Polycrystalline Pentacene Thin-film Transistors Research Articles

A study on the effect of film crystallinity and morphology on charge carrier concentration-dependent hole mobility in pentacene thin-film transistors: advantages of high deposition rate

The combined effect of deposition rate and substrate temperature on the film crystallinity, morphology, and electronic properties of pentacene is studied. It is shown that the channel mobility in polycrystalline pentacene thin-film transistors is relatively immune to substrate temperature, and the films offer good hole mobility when grown at a high rate. This is advantageous when high throughput with low deviation in electrical parameters over devices are required. The surface morphology is characterized by atomic force microscopy measurements and the crystallinity is studied using x-ray diffraction. The effect of growth parameters on the crystalline phases of pentacene is correlated to the charge carrier transport. It is found that the field-effect mobility is primarily affected by the crystalline phases of the film rather than the grain size. The charge carrier dependence of the hole mobility is used to parameterize the dispersion (width) in the density of states (DOS) of the highest occupied molecular orbital of the films in the transistor channel region. It is found that the presence of multiple phases in the path of the charge carrier flow increases the dispersion of the DOS.

Microscopic gate-modulation imaging of charge and field distribution in polycrystalline organic transistors

In this work, a high-resolution microscopic gate-modulation imaging (μ-GMI) technique is successfully developed to visualize inhomogeneous charge and electric field distributions in operating organic thin-film transistors (TFTs). We conduct highly sensitive and diffraction-limit gate-modulation sensing for acquiring difference images of semiconducting channels between at gate-on and gate-off states that are biased at an alternate frequency of 15 Hz. As a result, we observe unexpectedly inhomogeneous distribution of positive and negative local gate-modulation (GM) signals at a probe photon energy of 1.85 eV in polycrystalline pentacene TFTs. Spectroscopic analyses based on a series of μ-GMI at various photon energies reveal that two distinct effects appear, simultaneously, within the polycrystalline pentacene channel layers: Negative GM signals at 1.85 eV originate from the second-derivative-like GM spectrum which is caused by the effect of charge accumulation, whereas positive GM signals originate from the first-derivative-like GM spectrum caused by the effect of leaked gate fields. Comparisons with polycrystalline morphologies indicate that grain centers are predominated by areas with high leaked gate fields due to the low charge density, whereas grain edges are predominantly high-charge-density areas with a certain spatial extension as associated with the concentrated carrier traps. Consequently, it is reasonably understood that larger grains lead to higher device mobility, but with greater inhomogeneity in charge distribution. These findings provide a clue to understand and improve device characteristics of polycrystalline TFTs.

Co-Actions of Ambient Pressure and Gas Molecular Adsorption on the Carriers’ Transport in Polycrystalline Pentacene Thin-Film Transistors

Organic transistors have proved to have potential applications in pressure sensors. However, few reports consider the coactions of pressure and ambient gas adsorption on the characteristics of the sensitive transistors. In this article, pentacene polycrystalline thin films were fabricated as the active layer of organic transistors, and the effects of ambient pressure and the gas adsorption on the carriers’ transport characteristics have been investigated. It was found that during the process from one atmosphere to vacuum (∼5 × 10–3 Pa) the device output, saturation source-drain currents (IDS), changed with pressure not monotonously but with an unexpected reversible minimum peak. Considering the variation of gas adsorption quantity and the distance between pentacene grains with pressure, we established models to understand the nature of the pressure sensitivity. We found that in low pressures the adsorption of gas molecules in grain boundaries was the main factor that affects device performance, whereas in high pressures, the shortening of the distance between pentacene grains was the main factor. Our research will benefit the understanding of charge-transport nature and, more importantly, give some instructions on using and designing highly sensitive pressure sensors.

Charge Transport Model Based on Single-Layered Grains and Grain Boundaries for Polycrystalline Pentacene Thin-Film Transistors

The charge transport process in organic thin-film transistors (OTFTs) has always been under intensive research while the mechanism is still controversial, even in the small-molecule OTFTs. In this study, we report on a transport model for interpreting the charge transport mechanism in polycrystalline pentacene TFTs. By investigating the growth mode of pentacene films and the distribution of charge carriers in the channel of OTFTs, a transport model involving two-dimensional single-layered grains and grain boundaries (GBs), namely the two-dimensional grain boundary (TDGB) model, is proposed for describing the charge transport process in polycrystalline pentacene TFTs. An analytical expression for the field-effect mobility could be obtained from the model. The model is applied to explain the mobility dependence on the grain sizes at the first monolayer of pentacene films, temperature and on gate voltage in polycrystalline pentacene TFTs, with good agreement obtained. In addition, the values of some crucial parameters are given by the simulations based on the model.

Spectroscopic Characterization of Charged Defects in Polycrystalline Pentacene by Time‐ and Wavelength‐Resolved Electric Force Microscopy

Spatial maps of topography and trapped charge are acquired for polycrystalline pentacene thin-film transistors using electric and atomic force microscopy. In regions of trapped charge, the rate of trap clearing is studied as a function of the wavelength of incident radiation.

A compact model for polycrystalline pentacene thin-film transistor

A compact physical model for the operation of polycrystalline pentacene thin-film transistors is presented here, valid in both linear and saturation regimes by including the Gaussian energy distribution of the grain-boundary traps. The effect of the temperature is also taken into account. The proposed model has been verified by comparing the simulated results with the experimental data.

Influence of gap states on the electrical stability of pentacene thin film transistors

The influence of oxygen induced gap states on the device operation and the stability of polycrystalline pentacene thin film transistors (TFTs) was investigated by electrical in situ measurements. Unexposed devices are stable, whereas devices exposed to dry oxygen exhibit a shift of the threshold voltage upon prolonged device operation. Stressing the transistor in the on-state (negative bias) leads to a shift of the threshold voltage towards negative gate voltages, whereas prolonged bias stress in the off-state causes a shift of the threshold voltage in the opposite direction. The gap states are formed 0.18eV (acceptor-like states) and 0.62eV (donor-like states) above the valence band maxima. The charge carrier mobility and the on/off ratio of the transistor are not affected by the gap states. A simple density-of-states model will be presented, which allows for the explanation of the experimental results.

Electrical stability of pentacene thin film transistors

The influence of environmental conditions on the device operation and the stability of polycrystalline pentacene thin film transistors (TFTs) were investigated. Electrical in-situ and ex-situ measurements of staggered pentacene TFTs were carried out to study the influence of dry oxygen and moisture on the device stability. The transistors were fabricated by organic molecular beam deposition on thermal oxide dielectrics. Oxygen exposure of the pentacene films lead to the creation of acceptor-like states in the bandgap. The acceptor-like states cause a shift of the onset of the drain current towards positive gate voltages. The charge carrier mobility and the on/off ratio of the transistor are not affected by the acceptor-like states. Furthermore, the acceptor-like states have an influence on the stability of the TFTs. Devices exposed to oxygen exhibit a shift of the threshold voltage upon prolonged biasing. Transistors characterized under vacuum conditions (no oxygen exposure) do not exhibit a shift of the threshold voltage (bias stress effect) as a consequence of prolonged biasing. The experimental results show a clear correlation between the device behavior upon oxygen exposure and the stability of the devices. The shift of the onset voltage upon oxygen exposure correlates with the shift of the threshold voltage upon prolonged bias. The influence of dry oxygen on the onset voltage, the threshold voltage, and the electrical stability will be described. Furthermore, the influence of bias stress on the operation of organic circuits like an active matrix addressed OLED displays will be discussed.

Influence of molecular structure and microstructure on device performance of polycrystalline pentacene thin-film transistors

The authors have fabricated the pentacene thin films on polymethylmethacrylate (PMMA) and on silicon dioxide dielectric surfaces featuring similar surface energy and surface roughness. On both surfaces the pentacene films displayed high crystal quality from x-ray diffraction scans, although the film on PMMA had significantly smaller grain size. The pentacene transistors with PMMA exhibited excellent electrical characteristics, including high mobility of above 1.1cm2∕Vs, on/off ratio above 106, and sharp subthreshold slope below 1V∕decade. The analysis of molecular microstructure of the pentacene films provided a reasonable explanation for the high performance using resonance micro-Raman spectroscopy.

Temperature dependence of Hall effects in organic thin-film transistors on plastic films

The authors investigated the temperature dependence of Hall effects on polycrystalline pentacene thin-film transistors manufactured on plastic base films. The temperature dependence of the charge amounts evaluated from the Hall effect measurements is systematically compared with that evaluated from capacitance-voltage (CV) measurements. Our result indicates that the conduction carriers are dominated by hopping between polycrystalline grain boundaries below 260K and tend to be delocalized above 260K.

Environmental stability and electronic transport of pentacene thin film transistors

The influence of environmental conditions on the electronic structure and the device stability of polycrystalline pentacene transistors were investigated. Electrical in situ measurements of pentacene TFTs were carried out to study the influence of dry oxygen on the device operation. The polycrystalline pentacene thin film transistors were fabricated by organic molecular beam deposition (OMBD) on thermal oxide dielectrics. Exposing the pentacene film to oxygen leads to the creation of acceptor-like states deep in the bandgap. The acceptor-like states cause a reduction of the subthreshold slope and a shift of the onset of the drain current to positive gate voltages. The threshold voltage and the mobility of the transistors are not affected by the dry oxygen. A simple model will be described which directly correlates the onset voltage of the transistors with the acceptor concentration in the pentacene film.

Ambipolar Carrier Transport in Polycrystalline Pentacene Thin-Film Transistors

We have investigated the dependence of polycrystalline pentacene thin-film transistors (TFTs) characteristics on work function of source-drain contact metals. The pentacene TFTs using Mg, Al, Ag, and Au source-drain electrodes showed only p-type characteristics and the field-effect hole mobilities were strongly dependent on their work function. On the other hand, the pentacene TFTs using Ca source-drain electrodes showed typical ambipolar characteristics. The field-effect hole mobility of 4.5 × 10−4 cm2/Vs and field-effect electron mobility of 2.7 × 10−5 cm2/Vs were estimated from saturation currents. Appearance of an electron enhancement mode in pentacene TFTs was ascribed to the lowering of barrier for electron injection at source electrodes.

Influence of organic gate dielectrics on the performance of pentacene thin film transistors

The electronic transport of polycrystalline pentacene thin film transistors is investigated. The thermally evaporated pentacene films prepared on organic dielectrics like benzocyclobutane exhibit mobilities comparable with inorganic dielectrics like thermal oxide and plasma enhanced chemical vapor deposited silicon nitride. To gain insights in the electronic transport behavior of the organic thin film transistors (TFTs) the I/ V characteristics were simulated by a one-dimensional density-of-states transistor model. The experimental data can be described by using a broad distribution of acceptor-like states deep in the bandgap and a narrow distribution of donor-like states close to the valance band. The influence of the different dielectrics on the defect distribution and the electronic transport will be discussed.

Pentacene thin film transistors on large area compatible gate dielectrics

The properties of polycrystalline pentacene thin film transistors (TFT) are reported, showing the influence of the deposition conditions with different inorganic and organic dielectrics. Dielectrics compatible with large area fabrication were explored to facilitate low cost electronics on glass or flexible plastic substrates. Transistors were realized on plasma-enhanced chemical vapor deposited (PECVD) silicon nitride, silicon oxide and spin casted poly vinyl phenol (PVP). The physical and chemical properties of the dielectric have a distinct influence on the morphology and the structural properties of the pentacene films. Polycrystalline films with large crystals and high structural order can be grown on smooth dielectrics. For rougher dielectrics the crystal size is reduced and the films exhibit no structural order. Furthermore, the growth is affected by the surface wetting properties of the dielectric. The surface chemistry can be controlled by a treatment of the dielectric with self-assembled monolayers (SAMS). The impact of the different dielectrics on the device parameters like mobility and subthreshold slope will be discussed.

Morphology and electronic transport of polycrystalline pentacene thin-film transistors

Temperature-dependent measurements of thin-film transistors were performed to gain insight in the electronic transport of polycrystalline pentacene. Devices were fabricated with plasma-enhanced chemical vapor deposited silicon nitride gate dielectrics. The influence of the dielectric roughness and the deposition temperature of the thermally evaporated pentacene films were studied. Although films on rougher gate dielectrics and films prepared at low deposition temperatures exhibit similar grain size, the electronic properties are different. Increasing the dielectric roughness reduces the free carrier mobility, while low substrate temperature leads to more and deeper hole traps.

Charge Transport in Pentacene Thin Film Transistors

ABSTRACTThe charge transport properties in polycrystalline pentacene thin film transistors is investigated. A potential barrier is formed at grain boundaries due charged trapping states. The influence of such grain boundaries on the hole mobility of the devices is analyzed for different grain sizes, trap concentrations, and carrier densities. The results reveal that room temperature mobilities exceeding 0.5 cm2/Vs can be obtained in thin films with large grains as well as in nanocrystalline material. Consequently, single crystal device limits can be reached also by polycrystalline pentacene thin film transistors.