Microcrystalline array structures induced by heat treatment of friction-transferred organic semiconductor films.

The correlation between molecular orientation and optoelectrical properties is most critical to the future design of molecular materials. We made highly-anisotropic microcrystalline array structures with an organic semiconductor, a methoxy-substituted thiophene/phenylene co-oligomer (TPCO), by depositing it on friction-transferred poly(tetrafluoroethylene) (PTFE) layers fabricated on substrates with several heat treatments. Polarising microscope observation, polarised emission and absorption spectra measurements indicated that the TPCO molecules aligned along the drawing direction of PTFE. Using these films, we fabricated two types of field-effect transistors (FETs) and compared them with those using non-heated TPCO films which provide aligned pleats structures. Ones had the channel length direction parallel to the drawing direction of PTFE and the others had the channel length direction perpendicular to that drawing direction. As for the microcrystalline array films, the mobility ratio of the former FET to that of the latter device was about 27 in the saturation region, while the emission polarisation ratio was 4.5. The heat treatment promoted the crystal growth to enhance the mobility while retaining the high anisotropy. The results demonstrate that the heat treatments of the TPCO films on the friction-transferred layers were useful for controlling crystallinity and orientation of the molecules.

Clarification of the Molecular Doping Mechanism in Organic Single-Crystalline Semiconductors and their Application in Color-Tunable Light-Emitting Devices.

Organic single-crystalline semiconductors with long-range periodic order have attracted much attention for potential applications in electronic and optoelectronic devices due to their high carrier mobility, highly thermal stability, and low impurity content. Molecular doping has been proposed as a valuable strategy for improving the performance of organic semiconductors and semiconductor-based devices. However, a fundamental understanding of the inherent doping mechanism is still a key challenge impeding its practical application. In this study, solid evidence for the "perfect" substitutional doping mechanism of the stacking mode between the guest and host molecules in organic single-crystalline semiconductors using polarized photoluminescence spectrum measurements and first-principles calculations is provided. The molecular host-guest doping is further exploited for efficient color-tunable and even white organic single-crystal-based light-emitting devices by controlling the doping concentration. The clarification of the molecular doping mechanism in organic single-crystalline semiconductor host-guest system paves the way for their practical application in high-performance electronic and optoelectronic devices.

Quantitative evaluation of light-matter interaction parameters in organic single-crystal microcavities.

Investigation of physics on light-matter interaction and strong coupling formation in organic microcavities is important to characterize the device structure enabling efficient room-temperature polariton condensation. In this study, we evaluate quantitatively the light-matter interaction parameters for three types of organic single-crystal microcavities and discuss the effects of microcavity structures on the strong coupling formation. We found that improvement in cavity quality factor causes a reduction in the photon damping constant, which results in an increase in the Rabi splitting energy. Moreover, when we used a metal thin film as the cavity mirror, it was revealed that the exciton damping became 30 times stronger than that in a dielectric mirror cavity. These experimental findings are very intriguing to achieve low-threshold or electrically pumped organic polariton devices.

The entangled triplet pair state in acene and heteroacene materials.

Entanglement of states is one of the most surprising and counter-intuitive consequences of quantum mechanics, with potent applications in cryptography and computing. In organic materials, one particularly significant manifestation is the spin-entangled triplet-pair state, which mediates the spin-conserving fission of one spin-0 singlet exciton into two spin-1 triplet excitons. Despite long theoretical and experimental exploration, the nature of the triplet-pair state and inter-triplet interactions have proved elusive. Here we use a range of organic semiconductors that undergo singlet exciton fission to reveal the photophysical properties of entangled triplet-pair states. We find that the triplet pair is bound with respect to free triplets with an energy that is largely material independent (∼30 meV). During its lifetime, the component triplets behave cooperatively as a singlet and emit light through a Herzberg-Teller-type mechanism, resulting in vibronically structured photoluminescence. In photovoltaic blends, charge transfer can occur from the bound triplet pairs with >100% photon-to-charge conversion efficiency.

Organic Light-Emitting Diodes with Heterojunction of Thiophene/Phenylene Co-Oligomer Derivatives.

Organic light-emitting diodes are fabricated by heterojunction of thiophene/phenylene co-oligomer films using biphenyl-capped bithiophene (BP2T) and its cyano-substituted derivative (BP2T-CN). Strong electron-withdrawing cyano-groups in BP2T-CN transform the p-type BP2T into n-type. Photoluminescence and electroluminescence from their bilayered films dominantly result from the BP2T-CN layer since the lying molecular orientation of BP2T-CN facilitates surface emission while the standing orientation of BP2T is not suitable for the device configuration. The current density and electroluminescence intensity are considerably increased by carrier doping with MoO3 and Cs2CO3 into the BP2T and BP2T-CN films, respectively.

Amplified Emission and Field-Effect Transistor Characteristics of One-Dimensionally Structured 2,5-Bis(4-biphenylyl)thiophene Crystals.

One-dimensional (1D) structures of 2,5-bis(4-biphenylyl)thiophene (BP1T) crystals are fabricated for light amplification and field-effect transistor (FET) measurements. A strip-shaped 1D structure (10 µm width) made by photolitography of a vapor-deposited polycrystalline film shows amplified spontaneous emission and lasing oscillations under optical pumping. An FET fabricated with this 1D structure exhibits hole-conduction with a mobility of µh = 8.0 x 10(-3) cm2/Vs. Another 1 D-structured FET is fabricated with epitaxially grown needle-like crystals of BP1T. This needle-crystal FET exhibits higher mobility of µh = 0.34 cm2/Vs. This improved hole mobility is attributed to the single-crystal channel of epitaxial needles while the grain boudaries in the polycrystalline 1 D-structure decrease the carrier transport.

Emission Behavior of Crystalline 1,4-Bis(4-phenylthiophene-2-yl)benzene Film Under Optical Excitation with Ultra Short Pulses.

We evaluated emission behaviors of crystallized films of 1,4-bis(5-phenylthiophene-2-yl)benzene (AC5) in detail which was a representative thiophene/phenylene co-oligomer. The crystallized AC5 films were prepared by vapor deposition onto a substrate and thermal treatment. The AC5 films consisted of a crystalline domain with the size of several tens of micrometers. We used femtosecond laser pulses for the excitation of the AC5 films. As a result, the femtosecond laser pulses did not induce re-absorption above excitation energy densities of their laser threshold. The obtained gain value for AC5 crystallized film was large, over 150 cm-1. Furthermore, the emission cross section of the crystallized AC5 film was nearly 10(-16) cm2.

Spectrally-Narrowed Emissions from Organic Crystals Having a One-Dimensional Grating on Their Surface.

We have succeeded in directly engraving one-dimensional diffraction gratings on the surface of organic semiconducting oligomer crystals by using focused ion beam (FIB) lithography and laser ablation (LA) methods. The FIB method enabled us to shape the gratings with varying periods down to ~150 nm. With the LA method a large-area grating with a ~500-nm period was readily accessible. All the above crystals indicated spectrally-narrowed emission (SNE) lines even in the case of shallow groove depths ~2-4 nm. In particular, we definitively observed the SNE pertinent to the first-order diffraction with the crystal having the diffraction grating of a 148.3-nm average period. The present results indicate utility of the built-in gratings that can directly be fabricated on the surface of the crystals.

Intrinsic Polarization and Tunable Color of Electroluminescence from Organic Single Crystal-based Light-Emitting Devices.

A single crystal-based organic light-emitting device (OLED) with intrinsically polarized and color-tunable electroluminescence (EL) has been demonstrated without any subsequent treatment. The polarization ratio of 5:1 for the transversal-electric (TE) and transversal-magnetic (TM) polarization at the emission peak of 575 nm, and 4.7:1 for the TM to TE polarization at the emission peak of 635 nm, respectively, have been obtained. The emitting color is tunable between yellow, yellow-green and orange by changing the polarization angle. The polarized EL and the polarization-induced color tunability can be attributed to the anisotropic microcavity formed by the BP3T crystal with uniaxial alignment of the molecules.

Ambipolar light-emitting organic single-crystal transistors with a grating resonator.

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.

Exotic optoelectronic properties of organic semiconductors with super-controlled nanoscale sizes and molecular shapes.

We present several aspects of thiophene/phenylene co-oligomers (TPCOs). TPCOs are regarded as a newly occurring class of organic semiconductors. These materials are synthesized by hybridizing thiophene and phenylene rings at the molecular level with their various mutual arrangements. These materials are characterized by the super-controlled nanoscale sizes and molecular shapes. These produce peculiar crystallographic structures and high-performance optical and electronic properties. The crystals of TPCOs were obtained through both vapor phase and liquid phase. In the TPCO crystals, the molecules take upright configuration. These cause large carrier mobilities of field-effect transistors and laser oscillations under optical excitations. Spectrally-narrowed emissions (SNEs) were also achieved under weak optical excitation using a mercury lamp. The light-emitting field-effect transistors using these crystals for an active layer have shown the current-injected SNEs when the device was combined with an optical cavity and operated by an alternating-current gate-voltage method. Thus the TPCO materials will play an important role in the future in the fields of nanoscale technology and organic semiconductor materials as well as their optoelectronic device applications.

Triplet dynamics in pentacene crystals: applications to fission-sensitized photovoltaics.

The decay and transport of triplet excitons photogenerated via singlet exciton fission in polycrystalline and single-crystalline pentacene is reported. Using transient absorption spectroscopy, we find evidence for diffusion-mediated triplet-triplet annihilation. We estimate monomolecular lifetimes, bimolecular annihilation rate constants, and triplet exciton diffusion lengths. We discuss these results in the context of current solar cell device architectures.

Light-emitting field-effect transistors having combined organic semiconductor and metal oxide layers.

A new organic light-emitting field-effect transistor characterized by a metal oxide layer inserted between the organic layer and the gate insulator is proposed. The metal oxide is indirectly connected with source and drain electrodes through the organic layer. Upon increasing the potential difference between the source and drain electrodes, the emission becomes exceedingly strong and the emission region encompasses the whole channel zone.

Organic single-crystal light-emitting transistor coupling with optical feedback resonators.

Organic light-emitting transistors (OLETs) are of great research interest because they combine the advantage of the active channel of a transistor that can control the luminescence of an in-situ light-emitting diode in the same device. Here we report a novel single-crystal OLET (SCLET) that is coupled with single crystal optical feedback resonators. The combination of single-crystal waveguides with native Fabry-Perot cavities, formed by parallel crystal edges, drastically lowers the threshold energy for spectral narrowing and non-linear intensity enhancement. We apply this structure to SCLETs and demonstrate the first fabrication of a SCLET with the optical feedback resonators.

Current-confinement structure and extremely high current density in organic light-emitting transistors.

Extremely high current densities are realized in single-crystal ambipolar light-emitting transistors using an electron-injection buffer layer and a current-confinement structure via laser etching. Moreover, a linear increase in the luminance was observed at current densities of up to 1 kA cm(-2) , which is an efficiency-preservation improvement of three orders of magnitude over conventional organic light-emitting diodes (OLEDs) at high current densities.

Single crystals of 5,5'-Bis(4'-methoxybiphenyl-4-yl)-2,2'-bithiophene for organic laser media.

Room temperature lasing was stably observed in air from an orthorhombic crystal of 5,5'-bis(4'-methoxybiphenyl-4-yl)-2,2'-bithiophene (BP2T-OMe). A pair of parallel {110} facets of the single crystal acted as effective Fabry-Pérot mirrors. This prominent lasing is based on high group refractive index (3.5) and Q factor (4500) of the orthorhombic crystal resonator indicating a promising potential of BP2T-OMe for organic laser media.

Ambipolar organic single-crystal transistors based on ion gels.

Ambipolar electric double-layer transistors (EDLTs) using organic single crystals and ion-gel electrolytes are successfully created by optimising the fabrication of gel films. The p- and n-type EDLTs enable us to investigate the HOMO-LUMO gap energy of semiconductors, offering a new method with which to measure it.

Lasing from epitaxially oriented needle crystals of a thiophene/phenylene co-oligomer.

Optically pumped lasing of epitaxially oriented molecular crystals is presented. Needle-like crystals of a thiophene/phenylene co-oligomer is grown along the [110] direction of a KCl substrate. The fluorescence light is one-dimensionally confined and amplified by the self-waveguiding effect in the elongated needles. The Fabry-Pérot resonation by the terminated ends results in laser oscillations corresponding to the needle length.