Pentacene OTFTs Research Articles

Quasi-Static Capacitance-Voltage Characterizations of Mobile Ion Accumulation and Depletion Phenomena in Organic Thin-Film Transistors

By conducting C-V profile measurement, we analyzed the charge transport behavior in polycrystalline pentacene OTFTs. We found that carrier accumulation and depletion processes in the semiconductor channel layer is caused by a trap and de-trap mechanism. We apply such processes to explain C-V profiles at various gate voltages, ramp rates and sweep directions. At faster ramp rates, hysteresis can be observed from C-V profile. The phenomenon can be attributed to the asymmetry of the relaxation time of carrier trap and detrap mechanism. At the slower ramp rate, most carriers are able to interact with the gate stress, so the hysteresis disappears. And beyond this ramp rate, the corresponding threshold voltages are then kept at constant values, despite the existence of traps. Furthermore, low-mobility mobile ions or impurities participate in the charge accumulation and depletion when the ramp rate is much slower, which results in a skew of C-V profiles.

Quasi-static capacitance–voltage characterizations of carrier accumulation and depletion phenomena in pentacene thin film transistors

We analyze charge accumulation and depletion behaviors from C–V curves at various gate voltages, ramp rates and sweep directions in pentacene OTFTs. The polycrystalline property of the pentacene layer has lead to carrier charge and discharge processes in the channel layer by a trap-controlled transport mechanism. We apply such processes to describe C–V profiles at various ramp rates. Hysteresis can be observed from C–V curves at faster ramp rates. The phenomenon can be attributed to the difference of the relaxation time of carrier trap and detrap processes. As the ramp rate becomes slower, hysteresis disappears since most carriers are able to interact with the gate stress. And beyond this ramp rate, the corresponding threshold voltages are then kept at constant values, despite the existence of traps. Furthermore, low-mobility carriers (mobile ions or impurities) participate in the charge accumulation and depletion when the ramp rate is even slower, which results in a skew of C–V profiles. Finally, we extend the C–V measurement to devices with different channel lengths.

63.3: High Resolution OTFT‐OLED on Plastic Substrate Using Self‐organized Process

AbstractA high resolution (2 inch, 80 dpi) organic thin‐film transistor driven light‐emitting diode has been developed on plastic substrate. The pentacene OTFT array was fabricated using self‐organized process. The TFT in the array exhibited an average field‐effect mobility of ∼1 cm2/Vs, an on/off current ratio of 107, and a threshold voltage of ∼ −8 V after the bank formation with parylene‐C / SiNx, which is good enough to drive OLEDs. An AMOLED was driven in the bent state of 100 mm radius.

Aging effects and electrical stability in pentacene thin film transistors

We have studied the transfer characteristic variations induced by aging effects and applied voltage in top contact pentacene thin film transistors (OTFTs) fabricated by using Polymethylmetacrylate buffer layer. The electrical stability of pentacene OTFTs was tested by applying prolonged bias stress (up to 10 4 s) with gate voltage V gstress = − 30 V and + 30 V. The environmental effects were analysed by measuring the degradation of electrical characteristics of OTFT exposed to air. The results have been analysed in terms of trap state model, evaluating the channel conductance using a one-dimensional approach. This allows us to correlate the transfer characteristics variations to changes in localised state distribution.

Improved carrier mobility for pentacene TFT by NH3 annealing of gate dielectric

The carrier mobility of pentacene OTFT is enhanced by annealing its gate dielectric (SiO2) in NH3. The device has a field-effect mobility of 0.53cm2/Vs, with on/off current ratio of 106, and subthreshold slope of 2.4V per decade. When compared with the control sample with N2-annealed SiO2 as gate dielectric, the mobility of the proposed pentacene OTFT is increased by over 50%. AFM micrographs show that the higher mobility should be due to the smoother gate-dielectric surface passivated by the NH3 annealing, and also larger pentacene grains grown on the smoother gate-dielectric surface.

Electrical Characteristics of Pentacene Thin Film Transistors in Volatile Compound Vapors

We have systematically investigated the real-time I DS responses and electrical characteristics of pentacene OTFTs exposed to six volatile compound vapor flows of acetone, ethanol, methanol, water, chloroform, and toluene, which could be classified into two group according to the influence on the electric performance of OTFTs. The first three compounds caused remarkable and irreversible degradation of electric performance of OTFTs. However, the latter three compounds caused slight and reversible changes of their electric performance. Moreover, the good linearity existed in plot of changes of the normalized I DS as a function of their concentrations in N2 flow.

High-performance OTFT and its application

In this paper we address the issues related with OTFTs on plastic substrates: (1) the growth of organic semiconductors by organic chemical vapor deposition; (2) organic insulator; (3) electrodes for gate, source/drain; (4) island formation of organic semiconductor; (5) improvement of OTFT performance; (6) Bias-stress effect and (7) passivation effect. We achieved a field-effect mobility of 1.8 cm2/V s, threshold voltage of −7.5 V and on/off current ratio of >108 for the pentacene OTFT on plastic. The OTFT can be applied to flexible displays and low-cost circuits on plastic.

10.2: Low-Voltage Pentacene OTFTs with High-K Gate Dielectric and its Application to AMOLED

We recently developed room-temperature deposited pyrochlore-structure Bi1.5Zn1.0Nb1.5O7 (BZN) insulator having the high dielectric constant (∍=50) and low leakage current. Using the high-K BZN film as a gate insulator, we fabricated pentacene OTFTs operating at low voltage. We fabricated 12 × 12 OTFT-driven OLED. We are integrating the OLED array with the low-voltage high-K pentacene OTFT.

10.1: Invited Paper: Electrical and Chemical Stabilities of Organic Thin-Film Transistors for Display Applications

We studied chemical and electrical stabilities for the pentacene OTFT fabricated on plastic with self-organized process. The degradation in on-current, threshold voltage and field-effect mobility of the OTFT under air exposure can be expressed as exponential form on time and it can be reduced by multilayer passivation on the organic semiconductor, which reduces the penetration of H12O and O2 into the pentacene. The threshold voltage degrades significantly after gate bias-stress, which can be greatly reduced by optimizing the organic gate insulator used for the TFT.

High mobility top-gated pentacene thin-film transistors

A common device geometry for measuring the electrical characteristics of organic semiconductors is the thin-film organic field-effect transistor (OTFT). Mostly for reasons of cost, convenience, and availability, this usually involves depositing the organic material on a prefabricated gate structure such as Si∕SiO2, the surface chemistry of which is often modified with self-assembled monolayers. The interactions between these surfaces and the deposited organic can have a profound effect on thin-film growth and the resulting electrical characteristics since most of the charge transport in these structures occurs near the organic-insulator interface. An alternative to this traditional technique is to assemble the transistor on top of the organic semiconductor. We have used chemical-vapor deposition of the polymeric dielectric material parylene to create pentacene OTFTs with gate electrodes both on top of and below the semiconductor film, with field-effect mobilities as high as 0.1cm2∕Vs and Ion∕Ioff ratios greater than 106 in the top-gated OTFTs. Comparing the electronic properties of top- and bottom-gate structures yields insight into the charge transport characteristics as well as the effects of device geometry, contacts, and surface roughness of the organic thin film.

3.2: Invited Paper: High-Performance OTFTs on Flexible Substrate

In this paper we address the issues related with OTFTs on plastic substrates: 1) the growth of organic semiconductors by organic chemical vapor deposition; 2) organic insulator; 3) electrodes for gate, source/drain; 4) island formation of organic semiconductor; 5) improvement of OTFT performance; 6) Bias-stress effect; and 7) passivation effect. We achieved a field-effect mobility of 1.8 cm2/Vs, threshold voltage of −7.5V and on/off current ratio of >108 for the pentacene OTFT on plastic

34.2: PMMA Buffer‐Layer Effects on Electrical Performance of Pentacene OTFTs with a Cross‐linked PVA Gate Insulator on a Flexible Substrate

AbstractIn this paper we proposed for the first time a technique of poly‐methylmethacrylate (PMMA) buffer layer insertion to overcome unsaturated output characteristics of OTFTs with cross‐linked poly‐vinylalcohol (PVA) gate insulators on a PET substrate. A 3:1 diluted PMMA buffer layer insertion resulted in saturated output characteristics and small shift of a threshold voltage in successive I‐V measurements for OTFTs due to the enhancement of surface hydrophobicity on the gate insulator. The electrical performances of a threshold voltage, a subthreshold slope and on‐off current ratio have been noticeably improved after PMMA buffer layers insertion. To our best knowledge, the highest mobility of 0.32 cm2/Vsec has been obtained among OTFTs fabricated with polymer gate insulators by spin coating processes on a PET substrate.

P‐7: Performance Improvement of Pentacene OTFT for Display Application

AbstractIn this paper we investigated the effects of various surface treatments such as the selective doping on the contact metals with self‐assembly and especially gate surface treatment with O2 plasma as well as organic molecules on the performance of pentacene‐based OTFTs. It has been found that the selective doping on metal contacts is a good way to improve the contact property. And the usage of OTS as the organic molecule for surface treatment of gate insulator is the most effective way to enhance the field effect mobility to 0.3cm2/V⋅sec and the on/off ratio to 106, which is good enough for display application.

Fabricaton Ofactive Devices and Logic Gates on Fibers

AbstractTextiles are a suitable substrate for large area, flexible and wearable electronics because of their excellent flexibility, mechanical properties and low cost manufacturability. The ability to fabricate active devices on fiber is a key step for achieving large area and flexible electronic structures. We fabricated transistors and inverters with a-Si film and pentacene film on Kapton film and cut them into fibers. The a-Si TFT showed a threshold voltage of 8.5 V and on/off ratio of 103 at a drain voltage of 10 V. These are similar to the characteristics of a TFT fabricated on a glass substrate at the same time. The maximum gain of the inverter with an enhancement n-type load was 6.45 at a drain voltage of 10 V. The pentacene OTFT showed a threshold voltage of -8 V and on/off ratio of 103 at a drain voltage of -30 V. The inverter with a depletion p-type load showed a voltage inversion but the inversion occurred at the wrong voltage. The antifuse was successfully programmed with a voltage pulse and also a current pulse. The resistance decreased from 10 GΩ to 2 kΩ after the programming.