Modification of the quasi two-dimensional surface state on Au(111) surface with adsorption of n-alkane molecule, tetratetracontane: TTC was investigated by angle-resolved photoemission spectroscopy (ARUPS) and X-ray photoemission spectroscopy (XPS). From ARUPS the energy band structure of the surface state of Au(111) surface is found to be significantly affected by TTC adsorption. The modified surface potential on Au(111) surface by the adsorption confines the surface electrons into the quantum well newly established between TTC wide energy gap and sp band gap of the Au(111) surface increasing its localization nature. Clear chemical shift in C 1s core level was observed in XPS spectrum for TTC layer right at the interface and small back-donation from Au(111) surface to TTC unoccupied states is suspected. On the other hand, a large interface dipole layer about 0.72 eV was observed by TTC adsorption. From these results, Pauli repulsion effect is expected not to have dominant effect on such large dipole formation on TTC adsorption.

Organic field effect transistors (OFETs) fabricated on the polymer substrate including conductive organic materials and polymeric gate insulator is one of promising devices for the flexible electronic devices. For OFETs with a conductive organic layer formed on the gate insulating materials, the performance of devices strongly depends on the molecular structure of the gate insulators. In this study, we investigated the effect of hydroxyl group of gate insulating materials on the characteristics of n- and p-type OFETs utilizing [6,6]-phenyl C61-butyric acid methyl ester (PCBM), α, ω-dihexylsexithiophene (DH6T) and pentacene OFETs as a conducting layer, respectively. Poly(p-silsesquioxane) (PSQ) derivatives are used as a polymeric gate insulator, which containes various ratios of phenol-group with a hydroxyl group bonded to a phenyl ring in the side chain of their molecular structures.

A Metal / Organic / Inorganic semiconductor heterostructure was built and characterized in situ under ultra-high vacuum conditions (UHV). The aim was to investigate the influence of a perylene-derivative organic thin film on the transport electronic properties of Schottky Ag / GaAs diodes. The device was studied using a combination of photoemission spectroscopy (PES) and electrical measurements. The obtained results were discussed using the analytical expressions of a trapped charge limited current (TCLC) model.

The near-edge electronic structures of several dyes and phtalocyanines were investigated using Photo-Electron Spectroscopy in Air (PESA). The ionization potentials (IPs) of N3, N719, Z907, D102, D149 and D205 were derived from observed photoemission threshold energies to be 5.88, 5.47, 5.44, 5.53, 5.44 and 5.49eV, respectively. Density of states (DOS) of Fe-, Ni-, Cu- and H2-phthalocyanines were deduced from differentiation of observed photoemission yields with incident photon energies. DOS thus obtained were well compared with calculated ones estimated with use of the DV-Xα molecular orbital calculation method.

We have investigated the evolution of both the occupied and unoccupied states for alkali metal (Cs and Na) doped Copper-Phthalocyanine (CuPc) with photoemission and inverse photoemission spectroscopy. As the doping ratio increases, the lowest unoccupied molecular orbital (LUMO) of CuPc shifts downward, reaching the Fermi level. After the saturation, the LUMO intensity decreases monotonically, while a gap state grows in the valence spectra, which gives direct evidence for the origin of the doping-induced gap state in CuPc molecules.

In this report, we demonstrate that the performance and stability of pentacene top-contact field-effect transistor can be greatly improved with post-annealing treatment. After post-annealing at 90°C for 12 hours in nitrogen environment, the hole field-effect mobility of 0.3 cm2/Vs and the on/off current ratio of 107 were achieved, demonstrating 100% improvement in performance after the post-annealing treatment. The decay rate of drain current at constant gate and drain-source voltage was found to be decreased by more than 40%. The improved performance is attributed to the elimination of trapped holes and lattice defects in the organic semiconductor layer due to the post-annealing process.

Qualitative and quantitative studies of the oxidation of molecularly modified polycrystalline copper (Cu) thin films upon exposure to ambient air conditions for long periods (on the order of several months) are reported in this work. Thin films of Cu, prepared by thermal evaporation, were analyzed by means of x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) to gain an understanding on the growth mechanism of oxide on bare and molecularly modified Cu surfaces. The results from all techniques points for an unstable behavior of bare Cu surfaces characterized in very fast and continuous growth of Cu oxide layers during first 60 days of exposure to overall 6 nm oxide thickness. However, Cu films prepared under the same conditions, but covered with a self-assembled organic thin film layer of 1,4-phenylene diisocyanide (PDI) molecules adsorbed from solution, showed a decrease in the thickness of the copper oxide layer on the Cu surface. Our findings imply that chemisorbed PDI monolayers can serve as protective coatings for Cu.

The electronic structure of the interfaces between rubrene and various metals, including Au, Ag, Al, and Ca, have been investigated with photoemission and inverse photoemission spectroscopy. The formation of the interface dipole is observed at all interfaces. The Fermi level shifts linearly within the band gap as a function of metal workfunction, until it is pinned at the lowest unoccupied molecular orbital (LUMO) by Ca. Strong interactions take place at the interface between rubrene and Ca, evidenced by the evolution of the valence features.

Organic Thin-Film Transistors (OTFTs) in top-contact configuration and Metal-Insulator-Semiconductor (MIS) structures with different organic dielectrics as the gate insulator have been fabricated using the same organic semiconductor layer, pentacene, in order to investigate the changes in the electrical behavior by varying the interface properties. A gold bottom gate electrode was sputtered on a glass substrate whereas gold source and drain were thermally evaporated onto the pentacene layer. Several organic dielectrics have been tested as insulating layers, namely were poly(vinyl alcohol) (PVA), polyvinyl alcohol with ammonium dichromate (PVAad) as a cross-linking agent, poly (4-vinyl phenol) (PVP), poly(dimethylsiloxane) (PDMS) and poly(methylsilsesquioxane) (pMSSQ). The interesting differences found out by varying the interface confirm that the chemical-physical interaction between semiconductor and dielectric is crucial for the conduction mechanisms of the charge carriers. In particular we observed that the electron traps can influence not only the electron transport, therefore allowing a more or less marked ambipolar behavior, but also affect the hole transport.

The processes underlying degradation of organic light emitting diodes (OLEDs) are gradually becoming understood. In ruthenium-based ionic transition metal complex (iTMC) OLEDs, a dimeric species forms during device operation that quenches light emission [1]. Water has been implicated in this degradation process [2]. We report recent studies on degradation of OLEDs fabricated with Ir(ppy)2(dtb-bpy)PF6 [ppy = 2-phenylpyridine, dtb-bpy = 4,4'-di-tert-butyl 2,2'-bipyridine [3]. We have found that applying a thicker-than-usual metal electrode results in shorter turn-on times and higher light emission, though little improvement in lifetime. It appears that the degradation of these devices occurs by a different mechanism from that of the ruthenium-based devices and may involve local heating leading to chemical decomposition of the organic material.

Observation of recurring but often transient dark-colored substances in both the Ru(bpy)3(PF6)2 and Ir(ppy)2(dtb-bpy)PF6 systems, seen both in the solid state and in solution samples, may also be indicative of decomposition.

Considerable attention has recently been paid to light-emitting organic filed-effect transistors (LEOFETs) because of their potential applications to novel opto-electronic devices. In most LEOFETs, light emission occurs through the recombination of carriers injected from electrodes. Various efforts have been, thus, made to improve injection and transport properties of minority carriers in organic materials. On the other hand, light emission was frequently observed when we applied reverse bias voltages, that is, positive bias voltages, to the drain electrode for p-type FETs with grounded source and negatively biased gate electrodes. In this circuit, minority carriers were not injected into organic semiconductors. Nevertheless, the luminous intensity increased with drain voltage.

In the present work, we have prepared such novel type LEOFETs with various device structures. We have inserted thin metal wires between the source and drain electrodes in the channel region of pentacene FETs. It was found that the light emission was observed around the metal wires inserted and the luminous efficiency was improved with the number of wires. The luminescence spectrum was identical to that of pentacene films. The light emission was thought to be generated by via energy transfer to the pentacene films from plasmon excited on metal surfaces by current injection.

Pentacene thin-film transistors use either bottom-contact or top-contact electrodes. Transistors with bottom-contact electrodes are fabricated in such a way that metal is deposited and patterned before pentacene deposition while those with top-contact electrodes are fabricated in reverse order. Hole mobility is closely related to pentacene grain sizes and grain sizes of pentacene depend heavily on the surface roughness. When the surfaces are flat and smooth enough, the grains of pentacene are still small if metal is deposited before pentacene, i.e., bottom-contact configuration. Pentacene inside the channels formed by source and drain electrodes has much smaller grains than those far away from metal electrodes.

To further understand this phenomenon, we first studied pentacene grain growth on different substrates (oxide, Poly-4-vinylphenol or PVP, and SU8-2.) but with same metal electrodes (500 Ö Au). Next, we studied pentacene grain growth on the same substrate (oxide) but with different metal electrodes (500 Ö Au, 500 Ö Al, and 500 ÖCr). To avoid contaminating and damaging substrate surfaces, we used shadow masks for metal deposition.

For substrate studies, the fabrication process of testing samples began by oxidation of silicon wafers. PVP and SU8-2 were spin-coated on oxide wafers and cured. 500 Ö Au was thermally evaporated through shadow masks onto wafers to form source and drain. Pentacene was then thermally evaporated onto the wafers.

Likewise, the fabrication process of the second study began by oxidation of silicon wafers. Different metals were thermally evaporated through shadow masks onto oxide wafers. Finally, pentacene was thermally evaporated onto the wafers.

To further study the relation between pentacene grain sizes and distance from metal electrodes, we also made metal ring structures. The substrates were kept at room temperature during pentacene deposition for all samples.

Grain diameters were calculated from scanning probe microscopic or atomic force microscopic images. Images taken with Veeco Digital Instrument Dimension 3100 AFM were analyzed with Scion Image program. After counting particle numbers, average grain diameters were calculated assuming grains are circular.

Our data show that pentacene has drastically smaller grains inside the channels electrodes than that far away electrodes for all 3 kinds of substrates: oxide, PVP and SU8-2. EDS spectrum confirms that there is no Au inside the channels. Not only Au, but also Al and Cr show the same phenomenon. Across the 400-um metal rings, pentacene grain diameters increase first and then decrease, confirming that pentacene grain growth is somehow suppressed by the presence of metal.

This phenomenon adds another reason supporting that pentacene thin-film transistors with bottom-contact electrodes have worse performances due to smaller grains. Currently, we are working on modeling this phenomenon and finding theoretical explanations.

We report on the realization of flexible and transparent all-organic Ambipolar Field Effect Transistors. The devices were assembled on a flexible plastic foil, i.e. Mylar®, and the contacts were realized with poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and patterned by means of Soft Lithography, MicroContact Printing (μCP). As semiconductor layer we used organic bulk heterojunctions of pentacene/C60 realized either by co-deposition of the two different molecules or by a double layer structure in which pentacene was used as buffer layer at the interface with the gate dielectric. Interestingly, all devices (co-deposited and double layer), measured in air, worked in accumulation mode as ambipolar OFETs, however some interesting differences between the two structures can be pointed out. Supported by Atomic Force Microscopy, we demonstrated that growing C60 on a pre-deposited pentacene buffer layer leads to a clear improvement in the morphology and crystallinity of the deposited film allowing to improve n-type conduction by two orders of magnitude. This work is particularly interesting because on one hand it confirms the importance of the substrate properties for the ordered growth of organic semiconductors, which determines the transport properties of organic materials; moreover, it demonstrates, also for n-type and ambipolar transistors, the possibility of avoiding problems normally associated to metal contacts: the lack of mechanical robustness, flexibility, and the unfeasibility of realizing contacts with low cost techniques like printing or soft lithography. The flexibility and transparency of the final OFET structure and the simple low cost fabrication technique employed pave the way for an economic mass production of large area transparent “Plastic Electronics”.