Time-resolved Microscopic Optical Second-harmonic Generation Research Articles

Transient carrier visualization of organic-inorganic hybrid perovskite thin films by using time-resolved microscopic second-harmonic generation (TRM-SHG)

Abstract Based on the transient electric field migration directly probed by the time-resolved microscopic optical second-harmonic generation (TRM-SHG) technique, we successfully detected the transient hole transport in the perovskite for the first time. From the spectroscopic point of view, SHG signal resonantly enhanced at the fundamental wavelength of around 1560 nm through the band-transition under the voltage application. Square dependence of the SH intensity on the applied voltage at the wavelength of 1560 nm clearly indicated the electric field induced process of the SHG. We could visualize the carrier motion in the channel of perovskite field effect transistor (FET) with this wavelength. Carrier mobility was estimated as 0.02 cm2/V by analyzing the transient carrier motion. TRM-SHG technique was also employed to investigate the effect of traps on the transient carrier motion. Based on the peak of the transient electric field distribution, trap density, and dynamic carrier mobility were separately estimated.

Direct evaluation of anisotropic carrier mobility in uniaxially aligned polymer semiconductor film by time-resolved microscopic optical second-harmonic generation measurement

Mobility anisotropy in uniaxially-aligned fluorene co-polymer thin film was directly observed by using time-resolved microscopic optical second-harmonic generation (TRM-SHG) imaging. Main-chain orientation of fluorene co-polymer was determined by polarized absorption measurement, and the mobilities in the direction parallel and perpendicular to the main-chain were respectively estimated as cm2 Vs−1 and cm2 Vs−1 from the visualized carrier motion starting from a round-shape electrode. These results indicate that the mobility anisotropy of this sample was 4.0. Activation energy for each direction was also evaluated by the temperature dependence measurement as 117 and 94 meV, respectively. The TRM-SHG method enables us to estimate mobility and activation energy of the oriented polymer film in all directions at once.

Band-like transport observed in TIPS-pentacene thin film by time-resolved microscopic optical second-harmonic generation imaging

Carrier transport in single-crystalline grains of 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene was studied using time-resolved microscopic optical second-harmonic generation (TRM-SHG). TRM-SHG is used to directly visualize carrier injection and transport behavior in TIPS pentacene. A strong SHG signal observed at the edge of a FET source electrode at low temperature suggests the presence of a potential drop associated with the carrier injection process. An accurate carrier mobility calculation, taking into account the potential drop, shows a negative slope with temperature that indicates band-like transport.

Modeling carrier transport and electric field evolution in Gaussian disordered organic field-effect transistors

Stimulated by the time resolved microscopic optical second-harmonic generation (TRM-SHG) experiment, we model the carriers transport and electric field evolution in the channel of three-dimensional Gaussian disordered organic field-effect transistors (OFETs) by the coupled time-dependent master equation and Poisson equation. We show that this model with the Miller-Abrahams rate can satisfactorily account for the experimental observations that include different profiles of the electric field in the channel and the diffusionlike migration of the field peaks with respect to time. Particularly, we find that the dynamic mobility proposed by us earlier is distinct from the standard one in the presence of a typical disorder, which is attributed to the uncompleted energy relaxation of the carriers in the transport process.

Study of trap-filling effect on transient carrier transport in pentacene field effect transistors by time-resolved optical second harmonic generation

By using time-resolved microscopic optical second-harmonic generation (TRM-SHG) imaging, we studied the trap-filling effect on transient carrier transport in pentacene field effect transistors (FETs). We showed that the transient carrier transport was strongly dependent on trap-filling condition at the pentacene and gate-insulator interface. The TRM-SHG imaging caught the transient electric field migration which was caused from traveling carriers along the channel. Results showed that the effective carrier mobility increased with increase of the filled-trap density, i,e., with decrease of empty trap-sites, and finally saturated after all trap-sites were filled. Saturation voltage of the mobility indicates the filled trap density of approximately 1.4 × 10 12 / cm 2 .

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.

Electron Injection into Pentacene Field-Effect Transistor Observed by Time-Resolved Optical Second Harmonic Generation Imaging

Electron injection from a gold (Au) electrode into a pentacene field-effect transistor (FET) was investigated by time-resolved microscopic optical second harmonic generation (TRM-SHG) imaging. The evolution of the TRM-SHG images below the source electrode evidently showed the presence of electron injection. The transient electric field migration in the FET channel indicated an inhibited electron transport attributed to a high electron trap density. The theoretical approach based on the two-dimensional coupled diffusion-drift and Poisson equations with the multiple-trapping and releasing process clearly supports the trap-inhibited transport.

Influence of traps on transient electric field and mobility evaluation in organic field-effect transistors

A significant difference between the transient electric field profiles of the pentacene organic field-effect transistors (OFETs) with SiO2 and poly(methyl-methacrylate) (PMMA) insulators was found by the time-resolved microscopic optical second-harmonic generation (TRM-SHG) experiment. The profile of former device was broad and changed smoothly, while the latter one had a sharp peak. Particularly, the peak of the transient electric field in SiO2-insulated devices moved much faster than that in the PMMA-insulated one. Based on several experimental evidences and computational simulations, we proposed that these differences might arise from a higher trapped carrier density in the conductive channel on the PMMA insulator. Simple approaches were developed to evaluate the trap density and define dynamic carrier mobility in terms of the transient electric field measured by the TRM-SHG technique. This mobility quantitatively depicts that the transient hole transport in the OFET with the PMMA insulator is trap controlled.

Transient charge accumulation in pentacene field effect transistor with silver electrode

Time-resolved microscopic optical second harmonic generation (TRM-SHG) imaging was employed to study a transient charge accumulation in top-contact pentacene field effect transistor (FET) with Ag electrodes. It was demonstrated that the SHG signal at the edge of the Ag electrode decayed but remained in a steady state depending on biasing condition. An electric field formed in pentacene layer below Ag electrode activates the SHG, indicating the insufficient accumulation of injected carriers in the FET channel. By using the TRM-SHG technique transient change of the carrier density in the OFET is obtained.

Studying Transient Carrier Behaviors in Pentacene Field Effect Transistors Using Visualized Electric Field Migration

Time-resolved microscopic optical second harmonic generation (TRM-SHG) imaging was used to study the transient carrier behaviors in pentacene field effect transistor (FET) with Au and Ag electrodes on the basis of the migration of the electric field profile. We found that the time evolution of the electric field distribution in the pentacene FET channel was quite different between the devices using Ag and Au electrodes: enhanced SHG signal representing insufficient carrier accumulation decayed rapidly at the edge of the Au source electrode, while the signal decayed but remained at the edge of the Ag electrode. Interestingly, the peaks of the SHG signal depicting the edge of the carrier sheet in the channel region in both cases migrated in a way of the square root of time. A simple model was proposed to uniformly account for the experimental observations, and a numerical simulation on the transient carrier behaviors with the electric field migration using two-dimensional coupled diffusion-drift and the Poi...

Potential Drop at Electrode Contact of Organic Field-Effect Transistors Evaluated by Optical Second Harmonic Generation

Time-resolved microscopic optical second harmonic generation (TRM-SHG) imaging measurement revealed quantitatively the potential drop at the electrode contact of pentacene field effect transistors (FET). An activation of the SH signal at the edge of Ag-source electrode indicates the presence of large potential drop at pentacene-Ag contact during device operation, whereas negligible potential drop was observed at pentacene-Au contact. These findings agree with the injection characteristics of electrodes owing to the relationship between the work function of the metal and the HOMO level of pentacene.

Analysis of the Contact Resistance Effect on Pentacene FET Characteristics and Optical Second Harmonic Generation Measurements

mobility, eff , due to an increase in the response time required for charge accumulation at the interface. We show that the contact resistance Rc can be determined using the time-resolved microscopic optical second harmonic generation (TRM-SHG) measurement in combination with the typical FET characteristics because the TRM-SHG measurement directly probes carrier motion in the FET channel. With these results we discuss the carrier injection, transport and accumulation mechanisms in detail.

Diffusionlike electric-field migration in the channel of organic field-effect transistors

Time-resolved microscopic optical second-harmonic generation (TRM-SHG) technique provides a different approach to study the dynamics of carriers in organic field-effect transistor (OFET). Different from many common methods, the TRM SHG directly probes the transient electric-field distribution in the channel of the OFET in high temporal and spatial resolutions. In this work we used this technique to quantitatively study migration of the peak of the electric field. We found that under a broad range of experimental conditions, the migration of the peak of the electric field follows a diffusionlike behavior; namely, the square of the peak position of $\overline{x}$ is proportional to time $t$. Based on a two-dimensional computational simulation and general carrier transport mechanism, we proposed that this behavior arises from a simple relation, $\ensuremath{\mu}\ensuremath{\simeq}\ensuremath{\gamma}{\overline{x}}^{2}/(t|{V}_{\text{gs}}|)$, where $\ensuremath{\mu}$ is the carrier mobility, $\ensuremath{\gamma}$ is constant about 0.5, and ${V}_{\text{gs}}$ is the gate voltage with respect to the source voltage. The experimental and simulation data well supported this relationship.

Direct imaging of carrier motion in organic transistors by optical second-harmonic generation

Interest in the dynamic behaviour of carriers in organic materials is motivated by possible applications that include organic thin-film transistors, organic electro-luminescent devices and organic photoconductors. It can also provide an insight into the modelling of carrier transport and trapping in organic semiconductors and insulators. Here, we use an advanced second-harmonic generation technique to probe and visualize real carrier motion in organic materials. This is a time-resolved microscopic optical second-harmonic generation technique that allows direct and selective probing of dynamic carrier motion in organic materials. Experiments making use of this technique and using pentacene field-effect transistors have revealed dynamic changes of second-harmonic-generation intensity profiles arising from pentacene. Carrier velocity in organic solids is thus determined from the visualized carrier motion. We anticipate that this direct visualization technique will find wide application in the illustration of space-charge field formation in organic and inorganic materials, including biomaterials and polymers.