In this study, we have investigated the effect of surface treatment on the orientation and mobility of pentacene by using a photoreactive polyimide film to modify the gate-insulator surfaces of organic field effect transistors (OFETs). Surface modification includes a photoreactive polyimide film, presenting a passivated interface on which the semiconductor can grow. This polyimide film can control of the orientation of semiconductor by using linearly polarized deep UV (LPDUV) irradiation. Fabricated OFETs include stacked structures of Ta2O5 as the gate insulators and the photoreactive polyimide. Most of the characteristic parameters of the OFETs, such as carrier mobility and on/off current ration, have been improved by using the photo-alignment treatment achieved with LPDUV irradiation.

An organic complementary-metal-oxide-semiconductor (CMOS) inverter based on pentacene acting as both n- and p-type organic semiconductor is presented. The circuit consists of two spatially separated transistors which are realized within one continuous pentacene layer. Both transistors act exclusively in unipolar mode with electron and hole mobilities of 0.11 cm2V-1s-1 and 0.10 cm2V-1s-1, respectively. In the domain of the n-channel, electron accumulation in the pentacene is enabled by deposition of traces of calcium acting as electron donator. The CMOS inverter works reliably within the range of the supply voltage (60 V) with a gain in between 17 and 24 which is among the highest values observed in organic systems. Nevertheless, the circuit shows hysteresis, which is explained by a gate voltage depending trap occupation in the n-channel.

Turn-on dynamics of polymer field effect transistors were examined experimentally over a wide timescale. We found that the source current dependence on time following switch on of the gate bias exhibits a power law at the short time range, and an exponential decay at the intermediate to long time range. We demonstrate that the transistor dynamic behavior is governed by the channel charge build-up, and can be described accurately by a simple capacitor-resistor distributed line model.

A molecular tool box is presented, allowing the efficient synthesis of a broad variety of bis(tetracenyl) aromatics. These materials might be of considerable interest as semiconductors in organic field-effect transistors. The general synthetic strategy applied here involves Diels-Alder reactions of bromobenzocyclobutene derivatives and arene-1,4-endoxides[1], followed by transition-metal catalyzed condensation reactions of the formed 9-bromo-6,11-dihydrotetracene derivatives with aryldiboronic and -stannyl derivatives, respectively. In the final step, the resulting bis(dihydrotetracenyl) derivatives were dehydrogenated, giving the bis(tetracenyl) aromatics in good yields. They were characterized using NMR, UV-vis and mass spectrometry. Moreover, for some of these semiconductors the charge carrier mobility in thin film transistors has been determined.

Strong substrate dependence of interfacial electronic structure of 11, 11, 12, 12-tetracyanonaphtho-2, 6-quinodimethane (TNAP) thin film was observed by infrared reflection-absorption spectroscopy and ultraviolet photoemission measurements. It was found that an apparent charge transfer interaction exists in the interface between the TNAP film and Ag substrate and Cu complex is formed by deposition of TNAP onto Cu substrate.

A Self-Assembly oriented technique from the vapor-phase, Molecular Layer Epitaxy (MLE), was utilized for the buildup of organic multilayers as the active channel in organic field effect transistors (OFET). Carrier gas-assisted chemical vapor deposition (CVD) of 1,4,5,8-naphthalene-tetracarboxylic-dianhydride (NTCDA) and an aliphatic spacer are used in a pulsed mode for the covalent attachment of a single monolayer at a time resulting in an ordered dense multilayer film. The MLE approach uses a template layer to promote coupling between the substrate and the precursors deposited from the vapor phase. Interlayer epitaxy is governed by self-limiting vapor-phase condensation reactions while intra-layer ordering is achieved via horizontal π-stacking. Resulting multilayers were characterized by means of contact angle, variable angle spectroscopic ellipsometry (VASE), AFM, absorbance in the UV-vis.-NIR and FTIR. Multilayer structures are also built on a silicon substrate with predefined gold electrodes, using a self assembled template layer on the electrodes and on the thin gate oxide, thus allowing for the buildup of a multilayer structure covering both the electrodes and the channel area while enhancing the nature of the contact between the multilayer and the source and drain electrodes. Resulting OFET devices show n-type conductivity with a mobility of 0.031 cm2 V-1 s-1 for a 6nm thickness MLE film, thus justifying the utilization of the technique in OFETs research and applications.

The application of organic field effect transistors (OFETs) for large scale low-cost electronic devices has lead to intense research. Diindenoperylene (DIP) thin films on SiO2 are a prominent system due to their high structural out-of-plane order. While bottom contact OFET structures can be realized easily, preparation of top contacts might cause diffusion of metal atoms (typically Ag or Au) deep into the organic film changing the injection properties at the interface. These properties are of great importance for device fabrication. Therefore, only by understanding the diffusion behaviour of metals into the organic layer, formation of well defined interfaces and control of their properties will become possible. For a better understanding of the diffusion of noble metal atoms into crystalline organic films, a radiotracer technique has been used to obtain diffusion profiles for Ag and Au diffusion in crystalline DIP films. For Ag diffusion in DIP, the decrease in Ag concentration of four orders of magnitude within the first few nanometers indicates that most of the metal atoms remain near the surface while small amounts can penetrate deep into the thin film and can even accumulate at the interface between organic film and the silicon substrate. A comparison with diffusion profiles obtained for polymers indicates that the interplay between diffusion and immobilization by aggregation also determine the diffusion behaviour of metals in organic crystalline materials. Latest experiments support this interpretation of the diffusion profiles. Single atoms are highly mobile in the organic crystalline material due to the weak interaction between the metal and the organic material. Therefore, most of the single atoms that penetrate into the material do so during the initial phase of the deposition. When more and more atoms arrive at the surface, cluster formation sets in. Due to the high cohesive energy of the metal the atoms can not leave the cluster and become immobilized. After deposition of a closed surface layer no further metal diffusion should be observed. With the knowledge about the diffusion processes gained by the radiotracer measurements control of process parameters and development of barrier layers in sub-monolayer range should be possible.

A regioregular copolymer of 3–hexylthiophene and 3–(6–hydroxyhexan–1yl)thiophene has been functionalised with biotin hydrazide; binding of avidin to the biotin moieties causes drastic changes to the absorption spectrum of the polymer in solution, and to the electrochemistry and conductivity of the polymer in thin films.

The effect of oxygen doping on titanyl phthalocyanine (TiOPc) films was investigated by ultraviolet photoelectron spectroscopy (UPS). The energy of the electronic levels of TiOPc films probed by UPS was clearly different between the film deposited in ultrahigh vacuum (UHV) and in oxygen atmosphere. The film deposited in UHV showed downward band bending characteristic of n-type semiconductor. On the other hand, the film deposited in oxygen atmosphere showed upward band bending characteristic of p-type semiconductor. Such trends are in excellent correspondence with reported field effect transistor characteristics. In order to examine the Fermi level (EF) alignment between TiOPc film and the substrate, the HOMO energy of TiOPc relative to the EF of the metal substrate was determined for various substrates. Although EF alignment was not achieved for the TiOPc film prepared in UHV, possibly because of insufficient charge density in the TiOPc film, it was achieved in the case of TiOPc film exposed to oxygen, probably by p-type doping effect of oxygen.

We report a new Iridium(III) complex and the study of its optical, electrochemical and electroluminescence properties. The crystal structure shows an octahedral environment around Ir(III) center. Density functional theory (DFT) calculations indicate the contribution of the d-orbitals of Ir and the π-orbitals of the cyclometalating and ancillary ligands toward HOMO, whereas LUMO is concentrated on only the cyclometalating ligand. These complexes emit in the sky blue color region from an admixture of triplet metal-to-ligand-charge-transfer (3MLCT) and ligand π-π* states. A maximum external (ηex) quantumefficiency and luminance efficiency of 2.4% and 5.5 cd/A at 0.12 mA/cm2 was obtained from the device consisting of a 5% doped polymeric and low molecular host. A maximum brightness of 10,200 cd/m2 at 14.8 V was obtained from the device.

Capacitance measurements were conducted to determine the dielectric constant of electrostatic self assembly (ESA) films over various temperature and frequency ranges at 1 mV and 15% relative humidity. Measurement electrodes were fabricated on the ESA films using silver grease and a brass bar. In these tests, polymeric nanofilms were fabricated on gold-coated glass slides using the ESA process. Thicknesses of the films were between 100 nm and 600 nm obtained by depositing different number bilayers of negatively charged Poly s-119 (PS-119) or heparin and positively charged poly(diallyldimethylammonium chloride) (PDDA). The test results showed that dielectric constant values were around 2. Based on the test results, we concluded that this is a technique that might prove useful to estimate the capacitance and dielectric constant values of nanostructured ESA films, which can be largely used in the near future.

An important challenge for Organic Light-Emitting Diodes (OLEDs) manufacturing is patterning method of the organic materials used for different colors. In this talk, a Large-Area wet Micro-Printing (LAMP) technique is proposed and demonstrated for organic device patterning. A printing plate is first prepared by surface engineering so that a designed surface energy pattern is achieved. The printing plate is then coated with “ink,” brought into contact the device substrate, and the “ink” is transferred. With this approach, the red (R), green (G) blue (B) sub-pixel arrays needed in a full-color display can be printed in three successive steps, one step for each color. Both single-color pixel arrays and R, G, B sub-pixel arrays have been patterned as a demonstration of the feasibility of this method. The technique has the potential advantages of low-cost and high-throughput and it avoids some of the practical problems associated with the design and operation of an ink-jet apparatus.

Monolayers of tin complexes of phenoxide ligands spanning a range of dipole moments were prepared on the surface of ITO via simple metathesis reactions. They were characterized by quartz crystal microgravimetry (QCM) and a Kelvin probe. A nearly linear relationship was found between the measured ITO work functions and dipoles of the surface complexes. Measurements of current densities of diode devices built on surface modified ITO anodes were made, and a correlation was found between the total surface dipole per unit area and these current densities. Simple OLED devices were also constructed using these modified anodes.

We present an analytic description for the loss of photocurrent efficiency at moderate light intensities and demonstrate a simple technique for extracting the mobility of electrons in semiconducting polymer layers. The underlying theoretical analysis, which is based on a simple drift-recombination scheme, shows good agreement with the measured intensity dependent photocurrent quantum efficiency. The electron mobility extraction is demonstrated for pristine MEH-PPV. The same physical framework is then used to describe the switch on of the photocurrent and the buildup of electron and hole concentrations inside the device. By analyzing the time evolution of the measured photocurrent transient we find an independent verification for the results extracted from the intensity dependence measurements. We use the combination of theoretical and experimental study to discuss the role recombination and space charge effects in reducing photocurrent efficiency.

In various organic electronic devices, interfaces formed by organic layers can play important roles. We have been studying various organic interfaces for clarifying their structure and electronic structure. In this talk, we will report our recent study of the effect of various types of doping for a variety of dopants – residual impurity, atmospheric gases, and metallic and organic intentional dopants. In particular, detailed and quantitative information about the effect of oxygen from the viewpoint of electronic structure was obtained for titanyl phthalocyanine (TiOPc), and the results corresponded well with the recent report of atmospheric effect on orga nic field effect transistor.

Photoactive organic field-effect transistors, photOFETs, based on a conjugated polymer/fullerene blend, MDMO-PPV: PCBM (1:4), and polymeric dielectrics as polyvinylalcohol (PVA) or divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) with top source and drain electrodes were fabricated and characterized in dark and under AM1.5 illumination. With LiF/Al as top source and drain contacts the devices feature n-type transistor behavior in dark with electron mobility of 10-2 cm2/Vs. Under illumination, a large free carrier concentration from photo-induced charge transfer at the polymer/fullerene bulk heterojunction (photodoping) is created. The device performance was studied with different illumination intensities and showed to be strongly influenced by the nature of the organic dielectric/organic semiconductor interface resulting in phototransistor behavior in BCB-based photOFETs and in phototransistor or photoresistor behavior for PVA-based photOFETs.

Novel poly(3,6-hexyl-2,7-N-octylcarbazole) derivatives and poly(diindolocarbazole)s were synthesized. Optical, electrochemical, electrical and thermoelectric properties were investigated. Band structure calculations were used to predict which polymers were promising as thermoelectric materials. The best combination of Seebeck coefficient and conductivity (power factor) was 9,4 x10-8 Wm-1K-2 with a copolymer of carbazole and thiophene. This corresponds to a ZT at room temperature of 0.0003. Optimization of the polymer structure and doping level should lead to an increased ZT.

By blending poly(ethylene glycol) (PEG) into an electroluminescence (EL) polymer, significantly enhanced EL efficiency in a polymer light–emitting diode (PLED) with aluminum electrode was achieved. An orange-color-emitting PLED with 10 wt% PEG blending achieved device efficiencies exceeding 2.6 cd/A for a wide range of bias voltage, which is more than two orders of magnitude higher than that for a similar PLED without the PEG blending. A similar enhancement was observed by introducing PEG as an ultra-thin interfacial layer between the emissive polymer and the cathode. The enhancement was also observed for several different species of emissive polymers, and for blending with either PEG or poly(ethylene oxide) (PEO) with different molecular weights. The enhanced efficiency was a result of the reduction of electron injection barrier height at the cathode-polymer interface. It is believed that interfacial interaction that is specific to Al plays an important role in the enhancement mechanism.

The oligothiophene derivative (4-(5″″-tetradecyl-[2,2′;5′,2″;5″,2″′;5″′,2″″] pentathiophen-5-yl)-butyric acid (C14-5TBA) was synthesized and the structural and mechanical properties of self-assembled monolayers on mica have been studied by atomic force microscopy (AFM). The films were prepared by drop casting a dilute THF solution (1mM) of the oligothiphene on mica. Islands containing primarily monolayers with a very small percentage of multilayers were formed. The molecules adsorb through the carboxylic group, and expose the alkyl chain (CH2)13CH3. High resolution AFM scans reveal a well ordered structure of molecules with unit cell dimensions of 0.65 and 0.46 nm. Applying load to the tip, the molecular film was gradually compressed from an initial height of 4.1nm to a final one of 2.6 nm, corresponding to atilt of the alkyl chains. In regions covered with bilayers the molecules in the second layer were oriented opposite to those in the first layer, thus exposing the carboxylic end group to the air. These second layer was easily removed as the tip pressure increased.

One of the potential application areas for organic and polymers transistors is in radiofrequency identification (RFID) tags. One of the key components of an RFID tag is the front-end rectifier that must rectify a 13.56 MHz AC signal received from a resonant tuned antenna. The rectifier supplies operating power to the tag. Organic transistor circuits have hitherto not operated at this high frequency. We show that by operating pentacene transistors in the non-quasi-static (NQS) regime such operating speeds can be achieved in rectifier circuits. The circuits were fabricated on flexible plastic substrates and employed a solution-cast dielectric. The pentacene mobilities are in the range 0.1-1.5 cm2/V-s. The channel lengths of the transistors are in the range 2-4 μm. Full-wave NQS mode rectifiers were measured to have voltage rectification efficiency in excess of 28% at 14 MHz, demonstrating that such circuits can be used in RFID tags. These circuits operated successfully at speeds up to 20 MHz.