Dai Taguchi

Probing of carrier behavior in organic electroluminescent diode using electric field induced optical second-harmonic generation measurement

By using the electric field induced optical second-harmonic generation (EFISHG) measurements, we probed the transient electric field in a double-layer indium zinc oxide (IZO)/N, N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4, 4′-diamine(α-NPD)/tris(8-hydroxy-quinolinato)aluminum(III) (Alq3)/Al electroluminescent (EL) diode. Results evidently showed that EL was initiated by the injected hole transport across α-NPD layer, and holes accumulated at the α-NPD/Alq3 interface while EL was enhanced. Analysis based on the Maxwell–Wagner effect model well accounted for the hole accumulation. EFISHG measurement is useful as a tool for probing carrier behavior in organic EL devices.

Catalyst-Free Plasma Enhanced Growth of Graphene from Sustainable Sources

Details of a fast and sustainable bottom-up process to grow large area high quality graphene films without the aid of any catalyst are reported in this paper. We used Melaleuca alternifolia, a volatile natural extract from tea tree plant as the precursor. The as-fabricated graphene films yielded a stable contact angle of 135°, indicating their potential application in very high hydrophobic coatings. The electronic devices formed by sandwiching pentacene between graphene and aluminum films demonstrated memristive behavior, and hence, these graphene films could find use in nonvolatile memory devices also.

Analyzing carrier lifetime of double-layer organic solar cells by using optical electric-field-induced second-harmonic generation measurement

By using optical electric-field-induced second-harmonic generation (SHG) measurement, we directly probed charge accumulation and decay processes in double-layer [copper-phthalocyanine (CuPc)/C60] organic solar cells (OSCs). Results showed that negative excess charges accumulated at the CuPc/C60 interface under photoillumination and the charges decayed in a two-step process in dark. Analysis using the filtering method showed that 24% of the accumulated negative excess charges decayed with a lifetime τL1=5.3×10−5 s and the other 76% of charges decayed with a lifetime τL2=6.2×10−2 s. We conclude that analyzing SHG signal response provides a direct way to investigate carrier lifetime in multilayer OSCs.

Modeling of threshold voltage in pentacene organic field-effect transistors

To understand the physical meaning of threshold voltage in organic field-effect transistors (OFETs), we studied the threshold voltage (shift) dependence on gate-insulator thickness as well as active-layer thickness, by using pentacene OFETs with and without a dipole interlayer between pentacene active layer and SiO2 gate insulator. Results showed that the presence of dipole monolayer caused a large threshold voltage shift and there was a linear relationship between the threshold voltage shift and the layer thickness of pentacene as well as SiO2. Assuming the pentacene film is a dielectric layer and the threshold voltage in pentacene OFET is determined from a zero-electric-field condition at the gate insulator interface, we propose a model based on compensation of the local electric field in the vicinity of semiconductor and gate insulator interface. The model well accounts for both the large negative threshold voltage shift and the linear relation. These findings reveal the importance of interfacial elect...

Direct Probing of Photovoltaic Effect Generated in Double-Layer Organic Solar Cell by Electric-Field-Induced Optical Second-Harmonic Generation

By using time-resolved electric-field-induced optical second-harmonic generation (EFISHG) measurement, we directly probed photovoltaic effect generated in double-layer (pentacene/C60) organic solar cells (OSCs). The electric field in C60 layer was selectively probed by using incident laser with a wavelength of 1,000 nm. Results showed that excess positive charges Qs=1.5×10-8 C/cm2 accumulated at the pentacene/C60 interface under photoillumination. These charges generated the interfacial voltage Vs=0.16 V and deformed a potential profile in the OSCs. The EFISHG measurement is an effective way for directly catching carrier behavior at the double-layer interface in OSCs in terms of the photovoltaic effect.

The Charge Transport in Organic Field-Effect Transistor as an Interface Charge Propagation: The Maxwell–Wagner Effect Model and Transmission Line Approximation

By analyzing electric field migration in the pentacene organic field-effect transistor channel (OFET), visualized using the time-resolved microscopic optical second harmonic generation (TRM-SHG) is analyzed the propagation of injected carriers. We find that the accumulated charge propagation on the pentacene–gate insulator interface of the three-electrode system is clearly different from the drift in electric field of the two-electrode system. The propagation of injected carriers is evaluated on the basis of the Maxwell–Wagner effect model and the transmission line approximation. We show that the interface charge accumulation has a significant contribution to the charge transport in OFET. Proposed model for the transient state is beyond the limits of common used impedance spectroscopy models and represents extension of the small-signal analysis. Found relation between mobility and transit time helps in analysis of OFET transit time sensitive experiments such as the time-of-flight technique (TOF) or TRM-SHG.

Interaction of interfacial charge and ferroelectric polarization in a pentacene/poly(vinylidene fluoride-trifluoroethylene) double-layer device

A two-step polarization reversal process was identified in the pentacene/poly(vinylidene fluoride-trifluoroethylene) double-layer device. Displacement current measurement showed that three peaks generated non-symmetrically in the current-voltage characteristics. Accordingly, optical electric-field induced second-harmonic generation measurement displayed two hysteresis loops. A proposed model based on a two-step polarization reversal mechanism accounted for these results, and suggested that interaction of interfacial charge and ferroelectric polarization governed the mechanism. The proposed model is useful to explain the reduced remanent polarization in ferroelectric field-effect transistors, and will be helpful for developing organic devices with a ferroelectric layer.

Probing of interfacial charging and discharging in double-layer devices with a polyimide blocking layer by time-resolved optical second harmonic generation

By using time-resolved electric field induced second harmonic generation, we directly probed interfacial charging and discharging in double-layer devices with an indium tin oxide (ITO)/polyimide(PI)/N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD) (or pentacene)/Au structure, where PI serves as a blocking layer. The results showed that carriers injected from Au electrodes were responsible for interfacial charging and discharging, but different carrier behaviors were observed for pentacene and α-NPD devices, which implies that two devices have the different electrical properties. By analyzing the devices as a system of a Maxwell–Wagner effect element, we proposed a model that accounted for the difference in the charging and discharging processes in terms of carrier behavior.

Probing and modeling of interfacial carrier motion in organic devices by optical second harmonic generation

Here the authors report a novel optical second harmonic generation (SHG) measurement that allows carrier motion in solids to be probed directly. By catching nonlinear polarization induced in the solids by coupling with incident electromagnetic waves (laser beam) and dc electric field from moving carriers, carrier motion is visualized. Experiments making use of time-resolved SHG technique has revealed dynamic changes of SHG intensity profiles arising from pentacene field effect transistors in accordance with the carrier transport in the channel. The observed SHG intensity profiles are strongly dependent on the physical condition of the organic field effect transistors channel and well accounts for the effect of carrier traps on gate insulator such as silicon dioxide (SiO2) and poly(methyl methacrylate). The authors anticipate that a technique using time-resolved SHG can be a powerful tool to characterize interface states of organic devices.

Insight into the contact resistance problem by direct probing of the potential drop in organic field-effect transistors

The microscopic electric field induced second-harmonic generation technique is used for direct observation of electric field profiles and evaluation of the potential drop on the injection electrode in the organic field-effect transistors with various channel lengths L. It is found that the potential drop on injection electrode is not a function of L. We show that the analysis of the transmission line model (TLM) cannot distinguish channel length independent contact resistance and potential drop. Tracing back to the conceptional idea of contact resistance proposed by Shockley in 1964, the TLM approach is discussed to explain L-dependent contact resistance.