All-Polymer Organic Field-Effect Transistors with Memory Element

K. A. Mohamad, A. Alias, I. Saad K. Uesugi, H. FukudaNanoelectronics Device & Materials Research Group, SEIT Division of Engineering for Composite Functions

Universiti Malaysia Sabah Muroran Institute of Technology88400 Kota Kinabalu, Sabah, MALAYSIA 27-1 Mizumoto, Muroran 050-8585 Hokkaido, JAPAN

Abstract-We introduce an hole-accepting layer on apoly(methyl methacrylate) (PMMA) dielectric to investigatethe reversible threshold voltage (Vth) shifts in all-polymer n-channel organic field-effect transistor (OFET) using an organicsemiconductor of an poly[N,N’-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5’-(2,2’-bithiophene) (P(NDI2OD-T2)). Top drain-source with abottom-gate contact structure device exhibited a unipolarproperty with n-channel behavior. Furthermore, the existenceof poly(3-hexylthiophene) (P3HT) films as a hole-accepting-likestorage layers resulted in a reversible Vth shifts upon theapplication of external gate bias (Vbias). Hence, all-polymerorganic transistor with the hole-accepting layer exhibited alarge memory window (∆Vth = 10.7 V) for write and eraseelectrically without major degradation in saturation mobility(μsat = 1.8~2.8×10-4 cm2 V-1 s-1).

Keywords-organic semiconductor; n-channel; organic field-effect transistor; nonvolatile memory

I. INTRODUCTION

Research on organic semiconductor materials for theirbroad range of applications in the electronic industry hasattracted scientific and technological interest. Organicsemiconductors have been the subject of interest for the pastfew years [1]. During this decade, the use of organicsemiconductors in electronic applications has seen aspectacular evolution which provides a unique opportunityto enable low-cost, flexibility, solution processable, andeasier fabrication procedure. However, organic materialsbecame more than a curiosity when it was recognized tohave excellent electronic properties compared withinorganic semiconductors. Recently, there has beenimpressive progress in the development of electronicdevices, particularly high performance organic field-effecttransistors (OFETs) which been utilized in sensors, light-emitting displays, memory elements, and integrated circuits[2-5]. In conventional silicon technology, research anddevelopment for silicon nonvolatile memory is fueled by theserious drawback in the physical limitation of the devicestructure toward nanosized transistors. Hence, this could bean alternative or complement technology to the conventionalsemiconductor technology in the micro- and nano-scaledevices.

II. BACKGROUND AND LITERATURE REVIEW

Despite major development in the OFETs based on p-channel organic semiconductors, there are still problemsconcerning the development of n-channel organic devicesdue to the selection of n-channel organic semiconductormaterials, which are limited to a very small number ofmolecules and polymers [6]. There are also seriousdrawbacks in the implementation of organic electronicdevices, including poor solubility, difficulty of synthesis and

unstable transistor operation under the ambient atmosphere[7,8]. However, since the first air-stable n-channel OFETsbased on naphthalenetetracarboxylic diimides was reported[9], a large number of n-channel organic semiconductorshave been based on either naphthalene diimide (NDI)- orperylene diimide (PDI)-based polymers for organictransistors [10-13]. Therefore, clearly n-channel organicsemiconductors have an important role in the continuingdevelopment of organic semiconductor-based circuits andelectronic products.

Initial attempts at nonvolatile organic memory werereported using p-channel OFET memory with chargestorage in polymer electret [14] and nanoparticle-embeddedgate dielectrics [15]. However, recently, nonvolatile n-channel organic memory using a block copolymer-nanoparticle hybrid system has been reported [16]. W. L.Leong et al. demonstrated programmable-erasableproperties with a large memory window (~9–11 V) using n-channel (perfluorinated copper phthalocyanine) OFETsmemories where in-situ synthesized gold (Au) nanoparticlein self-assembled polystyrene-block-poly-4-vinylpyridine(PS-b-P4VP) block copolymer nano-domains as chargestorage elements. Thus, the use of n-channel organicsemiconductors has become important for the continuousdevelopment of OFETs with memory element.

In this study, we demonstrate fabrication andcharacterization of solution processed all-polymer OFETswith heterojunction thin films of P(NDI2OD-T2) andpoly(3-hexylthiophene) (P3HT). Moreover, we also presentall-polymer organic transistor based on P(NDI2OD-T2) asan n-channel active layer with a hole-accepting layer onpoly(methyl methacrylate) (PMMA) organic dielectric formemory element.

III. DEVICE CONCEPT AND FABRICATION

A. Organic semiconductor solutionsThe poly[N,N’-bis(2-octyldodecyl)-naphthalene-1,4,5,8-

bis(dicarboximide)-2,6-diyl]-alt-5,5’-(2,2’-bithiophene)[P(NDI2OD-T2)] was purchased from Polyera Corporationunder the trade name of ActiveInk N2200. The P(NDI2OD-T2) is highly soluble in chlorinated solvents and for thiswork, P(NDI2OD-T2) was diluted in chloroform withoutfurther purification to form 0.5 wt% solution. The LUMO(lowest unoccupied molecular orbital) and HOMO (highestoccupied molecular orbital) energy levels of P(NDI2OD-T2)are approximately 4.0 eV and 5.6 eV below the vacuumlevel, respectively.

The P3HT with regioregularity of 98.5%, was purchasedfrom Rieke Metals. This conjugated polymer is one of themost widely used p-type semiconductors, especially inorganic solar cells. For this work, P3HT was diluted in

2012 Third International Conference on Intelligent Systems Modelling and Simulation

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DOI 10.1109/ISMS.2012.96

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2012 Third International Conference on Intelligent Systems Modelling and Simulation

978-0-7695-4668-1/12 $26.00 © 2012 IEEE

DOI 10.1109/ISMS.2012.96

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dichloromethane to form a 0.05 wt% solution. The HOMOand LUMO energy levels of P3HT are approximately 4.8 eVand 3.0 eV below the vacuum level, respectively.

B. Fabrication of organic field-effect transistorsA (100)-oriented B-doped p+-type si wafer (< 0.1 Ω cm)

was used as the substrate and gate contact. The substrateswere rinsed ultrasonically in deionized water, ethyl alcohol,acetone, and then methyl alcohol, and cleaned by thestandard RCA cleaning procedure. Poly(methylmethacrylate) (PMMA) with molecular weight of 120 000,was purchased from Sigma Aldrich and was diluted in ethylacetate with a concentration of 80 mg/ml without furtherpurification and then spin-coated directly onto the siliconsubstrate to form an organic dielectric layer. P3HT film wasdeposited by spin-coating at 5000 rpm onto the PMMAdielectric layer to form the charge trapping layer, followedby spin coating of P(NDI2OD-T2) in chloroform solution at5000 rpm to form the n-channel active semiconducting layer.0.1 ml of each solution was filtered through a 0.2 μmsyringe filter prior to deposition. Finally, a gold (Au) filmwas deposited through a designated shadow mask in avacuum chamber for the top source and drain contacts. Thechannel dimension of these devices was W/L = 2.5 mm/50μm. The schematic structure of the bottom gate with topdrain-source contact structure n-channel OFETs is shown inFig. 1. The electrical measurements of the devices weremeasured in a probe station with a computer-controlledautomatic electrical analyzer at room temperature in thedark under the ambient atmosphere.

Figure 1. Schematic diagram of OFETs device structure.

IV. DEVICE CHARACTERIZATION RESULTS

A. Output characteristicsFigure 2 shows representative output characteristics of

OFETs with P3HT film on the highly doped p-type siliconsubstrate, respectively. The device shows a typical outputcurve of a field-effect transistor (FET) and undoubtedlyindicates that only electrons are accumulated at thesemiconductor-dielectric interface and the current flow fromthe source to the drain through the channel region whenpositive gate voltages (Vg) are applied. Therefore, the OFETis functioned in the n-channel operation in accumulationmode with increasing positive drain current (Vds). A bottomsource-drain with a bottom-gate structure ambipolar organicfield-effect transistors (OFETs) based on a bulkheterojunction layer of P(NDI2OD-T2) and P3HT havebeen reported [17], in which the p-channel and n-channelsemiconductor layers have to be deposited using orthogonalsolvents in order to prevent mixing of the layers. In fact, thetop contact structure device based on heterojunction thinfilms did not interfere with the nature of charge transport, as

the devices were found to behave as n-channel operationmode similarly to the unipolar P(NDI2OD-T2)-OFETs [10].However, a partial re-dissolution of the deposited P3HTlayer occurs at the P(NDI2OD-T2)-P3HT interface upon thespin-coating of P(NDI2OD-T2) solution. Thus, the presenceof very thin P3HT domains in P(NDI2OD-T2) could slowdown the holes, and cause more electrons to travel into theactive semiconductor layer inducing an increase in the draincurrent. On the other hand, the P3HT film could alsoprovide additional protection to the NDI-based polymerfrom possible degradation by oxygen and humidity, thuspreventing from device performance decline.

Figure 2. Output characteristics of all-polymer OFET.

B. Transfer characteristicsFigure 3 shows the transfer characteristics of the

P(NDI2OD-T2)-OFETs with P3HT film at a constantpositive Vds (Vds = +20 V). Transistor parameters such ascharge carrier mobility (μ), threshold voltage (Vth), on/offcurrent ratio (Ion/Ioff), and subthreshold slope (SS) are1.1~2.8×10-4 cm2 V-1 s-1, 30.6~34.6 V, 102, and 3.5 V/dec,respectively. The charge carrier mobilities of these deviceswere extracted in either the linear or saturation region usingthe standard transistor equation for FETs [18]. In fact, themobility was in the range of the reported values for otherunipolar OFETs based on NDI-polymer [10], and typically,spin-coated n-channel polymer devices displayed mobilityin the range of 10-3-10-5 cm2 V-1 s-1. This result indicates thatthe additional P3HT layer influenced the performance of then-channel P(NDI2OD-T2)-OFETs. In addition, the Ion/Ioffwas relevant and the SS was also in the range of 0.1-10V/dec, which is typical of conventional OFETs [19].

Figure 3. Semilogarithmic plots of both drain current and square root ofdrain current versus gate voltage showing the transfer characteristics of all-polymer OFET.

p+-type SiPMMAP3HT

P(NDI2OD-T2)Au Au

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C. Memory elementAn external gate bias (Vbias) was applied to the gate

contact to cause reversible shifts in Vth in order to determinewhether the device can sense for memory state, i.e. writeand erase states, which is similar to the characteristicsdescribed by S. Tiwari et al. for nonvolatile memory [20].Figure 4 shows the transfer characteristics of theP(NDI2OD-T2)-OFETs with a P3HT-charge storage layerfollowing the application of an external Vbias, whichexhibited a significant shift depending on the polarity of theVbias. The initial transfer characteristic was clearly shifted ina negative direction after the application of external Vbias = -60 V (writing voltage) for Tbias = 5 min. The shift in the Vth(∆Vth) or memory window was observed to be 10.7 V.Thereafter, the application of an external Vbias = +60 V(erasing voltage) for Tbias = 5 min caused the positivelyshifted transfer characteristic to return nearly to its initialposition. The device parameters after the application ofexternal bias; μsat = 2.8×10-4 cm2 V-1 s-1, Vth = 30.6 V(initial), μsat = 1.8×10-4 cm2 V-1 s-1, Vth = 19.9 V (negativeexternal bias), and μsat = 1.9×10-4 cm2 V-1 s-1, Vth = 27.5 V(positive external bias). The writing voltage caused negativeshifts in Vth, which indicates that positive charges are storedfrom the channel into the P3HT-holes-acceptor layer. On theother hand, the reversibility of the Vth shift in a positivedirection was caused by an erasing operation of negativeVbias, which indicates that the detrapping process at theorganic interface. Therefore, the position of the transfercurve in the device could be controlled by the properselection of applied external Vbias with a certain amount oftime bias. Here, one should distinguish the memorybehavior upon bias-stress effects in OFETs by defect statesor impurities in semiconductor and/or gate dielectrics, whichalso lead to Vth shifts [21]. Although this external effectwould enhance to be opened the memory window, it cannotbe controllable to use as a practical memory as well as notpermanent [22].

Figure 4. Transfer characteristics obtained upon application of differentexternal gate bias conditions at constant positive Vds = +20 V for all-polymer OFET; Vbias = 0 V (initial state). Vbias = -60 V was applied for Tbias

= 5 min (write or “1” state), and Vbias = +60 V was applied for Tbias = 5 min(erase or “0” state).

V. CONCLUSIONS

We fabricated and characterized all-polymer OFETsbased on P(NDI2OD-T2) as an active layer, P3HT as a hole-

accepting layer, and PMMA as an organic gate dielectric.Furthermore, we also demonstrated memory element in all-polymer OFETs in which P3HT functioned as the chargestorage layer. The all-polymer OFETs manifested a memorywindow of 10.7 V (∆Vth = 10.7 V) upon the application ofan external gate bias without suffering any majordegradation of performance properties. The incorporation ofP3HT chains onto the PMMA dielectric films as chargestorage sites for holes resulted in a reversible shift in thethreshold voltage. After successfully demonstrating the n-channel organic transistor memory, currently we areworking on the investigation of the design and fabrication ofan organic nonvolatile integrated circuit memory elementcapable of storing both positive and negative polarity pulsesto define distinguishable memory states, i.e., dual-polaritynonvolatile memory, for application in a neural circuitmemory element. However, further improvement of thedevice performance is needed, especially the reliability ofthe memory operation, lowering of the high operatingvoltage and retention time, in order to complement our p-channel organic transistor memory with an electron-acceptorlayer.

ACKNOWLEDGMENT

This study was supported by a Grant-in-Aid for ScientificResearch (No. 17510107) from the Ministry of Education,Culture, Sports, Science and Technology of Japan.

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