Yusuke Tsutsui

Photoinduced Charge-Carrier Generation in Epitaxial MOF Thin Films: High Efficiency as a Result of an Indirect Electronic Band Gap?

For inorganic semiconductors crystalline order leads to a band structure which gives rise to drastic differences to the disordered material. An example is the presence of an indirect band gap. For organic semiconductors such effects are typically not considered, since the bands are normally flat, and the band-gap therefore is direct. Herein we show results from electronic structure calculations demonstrating that ordered arrays of porphyrins reveal a small dispersion of occupied and unoccupied bands leading to the formation of a small indirect band gap. We demonstrate herein that such ordered structures can be fabricated by liquid-phase epitaxy and that the corresponding crystalline organic semiconductors exhibit superior photophysical properties, including large charge-carrier mobility and an unusually large charge-carrier generation efficiency. We have fabricated a prototype organic photovoltaic device based on this novel material exhibiting a remarkable efficiency.

Unraveling unprecedented charge carrier mobility through structure property relationship of four isomers of didodecyl[1]benzothieno[3,2-b][1]benzothiophene

The structural and electronic properties of four isomers of didodecyl[1]-benzothieno[3,2-b][1]benzothiophene (C12-BTBT) have been investigated. Results show the strong impact of the molecular packing on charge carrier transport and electronic polarization properties. Field-induced time-resolved microwave conductivity measurements unravel an unprecedented high average interfacial mobility of 170 cm(2) V(-1) s(-1) for the 2,7-isomer, holding great promise for the field of organic electronics.

Complex permittivity analysis revisited: Microwave spectroscopy of organic semiconductors with resonant cavity

The complex permittivity analysis of microwave dielectric loss spectroscopy has been revisited to deduce the intrinsic values of the mobility of charge carriers injected at the interface between an organic semiconductor and a gate insulator. A perturbation model with a simple parabolic function for frequency-dependent microwave resonance in the cavity enables us to experimentally determine the real and imaginary parts of the permittivity change caused by the injected charge carriers based on the Drude–Zener model, leading to the simultaneous assessment of the intrinsic charge-carrier mobility, the averaged trap depth/density, and the ratio of free-trapped charge carriers. The present frequency-dependent precise analysis of complex permittivity provides rapid and non-destructive screening of the total landscape of the charge-carrier transport at interfaces.

Rapid Evaluation of Electron Mobilities at Semiconductor–Insulator Interfaces in an Ambient Atmosphere by a Contactless Microwave-Based Technique

Intrinsic mobility of electrons at the interfaces between crystalline organic semiconductors and insulating dielectric polymer films was rapidly evaluated in an ambient atmosphere by TRMC@Interfaces, a noncontact and nondestructive method based on dielectric loss spectroscopy of microwaves. By just preparing simple metal–insulator–semiconductor devices, local-scale motions of charge carriers injected into the interface by pulses of gate bias voltage were monitored through reflected microwave changes, resulting in the evaluation of local-scale charge carrier mobilities together with the value of trap density at the interface. The evaluated high electron mobilities of 12 cm2 V–1 s–1 for N,N′-bis(cyclohexyl)naphthalene-1,4,5,8-bis(dicarboximide) (DCy-NDI) and 15 cm2 V–1 s–1 for N,N′-dioctylperylene-1,4,5,8-bis(dicarboximide) (DC8-PDI) are the benchmarks for organic semiconducting materials that are comparable with the highest ones reported from the field-effect transistor devices. The present TRMC@Interfaces was found to serve as a rapid screening technique to examine the intrinsic performance of organic semiconducting materials as well as a useful tool enabling the precise discussion on the relationship among their local-scale charge carrier mobility, thin-film morphology, and packing structure.

Preferential formation of columnar mesophases via peripheral modification of discotic π-systems with immiscible side chain pairs

When sufficient volume of dodecyl chains are attached at one imide position of a perylenediimide (PDI) or naphthalenediimide (NDI) core and triethyleneglycol (TEG) chains on the other side, the resulting molecules PDIC12/TEG G0, PDIC12/TEG G1, and NDIC12/TEG G0 self-assemble into a rectangular columnar mesophase with p2mg symmetry, forming hydrophobic/hydrophilic nano-segregation of side chains. The driving force of PDIC12/TEG G0 to form preferentially the rectangular columnar mesophase is given by the immiscibility between the side chain pairs—exclusion of other phases such as cubic, crystalline and amorphous phases, where thermodynamically unstable contacts between hydrophobic and hydrophilic chains considerably take place. In contrast, this preference is less found in the analogous molecules decorated with either dodecyl or TEG chains at both termini. PDIC12/C12 G0 and PDITEG/TEG G0 form a hexagonal columnar mesophase because of the optimized chain/core volume. However, if the side chain volume grows, PDITEG/TEG G1 does not form a mesophase but undergoes a soft crystalline–isotropic phase transition, while PDIC12/C12 G1 was revealed to destabilize its columnar mesophase but forms a micellar cubic phase. NDIC12/C12 G0 resulted in a strong crystallization, while NDITEG/TEG G0 formed amorphous liquid. The molecular design strategy using immiscible side chain pairs potentially enables a variety of π-systems to stack up to form a columnar phase rather than other ordered phases, regardless of the chain/core volume balance.

A phase transformable ultrastable titanium-carboxylate framework for photoconduction

Porous titanium oxide materials are attractive for energy-related applications. However, many suffer from poor stability and crystallinity. Here we present a robust nanoporous metal–organic framework (MOF), comprising a Ti12O15 oxocluster and a tetracarboxylate ligand, achieved through a scalable synthesis. This material undergoes an unusual irreversible thermally induced phase transformation that generates a highly crystalline porous product with an infinite inorganic moiety of a very high condensation degree. Preliminary photophysical experiments indicate that the product after phase transformation exhibits photoconductive behavior, highlighting the impact of inorganic unit dimensionality on the alteration of physical properties. Introduction of a conductive polymer into its pores leads to a significant increase of the charge separation lifetime under irradiation. Additionally, the inorganic unit of this Ti-MOF can be easily modified via doping with other metal elements. The combined advantages of this compound make it a promising functional scaffold for practical applications.Porous TiO2 materials are attractive for energy-related applications owing to their accessible active sites, but suffer from poor stability. Here the authors synthesize a highly stable and porous metal–organic framework containing polymeric 1D Ti–O subunits, which displays a high condensation degree and high photoconductivity.

Hash-Mark-Shaped Azaacene Tetramers with Axial Chirality

The tetramers of azapentacene derivatives with unique hash mark (#)-shaped structures were prepared in a quite facile manner. The #-shaped tetramers are optically active due to possessing extended biaryl skeletons, and the structure of the tetramer composed of four dihydrodiazapentacene (DHDAP) units (1) was investigated as the first example of this kind of molecule. The tetramer 1 showed characteristic chiroptical properties reflecting its orthogonally arranged quadruple DHDAP moieties, as well as redox activity. The solution of enantiopure 1 exhibited intense circularly polarized luminescence (CPL) with a dissymmetry factor of 2.5 × 10-3. The absolute configuration of the enantiomers of 1 was experimentally determined by X-ray crystal analysis for the dication salt of the enantiomer of 1 with SbCl6- counterions. The solutions of enantiopure 12+·2[SbCl6-] also showed NIR circular dichroism (CD) spectra over the entire range from visible to 1100 nm, enabling the modulation of the chiroptical properties by redox stimuli.

Formation and Photodynamic Behavior of Transition Metal Dichalcogenide Nanosheet-Fullerene Inorganic/Organic Nanohybrids on Semiconducting Electrodes

Composite films that consisted of C60 and well-exfoliated nanosheets of transition metal dichalcogenides (TMDs), such as MoS2 or WS2 , with a bulk heterojunction structure were easily fabricated onto a semiconducting SnO2 electrode via a two-step methodology: self-assembly into their composite aggregates by injection of a poor solvent into a good solvent with the dispersion, and subsequent electrophoretic deposition. Upon photoexcitation, the composites on SnO2 exhibited enhanced transient conductivity in comparison with single components of TMDs or C60 , which demonstrates that the bulk heterojunction nanostructure of TMD and C60 promoted the charge separation (CS). In addition, the decoration of the TMD nanosheets with C60 hindered the undesirable charge recombination (CR) between an electron in SnO2 and a hole in the TMD nanosheets. Owing to the accelerated CS and suppressed CR, photoelectrochemical devices based on the MoS2 -C60 and WS2 -C60 composites achieved remarkably improved incident photon-to-current efficiencies (IPCEs) as compared with the single-component films. Despite more suppressed CR in WS2 -C60 than MoS2 -C60 , the IPCE value of the device with WS2 -C60 was smaller than that with MoS2 -C60 owing to its inhomogeneous film structure.

Optical and Structural Properties of ESIPT Inspired HBT–Fluorene Molecular Aggregates and Liquid Crystals

In bulk materials, positional isomers not only help in understanding how slight difference in molecular structure alters the crystal packing and optical properties, but also play a key role in developing new type of materials for functional applications. A detailed study on the photophysical properties of fluorene-HBT positional isomers in solution and in the solid state providing a molecular level understanding of the factors which influence fluorescence behavior is reported. Two molecules Ia and IIa were synthesized by Suzuki coupling reaction and their photophysical properties were compared to positional isomers Ib and IIb. Crystal structure analyses and density functional theory (DFT) computation studies were performed to understand structure-properties relation and the results reveal that changing substitution pattern has a marked influence on their packing modes and luminescence properties. Strong noncovalent interactions (π-π) in the solid state hamper the excited state intramolecular proton transfer (ESIPT) process which causes fluorescence quenching in the solid state (Ia and IIa = Φf, 28-40%; Ib and IIb = Φf, 55-67%). Compounds show solvent-responsive and aggregation induced emission (AIE) properties. Bent structures of Ia with double and symmetric substitution of ESIPT motifs exhibit particularly unique condensed phase upon heating, confirmed as a nematic liquid crystalline phase, and this is the first report on the ESIPT and AIE active liquid crystalline materials with a banana-shaped molecule.

In-situ analysis of microwave conductivity and impedance spectroscopy for evaluation of charge carrier dynamics at interfaces

A unique concerted analysis comprising non-contact microwave conductivity measurements and impedance spectroscopy was developed to simultaneously assess the charge carrier mobility and injection barriers. The frequency dependence of the microwave conductivity as well as the electrical current was analyzed by applying sinusoidal voltage to determine the equivalent circuit parameters. Based on the temperature dependence of the circuit parameters, the energy of the injection barrier was estimated to be 0.4 eV with the Richardson–Schottky model, and the band-like transport was confirmed with the negative temperature coefficient with the β value of 1.4 in the intra-layer conduction of C8-BTBT. In contrast, the increase in the resistance of the C8-BTBT layer with decreasing temperature implied the occurrence of hopping-like transport in the inter-layer conduction of C8-BTBT.