Kelvin probe force microscopy was used for extraction of the threshold and the pinch off voltages in organic thin film transistors. The first was determined by direct detection of the charge accumulation onset and the latter by a direct observation of the pinch off region formation. In addition, an effective threshold voltage shift can be extracted from the pinch-off voltage as a function of charge concentration. The dependence of the effective threshold voltage on the gate voltage must be considered when calculating charge carrier concentrations in organic thin film transistors.

Al was thermally evaporated onto the conjugated semiconductor polymer, poly(2-methoxy-5,2′- ethyl-hexyloxy-phenylene vinylene) (MEH-PPV) in successive steps. Interface formation and chemical interactions between the polymer and Al were investigated by X-ray Photoelectron Spectroscopy (XPS) and synchrotron source Ultraviolet Photoelectron Spectroscopy (UPS). XPS analyses (angle resolved and sputtering depth profile) proved the formation of an insulating aluminum oxide layer at the polymer metal interface. The C-O bonds in the polymer decreased as more Al was evaporated on the MEH-PPV. Also reaction between Al and the carbon backbone of the polymer leading to the creation of carbon-metal bonds was observed. UPS analysis suggested the possibility of Al reacting with the phenylene ring of the polymer.

Spin polarized charge carrier injection and transport in non-magnetic semiconductors is a key enabling mechanism for spin based electronic data processing. We present theoretical models to describe spin injection and spin transport in structures consisting of a ferromagnetic metal injector, a thin semiconductor layer, and a ferromagnetic metal collector. The semiconductors considered are conjugated polymers (e.g. PPV), small-molecule organic molecular crystals (e.g. pentacene), and inorganic semiconductors (e.g. silicon). In thermal equilibrium the charge carriers in these semiconductors are not spin polarized. Efficient spin injection requires that the semiconductor be driven far out of local thermal equilibrium. Since carrier mobilities (and other relevant parameters) in polymers, organic molecular crystals, and inorganic semiconductors differ by many orders of magnitude, their charge carrier injection characteristics differ significantly.

In this paper we address the optical process occurring in a standard Alq3 based OLED device using a classical electromagnetic framework. A comprehensive comparison with experimental results is made with an excellent agreement. The insertion of a hole blocking layer (HBL) is used to confine excitons formation and relaxation within a well defined region allowing us to separate optical effects from other mechanisms. By tuning the cavity thickness, we can demonstrate destructive interference into the wavelength region corresponding to Alq3 peak emission. Electroluminescence measurements in an integrating sphere further supports the idea that the interferences scheme used here describes well the optical behaviour of our samples.

We report on an approach towards integrated complementary-like circuits based on organic ambipolar transistors. In particular, we show that ambipolar transport can be achieved within a single transistor channel using gold electrodes and a solution processable polymer-small molecule blend as the electroactive material. To demonstrate the suitability of these devices for practical utilisation in logic circuits we realise complementary-like voltage inverters comprised entirely of ambipolar transistors. Moreover, by integrating several such inverters we are able to demonstrate more complex circuits such as ring oscillators.

Significant progress has been made in the area of p-type organic field effect transistors while progress in developing n-type materials and devices has been comparatively lacking, a limiting factor in the pursuit to develop complementary organic electronic circuits. Given the need for n-type organic semiconductors we have carried out studies using two different fullerene molecules, C60 and C70. Here, we report mobilities for C60 ranging from 0.02 cm2/Vs up to 0.65 cm2/Vs (depending on channel length), and mobilities from 0.003 cm2/Vs up to 0.066 cm2/Vs for C70. All devices were fabricated with organic films deposited under high vacuum but tested at ambient pressures under nitrogen.

We report the structure of 5,5′-bis(4-hydroxyhexylphenyl)-2,2′-bithiophene (HO6PTTP60H) powder and sublimated films studied by X-ray diffraction (XRD) and Infrared absorption spectroscopy (IRAS). From XRD, herringbone packing is observed for the HO6PTTP60H powder. Different layer spacing is found for the powder (33.1Å) and the sublimated film (26.5Å). This slight difference indicates that the HO6PTTP60H molecule is tilted in the film. IRAS results confirm the presence of the HO6PTTP6OH molecule on the surface and its good stability over a few days.

In order to enable low cost roll-to-roll or sheet-processing of organic light-emitting diode (OLED) devices, completely new deposition methods for both polymer and smallmolecule layers are being developed in place of the classic semiconductor manufacturing methods. In evaluating the utility of such methods, it is advantageous to have a robust and fast method to measure the thickness uniformity of the deposited organic layers. Non-uniformities at all spatial length scales from sub-mm to several cm can occur and so need to be understood as a function of the relevant parameters for each deposition method. Here we demonstrate a simple and fast method to quantify non-uniformities in thin films over arbitrarily large length scales. Our method utilizes the color of light reflected from the coated substrate and its variation with polymer layer thickness. This concept of color change is well known, and is due to constructive interference of light of particular wavelengths related to polymer layer thickness and optical constants. In our modification, a digital camera is used to capture images of the coated substrates, and hue is extracted from the image data files. We show that hue can be linearly correlated with polymer thickness. We demonstrate this for polymer based OLEDs using poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) and a light-emitting polymer (LEP) deposited on transparent substrate. The correlations were successfully used for 40-140nm PEDOT:PSS layers and 20-110nm LEP layers over length scales greater than 1 inch. The method sensitivity is estimated to be better than 5 nm. We show examples of non-uniformity analysis and how it relates to OLED performance.

We present our latest results on the design and fabrication of very high capacitance dielectric materials for organic field-effect transistors. We will show that utilization of appropriate self-assembling siloxane building blocks and polymer matrices allows solution-processed, pinhole-free organic dielectrics. Electrical (MIS, TFT) data demonstrate that these insulators can be efficiently integrated into large TFT structures. These devices function for both p- and n-channel semiconductors, the molecular components of which exhibit greatly different core structures and substituent functionalities. Substantial TFT response is achieved at very low operational biases (<1V), without serious leakage currents (<10-8 A/cm at 1V).

We describe a recently discovered magnetoresistance (MR) effect in semiconducting polymer and small molecule sandwich devices. The MR effect reaches up to 10% in a magnetic field of 10mT at room temperature. This MR effect is therefore amongst the largest of any bulk material. We characterize this effect and discuss its dependence on voltage, film thickness, temperature, electrode materials and (unintentional) impurity concentration in three different organic semiconductors. We found that the MR effect is only weakly temperature dependent and does not depend on sign and direction of the applied magnetic field. To the best of our knowledge, the discovered MR effect is not adequately described by any of the mechanisms known to date.

Diodes were formed by intermediating a thin film of PEDOT:PSS between aluminum and heavily doped silicon. Both p-type and n-type Si substrates were used. Hysteresis loops were observed in their current-voltage (I-V) characteristics. A ‘state’ can be written by applying a voltage pulse to aluminum electrode. The state of the device can be read out from the current under a small probe voltage (0.3-0.6 V, to Al electrode). Appling +4.0 V induces a ‘low’ conductance state while applying -4.0 V switches the device back to the ‘high’ conductance state. The current difference between two states is up to 3 orders of magnitude. The space charge storage in the polymer is believed to be responsible for the memory effect. Upon positive voltage bias the charges are injected into the place near the Al/PEDOT:PSS interface. The charges are stored there and will resist subsequent charge injection, resulting in the ‘low’ conduction state. The redox reactions further reduce the conductance of the device. Negative bias can remove the stored charges and reverse the redox reactions, thus recover the device back to ‘high’ conduction state.

Electron mobilities were studied as a function of thin-film growth conditions in hot wall epitaxially grown C60 based field-effect transistors. Mobilities in the range of ∼ 0.5 to 6 cm2/Vs are obtained depending on the thin-film morphology arising from the initial growth conditions. Moreover, the field-effect transistor current is determined by the morphology of the film at the interface with the dielectric, while the upper layers are less relevant to the transport. At high electric fields, a non-linear transport has been observed. This effect is assigned to be either because of the dominance of the contact resistance over the channel resistance or because of the gradual move of the Fermi level towards the band edge as more and more empty traps are filled due to charge injection.

Organosilane self-assembled monolayers, embedded in organic single-crystal field-effect transistors, significantly affect subthreshold properties in the transfer characteristics. The monolayer of either polarized or nearly unpolarized molecules is deposited on a SiO2/doped Si substrate before softly attaching a rubrene thin single crystal to form the “laminated crystal” transistors. As a result of effective passivation of the SiO2 surface, the device has achieved a subthreshold swing as low as 0.11 V/decade. It is also demonstrated that threshold gate voltage is shifted by polarization of the monolayers, indicating that threshold of the device is adjustable by the choice of silane materials. Both of the functions are closely associated with low-power applications such as logic-circuit components.

We use ultraviolet photoelectron spectroscopy to investigate the effect of oxygen and air exposure on the electronic structure of pentacene thin films. It is found that O2 and water do not react noticeably with pentacene, whereas singlet oxygen/ozone readily oxidize the organic compound. Also, we obtain no evidence for considerable p-type doping of pentacene by O2 at low pressure. However, oxygen exposure lowers the hole injection barrier at the interface between Au and pentacene by 0.25 eV, presumably due to a modification of the Au surface.

Organic thin-film transistors (OTFTs) appear to have become strong contenders to silicon based MOSFET devices whenever low-cost and relatively low performance circuits are required in applications such as radio frequency identification (RFID) for large volume supply chains. In order to develop circuits based on OTFTs, circuit designers require circuit models that predict the operation of OTFT with a reasonable accuracy. Although, generally, OTFT operation is similar to ordinary silicon MOSFET devices, there are several characteristics that clearly differentiate them. One important difference between the operation of the OTFT and the silicon MOSFET (that is a direct consequence of the physical implementation of OTFT) is that the organic transistor is normally operated in the accumulation mode, while the silicon transistor regularly operates in the inversion mode. Due to the molecular nature of the semiconductor, the carrier mobility is orders of magnitude lower than for the silicon MOSFET. Variable carrier mobility law, low on/off ratio, and the Schottky barrier at the interface between the source/drain metal contact and the organic semiconductor are among other important effects that had to be considered for developing of an accurate circuit model of the organic transistor. The developed model has been used to simulate DC characteristics and also simple circuits such as logic gates, ring oscillators, rectifiers, etc.

This paper presents the developed model as well as a comparison between the simulated data and the experimental data. The experimental circuits were fabricated on flexible plastic substrates and employed a solution-cast dielectric. Pentacene was the semiconductor of choice with carrier mobility in the range of 0.1 – 1.5 cm2/V.s.

In this paper, we present research results that explore the basic molecular structure, orientation, and electrical properties of the conjugated polymer poly(2-methoxy-5- (2,9-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV). The bandgap structure of MEH-PPV was investigated through optical absorption and emission spectra, and ultraviolet photoemission spectroscopy (UPS). Based on the optical absorption of MEH-PPV, the π-π* bandgap energy is 2.14eV. The emission spectrum of ITO/MEH-PPV/Al light emitting diode has a peak at 590nm in wavelength which corresponds to photon energies of 2.1eV. By using ultraviolet synchrotron radiation to investigate the highest occupied molecular orbitals (HOMO) of the MEH-PPV, the relative change in photoemission cross-section for various electron states was measured. The HOMO to Fermi level gap obtained from UPS spectra is about 3eV. The UPS measurements include angle-resolved and incident energy dependent spectra. Some indication of bulk film orientation was inferred by dispersion of incident energy dependent UPS. Disparities in these spectra imply some role for defect states, extrinsic carrier involvement, or Fermi-level pinning.

A series of tetradonor substituted [2.2]paracyclophane-based two-photon absorption (TPA) fluorophores were designed, synthesized and characterized. Different substituents were chosen to modulate the conjugation and the donor strength and to allow the molecules to be neutral (N series, organic-soluble) or charged (C series, water-soluble). The overall set of compounds series allows us to examine the correlation among the solvent polarity, donor ability of the substituents, and TPA action cross section (ηδd, where δ is the TPA cross section and η is the quantum yield). In water, a significant drop of both δ and η is observed, approximately one-third relative to those in toluene. Weaker donors and/or weaker acceptors with enhanced conjugation are a better choice due to a minimized quenching related to strong charge transfer (CT) upon excitation. One thus needs to fine-tune the magnitude of CT and find an optimal balance among CT,δ and η.

Regioregular poly(3-hexylthiophene) (RR-P3HT) is a commercially available semiconducting polymer. Its high processability makes it favorable for fabrication of organic thin film transistors (OTFTs). Depending on the processing technique and device configuration, the field effect mobility of this polymer ranges from 0.01 to 0.1 cm2/Vs. The mobility also shows a correlation with the choice of gate dielectric material. The most commonly reported dielectric materials for OTFTs are SiO2, Al2O3 and Ta2O5. In this work, we report a new fully encapsulated top-gate RR-P3HT-based TFT structure with a-SiNx implemented as the gate dielectric and passivation material. The fabrication process enables realization of discrete transistors and transistor circuits through four consecutive photolithographic steps. The process is compatible for various substrates including Corning glass, Si wafers, and any appropriate plastic substrates. This paper addresses a number of critical technological issues such as substrate surface treatment to improve film adhesion, optimal spin coating conditions for uniform polymer film formation, preparation of device quality a-SiNx films by plasma-enhanced chemical vapor deposition (PECVD) at 75°C substrate temperature, and a tailored etch process for patterning of the polymer film. Current-voltage characteristics of the fabricated transistors are analyzed to evaluate the quality of the polymer/a-SiNx interface.

We present data on the transport of charge carriers in the organic semiconductor tetracene. Comparative measurements by time-of-flight (TOF) spectroscopy and measurements in field-effect transistor (FET) geometry reveal hole mobilities of about 1 cm2/Vs. Whereas for FETs only hole transport can be detected, from TOF a strong dispersive transport for negative charge carriers is observed. This observation is mainly caused by deep-level trapping of electrons. By fitting the temperature dependent hole mobility to a model of multiple-trapping and release of charge carriers the trap energy and the relative trap density can be adjusted to 130 meV and 5·10-3, respectively. Comparative chemical and structural analysis of inhomogeneities show that the traps affecting the transport are mainly caused by chemical defects rather than by structural imperfections.

We describe how the composition of an organic - based magnet can be controlled by varying the Chemical Vapor Deposition (CVD) conditions. A study was conducted for the Co2(CO)8/ TCNE system to form cobalt tetracyanoethylene [Co(TCNE)x, x∼2, a paramagnetic material], and for the V(CO)6/ TCNEx system to form vanadium tetracyanoethylene [V(TCNE)x, x∼2, a ferrimagnetic material]. Thin V(TCNE)x, x∼2 films (∼0.05-0.5 μm) with room temperature conductivity of 10-4<σRT<10-3S/cm and magnetic ordering temperature Tc of up to ∼400K were deposited. The V(TCNE)x, x∼2 thin films have the potential for incorporation in a spin-valve device as one of the magnetic contacts, and are promising candidates to form optically controlled magnetic-based structures.