Charge carrier transport under reverse voltage conditions is of major relevance in devices like organic photo-detectors, organic solar cells (tandem cells), organic light emitting diodes (generation contacts), and organic Zener diodes. We present organic pin-diodes comprising molecular doped layers of pentacene and C60 with an adjustable and reversible reverse breakdown behavior. We discuss the electric field and temperature dependence of the breakdown mechanism and propose a coherent charge transport scenario to describe the experimental findings. Within this model a field assisted tunneling of charge carriers over a rather large distance from valence to conductance states (and vice versa) governs the breakdown behavior. This is in accordance to experimental observations where charge carriers can overcome a layer thickness of 110 nm in the breakdown regime.Graphical abstractHighlights► Reversible reverse breakdown in pentacene/C60 pin-diodes. ► Reverse breakdown condition tunable by the electric field within the junction. ► Activation energy of transport within the breakdown <10 meV. ► Field and temperature dependence of breakdown behavior suggest a tunneling process. ► A proposed model assuming coherent transport can explain the experimental findings.

Direct structuring techniques are an indispensable need for future low-cost applications of organic semiconductor materials in e.g. active matrix displays or integrated circuits. We demonstrate direct structuring of a small molecule organic semiconductor by a photo-lithography lift off process under ambient conditions. To show compatibility of this process, we fabricate organic thin film transistors (OTFT) containing the benchmark electron transporting semiconductor C60 as active material in a top-contact geometry. C60 as electron transporting semiconductor serves as good indicator for contamination and degradation caused by the structuring procedure. To disclose influences of structuring, we discuss the OTFT performance for different channel lengths from 100 μm down to 2.7 μm. In particular, we show that lithography processing gives rise to increased contact resistances. Apart from that, mobility of C60 as material parameter is only weakly affected which underlines the compatibility of the suggested structuring procedure. The potential of this structuring procedure for future integration of driving transistors in active matrix displays is demonstrated.Graphical abstractHighlights► Fluorine based photo-lithography for direct patterning of C60 thin film transistors. ► Structuring under ambient conditions feasible without loss in OTFT performance. ► Increased contact resistance related to resist residuals. ► Strong derivations of OTFT scaling laws observed by channel length variation.

The structural properties and charge carrier mobility of pentacene doped by 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) and 2,2-(perfluoronaphthalene-2,6-diylidene) dimalononitrile (F6-TCNNQ) are studied by X-ray diffraction, scanning electron microscopy, field effect transistor measurements, and space charge limited currents (SCLC). We observe the presence of polycrystalline and amorphous domains within the doped pentacene film grown under co-deposition conditions. The appearance of the amorphous phase is induced by the molecular dopants F4-TCNQ and F6-TCNNQ. A strong drop of crystallite size is obtained at a doping concentration of around 7 and 4 wt.%, respectively. The loss of the polycrystalline structure is correlated to a strong decrease of the charge carrier mobility in pentacene in horizontal and vertical film structures. We discuss typical scenarios of charge transport for polycrystalline and amorphous thin films in order to explain the observed loss of mobility originated by the doping induced structural phase transition. In this way an optimum doping concentration for highest conductivity with acceptable mobility is determined which can help to improve the performance of organic solar cells and organic high-frequency rectification diodes.Graphical abstractHighlights► We investigate structural and electronic properties of molecular doped pentacene. ► We employ two strong acceptor materials F4-TCNQ and F6-TCNNQ. ► We reveal a structural phase transition for doping ratios above 4 wt.%. ► The structural transition is correlated to a strong loss in charge carrier mobility. ► Different scenarios of charge carrier transport are discussed for explanation.

Organic pin-diodes employing molecular doped hole and electron transport layers working at ultra-high-frequencies (UHF) are presented. In comparison to undoped organic Schottky diodes reported for ultra-high-frequency applications, the pin-concept is superior since the doping concentration and the intrinsic interlayer thickness can be adjusted in order to control the performance of such pin-diodes. We investigate the influence of both parameters on basic diode parameters like forward resistance and rectification ratio. In particular, we discuss the role of molecular doping and its influence on the AC signal response and we present an optimized doping ratio in order to accomplish highest charge carrier mobility and conductivity. Optimized devices are working as rectifiers up to 300 MHz at a small operating voltage of 2 V. The frequency limit is estimated to be at 1 GHz.Graphical abstractHighlights► We report molecular doped pin-diodes working in the ultra-high-frequency region. ► Diodes with optimized rectification, forward resistance, and reverse capacitance. ► DC output voltage of 1.5 V for an AC input signal of 2 V amplitude at 20 MHz. ► Cut-off frequency estimated to be 1 GHz.

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.