Katsuichi Kanemoto

Temperature effects on quasi-isolated conjugated polymers as revealed by temperature-dependent optical spectra of 16-mer oligothiophene diluted in a sold matrix

Temperature dependences (4-300 K) of photoluminescence (PL) and absorption spectra of 16-mer oligothiophene (16 T) extremely diluted in polypropylene (PP) have been investigated in order to clarify temperature effects on quasi-isolated conjugated polymers. The PL and absorption spectra are found to blueshift with increasing temperature. The reason for the blueshift is discussed by comparing models based on the refractive index of the solvent (PP) and on the thermal conformational change of 16 T. The blueshift is concluded to result from the thermal conformational change. Time-resolved PL spectra show a redshift of PL band following photoexcitation (spectral migration). The amount of the migration is shown to increase with increasing temperature. The increased migration is concluded to be due to the thermal conformational change. The temperature dependence of the effective conjugation length (ECL) of 16 T is calculated for the absorption and PL transitions. The calculation suggests that ECL is reduced at room temperature to two-thirds of the intrinsic chain length. The activation energy of the conformational change is estimated to be 22.4 meV from the temperature dependence of ECL. We demonstrate that the steady-state PL spectra are well reproduced by simple Franck-Condon analyses using a single Huang-Ryes factor over a wide temperature range. The analyses reveal features of temperature dependence in important spectral parameters such as the Stokes shift, linewidth, and Huang-Ryes factor.

Direct optical probing of negative carriers from an operating [6,6]-phenyl C61 butyric acid methyl ester diode

We have performed spectroscopic measurements combining with the diode operation of [6,6]-phenyl C61 butyric acid methyl ester (PCBM) for directly characterizing the nature of its n-carriers. The measurements in the visible region reveal that electrons for the n-carrier generation are primarily accommodated into the t1g and t1u levels. The measurements in the near-infrared region indicate that the n-carriers of the PCBM diode are delocalized compared to the PCBM anions in solution. We also show that the frequency-dependence of the spectroscopic signals can estimate the lifetime of the n-carriers in the PCBM diode (23 μs).

Polaron dynamics of heavily doped regioregular and regiorandom poly(3-alkylthiophenes) revealed by electron spin resonance spectroscopy.

Electron spin resonance (ESR) features in heavily doped conjugated polymers are investigated through the comparison of temperature dependences of ESR spectra between head-to-tail coupled regioregular (RR) and regiorandom (RRa) poly(3-octylthiophenes) (P3OTs). RR-P3OT, used as a model of having crystalline grains in the solid film, is found to exhibit anisotropic ESR spectra, whereas RRa-P3OT gives almost isotropic ESR spectra similar to those of usual heavily doped conjugated polymers. This difference in the degree of spectral anisotropy primarily arises from a difference in their film morphology. Spectral simulations show the anisotropy observed in RR-P3OT to be caused by g-anisotropy. The presence of the g-anisotropy in RR-P3OT indicates that its polarons spend most of the time within a single crystalline grain that has some domains with a common direction of the g-tensor. The g-anisotropy turns out to decrease with increasing temperature. This result is explained by thermally activated hopping motions between crystalline grains. We emphasize that the decrease in the g-anisotropy with temperature should be associated with its activated type of temperature dependence of conductivity. In RRa-P3OT, its isotropic ESR spectra are suggested to be caused by the interchain motion as well as the intrachain one.

Direct monitoring of bias-dependent variations in the exciton formation ratio of working organic light emitting diodes

In typical operation of organic light emitting diodes (OLEDs), excitons are assumed to generate with a ratio of 1:3 for singlet and triplet excitons, respectively, based on a simple spin statistics model. This assumption has been used in designing efficient OLEDs. Despite the larger generation ratio of triplet excitons, physical properties of fluorescent OLEDs are usually evaluated only through the electroluminescence (EL) intensity from singlets and the behaviors of triplets during the LED operation are virtually black-boxed, because the triplets are mostly non-emissive. Here, we employ transient spectroscopy combined with LED-operation for directly monitoring the non-emissive triplets of working OLEDs. The spectroscopic techniques are performed simultaneously with EL- and current measurements under various operation biases. The simultaneous measurements reveal that the relative formation ratio of singlet-to-triplet excitons dramatically changes with the magnitude of bias. The measurements also show that the generation efficiency of singlets scales with the bias, whereas that of triplets is nearly bias-independent. These features of the formation ratio and efficiency are compatibly explained by considering the yield of intersystem crossing and the energy separation of excitons from electron-hole pairs. The obtained findings via the spectroscopic measurements enable prediction of the formation pathways in OLEDs.

True Vapor–Liquid–Solid Process Suppresses Unintentional Carrier Doping of Single Crystalline Metal Oxide Nanowires

Single crystalline nanowires composed of semiconducting metal oxides formed via a vapor-liquid-solid (VLS) process exhibit an electrical conductivity even without an intentional carrier doping, although these stoichiometric metal oxides are ideally insulators. Suppressing this unintentional doping effect has been a challenging issue not only for metal oxide nanowires but also for various nanostructured metal oxides toward their semiconductor applications. Here we demonstrate that a pure VLS crystal growth, which occurs only at liquid-solid (LS) interface, substantially suppresses an unintentional doping of single crystalline SnO2 nanowires. By strictly tailoring the crystal growth interface of VLS process, we found the gigantic difference of electrical conduction (up to 7 orders of magnitude) between nanowires formed only at LS interface and those formed at both LS and vapor-solid (VS) interfaces. On the basis of investigations with spatially resolved single nanowire electrical measurements, plane-view electron energy-loss spectroscopy, and molecular dynamics simulations, we reveal the gigantic suppression of unintentional carrier doping only for the crystal grown at LS interface due to the higher annealing effect at LS interface compared with that grown at VS interface. These implications will be a foundation to design the semiconducting properties of various nanostructured metal oxides.

Displacement Current Induced by Electron Spin Resonance in Organic Semiconductor

A current induced by electron spin resonance (ESR) transitions is investigated for polymer diodes using electrically detected magnetic resonance techniques. Polymer diodes under photoexcitation are shown to exhibit a strong ESR-induced current. The ESR current exhibits a spiked time response for modulation of the resonance condition and is concluded to result from a displacement current. The magnitude of the displacement current is found to vary depending on polymers used for active layers. Trapping and de-trapping processes of carriers are expected to determine the time profile of the displacement current, suggesting that materials with shallow trap levels would be suitable for applications to spin-based devices.

Optical Properties of Hexanuclear 6-Methylpyridinethiolato-Copper (I) Crystals

Luminescent material Hexanuclear 6-Methylpyridinethiolato-Copper (I) has been investigated by measuring the photoluminescence spectra, their temperature dependence, temporal profiles and excitation spectra. Luminescence peaks appear at 1.88 v eV at room temperature, and at about 2.2 v eV below the liquid nitrogen temperatures. The origin of these two bands have been assigned to the transition within the metal-cluster center (CC) and that due to the metal to ligand charge transfer (CT). These origins have been examined by changing the ligand constitution from [Cu 6 (6-mpyt) 6 ] to [Cu 6 (4-6-mpyt) 6 ]. The observed change in the luminescence spectra confirms the validity of the assignment. The configuration coordinate model based on the experimental results is presented.

Spectroscopic observation of triplet exciton dynamics during operation in polymer light emitting diodes

Abstract In typical fluorescent organic light emitting diodes (LEDs), the observation of triplet excitons is difficult because of their non-radiative features of decay. The generation process of triplet excitons was successfully monitored by employing spectroscopic techniques combined with LED operation. The generation dynamics of singlet excitons are then simultaneously examined by time-resolved electroluminescence measurements, enabling relative comparison of the generation processes of singlet and triplet excitons. As a result, the generation ratio of singlet excitons is shown to be enhanced by applied voltage. The enhancement is concluded to result from the increase of the generation rate in signlet excitons induced by electric field.

Determining internal screening electric field of working polymer light emitting diodes

This study provides a method of determining the internal electric field of polymer light emitting diodes (LEDs) in the working condition. The method employs Stark signals induced by triangular shaped pulse biases and enables estimates of the internal field in a broad voltage region. The internal field under forward bias is shown to be determined by the screening effect caused by injected carriers. Spatial distribution calculated for the LED suggests the presence of strong electric field formed by accumulated carriers near the electrodes. The proposed method is applicable to a variety of devices and can promote understanding of veiled roles of internal fields on device operation.

Determination of photocarrier density under continuous photoirradiation using spectroscopic techniques as applied to polymer: Fullerene blend films

We propose a method to determine the density of photocarrier under continuous photoirradiation in conjugated polymers using spectroscopic signals obtained by photoinduced absorption (PIA) measurements. The bleaching signals in the PIA measurements of polymer films and the steady-state absorption signals of oxidized polymer solution are employed to determine the photocarrier density. The method is applied to photocarriers of poly (3-hexylthiophene) (P3HT) in a blended film consisting of P3HT and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). The photocarrier density under continuous photoirradiation of 580 mW/cm2 is determined to be 3.5 × 1016 cm−3. Using a trend of the carrier density increasing in proportion to the square root of photo-excitation intensity, we provide a general formula to estimate the photocarrier density under simulated 1 sun solar irradiation for the P3HT: PCBM film of an arbitrary thickness. We emphasize that the method proposed in this study enables an estimate of carrier density...