Recently much interest has been directed toward solution processable polymeric semi-conductor materials containing thiophene based moieties.[1] In particular, dialkylated tetrathienoacene (fused thiophene) copolymers have been used as high mobility semiconductors in field-effect transistors with a field-effect hole mobility exceeding 0.3 cm2/Vs.[2] Expansion of this class of materials to include new materials with either linear (DCnFT4) or branched (DbCnFT4) di-alkyl-substituted fused thiophenes has been achieved with the development of two distinct synthetic routes for ring formation and the introduction of the side-chains.

DC17FT4 was prepared from tetrabromothieno[3,2-b]thiophene through a sequence of diketone formation, cyclization, hydrolysis and decarboxylation. Synthesis of the new compound di-2,4,4-trimethylpentyl-FT4 (DbC8FT4) has been accomplished in a more complex 8 step process, featuring a mono-ketone-ester as a key intermediate.

DCnFT4 can be brominated in the 2- and 6-positions with NBS. The dibromide can then be used to form copolymers through cross coupling reactions. These polymers are solution processable semi-conductors. The polymers have been characterized by GPC and within organic thin-film transistor (OTFT) devices.

A new class of high-vacuum organic deposition chamber was developed to study the structure and growth of organic semiconductor thin films. Using the chamber in situ real-time 2-dimensional grazing incidence x-ray diffraction (2D-GIXD) was measured during thin film growth of distyryl-oligothiophenes derivatives (DS2T) on SiO2 surface. An evolution of 2D-GIXD pattern was clearly observed during the growth due to the crystallization of the molecular domains consisting of standing-up orientation. A theoretical simulation on the experimental 2D-GIXD map provided a potential use of this system to determine the unit cell parameters in thin films. Thickness dependence and influence of air exposure on the film structure were also studied.

In this paper, low-cost rectifier based on an organic diode for use in organic radio frequency identification (RFID) tags is proposed. Pentacene is the electroactive layer, with 7,7,8,8-tetracyanoquinodimethane (TCNQ) modified low-cost copper (Cu) and aluminum (Al) as the Ohmic and Schottky contacts, respectively. Hole injection barrier between Cu and pentacene can be decreased by forming the self-assembled layers of Cu-TCNQ. The diode shows a high rectification ratio of approximately 2×106 at 5V and the organic diode based rectifier circuit generated a dc output voltage of approximately 2V at 13.56MHz, using an input ac signal with zero-to-peak voltage amplitude of 5 V. The results indicate that chemical modification of the low-cost electrodes could be an efficient way toward low-cost high performance organic electronics devices.

In an effort to realize high-speed organic logic components, p- and n-type single-crystal organic field-effect transistors (SC-OFETs) were fabricated using air-gap structures with channel lengths as short as several μm. High carrier mobility of about 10 cm2/Vs is demonstrated in rubrene SC-OFETs even with the short channel length of 6 μm, using Si-based microstructures. The contact resistance is estimated to be 450 Ohm cm, which is only 5% of the total channel resistance between source and drain electrodes. Performances of n-type air-gap devices based on PDIF-CN2 are also demonstrated exhibiting electron transport with the carrier mobility of about 2 cm2/Vs. Furthermore, micron-scale air-gap structures are fabricated using insulating materials on glass substrates to reduce parasitic gate capacitance. The cut-off frequency of this rubrene air-gap device is measured to be as high as 8 MHz with applied drain voltage VD of 15 V. These techniques are promising to be applicable to next-generation organic high-speed logic circuits.

Time of flight (ToF) is the most straightforward technique to determine polymeric semiconductor mobility for electronic applications. We demonstrate ToF limits of applicability to amorphous PPV derivatives, such as poly[2-methoxy-5-(3’,7’-dimethylloctyloxy)-1-4-phenylene vinylene] (MDMO-PPV) and poly[2-methoxy-5-(2’-ethylhexyloxy)-1-4-phenylene vinylene] (MEH-PPV), and polycrystalline poly(3-hexylthiophene) (P3HT). Hole and electron mobility (μ) in submicrometric films (200 – 500 nm) is overestimated compared to casted layers, due to reduced absorption capability, which is confirmed by Charge Extraction by Linearly Increasing Voltage (CELIV) measurements. Charge transport properties in nanometric films, such as for Field-Effect Transistors (FET), can not be studied by current-mode ToF. Hole mobility of ca. 10-5 cm2/Vs with Poole-Frenkel behavior for PPV derivatives and 10-3 cm2/Vs for P3HT is at least one order of magnitude higher than ToF results.

Thin films (50 nm) of 2,5-di-4-biphenylthiophene (PPTPP), 5,5´-di-4-biphenylyl-2,2´-bithiophene (PPTTPP) and 4,4´-di-2,2´-bithienylbiphenyl (TTPPTT) were vapor-deposited on microstructured gold (source- and drain-) electrode arrays on thermal SiO2 as gate dielectric with underlying Si serving as gate electrode. The films were studied in their field-effect characteristics during film growth and subsequent to it. A decay of specific conductivity and of charge carrier mobility was observed in subsequent measurements. During annealing without an applied field the films recovered but showed a second decay as soon as an electric field was applied again for repeated characterization. Induced dipoles and subsequent structural changes as well as chemical interactions with the SiO2 interface are discussed as possible origins of these observations.

In this contribution we have reported about bi-component blends of readily accessible semiconducting molecular arylacetylenes with insulating high-density polyethylene (HDPE) and poly(vinylidene fluoride) (PVDF) that may exhibit electronic characteristics comparable to those of the neat semiconductors, as measured in field-effect transistors (FETs).

Short-chain oligomers of aniline are attractive semi-metallic materials for applications as organic electrodes or hole-transporting layers in organic photovoltaics. However, conventionally processed oligoanilines are often amorphous, which limits their conductivities and carrier transport mobilities. Here, we report a simple solvent-exchange method that can render a variety of oligoanilines and their derivatives into crystals of different shapes and dimensions, including 1-D fibers and wires, 2-D ribbons, and 3-D plates, hollow spheres, porous sheets, and flower-like structures. Dopant ions are also simultaneously incorporated into the crystals during self-assembly, allowing them to become conducting. Mechanistic studies suggest that the higher order crystals arise from the most primitive nanofibrillar morphology via hierarchical assembly, providing insights into a general approach to control organic crystal morphologies. Selected area electron diffraction studies reveal their single crystalline nature.

In the past few years, organic vapor phase deposition (OVPD) has been demonstrated to be an effective deposition method for high-performance monochrome and white organic light emitting diodes (OLEDs) [1-4]. OVPD provides good material utilization efficiency and large achievable deposition rates.

An application of p-type doping is the improvement of hole injection either from the anode contact or from a charge generation layer in stacked OLEDs [5]. Nevertheless, no reports on p-type doping using OVPD can be found in literature, in part due to the thermal instability and high chemical sensitivity of organic dopants.

In this work, p-type doping using an AIXTRON Gen-1 OVPD tool with two different show-erhead designs is examined. NDP-2 (NOVALED) and N,N‘-diphenyl-N,N‘-bis(1-naphthylphenyl)-1,1‘-biphenyl-4,4‘-diamine (NPB) were used as p-type dopant (guest) and hole-conducting host, respectively. p-Type doped hole-only devices were fabricated and compared with undoped ones.

Two different showerhead designs (made either of aluminum or stainless steel) were investi-gated with respect to OLED performance to determine possible side reactions.

Highly efficient monochrome red OLEDs including a p-type doped hole transport layer were demonstrated exhibiting a current efficiency of 31 cd/A, a power efficiency of 26 lm/W and a driving voltage of 3.7 V without improved light outcoupling (all values at 1000 cd/m2).

The application of hopping theory for the prediction of charge (hole) mobility in amorphous organic molecular materials is studied in detail. Application is made to amorphous cells of N,N’-diphenyl-N,N’-bis-(3-methylphenylene)-1,1’-diphenyl-4,4’-diamine (TPD), N4,N4’-di(biphenyl-3-yl)-N4,N4’-diphenylbiphenyl-4,4’-diamine (mBPD), 1,1-bis-(4,4’-diethylaminophenyl)-4,4-diphenyl-1,3,butadinene (DEPB), N1,N4-di(naphthalen-1-yl)-N1,N4-diphenylbenzene-1,4-diamine (NNP), and N,N’-bis[9,9-dimethyl-2-fluorenyl]-N,N’-diphenyl-9,9-dimethylfluorene-2,7-diamine (pFFA). Detailed analysis of the computation of each of the parameters in the equations for hopping rate is presented, including studies of their convergence with respect to various numerical approximations. Based on these convergence studies, the most robust practical methodology is applied to investigate the dependence of mobility on such parameters as the monomer reorganization energy, the monomer-monomer coupling and the material density. The results give insight into what factors should be controlled to develop materials with higher (or lower) charge (hole) mobility, and what will be required to improve the accuracy of predictions of mobility in amorphous organic materials.

Optimization of the interface between the organic semiconductor (OSC) & the source-drain (S/D) electrode is critical in order to improve organic thin film transistor (OTFT) device performance. This process typically involves coating the metal S/D electrodes with an optimal self-assembled thiol layer; a process that requires pristine metal surfaces for successful treatment. Obtaining contamination free surfaces can be challenging in the case of printed metal electrodes. Here we demonstrate an effective strategy to address this issue by introducing a brief low power forming gas plasma treatment prior to the surface coating step. We show a two orders of magnitude decrease in the contact resistance as a result of this treatment.

The effect of the electron blocking layer on the performance of white organic light-emitting diodes is studied. A variation of the material influences not only the carrier transport, but also the light distribution from the different emitters. Highest external quantum efficiency is reached for the material with the worst electrical properties, while highest luminous efficacy is obtained for the material with the best transport characteristics.

The work function of the indium tin oxide electrode (ITO) modified by a self-assembled monolayer (SAM), which is used as an electrode in organic electroluminescent (EL) devices, was investigated in this study. It is revealed that chemical modification of ITO with p-substituted with different terminal groups (NH2−, Cl−, and CF3−) benzoic acids as a SAMs material with carboxyl binding group is caused to increase the work function of the ITO electrode. Through a self-assembly process, the transmittance of the ITO with a SAM was not changed. The work function change of ITO with various SAM was measured by using cyclic voltammetry. Characteristics of EL devices were increased because the energy barrier was decreased in an interface between the ITO and an organic layer in the EL devices. The correlation of the work function change and the performance of the chemically modified EL devices was estimated.

The light out-coupling potential of introducing a semitransparent Ag layer between the anode and the organic layer stack of monochrome bottom-emitting organic light emitting diodes (OLED) is examined. Red and green phosphorescent as well as deep-blue fluorescent resonant-cavity OLED (RC-OLED) comprising a semitransparent Ag layer are processed by means of organic vapor phase deposition (OVPD). An enhancement of the luminous efficiency of up to 81% can be observed.

The impressive efficiency enhancement can be explained by a reduced formation of substrate modes in combination with a strong narrowing of the emission spectrum leading to an increased true luminous efficiency.

Organic magnetoresistance (OMAR) of single-crystalline (SC) pentacene (C22H14) and perfluoropentacene (C22F14) was studied using field-effect transistor structures. The gate voltage effect showed that the OMAR originates from photo-induced current and requires both electrons and holes in the transport channel. The temperature dependence showed the maximum magnetoresistance (MR) ratio up to -6% under light irradiation at approximately 200 K and magnetic field of 80 mT. The charge carrier mobility and the exciton diffusion length were not important factors to determine the MR ratios. The interaction between triplet excitons and traps was thought to govern the OMAR behaviors.

In this work, we report the results of solid state structural study of a series of oligothiophenes and ethylenedioxythiophenes (EDOT) endowed with bromine atoms at different positions and in the presence/absence of the strong electron acceptor, the tricyanovinyl group (TCV). Five new compounds were synthesized, crystallized and their single crystal structures were determined by X-ray diffraction. The impact of bromine atom(s) on solid state packing as viewed through analysis of intermolecular interactions is presented. Comparative study using experimental, computed and published data of closely related structures shows intra-molecular interactions involving Br in the presence of TCV and EDOT. Distances shorter that the summation of the Van der Waals radii were observed which indicate the role of Br…NC, S…S and Br…Br interactions in addition to pi stack formation and planarity of these compounds due to the presence of strong electron acceptors. Possible implications of how these structural features may impact charge transport in oligothiophenes will be discussed.

The carrier transport properties in the emissive layer of phosphorescent polymer organic light-emitting diodes (OLEDs) were observed by time-of-flight (TOF) mobility measurements. The hole and electron mobilities in carrier transport polymer without iridium complexes were measured with high levels, 3 × 10-4 cm2/Vs (hole) and 1 × 10-3 cm2/Vs (electron), of non-dispersive transport. The hole and/or electron transport properties were degraded when the iridium complexes were included in the phosphorescent polymers. The complexes acted as a trap in the phosphorescent polymers when the energy levels of the iridium complexes were lower than that of the carrier transport polymer.