Hans Kleemann

Doped organic transistors operating in the inversion and depletion regime.

The inversion field-effect transistor is the basic device of modern microelectronics and is nowadays used more than a billion times on every state-of-the-art computer chip. In the future, this rigid technology will be complemented by flexible electronics produced at extremely low cost. Organic field-effect transistors have the potential to be the basic device for flexible electronics, but still need much improvement. In particular, despite more than 20 years of research, organic inversion mode transistors have not been reported so far. Here we discuss the first realization of organic inversion transistors and the optimization of organic depletion transistors by our organic doping technology. We show that the transistor parameters—in particular, the threshold voltage and the ON/OFF ratio—can be controlled by the doping concentration and the thickness of the transistor channel. Injection of minority carriers into the doped transistor channel is achieved by doped contacts, which allows forming an inversion layer.

Organic Zener Diodes: Tunneling across the gap in organic semiconductor materials

Organic Zener diodes with a precisely adjustable reverse breakdown from -3 to -15 V without any influence on the forward current-voltage curve are realized. This is accomplished by controlling the width of the charge depletion zone in a pin-diode with an accuracy of one nanometer independently of the doping concentration and the thickness of the intrinsic layer. The breakdown effect with its exponential current voltage behavior and a weak temperature dependence is explained by a tunneling mechanism across the highest occupied molecular orbital-lowest unoccupied molecular orbital gap of neighboring molecules. The experimental data are confirmed by a minimal Hamiltonian model approach, including coherent tunneling and incoherent hopping processes as possible charge transport pathways through the effective device region.

Improvement of voltage and charge balance in inverted top-emitting organic electroluminescent diodes comprising doped transport layers by thermal annealing

We present investigations of top emitting organic light emitting devices (OLED) comprising n- and p-doped organic charge transport layers. It has been found previously that in comparison to noninverted p-i-n OLEDs, inverted n-i-p OLEDs show reduced device performances after fabrication. These differences can be eliminated by subsequent thermal annealing of the whole n-i-p OLED. After this process, the n-i-p OLED exhibits a superior low driving voltage of 2.9 V at 1000 cd/m2 and shows an increase in external quantum efficiency from 11% to almost 15% which we ascribe to a modified charge balance within the intrinsic organic emission layer.

High‐Performance Vertical Organic Transistors

Vertical organic thin-film transistors (VOTFTs) are promising devices to overcome the transconductance and cut-off frequency restrictions of horizontal organic thin-film transistors. The basic physical mechanisms of VOTFT operation, however, are not well understood and VOTFTs often require complex patterning techniques using self-assembly processes which impedes a future large-area production. In this contribution, high-performance vertical organic transistors comprising pentacene for p-type operation and C60 for n-type operation are presented. The static current-voltage behavior as well as the fundamental scaling laws of such transistors are studied, disclosing a remarkable transistor operation with a behavior limited by injection of charge carriers. The transistors are manufactured by photolithography, in contrast to other VOTFT concepts using self-assembled source electrodes. Fluorinated photoresist and solvent compounds allow for photolithographical patterning directly and strongly onto the organic materials, simplifying the fabrication protocol and making VOTFTs a prospective candidate for future high-performance applications of organic transistors.

Controlled formation of charge depletion zones by molecular doping in organic pin-diodes and its description by the Mott-Schottky relation

Molecular doping of organic semiconductors is a key technology for highly efficient organic light-emitting diodes. Nevertheless, the underlying fundamental mechanisms are under discussion. This is because of the complex situation of structural disorder and strong polaronic coupling in such systems. We provide for the first time a systematic study of the formation of charge depletion zones in organic pin-diodes comprising molecular doped hole and electron transport layers. Impedance spectroscopy is employed to study the capacitance of these depletion zones. In particular, we show that the voltage dependent capacitance function obeys the Mott-Schottky relation concerning the influence of doping and the effect of an additional depletion zone given by the intrinsic interlayer. From temperature dependent measurements of the depletion capacitance, we can deduce the amount of active dopant states, their activation energy, and the spatial field distribution within the junction. The measured activation energy of t...

Bidirectional operation of vertical organic triodes

Due to their effective short channel length of only a few hundred nanometers, vertical organic triodes (VOTs) have a high potential to overcome problems of low current densities and switching speed in current organic field effect transistors (OFETs). Furthermore, VOTs are easy to build because no sub-structuring of the base contact is necessary. Nevertheless, these devices are poorly investigated. In literature, two different working mechanisms are suggested: hot carrier transport through the metallic base or transport of charge carriers through a permeable base electrode. As a strong asymmetry is expected for function principle based on hot carriers, we are able to distinguish between both mechanisms by examining the bidirectional transmission properties of the VOT consisting of electron transporting materials. We show that high transmission values (>95%) are possible for both directions, suggesting a base contact with openings forming a grid electrode.

Density of states determination in organic donor-acceptor blend layers enabled by molecular doping

Charge carrier transport is a key parameter determining the efficiency of organic solar cells, and is closely related to the density of free and trapped states. For trap characterization, impedance spectroscopy is a suitable, non-invasive method, applicable to complete organic semiconductor devices. In order to contribute to the capacitive signal, the traps must be filled with charge carriers. Typically, trap filling is achieved by illuminating the device or by injecting charge carriers through application of a forward bias voltage. However, in both cases, the exact number of charge carriers in the device is not known and depends strongly on the measurement conditions. Here, hole trap states of the model blend layer ZnPc:C60 are filled by weak p-doping, enabling trap characterization in a blend layer at a controlled hole density. We evaluate impedance spectra at different temperatures in order to determine the density of occupied states (DOOS) directly from the capacitance-frequency spectra by assuming a ...

A charge carrier transport model for donor-acceptor blend layers

Highly efficient organic solar cells typically comprise donor-acceptor blend layers facilitating effective splitting of excitons. However, the charge carrier mobility in the blends can be substantially smaller than in neat materials, hampering the device performance. Currently, available mobility models do not describe the transport in blend layers entirely. Here, we investigate hole transport in a model blend system consisting of the small molecule donor zinc phthalocyanine (ZnPc) and the acceptor fullerene C60 in different mixing ratios. The blend layer is sandwiched between p-doped organic injection layers, which prevent minority charge carrier injection and enable exploiting diffusion currents for the characterization of exponential tail states from a thickness variation of the blend layer using numerical drift-diffusion simulations. Trap-assisted recombination must be considered to correctly model the conductivity behavior of the devices, which are influenced by local electron currents in the active ...

Elementary steps in electrical doping of organic semiconductors

Fermi level control by doping is established since decades in inorganic semiconductors and has been successfully introduced in organic semiconductors. Despite its commercial success in the multi-billion OLED display business, molecular doping is little understood, with its elementary steps controversially discussed and mostly-empirical-materials design. Particularly puzzling is the efficient carrier release, despite a presumably large Coulomb barrier. Here we quantitatively investigate doping as a two-step process, involving single-electron transfer from donor to acceptor molecules and subsequent dissociation of the ground-state integer-charge transfer complex (ICTC). We show that carrier release by ICTC dissociation has an activation energy of only a few tens of meV, despite a Coulomb binding of several 100 meV. We resolve this discrepancy by taking energetic disorder into account. The overall doping process is explained by an extended semiconductor model in which occupation of ICTCs causes the classically known reserve regime at device-relevant doping concentrations.Molecular doping is routinely used in organic semiconductor devices nowadays, but the physics at play remains unclarified. Tietze et al. describe it as a two-step process and show it costs little, energetically, to dissociate charge transfer complexes due to energetic disorder of organic semiconductors.

Non-volatile organic memory devices comprising SiO2 and C60 showing 104 switching cycles

We present a non-volatile organic memory device comprising a thin SiO2 layer, the organic semiconductor C60, and an organic n-type doped layer between two metallic electrodes. The memory device shows a stable hysteresis in the current-voltage characteristics with an ON/OFF ratio in the range of three or higher and reasonable switching behavior with 104 write-read-erase-read cycles. The data retention time reaches from several hours up to several days and depends on the read out frequency. We exclude a filamentary conduction mechanism as cause of the memory effect and propose that the presence of charge carrier traps at the interface of the C60 layer with the oxide causes the hysteresis of this organic non-volatile memory device.