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High performance top-gate field-effect transistors based on poly(3-alkylthiophenes) with different alkyl chain lengths Kenichiro Takagi a , Takashi Nagase a,b , Takashi Kobayashi a,b , Hiroyoshi Naito a,b,a Department of Physics and Electronics, Osaka Prefecture University, Sakai 599-8531, Japan b The Research Institute for Molecular Electronic Devices, Osaka Prefecture University, Sakai 599-8531, Japan article info Article history: Received 26 July 2013 Received in revised form 5 November 2013 Accepted 12 November 2013 Available online 26 November 2013 Keywords: Organic field-effect transistors Poly(3-alkylthiophene) Top-gate configuration CYTOP Poly(4-chlorostyrene) Poly(methyl methacrylate) abstract The device characteristics of top-gate field-effect transistors (FETs) based on typical poly- mer semiconductor regioregular poly(3-alkylthiophenes) (P3ATs) with different alkyl chain lengths are investigated. High field-effect mobilities of 10 2 cm 2 /Vs are obtained irre- spective of alkyl chain length even when polymer gate insulators with different dielectric constants (2.1–3.9) are used. This is attributed to the spontaneous formation of highly ordered edge-on lamellar structures at the surface of P3AT thin films that are the channel regions in top-gate FETs. In addition, top-gate P3AT FETs containing different gate insula- tors exhibit high operational stability, with low threshold voltage shifts of <0.5 V following prolonged gate bias stress, which is comparable to that of hydrogenated amorphous silicon thin film transistors. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Polythiophenes are one of the most promising classes of materials for use as active semiconducting layers in organ- ic field-effect transistors (organic FETs or OFETs). Regioreg- ular poly(3-alkylthiophenes) (P3ATs, Fig. 1), which contain only head-to-tail coupling, readily form planar conforma- tions, leading to highly conjugated structures [1–3]. The most well-known example is regioregular poly(3-hexylthi- ophene) (P3HT), which was the first semiconducting poly- mer to achieve a field-effect mobility of 0.1 cm 2 V 1 s 1 [4,5]. The high mobility of P3HT is attributed to its forma- tion of edge-on lamellar structures with the (1 0 0)-axis normal to the substrate, resulting in efficient two-dimen- sional carrier transport [6]. Because of such transport prop- erties, the transport characteristics of P3ATs, especially P3HT, have been extensively investigated in transistor con- figurations [7–11]. Several groups have reported the field-effect mobilities of FETs containing P3ATs with different alkyl chain lengths using bottom-gate configurations (Fig. 2(a)) [12–17]. The mobilities of bottom-gate P3AT FETs fabricated on un- treated or UV/O 3 -treated SiO 2 gate insulators with high surface energy decreased markedly with increasing alkyl chain length of P3AT [12–15]. This trend was caused by the disturbance of carrier transport by the alkyl side chains; the insulating alkyl chains extend in the direction of carrier transport (face-on P3AT orientation). In contrast, Sauve et al. [16] reported that the mobility of bottom-gate P3AT FETs is independent of alkyl chain length. They used octadecyltrichlorosilane (ODTS) to treat the surface of the SiO 2 substrate because such treatment is known to lower the substrate surface energy and improve the crystallinity of organic semiconductor molecules on the surface of SiO 2 substrates [9,18]. Oosterbaan et al. [17] also showed that mobility is independent of alkyl chain length in bottom- gate FETs with P3AT nanofiber layers fabricated by slow cooling of P3AT solutions. These chain-length-independent mobilities were caused by the enhancement of lamellar ordering of P3AT molecules on SiO 2 surfaces with low surface energies. 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.11.022 Corresponding author at: Department of Physics and Electronics, Osaka Prefecture University, Sakai 599-8531, Japan. Tel./fax: +81 72 254 9266. E-mail address: [email protected] (H. Naito). Organic Electronics 15 (2014) 372–377 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel

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Page 1: High performance top-gate field-effect transistors based on poly(3-alkylthiophenes) with different alkyl chain lengths

Organic Electronics 15 (2014) 372–377

Contents lists available at ScienceDirect

Organic Electronics

journal homepage: www.elsevier .com/locate /orgel

High performance top-gate field-effect transistors basedon poly(3-alkylthiophenes) with different alkyl chain lengths

1566-1199/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.orgel.2013.11.022

⇑ Corresponding author at: Department of Physics and Electronics,Osaka Prefecture University, Sakai 599-8531, Japan. Tel./fax: +81 72 2549266.

E-mail address: [email protected] (H. Naito).

Kenichiro Takagi a, Takashi Nagase a,b, Takashi Kobayashi a,b, Hiroyoshi Naito a,b,⇑a Department of Physics and Electronics, Osaka Prefecture University, Sakai 599-8531, Japanb The Research Institute for Molecular Electronic Devices, Osaka Prefecture University, Sakai 599-8531, Japan

a r t i c l e i n f o

Article history:Received 26 July 2013Received in revised form 5 November 2013Accepted 12 November 2013Available online 26 November 2013

Keywords:Organic field-effect transistorsPoly(3-alkylthiophene)Top-gate configurationCYTOPPoly(4-chlorostyrene)Poly(methyl methacrylate)

a b s t r a c t

The device characteristics of top-gate field-effect transistors (FETs) based on typical poly-mer semiconductor regioregular poly(3-alkylthiophenes) (P3ATs) with different alkyl chainlengths are investigated. High field-effect mobilities of �10�2 cm2/Vs are obtained irre-spective of alkyl chain length even when polymer gate insulators with different dielectricconstants (2.1–3.9) are used. This is attributed to the spontaneous formation of highlyordered edge-on lamellar structures at the surface of P3AT thin films that are the channelregions in top-gate FETs. In addition, top-gate P3AT FETs containing different gate insula-tors exhibit high operational stability, with low threshold voltage shifts of <0.5 V followingprolonged gate bias stress, which is comparable to that of hydrogenated amorphous siliconthin film transistors.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction Several groups have reported the field-effect mobilities

Polythiophenes are one of the most promising classes ofmaterials for use as active semiconducting layers in organ-ic field-effect transistors (organic FETs or OFETs). Regioreg-ular poly(3-alkylthiophenes) (P3ATs, Fig. 1), which containonly head-to-tail coupling, readily form planar conforma-tions, leading to highly conjugated structures [1–3]. Themost well-known example is regioregular poly(3-hexylthi-ophene) (P3HT), which was the first semiconducting poly-mer to achieve a field-effect mobility of 0.1 cm2 V�1s�1

[4,5]. The high mobility of P3HT is attributed to its forma-tion of edge-on lamellar structures with the (100)-axisnormal to the substrate, resulting in efficient two-dimen-sional carrier transport [6]. Because of such transport prop-erties, the transport characteristics of P3ATs, especiallyP3HT, have been extensively investigated in transistor con-figurations [7–11].

of FETs containing P3ATs with different alkyl chain lengthsusing bottom-gate configurations (Fig. 2(a)) [12–17]. Themobilities of bottom-gate P3AT FETs fabricated on un-treated or UV/O3-treated SiO2 gate insulators with highsurface energy decreased markedly with increasing alkylchain length of P3AT [12–15]. This trend was caused bythe disturbance of carrier transport by the alkyl sidechains; the insulating alkyl chains extend in the directionof carrier transport (face-on P3AT orientation). In contrast,Sauve et al. [16] reported that the mobility of bottom-gateP3AT FETs is independent of alkyl chain length. They usedoctadecyltrichlorosilane (ODTS) to treat the surface of theSiO2 substrate because such treatment is known to lowerthe substrate surface energy and improve the crystallinityof organic semiconductor molecules on the surface of SiO2

substrates [9,18]. Oosterbaan et al. [17] also showed thatmobility is independent of alkyl chain length in bottom-gate FETs with P3AT nanofiber layers fabricated by slowcooling of P3AT solutions. These chain-length-independentmobilities were caused by the enhancement of lamellarordering of P3AT molecules on SiO2 surfaces with lowsurface energies.

Page 2: High performance top-gate field-effect transistors based on poly(3-alkylthiophenes) with different alkyl chain lengths

Fig. 1. Molecular configuration of regioregular poly(3-alkylthiophene).

Table 1Weight-average molecular weight (Mw), polydispersity index (PDI), andregioregularity of poly(3-alkylthiophene) materials used in this study.

Polymer MW (k) PDI Regioregularity (%)

P3HT (x = 6) 72 2.5 91P3OT (x = 8) 83 1.9 91P3DT (x = 10) 82 2.1 –

K. Takagi et al. / Organic Electronics 15 (2014) 372–377 373

Despite intense research of carrier mobility using thebottom-gate configuration, the top-gate configuration ofP3AT FETs (Fig. 2(b)) has not been fully investigated. Wehave previously shown that solution-processed OFETs witha top-gate configuration based on dioctylbenzothieno[2,3-b]benzothiophene (C8-BTBT) exhibit high field-effectmobility and operational stability [19]. C8-BTBT is a prom-ising small molecule for use in OFETs that was developedby Takimiya et al. [20]. Top-gate C8-BTBT FETs are readilyfabricated using a simple, conventional spin-coating meth-od. On the other hand, for polymer semiconductors, highmobilities of top-gate P3HT FETs incorporating TiO2 nano-composite dielectrics [21] or flexible substrates [22] werereported. However, the mechanism of the high mobilitieshas not been mentioned clearly. In addition, high perfor-mance top-gate FETs with both high mobility and highoperational stability using polymer semiconductors hasnot been demonstrated so far.

In this study, we fabricate top-gate FETs containingP3ATs with different alkyl chain lengths and systematicallycharacterize their electrical performance. Different poly-mer insulating materials are used as gate insulators todemonstrate high performance top-gate P3AT FETs withgate insulators with different dielectric constants and togain insight into the orientation of P3AT molecules in thechannel regions. We also study the operational stabilityof P3AT FETs against prolonged gate bias stress.

2. Experimental

P3HT (carbon number of alkyl chain x = 6), poly(3-octyl-thiophene) (P3OT, x = 8), and poly(3-decylthiophene)(P3DT, x = 10) were used as P3AT materials. These poly-mers were purchased from Rieke Metals Inc. (Lincoln,NE). Physical parameters of the P3AT materials are sum-marized in Table 1. We used P3AT polymers with almostthe same weight-average molecular weight (Mw), polydis-persity index (PDI), and regioregularity, which all affectcharge transport [6–10], although the regioregularity ofP3DT was not shown in the data sheet from themanufacturer.

Fig. 2. Structures of (a) bottom-g

Top-gate P3AT FETs were fabricated as follows: Cr/Ausource-drain electrodes were defined on glass substratesby photolithography. The surface energy of the substratewas increased by UV/O3 treatment, which facilitates theformation of homogeneous organic semiconductor films.Thin films of P3ATs were spin coated onto the substratesfrom 1 wt% anhydrous chlorobenzene solutions of P3HT,P3OT, or P3DT at 2000 rpm. The films were annealed undervacuum at 100 �C for 1 h. The thicknesses of P3HT, P3OT,and P3DT thin films were �50 nm. A fluoropolymer insula-tor layer of CYTOP™ (CTL-809M, Asahi Glass Co., Ltd.),poly(4-chlorostyrene) (PCS, Sigma–Aldrich), or poly(methyl methacrylate) (PMMA, Sigma–Aldrich) was thenspin coated onto the P3AT layers as the gate insulator.The samples were dried under vacuum at 100 �C for 1 h.The dielectric constants of CYTOP, PCS, and PMMA layerswere 2.1, 3.2, and 3.9, respectively. The thicknesses of allpolymer insulating layers were �500 nm. Finally, Al gateelectrodes were evaporated on the gate insulator layers.The channel length and width of the FETs were 50 lmand 3 mm, respectively.

For comparison, we also fabricated bottom-gate P3ATFETs using highly doped n+-Si wafers with 300-nm-thickthermally grown SiO2 layers as substrates. The surface ofthe SiO2 layers was treated with UV/O3 or ODTS. For theODTS treatment, SiO2 substrates were immersed in a solu-tion of ODTS (50 mM) in toluene for 90 h. P3AT layers wereformed in the same manner as for the top-gate FETs, fol-lowed by vacuum deposition of Au source-drain elec-trodes. The channel length and width were identical tothose of the top-gate FETs. Electrical measurements ofthe fabricated devices were obtained in a N2-filled glove-box system using Keithley 6430 and 2400 source metersat room temperature.

3. Results and discussion

The typical output characteristics (drain current ID–drain voltage VD) of top-gate P3HT FETs with CYTOP gateinsulators are shown in Fig. 3(a). The output characteristicsshow well-defined saturation behavior, and characteristicssimilar to those presented on Fig. 3(a) were also obtained

ate and (b) top-gate FETs.

Page 3: High performance top-gate field-effect transistors based on poly(3-alkylthiophenes) with different alkyl chain lengths

Fig. 3. (a) Output characteristics of the top-gate P3HT FET with a CYTOP layer and (b) transfer characteristics of top-gate P3HT, P3OT, and P3DT FETs withCYTOP gate insulator layers.

374 K. Takagi et al. / Organic Electronics 15 (2014) 372–377

for the P3OT and P3DT FETs (data not shown). Fig. 3(b)shows the transfer characteristics (ID-gate voltage VG) oftop-gate P3AT FETs with CYTOP gate insulators. The trans-fer characteristics exhibit a high on/off ratio (�105–106)and negligibly small hysteresis when VG is swept from+20 to �60 V and then from �60 to +20 V, similar to ourprevious work [19].

We extracted the field-effect mobilities of the FETs fromtheir transfer characteristics using the following equation:

ID ¼WCi

2LlðVG � VthÞ2; ð1Þ

where W is channel width, L is channel length, Ci is capac-itance per unit area of gate dielectric layers, Vth is thresh-old voltage, and l is field-effect mobility in the saturationregime (VD > VG � Vth). The mobilities of top-gate P3HT,P3OT, and P3DT FETs with CYTOP insulators were deter-mined to be 3.9 ± 0.2 � 10�2, 1.4 ± 0.0 � 10�2, and9.0 ± 0.7 � 10�3 cm2 V�1s�1, respectively. Fig. 4 plots thesemobilities as a function of alkyl chain length. Forcomparison, the data obtained for UV/O3- and ODTS-trea-ted bottom-gate FETs are also shown in Fig. 4. For the UV/O3-treated bottom-gate FETs, mobility decreased by over

Fig. 4. Field-effect mobilities of top-gate and bottom-gate P3AT FETs as afunction of alkyl chain length.

one order of magnitude with increasing alkyl chain lengthfrom x = 6 to x = 10. These results are consistent withthose of previous reports [12–16] and are related to theinterruption of carrier transport by the insulating alkylchains (face-on P3AT orientation). The top-gate P3AT FETswith different alkyl chain lengths exhibit higher mobili-ties compared with UV/O3-treated bottom-gate FETs byover two orders of magnitude, which are comparable tothose of ODTS-treated bottom-gate FETs. These highmobilities are attributed to enhancement of the lamellarordering of P3ATs at the air/P3AT interface [23,24]. Theseresults show that highly ordered lamellar structures ofP3AT molecules are formed irrespective of alkyl chainlength.

We also examined the surface topography and phaseimages, and absorption spectra of P3AT thin films. Topog-raphy and phase images were obtained by atomic forcemicroscopy (AFM). Note that for top-gate FETs, the regionof AFM scanning is exactly the same as the channel region.Fig. 5 shows the AFM images of the P3AT thin films. Theimages of P3HT, P3OT and P3DT are almost the same androot-mean-square (rms) roughness of all of the films is lessthan 1 nm. Fig. 6 shows the absorption spectra of P3AT thinfilms. The spectra are almost the same, indicating that theeffective p-conjugation length is almost the same in all ofthe P3AT thin films. The results presented in Figs. 5 and 6are consistent with the observation that the mobility oftop-gate P3AT FETs is independent of alkyl chain length.

Fig. 7 plots mobilities of top-gate P3AT FETs as a func-tion of dielectric constant of the gate insulating layer.The mobility of each P3AT FET is independent of the dielec-tric constant of the gate insulator, although P3DT FETs withPCS and PMMA insulators were not able to be prepared be-cause the P3DT layers dissolved in n-butyl acetate, whichwas the solvent for PCS and PMMA. The results depictedin Fig. 7 are quite different from the influence of the dielec-tric constant of the gate insulator on mobility of top-gateFETs based on an amorphous organic semiconductor ofpoly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA)[25], where mobility decreased by over an order of magni-tude when the dielectric constant of the gate insulator wasincreased from �2 to �4.

Page 4: High performance top-gate field-effect transistors based on poly(3-alkylthiophenes) with different alkyl chain lengths

Fig. 5. AFM images of (a and b) P3HT, (c and d) P3OT, and (e and f) P3DT thin films. The left side (a, c, and e) shows topographies and the right side (b, d, andf) depicts phase images. Images are 500 � 500 nm in size. The rms roughness of each film is also shown.

Fig. 6. Absorption spectra of P3HT, P3OT, and P3DT thin films.Fig. 7. Field-effect mobilities of top-gate P3AT FETs as a function ofrelative dielectric constant of the gate insulator.

K. Takagi et al. / Organic Electronics 15 (2014) 372–377 375

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376 K. Takagi et al. / Organic Electronics 15 (2014) 372–377

In the top-gate PTAA FETs, the mobility was reduced bybroadening of density of states of highest occupied molec-ular orbital level of PTAA caused by randomly orientedstatic dipole moments in the gate insulator. The insensitiv-ity of mobility to the dielectric constant observed in Fig. 7provides evidence for the edge-on orientation of P3AT mol-ecules at the air/P3AT interface. The formation of edge-onstructures increases the distance between the polymerbackbones and dipoles of the gate insulator by the insulat-ing alkyl chains, and this leads to electronic decoupling andsuppression of the broadening of the density of states, asshown by a numerical calculation [26]. In fact, the mobili-ties of our top-gate FETs containing P3ATs with differentalkyl chain lengths are comparable to those of ODTS-trea-ted bottom-gate P3AT FETs (Fig. 4), in which P3AT mole-cules in the channel regions adopt an edge-on orientation[9]. According to the calculation in Ref. [24], the broaden-ing of density of states due to the dipoles in the gate insu-lators is significantly suppressed within 0.5–1 nm from theinterface when PMMA gate insulator is used [26]. X-raydiffraction studies have shown that the distance betweenthe P3AT backbones and the gate insulator interfaces are0.8, 1.0, and 1.1 nm for P3HT, P3OT, and P3DT, respectively[27,28]. The high mobilities of the top-gate P3AT FETs withPMMA gate insulators reported in this study are consistentwith the numerical calculation [26]. We therefore concludethat P3AT molecules at the air/P3AT interface form highlyordered edge-on lamellar structures irrespective of alkyl

Fig. 8. Transfer characteristics of top-gate (a) P3HT, (b) P3OT, and (c) P3DT FETs w102, and 103 s.

chain length. Use of the top-gate configuration where theordered structures formed at the surface of P3AT thin filmswork as channel regions leads to high mobility in P3ATFETs containing gate insulators with different dielectricconstants.

Phenomena similar to those illustrated in Fig. 7 have re-cently been observed in FETs based on poly{[N,N9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)} with gate insulatorsof CYTOP, poly(t-butylstyrene), polystyrene, polyolefin-polyacrylate, and PMMA [29], and poly(2,5-bis(3-tetrade-cylthiophen-2-yl)thieno[3,2-b]tthiophene) with gate insu-lators of CYTOP, polystyrene, PMMA, and poly(vinylphenol) [30]. In both cases, the mobilities are not influ-enced by the dielectric constants of gate insulators becauseof the separation of the channel regions from dipoles in thegate insulators.

Finally, we investigated the operational stability of top-gate P3AT FETs with CYTOP gate insulators against pro-longed gate bias stress. In OFETs, the origin of operationalinstability caused by gate bias stress is not well understoodat present because a number of factors can affect theirelectrical characteristics [31]. However, in most cases, theinstability is caused by carrier trapping in deep localizedstates in the channel regions. One possible origin of thegeneration of deep localized states is the presence of waterat the semiconductor/insulator interface [32–35]. As a re-sult, the stability of OFETs can be enhanced remarkably

ith CYTOP gate insulators before and after bias stress of VG = �60 V for 10,

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K. Takagi et al. / Organic Electronics 15 (2014) 372–377 377

by using CYTOP, which is highly hydrophobic, as a gateinsulator [36–38].

Fig. 8 shows the transfer characteristics of top-gate P3ATFETs with CYTOP layers before and after gate bias stress ofVG = �60 V for 10, 102, and 103 s. During stressing with agate bias, VD was held at 0 V to apply a homogeneous electricfield throughout the channel region [39]. The ID–VG curvesobtained for the P3HT, P3OT, and P3DT FETs are identicalto the corresponding initial curve even after bias stress for103 s. Operational stability is often evaluated by measuringthe threshold voltage shift DVth, which is defined as Vth

(after gate bias stress) � Vth (before gate bias stress). Thetop-gate P3HT, P3OT, and P3DT FETs with CYTOP gate insu-lator layers exhibit high operational stability with DVth at103 s of �0.06, 0.2, and 0.3 V, respectively. These valuesare comparable to or lower than those of hydrogenatedamorphous silicon thin film transistors [40,41]. The top-gate P3AT FETs with PCS and PMMA layers also show smallthreshold voltage shifts of |DVth| < 0.5 V (data not shown).Note that in bottom-gate FETs, the coating of homogeneousorganic semiconductor films on polymer gate insulatorswith relatively low surface energies are difficult. The useof the top-gate configuration is effective to make the fabri-cation process simple and to improve the device perfor-mance including mobility and operational stability.

4. Conclusions

The electrical performance of top-gate FETs containingP3ATs with different alkyl chain lengths and gate insula-tors with different dielectric constants were systematicallycharacterized. Top-gate P3AT FETs fabricated on UV/O3-treated glass substrates exhibited mobilities of�10�2 cm2/Vs, which were comparable to those of ODTS-treated bottom-gate P3AT FETs. In addition, the mobilitieswere independent of the dielectric constant of the gateinsulator layer, which were attributed to the spontaneousformation of edge-on lamellar structures of P3AT mole-cules at the surface of P3AT thin films irrespective of alkylchain length. The top-gate P3AT FETs exhibited high oper-ational stability against prolonged gate bias stress, whichwas comparable to that of hydrogenated amorphous sili-con thin film transistors. Our results show use of the top-gate configuration enhances field-effect mobility as wellas operational stability of P3AT FETs by a simplified fabri-cation process.

Acknowledgments

This research is granted by the Japan Society for thePromotion of Science (JSPS) through the ‘‘Funding Programfor World-Leading Innovative R&D on Science and Technol-ogy (FIRST Program),’’ initiated by the Council for Scienceand Technology Policy (CSTP). This work is partly sup-ported by a Grant-in-Aid for Scientific Research (B) (No.23360140) from the Japan Society for the Promotion ofScience and by a Grant-in-Aid for Scientific Research onInnovative Areas ‘‘New Polymeric Materials Based on Ele-ment-Blocks (No. 2401)’’ (No. 24102011) of the Ministryof Education, Culture, Sports, Science, and Technology,Japan.

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