Bottom-contact Organic Thin-film Transistors Research Articles

Organic Thin-Film Transistors with Transfer-Printed Au Electrodes on Flexible Substrates

Bottom-gate, bottom-contact organic thin-film transistors (OTFTs) based on 6,13-bis(tri-isopropylsilylethynyl) pentacene (TIPS-pentacene) were fabricated on flexible substrates by transfer-printing and solution-based processes and their performances were characterized. Au gate and source/drain electrodes were transfer-printed from a release-layer-coated flexible mold at 2.4 MPa and 120 °C in ambient air. A polymer gate dielectric of poly(4-vinyl phenol) (PVP) and an active layer of TIPS-pentacene were deposited by solution processing. In addition, the TFT characteristics in channels of 40 and 4 µm length were compared. Also, the lack of saturation of the drain–source current (IDS) was investigated by considering the output characteristics in a short channel.

A study on the carrier injection mechanism of the bottom-contact pentacene thin film transistor

For an analysis of the mechanism of carrier injection in the structure of bottom contact organic thin-film transistor (OTFT), Al blocking layer was applied to the source/drain electrode in variety of ways in the fabrication of bottom contact OTFT. From the comparison of the transfer characteristics of the OTFTs with different electrodes, it is obvious that the main direction of carrier injection is mainly dependent on the thickness of electrode. When the electrodes become thicker and thicker, the main carrier injection path is expected to be located at the side of the electrodes as well as the top of the electrodes.

Origin of characteristics differences between top and bottom contact organic thin film transistors

The differences in drain current and drain voltage (Id-Vd) characteristics of top and bottom contact organic thin film transistors (OTFTs) are analyzed by an OTFT devices simulator, which makes it possible to derive Id-Vd characteristics, potential distribution, and hole concentration distribution by solving Poisson’s equation and current continuity equation. It is found that the intrinsic characteristics of top contact devices are superior to those of the bottom contact ones, which is usually believed to be due to poor contact characteristics and poor semiconductor quality of bottom contact OTFTs. The mechanism behind the intrinsic characteristics differences is the deficiency of carriers at the source-channel interface, resulting to a very high potential drop, which the bottom contact devices suffer more. Remarkable improvements in drain current are expected by only inserting high carrier concentration region around the source/drain contact area, which totally eliminates the potential drop.

Characteristics of Inverters Using Pentacene Organic Thin Film Transistors with Printed Ag Electrodes

Inverters were fabricated with pentacene organic thin film transistors (OTFTs) using inkjet printing method. The gate, source and drain electrodes were printed with Ag ink. The mobility and threshold voltage of the bottom-contact OTFT were 0.078 cm2/Vs and 2.8 V, respectively. Printed inverters operated well in 200 Hz at VDD = −20 V.

Ambient induced degradation and chemically activated recovery in copper phthalocyanine thin film transistors

The electrical degradation (aging) of copper phthalocyanine (CuPc) organic thin film transistors (OTFTs) was investigated. Thick (1000 ML) and ultrathin (4 ML) channel thicknesses were used in bottom contact OTFTs to correlate the electrical effects of aging with film microstructure. Proper TFT saturation behavior was unattainable in thick devices subject to ambient aging; however ultrathin devices were significantly less susceptible and maintained good saturation and subthreshold behavior. Therefore 1000 monolayer (ML) CuPc OTFTs were characterized in ambient air, clean dry air, clean humidified air, and NOx environments to isolate the ambient components that induce aging. Thick channel devices which had been aged in ambient air to the point of losing all saturation behavior could be restored to proper saturation behavior by exposure to clean humidified air. The data are consistent with aging resulting primarily from adsorption of strong oxidants from ambient air within the grain boundaries of the CuPc films.

Comparative analysis of unity gain frequency of top and bottom-contact organic thin film transistors

This work shows that placement of source/drain contacts on top of organic semiconductor film in top-contact organic thin film transistors (OTFT) can offer significant improvement in device speed as compared to bottom-contact devices. 2D numerical simulations and analytical models for unity gain frequency are used to quantify the differences between the top and bottom-contact devices. It is shown that while transconductance is lower in top-contact devices, the gate–drain capacitance is significantly lower as well resulting in improved frequency performance. It is also shown that there is an optimum semiconductor film thickness at which maximum unity gain frequency in top-contact OTFT is obtained.

Effect of O2-Plasma Treatment of Mo on the Crystal Growth Mode of Pentacene of Organic Thin-Film Transistors

We report the change in the crystallinity of pentacene film on -plasma-treated molybdenum (Mo) and the electrical properties of pentacene thin-film transistors using Mo as source–drain electrodes. The surface energy of the Mo film was increased with -plasma treatment. Synchrotron radiation photoemission spectroscopy results revealed that the valence band split from the metallic Fermi level as the surface of the Mo was treated with plasma. This provides evidence that the surface of the Mo film changed to Mo oxide . Grazing-angle incidence X-ray diffraction results showed that pentacene molecules on -plasma-treated Mo has well-aligned features in the vertical direction as well as in the lateral direction, leading to a high crystalline film growth. The growth mode of pentacene changed from a tilted bulklike structure to a thin-film phase type by the treatment. This change in the growth mode is attributed to the reduction in interaction energy between pentacene and Mo via the formation of oxide at the Mo/pentacene interface. Secondary electron emission spectra showed that the work function increased by 0.55 eV after -plasma treatment on the Mo surface. Thus, the Mo oxides lowered the potential barrier for the hole injection from Mo to pentacene, increasing the drain current of bottom-contact organic thin-film transistors.

Electrical stability of inkjet-patterned organic complementary inverters measured in ambient conditions

Complementary organic inverters were fabricated by inkjet patterning of both the metal contacts and the semiconductors. Bottom-gate, bottom-contact organic thin-film transistors with Ta2O5-polymer bilayer dielectrics, inkjet-printed silver electrodes, and inkjet-printed organic semiconductors exhibit hole and electron mobilities as high as ∼10−2 cm2/V s. Complementary inverters based on these transistors operate in ambient and exhibit a gain of −4.4 with supply voltage VDD=+20 V and −3 dB cutoff at 100 kHz with a load of 0.02 pF. The electrical stability of the inverters was evaluated for analog and digital operation, and a noise margin ≥1.1 V at VDD=+15 V was measured with bias-stress effects included.

Laser forward transfer of silver electrodes for organic thin-film transistors

Organic thin-film transistors (OTFTs) with top- and bottom-contact configurations were fabricated using silver nano-inks printed by laser forward transfer for the gate and source/drain electrodes with pentacene and poly-4-vinylphenol as the organic semiconductor and dielectric layers, respectively. The volume of the laser-printed Ag pixels was typically in the subpicoliter (0.2–0.4 pl) range. The top-contact OTFTs resulted in lower contact resistance compared to those obtained from the bottom-contact OTFTs, and showed improved overall device performance. The top-contact OTFTs exhibited field-effect mobilities of ∼0.16 cm2 V−1 s−1 and on/off current ratios of ∼105.

An analysis of the difference in behavior of top and bottom contact organic thin film transistors using device simulation

This paper presents a systematic analysis of an already reported phenomenon, namely, the difference in device performance of top and bottom contact organic thin film transistors (OTFT) by combining experiments and two-dimensional device simulations. The mobility of the as measured devices in the bottom contact OTFT is found to be lower by two orders of magnitude than the top contact structure, which is generally attributed to the higher metal-semiconductor contact resistance in the bottom contact devices due to lower contact area. However, we found that this large mobility difference exists even after correcting for the metal-semiconductor contact resistance through transfer line method (TLM). This result suggests that structural differences are playing a dominant role in lowering down the performance of bottom contact devices. This effect is then systematically investigated through two-dimensional physics-based numerical simulations by considering several structural inhomogenities around the contacts. The main reason for such an occurrence is attributed to the poor morphology (or comparatively low mobility) of pentacene films around the source/drain electrodes in the bottom contact devices. Finally, we also show a reasonable match between the simulated and experimental device characteristics, enabling calibration of the simulator for further use in design of OTFTs.

Bottom-Contact n-Channel Organic Thin-Film Transistors with Naphthalene-Based Derivatives

We investigated bottom-contact organic thin-film transistors based on n-benzyl naphthalene 1,4,5,8-tetracarboxylic diimides. The electrical characteristics were tested in both air and vacuum environments. n-Type semiconductors which are modified by fluorinated imide can be operated in ambient environment. The close packing of the fluorinated ester imide group leads to carrier mobility as high as 1.6 X 10 -2 cm 2 V -1 s -1 . The short distance between each molecule can be found via a single-crystal structure.

Displacement current in bottom-contact organic thin-film transistor

We have used the Shockley–Ramo theorem to calculate the displacement current of drifting charge carriers in bottom-contact organic thin-film transistors (OTFTs). The displacement current is quantified in terms of the weighting field of the sensing electrode. We demonstrated that for the transistors, with dielectric thickness below 100 nm and channel length above 1 µm, the weighting field near the sensing electrode exponentially decays with the distance from the electrode. This implies that the charge transport near metal contacts can be studied with a measurement of displacement current. The presented model approximation simplifies interpretation of, for example, transient photocurrent measurements and optical de-trapping measurements in OTFTs.

Performance Improvement and Application of Organic Thin-film Transistor

The interfaces, organic semiconductor/gate insulator and organic semiconductor/ source-drain (S-D) electrodes, in organic thin-film transistors (TFTs) are crucial for the improvement of transistor characteristics. We describe (1) the improvement of mobility and lowering driving voltage by treating the gate-insulator surface with self-assembled monolayers, and (2) the reduction of contact resistance by employing a carrier injection layer for the S-D electrodes in bottom-contact organic TFT.

Stability of polymeric thin film transistors for x-ray imaging applications

The effects of x-ray irradiation on the electrical performance of organic thin film transistors (OTFTs) have been examined up to equivalent lifetime doses for medical digital x-ray imaging backplanes. Transistor performance metrics have been assessed before and after exposures up to 500Gy for bottom-gate, bottom-contact OTFTs of p-channel polytriarylamine and polyfluorene semiconductors. There is no discernible degradation in device performance for either material and no indication of the creation of additional trap states. In situ measurements indicate that x-ray exposure tends to reverse undesirable shifts in the threshold voltage due to prolonged atmospheric exposure and bias stress.

Enhanced performance of solution-processed regioregular poly(3-hexylthiophene) thin-film transistors using planar bottom-contact architecture

The authors report on the solution-processed planar bottom-contact (pBC) organic thin-film transistors and contact effect on gate threshold voltage incorporating regioregular poly(3-hexylthiophene) active layer. By employing pBC configuration, the transistors on SiO2∕Si without surface modification show much higher mobility, lower threshold voltage, and narrower dispersion of threshold voltage when compared to the conventional bottom-contact counterparts. The high mobility and lower threshold voltage are attributed to an improved contact at the interface between the source/drain electrodes and the poly(3-hexylthiophene) active layer.

Thickness dependence of mobility of pentacene planar bottom-contact organic thin-film transistors

We have fabricated planar bottom-contact organic thin-film transistors as a function of the thickness of the pentacene active layer. The highest mobility of the planar bottom-contact transistors is 0.47 cm 2/Vs with only a 7 nm pentacene active layer. Our planar bottom-contact transistors show much higher mobility than conventional bottom-contact counterparts and even higher than the reported mobility values of top-contact counterparts for each thickness in the range from 2.5 to 10 nm. We find that spike at the edges of source and drain electrodes seriously deteriorates device performance.

High‐Performance Bottom‐Contact Organic Thin‐Film Transistors with Controlled Molecule‐Crystal/Electrode Interface

An improvement of the performance of bottom-contact organic thin-film transistors is demonstrated by embedding and planarizing the source/drain electrodes in a gate dielectric. The electric contact with the pentacene active layer is superior to conventional electrode configurations because of the favorable growth of pentacene grains adjacent to the source/drain electrode edges: these can be seen in the figure.

Perfluorocyclobutane containing polymeric gate dielectric for organic thin film transistors with high on/off ratio

A thermally cross-linkable perfluorocyclobutane (PFCB) polymer was synthesized and examined for use as the gate dielectric in organic thin film transistors (OTFTs). The PFCB polymer showed good solvent and process resistance during the photolithographic patterning of the electrodes. Bottom contact OTFTs were fabricated with poly(3-hexylthiophene)-2,5-diyl (P3HT) as the semiconductor, either spin cast or dip coated, on the PFCB gate dielectric. OTFTs fabricated with dip-coated P3HT showed a field effect mobility of 1.8×10−3cm2∕Vs and a high on/off current ratio of 8.3×106 in the saturation regime, with a source-drain voltage of VD=−50V.

An experimental study of contact effects in organic thin film transistors

We report on parasitic contact effects in organic thin film transistors (OTFTs) fabricated with pentacene films. The influence on the OTFT performance of the source and drain contact metal and the device design was investigated. Top contact (TC) and bottom contact (BC) gated transmission line model (gated-TLM) test structures were used to extract the combined parasitic contact resistance as a function of gate voltage swing and drain-source voltage for OTFTs with gold source and drain contacts. For comparison BC test structures with palladium contacts were studied. Differences in the bias dependence of the contact resistance for TC and BC OTFTs indicate that charge injection and device performance are strongly affected by the device design and processing. The results from this investigation show that TC and BC device performances may be contact limited for high mobility OTFTs with channel lengths less than 10μm.

Hysteresis mechanism and reduction method in the bottom-contact pentacene thin-film transistors with cross-linked poly(vinyl alcohol) gate insulator

The origin of the hysteresis phenomenon in bottom-contact pentacene organic thin-film transistors (OTFTs) with cross-linked poly(vinyl alcohol) (PVA) insulator is studied. From electrical measurements with various sweep ranges and two different sweep directions, the hysteresis effect is presumed to be caused by the electrons or holes that could be injected from the gate and trapped in the PVA bulk, rather than by the polarization or internally existing mobile ions. The assumption is confirmed by the clear reduction of hysteresis in OTFTs with a blocking oxide layer between gate and PVA insulator.