Torsten Fritz

Low voltage organic light emitting diodes featuring doped phthalocyanine as hole transport material

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.

Epitaxy and Molecular Organization on Solid Substrates

The recent emergence of molecular films as candidates for functional electronic materials has prompted numerous investigations of the underlying mechanisms responsible for their structure and formation. This review describes the role of epitaxy in molecular organization on crystalline substrates. A much-needed grammar of epitaxy is presented that classifies the various modes of epitaxy according to transformation matrices that relate the overlayer lattice to the substrate lattice. The different modes of epitaxy can be organized hierarchically to reflect the balance of overlayer–substrate and molecule–molecule energies. In the case of molecular overlayers, the mismatch of overlayer and substrate symmetries commonly leads to coincident epitaxy in which some of the overlayer lattice points do not reside on substrate lattice points. Analyses of numerous reported epitaxial molecular films reveal that coincidence is quite common even though, based on overlayer–substrate interface energies alone, not as energetically favorable as commensurism. The prevalence of coincidence can be attributed to overlayer elastic constants, associated with molecule–molecule interactions within the overlayer, that are larger than the elastic constants of the overlayer–substrate interface. This condition facilitates prediction of the epitaxial configuration and overlayer structure through simple and comparatively efficient geometric modeling that does not require the input of potential energies, while revealing the role of phase coherence between the overlayer and substrate lattices.

Very-low-operating-voltage organic light-emitting diodes using a p-doped amorphous hole injection layer

We demonstrate the use of a p-doped amorphous starburst amine, 4, 4′, 4″-tris(N, N-diphenyl- amino)triphenylamine (TDATA), doped with a very strong acceptor, tetrafluoro- tetracyano-quinodimethane by controlled coevaporation as an excellent hole injection material for organic light-emitting diodes (OLEDs). Multilayered OLEDs consisting of double hole transport layers of p-doped TDATA and triphenyl-diamine, and an emitting layer of pure 8-tris-hydroxyquinoline aluminum exhibit a very low operating voltage (3.4 V) for obtaining 100 cd/m2 even for a comparatively large (110 nm) total hole transport layer thickness.

Controlled doping of phthalocyanine layers by cosublimation with acceptor molecules: A systematic Seebeck and conductivity study

We investigate the doping of vanadyl–phthalocyanine by a fluorinated form of tetracyano-quinodimethane as an example of controlled doping of thin organic dye films by cosublimation of matrix and dopant. The electrical parameters of the films derived from conductivity and Seebeck measurements show that the results largely follow standard models used to describe the doping of crystalline semiconductors; e.g., a smooth shift of the Fermi level towards the valence states with increasing doping is observed. Other effects, like the superlinear increase of conductivity with the molar doping ratio, need the inclusion of additional effects like percolation.

Interface electronic structure of organic semiconductors with controlled doping levels

Abstract We investigate the properties of inorganic–organic interfaces by ultraviolet and X-ray photoemission spectroscopy (UPS and XPS) and transport experiments. In particular, we study the interface between inorganic conductive substrates and organic layers that are intentionally p-type doped by co-evaporation of a matrix material and acceptor molecules. The photoemission spectra clearly show that the Fermi levels shift due to the doping and that the space charge layer width changes with doping (high doping – small width). The changes in the electronic structure of the interface due to doping agree well with results of transport experiments.

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

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.

Enhanced Hole Injection into Amorphous Hole‐Transport Layers of Organic Light‐Emitting Diodes Using Controlled p‐Type Doping

We demonstrate enhanced hole injection and lowered driving voltage in vacuum-deposited organic light-emitting diodes (OLEDs) with a hole-transport layer using the starburst amine 4,4′,4″-tris(N,N-diphenyl-amino)triphenylamine (TDATA) p-doped with a very strong acceptor, tetrafluoro-tetracyano-quinodimethane (F4-TCNQ) by controlled coevaporation. The doping leads to high conductivity of doped TDATA layers and a high density of equilibrium charge carriers, which facilitates hole injection and transport. Moreover, multilayer OLEDs consisting of double hole-transport layers of thick p-doped TDATA and a thin triphenyl-diamine (TPD) interlayer exhibit very low operating voltages.

The Lowest Energy Frenkel and Charge-Transfer Excitons in Quasi-One-Dimensional Structures: Application to MePTCDI and PTCDA Crystals

Abstract We consider the exciton states in quasi-one-dimensional organic crystals with strong orbital overlap between neighboring molecules. In such crystals, the energy difference between the lowest Frenkel exciton and the nearest-neighbor charge-transfer excitons becomes small and their strong mixing determines the nature of the lowest energy states. We discuss these effects for crystalline N , N ′ -dimethylperylene-3,4,9,10-dicarboximide (MePTCDI) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). To model the exciton states, we use a Hamiltonian which includes the mixing of Frenkel excitons with several vibronic levels and charge-transfer excitons. With appropriate fitting parameters, we demonstrate that 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 absorption peaks. This polarization behavior is discussed as a qualitative proof for the varying contribution of the charge-transfer excitons, which have a transition dipole direction different from that of the Frenkel excitons.

Pyronin B as a donor for n-type doping of organic thin films

We present an approach to stable n-type doping of organic matrices using organic dopands. To circumvent stability limitations inherent in strong organic donors, we produce the donor from a stable precursor compound in situ. As an example, pyronin B chloride is studied as a dopant in a 1,4,5,8-naphthalene tetracarboxylic dianhydride matrix. Conductivities up to 2×10−4 S/cm are obtained, which is two orders of magnitude higher than obtained previously using bis(ethylenedithio)-tetrathiafulvalene as a dopant [A. Nollau, M. Pfeiffer, T. Fritz, and K. Leo, J. Appl. Phys. 87, 4340 (2000)]. Field-effect measurements are used to prove n-type conduction. Other matrices which can be doped are N,N′-dimethyl-perylene-3,4,9,10-tetracarboxylic diimide and fullerene C60, frequently used in organic solar cells. Visible light and Fourier-transform infrared spectroscopy confirm the donor properties of pyronin B.

Controlled p-doping of pigment layers by cosublimation: Basic mechanisms and implications for their use in organic photovoltaic cells

Abstract We present a systematic study on doping of vanadyl- and zinc-pathalocyanine by a fully fluorinated form of tetracyano-quinodimethane as an example of controlled doping of thin organic films by cosublimation of matrix and dopant. The films are characterized in situ by temperature dependent Seebeck and conductivity measurements. We observe a drastic increase of conductivity and a corresponding shift of the Fermi level towards the valence states with increasing dopant concentration. We thus conclude that doping has the potential of both reducing the series resistance and increasing the photovoltage of organic solar cells. As a first step to exploit this potential, we present two different ways of preparing diodes with rectification ratios in excess of 10 4 using doped phthalocyanines. By adding an undoped interlayer between the contact and the doped layer, we have produced diodes which work already in the strict absence of oxygen and are stable in air. To increase the efficiency of charge carrier generation in photovoltaic cells, we need to use photoactive donor–acceptor-heterojunctions. We present here first examples of pn- and pin-type heterojunctions combining p-doped and nominally undoped layers.