Dietrich Haarer

Spectral diffusion of a photochemical proton transfer system in an amorphous organic host: Quinizarin in alcohol glass

Experimental data describing the time evolution of photochemical holes in organic glasses are reported. The photochemical system is 1,4‐dihydroxyanthraquinone (quinizarin) in ethanol/methanol glasses; its photochemistry is based on proton or deuteron transfer processes. The experiments show a logarithmic increase of the hole widths in a time domain between minutes and about 104 min. The experimental results yield a pronounced deuteration effect and little variation with temperature between 1.35 and 4.2 K. The data are interpreted in a semiquantitative way using current theories of spectral diffusion in amorphous solids. The fastest measured photochemical rates are on the order of seconds, leaving a ‘‘time independent’’ linewidth of about 0.4 cm−1 at 1.35 K.

Novel hybrid solar cells consisting of inorganic nanoparticles and an organic hole transport material

A novel three-layer concept for efficient solid-state solar cells is presented. The hybrid devices consist of an inorganic nanocrystalline titanium dioxide layer for electron conduction, a surface-adsorbed ruthenium dye complex for light absorption, and an organic triphenyldiamine layer for the transport of holes. External quantum efficiencies of up to 0.2% have been achieved. Time-resolved current measurements with pulsed excitation of the dye layer reveal the charge transport to be totally controlled by the nanoparticles in the long-time regime. The high charge storage capability of the porous titanium dioxide layers is demonstrated by junction-recovery measurements.

Electroluminescence and electron transport in a perylene dye

Charge carrier transport in vapor-deposited films of 1,6,7,12-tetraphenoxy-N,N′-bis-(2,6-diisopropylphenyl)-perylene-3,4,9,10-bis(dicarboximide) was investigated using two different methods, the time-of-flight (TOF) technique and time-resolved electroluminescence. Electron mobilities of 10−5 cm2/V s were measured in the bulk using a time-of-flight technique. Hole transport was found to be dispersive and, thus, a transit time for holes could not be obtained. The above dye was also used to fabricate single layer light emitting diodes showing clearly visible red electroluminescence under ambient conditions. Our experiments on transit electroluminescence confirmed the measured electron mobility and ruled out the possibility that the transit time of holes is shorter than the time range investigated in our time-of-flight experiments.

Photochemical hole burning of phthalocyanine in polymer glasses: Thermal cycling and spectral diffusion

In this paper we report an analysis of hole burning line shapes yielding both reversible and irreversible contributions to the linewidth which are due to spectral diffusion. These terms show a linear T dependence and can, through cycling experiments, be investigated in the temperature range of 0.5<T<25 K. The range 1<T<25 K is particularly interesting because, under normal conditions, Debye contributions and local mode contributions overwhelm the small spectral diffusion terms at these temperatures. Comparison between optical data and specific heat data supports our model of spectral diffusion.

Stark effect of polar and unpolar dye molecules in amorphous hosts, studied via persistent spectral hole burning

A simple analytical model is presented describing the shape of a persistent spectral hole in a homogeneous electric field for random orientation of the dye molecules in the host matrix. We consider the two cases that the absorbers have either one fixed value or a distribution of values of the electric dipole moment difference Δμ between the ground and excited electronic states. Comparison with experiment yields the Δμ values for chlorin (Δμ=0.214 D) and a substituted Zn‐tetrabenzoporphin (∼(Δμ) =0.174 D) in poly(vinylbutyral). In the latter case the isolated dye molecule has inversion symmetry, yet there is nevertheless a dipole moment through the interaction with the polar matrix of low local symmetry.

Logarithmic decay of photochemically induced two-level systems in an organic glass

Abstract The recovery dynamics of photochemically labeled, metastable sites are investigated via photochemical hole burning in the visible spectral range. It is shown that, in a glass matrix, the recovery follows a logarithmic law. The experimental data are interpreted with the concept of two-level systems (TLSs). This interpretation is supported by the observed large isotope effect. The tunneling rates for protons and deuterons and the heights of the pertinent tunneling barriers are discussed.

Electroluminescence from multilayer organic light‐emitting diodes using poly(methylphenylsilane) as hole transporting material

We have investigated the optical and electrical properties of polysilane‐based multilayer electroluminescent (EL) devices, utilizing poly(methylphenylsilane) (PMPS) as the hole transporting material, in order to elucidate the mechanism of EL emission in these devices. The EL devices which we fabricated have two or three functional organic layers. These layers are composed of a PMPS layer as well as a 3‐(2’‐Benzothiazolyl)‐7‐diethylaminocoumarin (Coumarin 6) doped polystyrene (PS) layer and/or a tris‐(8‐hydroxyquinoline) aluminum (Alq3) layer. An indium‐tin‐oxide‐coated glass and an Al electrode were used as the hole and the electron injecting electrode, respectively. On the basis of a combined analysis of the basic characteristics of these devices, photoexcited fluorescence spectra and decay curves as well as the band diagram of these devices, we concluded that the recombination of charge carriers and the EL emission in the three‐layer device occur both in the Coumarin 6:PS and the evaporated Alq3 layers.

Poly(triarylamine)s- synthesis and application in electroluminescent devices and photovoltaics

Abstract Polymeric triphenyldiamines (p-TPD)s and naphthylphenyldiamines (p-NPD)s were synthesized by Ullmann reaction between bis(N,N-diaryl)amines and aryldiiodides in presence of phase-transfer catalyst, 18-crown-6. The polymers obtained are soluble in THF, CHCl 3 etc and possess Tgs in the range of 206°C to 232°C and form stable and amorphous thin films. The HOMO energy values determined from cyclic voltammetry measurements lie between −5.10 and −5.17 eV with respect to vacuum energy level. The p-TPD and p-NPD are effectively applied as hole transport material in Electroluminescent devices, ITO/p-TPD/Alq 3 /Al. A solid-state solar cell, consisting of an inorganic nanocrystalline TiO 2 layer coated on ITO for electron conduction, a surface-adsorbed ruthenium complex dye for light absorption and p-TPD as hole transport layer is also realized. Using appropriate counter electrode such as Au or Pt, quantum efficiencies of up to 0.2% and open circuit voltage of 550mV were realised.

One-dimensional hopping transport in a columnar discotic liquid-crystalline glass

Abstract The charge carrier transport in liquid-crystalline triphenylenes is completely different from the transport in amorphous photoconductors. The high degree of order leads to quasi-one-dimensional transport. In this paper we are proposing a model for a field-assisted incoherent hopping process in one-dimensional systems based on Monte Carlo simulations. The numerically determined field and temperature dependences were compared with experimental results of a butyloxy-substituted triphenylene dimer. In this glass-forming material the mobility could be measured by a conventional time-of-flight technique in the very large temperature range between 128 and 410K. This dimer is, at room temperature, ordered in, a discotic hexagonal plastic phase with a very high hole mobility of 10−2cm2V−1 s−1. We have observed a transition from a non-activated to a thermally activated behaviour of the charge carrier mobility at –20°C, which we assign to a phase transition from the liquid-crystalline phase to the anisotrop...

Novel functional materials based on triarylamines–synthesis and application in electroluminescent devices and photorefractive systems

A variety of new functional materials based on triarylamines, such as low molecular weight glasses which possess hole conducting/photoconductive properties as well as amorphous bifunctional materials which combine photoconductive and non-linear optical (NLO) properties in one compound, have been synthesized. The new hole transporting glasses belong to the class of 1,3,5-tris(triaryldiamino)benzenes (TTADB). The hyperbranched structure and the large aryl groups attached as substituents lead to high glass transition temperatures (Tg) of up to 141°C in these compounds. The TTADBs do not recrystallize upon cooling from the melt, but form stable glasses. Cyclic voltammetry studies reveal multi-oxidation stages for these compounds of which the first oxidation is reversible. The HOMO energy values determined from CV for TTADB-1 and TTADB-2 are -4.82 and -4.94 eV, respectively. Light emitting diodes with the structure ITO/TTADB-2/Alq3/Al (where ITO=indium tin oxide) show high efficiency and large current carrying capacity. Further, bifunctional compounds have been synthesized in which a photoconductive moiety such as bis(carbazolyl)triphenylamine or bis(diphenylamino)triphenylamine is covalently bound to different NLO chromophores. Some of these compounds are thermally and morphologically stable amorphous materials, possessing Tg in the range from 85 to 122°C. Cyclic voltammetry measurements reveal that the HOMO energy values are between -4.81 and -5.45 eV. In photorefractive measurements using 40 µm thick samples, a diffraction efficiency of 27%, which corresponds to a refractive index modulation (Δn) of 3.5×10-3, a maximum two beam coupling gain coefficient (Γ) of 90 cm-1 and a response time of 40 ms were obtained.