Showing papers by "Torsten Fritz published in 2000"

Controlled n-type doping of a molecular organic semiconductor: Naphthalenetetracarboxylic dianhydride (NTCDA) doped with bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF)

Abstract: We present a study of controlled n-type doping in molecular organic semiconductors: Naphthalenetetracarboxylic dianhydride is doped by cosublimation with the donor molecule bis(ethylenedithio)-tetrathiafulvalene. Electrical parameters are deduced from temperaturedependent measurements of the conductivity and the thermopower for various dopant concentrations. The results are compared to the predictions of a standard model commonly used for crystalline semiconductors. The Fermi level shifts towards the transport level, the conductivity is increased, and the mobility decreases with the doping ratio.

A simple measurement of the absolute internal quantum efficiency of thin organic films

Abstract: In this article, we present a simple method for the determination of the absolute internal quantum efficiency of thin organic dye layers. The basic idea is the comparison of the luminescence of the film with the reflection of a white diffusive reflectance standard measured at one angle with a simple spectrofluorometer. The method is compared to the procedure of de Mello [J. C. de Mello, H. F. Wittmann and R. H. Friend, Adv. Mater. 9, 230 (1997)], which uses an integrating sphere. As examples, the quantum yields of films of the two perylene derivatives N,N′-dimethylperylene-3,4:9,10-dicarboximide and 3,4,9,10-perylenetetracarboxylic dianhydride, are determined.

Improved method for determination of optical constants of organic thin films from reflection and transmission measurements

Abstract: A new technique for determining the optical properties of organic thin films is presented. A detailed evaluation of the accuracy of the determined optical constants has been performed, and the best combination of measured values yielding the smallest errors in the index of refraction for realistic experimental uncertainties has been found. The proposed method utilizes the fact that optical constants are smooth continuous functions, which reduces the possibility of encountering multiple solutions. The method consists of two steps. In the first step the optical constants at all wavelengths and the film thickness are determined. In the second step the thickness and the imaginary part of the index of refraction are kept fixed while we reevaluate the real part of the index of refraction by using a different objective function with improved sensitivity to the refractive index. After verifying that the proposed method is capable of an accurate estimation of optical constants, we determine the index of refraction data of vanadyl-phthalocyanine in the visible spectral range.

Modelling the optical constants of organic thin films: impact of the choice of objective function

Abstract: In this work, we model the optical constants of three organic materials using a modified Lorentz oscillator model. We have analysed the impact of the choice of the objective or merit function, which is employed for model parameter estimation, on the modelling of the optical constants. The objective function for model parameter determination should be chosen in such a manner that discrepancies between the experimental and calculated values for both parts of the index of refraction are minimized at the same time. We have found that the choice of objective function strongly influences the existence of multiple solutions. By combining a global optimization method, an appropriately chosen objective function and a modified oscillator model, we obtain good agreement with the experimental data for three organic materials.

The Mixing of Frenkel- and Charge-Transfer Excitons in 1D-Structures: Application to PTCDA and MePTCDI

Abstract: The exciton structure of crystalline PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI (N-N’-dimethylperylene-3,4,9,10-dicarboximide) is modeled by a one-dimensional Hamiltonian, which includes interactions between Frenkel excitons with several vibronic levels and charge transfer excitons. With appropriate fitting parameters, which are verified by quantum chemical calculations, this model can explain the main features of the low temperature absorption spectra. Polarized absorption spectra of MePTCDI show different polarization ratios for the various peaks. This polarization behavior is explained by the varying contribution of the CT transition dipole, which has a direction different from the Frenkel transition dipole.

Low Driving Voltage of Organic Light-Emitting Diodes Using p -Doping Starburst Amine as Hole Transporter

Abstract: We investigate the electrical properties and the OLED application of controlledly doped amorphous hole transporters. Thin films of starburst amine, 4,4',4“-tris(N,N-diphenyl- amino) triphenylamine (TDATA), doped by a fully fluorinated form of tetracyano- quinodimethane (F 4 -TCNQ), are characterized in situ by temperature dependent conductivity and Seebeck measurements. The conductivity and hole concentration increase with dopant concentration and are many orders of magnitude higher than those of undoped material. OLED devices with the layer sequence ITO/TDATA(200 nm)/Alq 3 (65 nm)/LiF(1 nm)/Al were fabricated. The use of p-doped TDATA thin films with high bulk conductivity and hole concentration reduces the resistance of the devices and leads to a thinner space charge layer which facilitates injection of holes from the ITO anode.