incorporated into a self- assembled matrix material, which leads to the formation of a regular hexagonal array of channels with a diameter of about 15 angstrom, in which the conjugated polymer chains are contained. The structure of the nano-composite in organic- light-emitting-diodes. A suitably structured m-LPPP waveguide shows a spectrally very narrow high directional blue-green light output when optically pumped. The high optical gain of m-LPPP is a results of the spectral separation of stimulated emission and photoinduced absorption bands, thus spectral narrowing is even observable in below cut-off waveguides. Under resonant excitation conditions, we observe strong stimulated Raman scattering.

Electrochemically and photoinduced infrared bands in PPV: a comparative study

Results of in situ Fourier transform infrared attenuated total reflection (FTIR-ATR) spectroscopy during electrochemical oxidation processes (electrochemically induced doping) as well as by photoinduced infrared absorption spectroscopy (photoinduced doping) of polyparaphenylenevinylene (PPV) are presented. Infrared active vibrational bands in the lower energy part of the infrared spectrum and infrared absorption due to electronic transitions at higher energies are observed and compared. The electrochemical doping of PPV occurs in two potential regions. In the `low doping region, the difference spectra obtained by in situ FTIR-ATR spectroscopy are similar to spectra obtained by chemical doping. In the `high doping region, the spectral behavior is different to the `low damping region and shows a higher similarity to the photoinduced absorption spectrum.

Electroluminescence devices with poly(paraphenylene) and derivatives as the active material

Conjugated polymers are promising materials for electro-luminescence devices. We have shown recently that blue light emission can be achieved, when poly(paraphenylene) is used as the active material in a Schottky-like device. The maximum of the electroluminescence emission is observed around 470 nm. Poly(paraphenylene), PPP is stable up to high temperatures and its electronic structure can be tailored by influencing the conjugation length and the tilt angle of consecutive phenyl rings. PPP was synthesized by soluble precursor polymers and as a soluble PPP ladder type polymer. Via the tailoring of the electronic structure, electroluminescence devices with different colors can be fabricated. In contrast to the blue emission of the PPP prepared by a precursor route, the PPP ladder polymer, which has planar structure exhibits a strong yellow-green emission. We report on the physical properties of the different devices, concerning the device structure and the active materials.

Conduction and degradation analysis of organic LEDs by current noise monitoring

The paper describes the use of noise current analysis to sense variations of the microscopic conduction process in organic Light Emitting Diodes and to track their evolution through time. The monitoring of current fluctuations has been made both in time and frequency domain and is of great value when one wants to correlate the conduction properties of the charge carriers and the changes in current flow with the corresponding changes in the microscopic morphology of the organic layers. The method reveals itself to be very effective also in sensing the initial state and the growth of catastrophic degradation of oLEDs in large advance with respect to current monitoring or other techniques. Microscopic damages within the device, as a result of microshorts and/or thermal breakdown, are shown to reveal a neat increase of the white noise component of about three orders of magnitude in the power spectral density, that can therefore be detected with very good time precision. This would allow to study the sources that may give reason of degradation, through structural or spectroscopic investigations for example, before the microscopic damages have sum up to a visible and irreversible macroscopic failure.

Quantitative analysis of the singlet exciton-polaron interaction in para-phenylene-type ladder polymers

The steady state photoinduced absorption (PA), photoluminescence (PL), PL detected magnetic resonance (PLDMR), and PA-detected magnetic resonance (PADMR) of poly- and oligo-(para-phenylenes) films is described. In particular, the excitation density (laser power) NO dependence of the PA, PL, and PLDMR signals are analyzed by means of a new rate equation model. It describes the dynamics of singlet excitons (SEs) and polarons in all three experiments with the same set of parameters. This yields the first quantitative analysis of the interaction of SEs and polarons in conjugated polymers. The model is based on the observations that mobile SEs are quenched by trapped and free polarons and that the spin 1/2 magnetic resonance conditions reduce the total polaron and triplet exciton (TE) populations. Since the sublinear NO dependence of the positive (PL-enhancing) spin 1/2 PLDMR and the polaron PA band are essentially the same, we conclude that that PLDMR is due to reduced quenching of SEs by polarons. The agreement between the model, the current results, and results from other spectroscopic techniques provides strong evidence for this quenching mechanism. It also suggests that it is a very significant process in luminescent (pi) - conjugated materials and organic light-emitting devices, which needs to be taken into account especially at high excitation densities such as in lasing action.