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Charge separation and transport behavior were investigated at an organic heterolayered interface consisting of a 4,4′,4″-tris[3-methylphenyl(phenyl)amino] triphenylamine (m-MTDATA) organic donor and a hexadecafluoro-copper-phthalocyanine (F16CuPc) organic acceptor. A two-dimensional charge sheet of electron-hole pairs was induced due to the formation of charge-transfer complexes at the heterointerface. The induced charges could be separated by application of an electric field perpendicular to the heterointerface. The charge-separation behavior was independent of the work function of the contact electrodes but was strongly dependent on the energy-level alignment between the highest occupied molecular orbital of the donor molecules and the lowest unoccupied molecular orbital of the acceptor molecules. This enabled the preparation of an organic light-emitting diode without injecting holes from the anode. The charge-transport behavior along the heterointerface was also investigated by fabricating laterally arranged contact electrodes. The F16CuPc/m-MTDATA heterointerface exhibited high conductivity of 0.001 S/cm, which is seven orders of magnitude higher than that of a single-layer F16CuPc or m-MTDATA film. In situ electrical measurements during formation of the heterointerface enabled the effective thickness of the charge sheet to be determined as being accumulated within 2–3 nm, which corresponds to the bilayer of both F16CuPc and m-MTDATA. [ABSTRACT FROM AUTHOR]

We investigated the electrical characteristics of single-component ambipolar organic field-effect transistors (OFETs) by controlling the device structure and preparation and the measurement conditions. Six organic semiconductor materials (copper-phthalocyanine, tris-(8-hydroxyquinoline)aluminum (Alq3), alpha-sexithiophene, 4-4′-bis-styrylphenyl, 2,7-diphenyl[1]benzothieno[3,2-b]benzothiophene, or a photopolymerized polydiacethylene derivative (PDA) were used as the active layer, and all were found to transport both holes and electrons. The PDA-based FETs had the highest hole and electron mobilities (0.12 and 0.025 cm2/V s, respectively). We also investigated the effect of air exposure on the OFETs. The hole mobility was barely affected by the exposure while the electron mobility was significantly affected. The threshold voltage for p-channel operation was shifted by the exposure while that for n-channel operation was not, indicating that the hole density in the active layer is increased by air exposure whereas the electron density is independent of air exposure. Furthermore, we prepared an Alq3-based p-channel OFET and investigated the effect of air exposure on it. While its operation was stable in vacuum, air exposure degraded its characteristics. These behaviors indicate that irreversible chemical reactions occur between cationic Alq3 species and oxygen or water molecules. [ABSTRACT FROM AUTHOR]

A series of perylene-3,4,9,10-bis(dicarboximide) (PDI) dimers linked through the bay regions was systematically synthesized to examine the electronic structures and photophysical properties in dependence on the distance and orientation between the two PDI units. The spectroscopic and electrochemical measurements suggested that the coupling value of a directly linked PDI dimer (PDI)2 is much larger than those of para- and meta-phenylene-bridged PDI dimers p-(PDI)2 and m-(PDI)2. The width of Davydov splitting was quantitatively evaluated to compare the coupling values between the two PDI units in these dimers by absorption spectroscopy in frozen 2-methyl-THF. Excimer formation of PDI dimers induced the strong fluorescence quenching and large red-shifts. Femtosecond transient absorption revealed a broad absorption derived from an excimer in the range from about 600 nm to the near-IR region. The rate constants of formation and decay of the excimer are strongly dependent on the coupling values. Time-resolved measurements on ferrocene-linked p-(PDI)2 revealed a competition between the photoinduced processes of electron transfer and excimer formation in PhCN, which is in sharp contrast with the sole electron-transfer process in toluene. [ABSTRACT FROM AUTHOR]

A series of fluorescent 'push-pull' tetrathia[9]helicenes based on quinoxaline (acceptor) fused with tetrathia[9]helicene (donor) derivatives was synthesized for control of the excited-state dynamics and circularly polarized luminescence (CPL) properties. In this work, introduction of a quinoxaline onto the tetrathia[9]helicene skeleton induced the 'push-pull' character, which was enhanced by further introduction of an electron-releasing Me2N group or an electron-withdrawing NC group onto the quinoxaline unit (denoted as Me2N-QTTH and NC-QTTH, respectively). These trends were successfully discussed in terms of by electrochemical measurements and density functional theory (DFT) calculations. As a consequence, significant enhancements in the fluorescence quantum yields ( ΦFL) were achieved. In particular, the maximum ΦFL of Me2N-QTTH was 0.43 in benzene (NC-QTTH: ΦFL=0.30), which is more than 20 times larger than that of a pristine tetrathia[9]helicene (denoted as TTH; ΦFL=0.02). These enhancements were also explained by kinetic discussion of the excited-state dynamics such as fluorescence and intersystem crossing (ISC) pathways. Such significant enhancements of the ΦFL values thus enabled us to show the excellent CPL properties. The value of anisotropy factor gCPL (normalized difference in emission of right-handed and left-handed circularly polarized light) was estimated to be 3.0×10−3 for NC-QTTH. [ABSTRACT FROM AUTHOR]

A series of quinoxaline-fused [7]carbohelicenes (HeQu derivatives) was designed and synthesized to evaluate their structural and photophysical properties in the crystal state. The quinoxaline units were expected to enhance the light-emitting properties and to control the packing structures in the crystal. The electrochemical and spectroscopic properties and excited-state dynamics of these compounds were investigated in detail. The first oxidation potentials of HeQu derivatives are approximately the same as that of unsubstituted reference [7]carbohelicene (Heli), whereas their first reduction potentials are shifted to the positive by about 0.7 V. The steady-state absorption, fluorescence, and circular dichroism spectra also became redshifted compared to those of Heli. The molecular orbitals and energy levels of the HOMO and LUMO states, calculated by DFT methods, support these trends. Moreover, the absolute fluorescence quantum yields of HeQu derivatives are about four times larger than that of Heli. The structural properties of the aggregated states were analyzed by single-crystal analysis. Introduction of appropriate substituents (i.e., 4-methoxyphenyl) in the HeQu unit enabled the construction of one-dimensional helical columns of racemic HeQu derivatives in the crystal state. Helix formation is based on intracolumn π-stacking between two neighboring [7]carbohelicenes and intercolumn CH ⋅⋅⋅N interaction between a nitrogen atom of a quinoxaline unit and a hydrogen atom of a helicene unit. The time-resolved fluorescence spectra of single crystals clearly showed an excimerlike delocalized excited state owing to the short distance between neighboring [7]carbohelicene units. [ABSTRACT FROM AUTHOR]

This review is focused on understanding of the charge-transport physics of high-mobility organic semiconductors at a molecular level. We review recent high-mobility small-molecule and conjugated polymer materials with a focus on crystalline materials that have been able to exceed mobilities of 0.5-1 cm2/V s. We discuss some of the main, competing factors that govern charge transport in these materials and present theoretical approaches that have been developed to describe systems in which moderately strong intermolecular electronic interactions and strong electron-phonon interactions are present. Finally, we review recent experimental results that have aimed to address the important question of whether at room-temperature charge carriers in these high-mobility organic semiconductors are in fact simply extended Bloch electrons that undergo occasional scattering processes or are localized on individual molecules and move by hopping. [ABSTRACT FROM AUTHOR]

The mobility μ of solution-processed organic semiconductorshas improved markedly to room-temperature values of 1–5 cm2 V−1 s−1. In spite of their growing technological importance, the fundamental open question remains whether charges are localized onto individual molecules or exhibit extended-state band conduction like those in inorganic semiconductors. The high bulk mobility of 100 cm2 V−1 s−1 at 10 K of some molecular single crystals provides clear evidence that extended-state conduction is possible in van-der-Waals-bonded solids at low temperatures. However, the nature of conduction at room temperature with mobilities close to the Ioffe–Regel limit remains controversial. Here we investigate the origin of an apparent ‘band-like’, negative temperature coefficient of the mobility (dμ/dT<0) in spin-coated films of 6,13-bis(triisopropylsilylethynyl)-pentacene. We use optical spectroscopy of gate-induced charge carriers to show that, at low temperature and small lateral electric field, charges become localized onto individual molecules in shallow trap states, but that a moderate lateral electric field is able to detrap them resulting in highly nonlinear, low-temperature transport. The negative temperature coefficient of the mobility at high fields is not due to extended-state conduction but to localized transport limited by thermal lattice fluctuations. [ABSTRACT FROM AUTHOR]

Distyrylbenzene(DSB) derivatives, trimer of phenylenevinylene, with cyano and alkoxy groups grew epitaxially on the friction-transferred poly(tetrafluoroethylene) (PTFE) layer. DSBs/PTFE double layers indicated the remarkable anisotropic feature in absorption and emission properties. [ABSTRACT FROM AUTHOR]

An active matrix backplane based on solution-processed organic thin-film transistors (OTFTs) has been developed for flexible displays having multiphoton organic light-emitting diodes (OLEDs). The OTFT device has a bottom-gate/bottom-contact type structure consisting of a polyethylene naphthalate-based flexible substrate covered with patterned gate electrodes and a gate dielectric of UV-cured cardo-polymer. Teflon AF was used for a hydrophobic bank structure that defines active regions of the OTFTs, and the organic semiconductor was coated by solution shearing. The OTFT backplane fabricated here had $384 \times 128$ pixels at a resolution of 300 dpi; images and video were successfully displayed on the resulting flexible multiphoton-emission OLED display. [ABSTRACT FROM AUTHOR]

We developed an alignment-free process for asymmetric contacts of Au and Al and applied it to light-emitting organic field-effect transistors. Because electrons were injected efficiently from Al contacts, the emission intensity and onset voltages for light were significantly better than those in a device with conventional Au/Cr contacts. Moreover, a device with 1 μm channel length asymmetric contacts of Au and Al showed about 50 times higher current than that of the device with conventional Au/Cr contacts. This significant improvement can be ascribed to both dual space-charge formation of holes and electrons and low carrier injection barriers. [ABSTRACT FROM AUTHOR]

The authors applied a wide-band-gap (2.9 eV) molecule of 4,-4′-bis(styryl)biphenyl (BSBP) as an active layer in light-emitting organic field-effect transistors. They found that BSBP provided both relatively high field-effect hole mobility of 0.01 cm2/V s and photoluminescence efficiency of 20% in thin film. They achieved ambipolar operation by without breaking vacuum through devices’ preparation and measurements, applying aluminum contacts, and inserting a hydroxyl-free poly(methylmethacrylate) layer, and light emission was observed when the device was operated in the ambipolar mode. The results presented here will open the way to fabricating efficient light-emitting transistors with high mobility. [ABSTRACT FROM AUTHOR]

Field-effect transistors (FETs) based on poly [2-methoxy, 5-(2′-ethyl-hexoxy)-1, 4-phenylenevinylene] (MEH-PPV) were prepared with bottom-contact type interdigital electrodes of Cr/Au and Al/Au on the SiO2/Si substrates. MEH-PPV exhibited a p-type semiconducting behavior and orange light emission was observed when the devices were operated in vacuum. It was found that the luminescence efficiency of the FETs with Al/Au electrodes was higher than that of Cr/Au electrodes. The simultaneous injection of holes and electrons into MEH-PPV occurred efficiently with the application of Al/Au heteroelectrodes. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Electrically driven organic lasers are among the best lasing devices due to their rich variety of emission colors as well as other advantages, including printability, flexibility, and stretchability. However, electrically driven lasing in organic materials has not yet been demonstrated because of serious luminescent efficiency roll-off under high current density. Recently, we found that the organic ambipolar single-crystal transistor is an excellent candidate for lasing devices because it exhibits less efficient roll-off, high current density, and high luminescent efficiency. Although a single-mode resonator combined with light-emitting transistors (LETs) is necessary for electrically driven lasing devices, the fragility of organic crystals has strictly limited the fabrication of resonators, and LETs with optical cavities have never been fabricated until now. To achieve this goal, we improved the soft ultraviolet-nanoimprint lithography method and demonstrated electroluminescence from a single-crystal LET with a grating resonator, which is a crucial milestone for future organic lasers. [ABSTRACT FROM AUTHOR]

Optically pumped amplified spontaneous emission (ASE) is demonstrated in the active layer of the polymer light-emitting electrochemical cells (LECs). The active layer blend of the light-emitting polymer and the ionic liquid retained a high degree of transparency, enabling its use as an optical slab waveguide, allowing a low ASE threshold energy of 4.1 μJ cm-2 to be achieved. We also demonstrate a planar LEC that showed fast turn on, bright linear shape emission in the middle of the two contacts, and a relatively high current density of 36 A cm-2 with a low driving voltage of 6 V. [ABSTRACT FROM AUTHOR]