Yoshihito Kunugi

Vapour deposited films of quinoidal biselenophene and bithiophene derivatives as active layers of n-channel organic field-effect transistors

n-Channel OFETs using oligoselenophene- and oligothiophene-based semiconductors as active layers have been successfully fabricated, and the field-effect mobilities for the selenophene-based compounds are found to be higher than those for the thiophene analogues.

An ambipolar organic field-effect transistor using oligothiophene incorporated with two [60]fullerenes

A solution-processed ambipolar organic field-effect transistor using [60]fullerene attached oligothiophene films was successfully fabricated.

Multifunctional π-Expanded Macrocyclic Oligothiophene 6-Mers and Related Macrocyclic Oligomers

Multifunctional π-expanded macrocyclic oligothiophene 6-mer 1, as well as 9- (2) and 12-mers (3), was synthesized using a McMurry coupling reaction as the key step. The 6-mer 1 was converted to cyclo[6](2,5-thienylene-ethynylene) (4) by using a bromination-dehydrobromination procedure. From X-ray analysis, the crystal structures of nonplanar 1 and round-shaped 2 and 4 were elucidated. STM showed that 4 formed a self-assembled monolayer at the liquid/solid interface to produce a hexagonal porous network. Chemical oxidation of 1, 2, and 4 with 1 and 2 equiv of Fe(ClO4)3 produced 1(•+) and 1(2+), 2(•+) and 2(2+), and 4(•+) and 4(2+), respectively. Although oligothiophene radical cations containing β,β-disubstituted thiophenes usually do not form π-dimers, 4(•+) clearly formed a π-dimer owing to its planar, round shape. As for the dications of 1, 2, and 4, 1(2+), which has 34π-electrons, exhibited a large diatropic ring current effect, whereas 34π dication 4(2+) only showed a medium diatropic ring current effect. In contrast to 1(2+) and 4(2+), 52π dication 2(2+) had biradical cationic character instead of Hückel-type cyclic conjugation. Interestingly, 6-mer 1 showed polymorphism and unusual melting point behavior due to the number of stable conformations in the solid state. Single crystals of 1 melted at 176 °C, whereas an amorphous film of 1 crystallized in the temperature range of 80-83 °C to form a lamellarly stacked microcrystalline film, which melted at 139 °C. The polymorphism of 1 was applied to either fluorescence switching or switching of field effect transistor (FET) activity and electrical conductivity.

Hole-transporting properties of organosilanylene–diethynylpyrene and diethynylanthracene alternating polymers. Applications to patterning of light-emitting images

Abstract Diethylsilanylene- and tetramethyldisilanylene-1,6-diethynylpyrene alternating polymers ( 1 and 2 ) were prepared by the reactions of 1,6-di(lithioethynyl)pyrene and the respective dichloroorganosilanes. The hole-transporting properties of the resulting polymers were evaluated by the performance of electroluminescent (EL) devices with the structure of indium–tin–oxide/polymer/Alq/Mg–Ag, in comparison with those of an organosilanylene-9,10-diethynylanthracene alternating polymer ( 3 ), reported previously. Among them, the device with polymer 1 exhibited the best performance with a maximum luminescence of 6000 cd m −2 , which increased to 16 000 cd m −2 when electron-blocking TPD layer was introduced between the polymer and Alq layers. Polymers 1 – 3 were found to be photoactive and irradiation of the polymer films with a UV lamp led to a drastic drop of the luminance of the device, being applicable to the patterning of EL images.

A relationship between driving voltage and the highest occupied molecular orbital level of hole-transporting metallophthalocyanine layer for organic electroluminescence devices

Abstract Effects of metallophthalocyanines (MPcs) are systematically investigated on electroluminescence (EL) characteristics of ITO/MPc/TPD/Alq3/Mg–Ag devices. A linear relationship is found between driving voltages of the devices and energies of the highest occupied molecular orbital (HOMO) level of MPcs, showing that the driving voltage of the device is substantially determined by the energy barrier for hole injection at the ITO/MPc interface. When an MPc having a higher HOMO level, such as CuPc or ZnPc, is selected as a hole transport material, the driving voltage of the device at a luminance of 100 cd m−2 significantly reaches as low as 5.5 V compared with 7.8 V for the device without an MPc layer. The insertion of MPc layer leads to increasing of the EL output, but does not affect the EL efficiency of the devices.

Influence of π-conjugation length on mobilities of charge carriers in conducting polymers

Mobilities of charge carriers in thin films of a series of copolymers with repeat units consisting of oligothiophenes bridged by Si atoms are measured over a range of oxidation stages in order to study the influence of π-conjugation length on the mobility. The number of thiophene units in oligothiophenes ranges from five to nine, the polymers being designated accordingly by PS5T to PS9T. Mobilities of charge carriers in these polymer films, where intrachain hopping transport is restricted within each oligothienylene unit, are constant in the low doping regions below 1%. As the doping level increases, mobilities for polymers with longer π-conjugation lengths start to rise at lower doping levels. The mobility enhancement follows also an increasing order of the π-conjugation length, although the 30 times enhancement of mobility for PS9T is still smaller than the four orders-of-magnitude increase of mobility observed earlier for electrochemically synthesized poly(3-methylthiophene). The results are discussed with electrochemical and spectroelectrochemical measurements from the viewpoint of the involvement of polarons, π-dimers, and bipolarons as possible charge carriers in the conduction mechanisms.

Fabrication of an Organic p-n Homojunction Diode Using Electrochemically Cation- and Photochemically Anion-Doped Polymer

An organic p-n homojunction has been successfully constructed in a single strip of poly(3-methylthiophene) (PMT) film by electrochemical cation doping on one side and photosensitized anion doping on the other side. The rectifying effect observed with the Al/(cation:anion)-doped PMT/Au cell provides evidence for the formation of a p-n homojunction in the PMT film since both Al/cation-doped PMT/Al and Au/anion-doped PMT/Au cells exhibit ohmic behavior. The formation of p-n homojunction can be inferred from measured work functions of the electrochemically cation-doped PMT, photochemically anion-doped PMT, Al and Au films. The p-n homojunction cell yields a short-circuit photocurrent of 0.16 µ A· cm-2, an open-circuit photovoltage of 0.23 V and a fill factor of 0.30. These voltaic and photovoltaic properties are found to be superior to those of the polymer diode based on a p-n junction made by pressure contact of the p-doped PMT film with the n-doped PMT film.

Synthesis of conjugated D–A polymers bearing bi(dithienogermole) as a new donor component and their applications to polymer solar cells and transistors

New donor–acceptor π-conjugated polymers composed of alternating bi(dithienogermole) (DTG2) and benzo-(BT) or pyridinothiadiazole (PT) units (pDTG2-BT or pDTG2-PT) were prepared and their optical, electrochemical, photovoltaic, and semiconducting properties were investigated. pDTG2-BT showed two absorption maxima at 738 and 686 nm, respectively. Of the two, one at longer wavelength likely due to the aggregated polymer segments was relatively intensified, when the spectrum was measured in film. In contrast, pDTG2-PT showed one broad band at 686 nm, which shifted to longer wavelength at 748 nm in film. These results indicate stronger interchain interaction for pDTG2-BT. Cyclic voltammograms of the polymer films showed pseudo-reversible profiles and the anodic peak of pDTG2-BT appeared at lower potential than that of pDTG2-PT, indicating the higher-lying HOMO (Highest Occupied Molecular Orbital) for pDTG2-BT. The polymer electronic states were also examined by the quantum chemical calculations on the polymer models. Of those polymers, pDTG2-BT exhibited the higher photovoltaic performance. The power conversion efficiency of the cell (ITO/PEDOT:PSS/pDTG2-BT:PC71BM/LiF/Al) reached 4.30% with Voc = 0.53 V, Jsc = 12.68 mA cm−2, and FF = 0.64, presumably reflecting the stronger interchain interaction, although pDTG2-PT afforded higher Voc, arising from its lower-lying HOMO. pDTG2-BT also exhibited higher performance as a thin film transistor material than pDTG2-PT. The performance of the cell with pDTG2-BT was also higher than that based on a similar alternating dithienogermole (DTG)-BT polymer, reported previously.

Transport and in situ ESR studies on polymer film composed of quinquethiophenes bridged by monosilanylene units

Abstract Apparent mobilities of positive charge carriers in a poly[(ethoxypropylsilanylene)quinque(2,5-thienylene)] (MS5T-OEt) film are evaluated over a wide range of doping levels. The mobility is found to decrease monotonously with the increase in doping level up to a few percents, in contrast to four order-of-magnitude increase of mobilities reported earlier for the poly(3-methylthiophene) (PMT) film. This clear difference indicates that the two kinds of diamagnetic species, cation radical π-dimers in the MS5T-OEt film and bipolarons in the PMT film, are related to the electrical conduction in completely different manners. A rather small difference between mobilities in almost dedoped MS5T-OEt and PMT films is discussed from a viewpoint of an interchain or intrachain transport of polarons. Anomalous changes of mobilities are observed in a relatively high doping region of the MS5T-OEt film. An in situ ESR study performed to characterize charge-carrying species in the MS5T-OEt film demonstrates that there are oligothienylene segments where polarons are isolated so as not to form π-dimers. A theoretical treatment is developed to evaluate a molar fraction of the isolated oligothienylene segments.

Electrochemical anion doping of poly[(tetraethyldisilanylene) oligot 2,5-thienylene)] derivatives and their p-type semiconducting properties

Abstract Chemically synthesized poly[(tetraethyldisilanylene)oligo(2,5-thienylene)] derivatives (DS/ m T; m = 3 to 5) have been successfully anion-doped by electrochemical oxidation. Band-gap energies of 2.52, 2.65, 2.82 and 3.27 eV were evaluated for DS5T, DS4T, DS3T and DS2T respectively. The DS5T, DS4T and DS3T films exhibited electrical conductivities of the order of 10 −3 to 10 −4 S cm −1 when doped with BF 4 − . The work functions of the films changed from 5.1 to ca. 5.5 eV with electrochemical anion doping. In cyclic voltammograms of the polymer films for anion doping and dedoping, an anodic peak potential and a cathodic one ( E pc .) shifted to the positive direction as the number m of thienylene units decreased. E pc s at a sweep rate of 100 mV s −1 were about 0.8, 0.9 and 1.0V for DS5T, DS4T and DS3T respectively. Reversible electrochemical doping and dedoping of the DS5T film were feasible when the potential was cycled between 0 and 1.2 V. At potentials more positive than 1.2 V, however, both overoxidation of the oligo(thienylene) unit and Si-Si bond cleavage took place, leading to decreases in conductivity and work function of the film.