Scholarly article on topic 'Solution-processed nanocomposite dielectrics for low voltage operated OFETs'

Solution-processed nanocomposite dielectrics for low voltage operated OFETs Academic research paper on "Materials engineering"

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Organic Electronics
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{"Low voltage OFETs" / "Organic–inorganic nanocomposite" / "Solution-processed high-k nanocomposite dielectric" / "Bilayer dielectric" / "Printed electronics"}

Abstract of research paper on Materials engineering, author of scientific article — Sheida Faraji, Teruo Hashimoto, Michael L. Turner, Leszek A. Majewski

Abstract A novel, solution processed high-k nanocomposite/low-k polymer bilayer gate dielectric that enables the fabrication of organic field-effect transistors (OFETs) that operate effectively at 1V in high yields is reported. Barium strontium titanate (BST) and barium zirconate (BZ) nanoparticles are dispersed in a poly (vinylidene fluoride-co-hexafluoropropylene) P(VDF-HFP) polymer matrix to form a high-k nanocomposite layer. This is capped with a thin layer (ca 30nm) of cross-linked poly(vinyl phenol) (PVP) to improve the surface roughness and dielectric–semiconductor interface and reduces the leakage current by at least one order of magnitude. OFETs were fabricated using solution-processed semiconductors, poly(3,6-di(2-thien-5-yl)-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione)thieno[3,2-b]thiophene) and a blend of 6,13-bis (triisopropylsilylethynyl) pentacene and poly (α-methylstyrene), in high yield (>90%) with negligible hysteresis and low leakage current density (10− 9 Acm− 2 at ±1V).

Academic research paper on topic "Solution-processed nanocomposite dielectrics for low voltage operated OFETs"

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Organic Electronics

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

Solution-processed nanocomposite dielectrics for low voltage operated OFETs

Sheida Farajia'*, Teruo Hashimoto b, Michael L. Turnerc, Leszek A. Majewskia

a School of Electrical and Electronic Engineering, University of Manchester, Sackville Street, Manchester M13 9PL, UK b Corrosion & Protection Centre, School of Materials, University of Manchester, The Mill, Sackville Street, Manchester M13 9PL, UK c School of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK

ABSTRACT

A novel, solution processed high-k nanocomposite/low-k polymer bilayer gate dielectric that enables the fabrication of organic field-effect transistors (OFETs) that operate effectively at 1 V in high yields is reported. Barium strontium titanate (BST) and barium zircon-ate (BZ) nanoparticles are dispersed in a poly (vinylidene fluoride-co-hexafluoropropylene) P(VDF-HFP) polymer matrix to form a high-k nanocomposite layer. This is capped with a thin layer (ca 30 nm) of cross-linked poly(vinyl phenol) (PVP) to improve the surface roughness and dielectric-semiconductor interface and reduces the leakage current by at least one order of magnitude. OFETs were fabricated using solution-processed semiconductors, poly(3,6-di(2-thien-5-yl)-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione)thi-eno[3,2-b]thiophene) and a blend of 6,13-bis (triisopropylsilylethynyl) pentacene and poly (a-methylstyrene), in high yield (>90%) with negligible hysteresis and low leakage current density (10 9Acm 2 at ±1 V).

Crown Copyright © 2014 Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4XI/).

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ARTICLE INFO

Article history:

Received 25 September 2014

Received in revised form 4 December 2014

Accepted 4 December 2014

Available online 16 December 2014

Keywords:

Low voltage OFETs

Organic-inorganic nanocomposite

Solution-processed high-k nanocomposite

dielectric

Bilayer dielectric

Printed electronics

1. Introduction

The rapid development of organic semiconductor materials that can be processed from solution opens up the possibility of printing organic field-effect transistors (OFETs) onto a wide range of substrates to enable the development of low-cost, large area electronics [1,2]. However, typical solution-processed OFETs operate at voltages that are too high for use in portable electronic devices or as aqueous sensors (V> 5 V) [3,4]. For these applications, transistors working in the range of 1.5-1 V are highly desirable. Lowering the operational voltage of OFETs can be achieved by reducing the threshold voltage and the subthreshold swing. These device parameters are largely controlled by the gate dielectric and the density of charge traps at the dielectric-semiconductor interface. Therefore, to achieve operational voltages approaching 1 V, high-capacitance,

* Corresponding author.

solution-processable gate insulators that form trap-free interfaces are essential.

In a field-effect transistor, the gate voltage (VG) required to switch the transistor "on" scales directly with insulator thickness, d, and inversely with insulator dielectric constant, k: VG ~ d/k. Usually, organic dielectrics have relatively low dielectric constants (k ~ 2-4) [5,6] and hence extremely thin layers (d < 20 nm) are required to obtain low-voltage transistor operation. Recently, organic transistors that operate at 4 V have been demonstrated using 10 nm cross-linked polymer and polymers blends [7,8], or self-assembled mono- [9] and multi-layer [10] (SAM) (d ~ 2.8 nm) insulators. Alternatively, 1 V OFETs using anodized TiO2 (k = 20-41, d = 7.5 nm) have been reported [11,12]. Organic transistors operating below 4 V have been demonstrated using solution-processed high-k dielectrics such as barium titanate (BT) nanocomposites (k ~ 12) [13] or relaxor ferroelectric polymers, i.e. poly (vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-

http://dx.doi.org/10.1016/j.orgel.2014.12.010 1566-1199/Crown Copyright © 2014 Published by Elsevier B.V.

This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

TrFE-CFE)) terpolymer (k ~ 60) [14]. Achieving OFET operation down to 1 V requires both minimizing d and maximizing k, and this leads to practical problems. For example, thin dielectric layers may be too fragile for flexible operation leading to an increase in the gate leakage current, whereas thicker films of high-k insulators introduce undesirable effects at the organic semiconductor-insulator interface. This can lead to increased charge trapping that causes a rise of threshold voltage (VT) and inverse subthreshold slope (SS), as well as, lowering the field effect mobility of charge carriers in OFETs [15,16]. Therefore, the fabrication of OFETs working at or below 1 V using both solution-processable semiconductors and high-k dielectrics is still very challenging.

Herein, we show that OFETs operating below 1 V can be successfully fabricated in high yield with low device hysteresis and gate leakage using a solution-deposited high-k nanocomposite/low-k polymer bilayer gate dielectric. The first layer is a high-k organic insulating polymer filled with very high-k ceramic nanoparticles that is partially capped by a second low-k polymer dielectric layer. One of the key challenges in the formation of high quality, high-k nanocomposite layers is to control the homogeneity of the nanoparticle dispersion and the stability of the nanocomposite suspension. The high surface energy and surface-to-volume ratio of nanoparticles usually results in agglomeration and phase separation from the polymer matrix, particularly at high filler loading, leading to an inhomogeneous mixture with poor processability, increased porosity and defect density [17]. Percolative pathways can be created through the aggregated fillers that leads to increased leakage current density, reduced dielectric breakdown strength and decreased dielectric constant [18]. Surface modification of the nanoparticles with a suitable coupling agent is one of the most widely used methods to prevent nanoparticle agglomeration. Such treatments promote interfacial interactions between the nanoparticles and the polymer matrix and thus improve the uniformity of the nanoparticle dispersion [19]. In this work, a stable, homogenous nanocomposite dispersion was prepared by incorporating high-k, perovskite-phase metal oxide nanoparticles of barium strontium titanate (BST, k ~ 16) [20] and barium zirconate (BZ, k ~ 32) [21] into a poly (vinylidene fluoride-co-hexafluoropropylene) P(VDF-HFP) polymer matrix (k ~ 11). By carefully identifying the best combination of fluorinated copolymer, organic solvents and nanoparticle-to-polymer volume ratio, a reproducible, uniform nanocomposite suspension was made without the need for further nanoparticle surface modification.

2. Experimental

P(VDF-HFP) pellets (50 mg ml"1) were dissolved in N,N-dimethylformamide (DMF) and stirred for a minimum of 6 h. Various volume ratios (7:1-1.25:1) of BST or BZ nano-particles were dispersed in P(VDF-HFP) solution, followed by a 2-h ultrasonication (80 W) and thereafter stirring for minimum 12 h to further promote uniform dispersion of nanoparticles. The nanocomposite suspension was then centrifuged at 6000 rpm for 5 min to separate any larger

particles and agglomerates to obtain a homogeneous suspension. The resultant nanocomposite suspension was stable and no apparent precipitation was observed for several weeks at room temperature.

To evaluate the transistor performance, bottom-gate, bottom-contact OFETs were fabricated using either solution-processed poly(3,6-di(2-thien-5-yl)-2,5-di(2-octyldode-cyl)-pyrrolo[3,4-c]pyrrole-1,4-dione)thieno[3,2-b]thiophene) (PDPPTT) [22] or a blend of 6,13-bis (triisopropylsilylethy-nyl) pentacene (TIPS-Pentacene) and poly(a-methylsty-rene) (PaMS) [23] as the active layer. OFETs were fabricated on Corning® glass substrates on which a 100 nm aluminum (Al) layer was thermally evaporated to serve as the gate electrode. The Al surface was briefly treated with UV-Ozone prior to spin-coating the nanocompos-ite dielectric layer. The nanocomposite dielectric film was formed by spin-coating P(VDF-HFP)-based nanocomposite suspension at 3000 rpm for 2 min and annealing at 90 °C for 90 min. A capping layer of poly(vinyl phenol) (PVP) (20 mgml"1) in propylene glycol monomethyl ether acetate (PGMEA) with added poly (melamine-co-formalde-hyde) (PMF) (10mgmr1) cross-linking agent was spin-coated on top of the nanocomposite layer at 5000 rpm for 2 min, followed by cross-linking at 130 °C for 90 min under nitrogen. Gold (Au) source and drain electrodes (50 nm) were deposited through a shadow mask by thermal evaporation. Au contacts are then modified by submersing samples in a 10 mM 2,3,4,5,6-pentafluorothi-ophenol (PFBT) solution followed by repeated washing with 2-propanol (IPA). For devices using the semiconductor blend, a 7:3 by weight (10 mgml"1) of TIPS-Penta-cene:PaMS solution was spin-coated at 500 rpm for 2 min, followed by heating the sample at 60 °C for 20 min under N2. PDPPTT was deposited by spin-coating a solution (10 mg ml"1) at 1000 rpm for 1 min and subsequently heating the sample at 110 °C for 30 min under N2.

Device characterization was performed in ambient air using an Agilent E5270B measurement mainframe with Karl Suss PH100 micromanipulator probes. AFM images were recorded on a Bruker Multimode 8 in Peak Force tapping mode at a resolution of 512 x 512 pixels. Cantilevers had a spring constant of approximately 0.350 N m"1, with a resonant frequency of approximately 50-80 kHz. A modulation frequency of 2 kHz was used. The TEM micrograph was obtained using aJEM2000FX2 QEOL) instrument operating at an acceleration voltage of 120 kV. The TEM specimen was prepared on an ultramicrotome EM UC6 (Leica) with a slice thickness of 50 nm.

3. Results and discussions

The dielectric properties of the prepared nanocompos-ite films were evaluated by the fabrication of parallel-plate capacitors. As shown in Fig. 1(a), solution processed BST-P(VDF-HFP) dielectric layers showed leakage currents at ±1 V up to 10"7 A cm"2. Similar leakage current behavior was observed in analogous BZ-P(VDF-HFP) films (Fig. 2(a)). To improve the electrical performance, a thin layer of PVP was applied to the surface of the nanocomposite films. Unlike reported previously [14,24], the thickness of the PVP capping layer was carefully tailored to only partially

T3 10-9

7Й 10

-0.5 0.0 0.5

Voltage [V] (a)

Fig. 1. (a) Leakage current density of BST-P(VDF-HFP) nanocomposite dielectric films. Tapping mode AFM images of adhesion profile of (b) uncapped and (c) PVP-capped BST-P(VDF-HFP). (d) Cross-sectional TEM image of a TIPS-Pentacene/PaMS OFET with PVP-capped BST-P(VDF-HFP) nanocomposite layer.

cover the topographic features of the rough nanocomposite surface (Figs. 1(b-c) and 2(b-c)), as this ensured that no significant loss of capacitance was observed. Deposition of a thin PVP layer (d = 20-30 nm) decreased the leakage current density by at least one order of magnitude to 10"9 A cm"2 at ±1 V, Figs. 1(a) and 2(a), and lowered the RMS surface roughness of the nanocomposite film from 28 nm to 20 nm and from 39 nm to 29 nm for PVP-capped BST-and BZ-nanocomposite films respectively. The capacitance (C, pF) of the nanocomposite bilayers was measured at a 500 Hz-1 MHz frequency range. At 1 kHz, the areal capacitance (C,-) decreased from 93.7 ± 0.2 nF cm"2 (d = 148 ± 0.3 and k = 15.7 ±0.4) for the uncapped BST-nanocomposite layer to 64.4 ± 0.2 nF cm"2 (d= 178 ±0.2 and k = 13.2 ± 0.2) for the bilayer dielectric. Thicker films of PVP (d >

100 nm) led to a significantly reduced Ci of 8.70 ± 0.06 nF cm"2. The contact angle of water was reduced from 89° to 77°, additionally confirming the presence of the PVP. Similarly, a reduction in Ci from 72.3 ± 0.8 nF cm"2 (d = 269.0 ±0.6 and k = 21.9 ±0.4) to 27.5 ± 0.3 nF cm"2 (d = 293.0 ± 0.1 and k = 8.9 ± 0.4) was measured when BZ-nanocomposite dielectric layer was capped with PVP.

A sectional TEM image of the completed TIPS-Pentacene and PaMS device is shown in Fig. 1(d), clearly showing the high surface roughness of the nanocomposite film. For pristine high-k nanocomposite layers, only poorly operating OFETs could be fabricated due to the large roughness and difficulties with deposition of the semiconductors from solution on the surface of the fluorinated polymer. Bilayer dielectric films of nanocomposites with thick PVP

2 10-8 E

c 10-10 0 10

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■ Uncoated

■ PVP-coated

-0.5 -1.0 -0.5 0.0 0.5 Voltage [V] (a)

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10-8 140

\ \ - 120

\ \ 100 g

10-1° - \ ____________________________ - 80 CN

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10-11 ■ \\ \y s - 40

10-12 r ' \\. ' 20

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-1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 Gate Voltage, VG [V]

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/ -1.4 V

-1.3 V

- -1.2 V

- ffjT^ -1.1 V -

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-0.9 V "

0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6 Drain Voltage, VSD [V]

Fig. 2. (a) Leakage current density of BZ-P(VDF-HFP) dielectric films. Tapping mode AFM images of adhesion profile of (b) uncapped and (c) PVP-capped BZ-P(VDF-HFP). (d) Transfer characteristics including leakage current (dotted line) and (e) output characteristic of TIPS-Pentacene/PaMS OFETs using PVP-capped BZ-P(VDF-HFP) dielectric, VSD = -1 V, channel length (L) = 2000 im and channel width (W) = 50 im.

ü.üü mV

films (d > 100 nm) produced OFETs that operated well above 5 V with a leakage current density <10-11Acm-2. Bilayer dielectrics with PVP layers of 20-30 nm resulted in high yields (>90%) of working OFETs that operate at 1 V.

Transfer and output characteristics of the BZ-P(VDF-HFP) OFETs using TIPS-Pentacene/PaMS are shown in Fig. 2(d-e). It was found that those devices could not be operated below 1 V due to a high threshold voltage (VT = -0.85 V). This is believed to be due to larger thickness and surface roughness and thus smaller Ci of BZ-P(VDF-HFP) nanocomposite layers. However, working OFETs operating at under 1.5 V with very small hysteresis, a leakage current density <10-9 A cm-2 at ±1.5 V and a mobility at 1.5 V of 0.08 cm2 V-1 s-1 were achieved. The full device characteristics for BZ-P(VDF-HFP) as the dielectric layer are

shown in Table 1. Subsequently, BST-P(VDF-HFP) OFETs using PDPPTT and TIPS-Pentacene/PaMS were fabricated (Fig. 3(a-b) and (c-d), respectively). In both cases the devices turned on below 1 V (VT ~ -0.5 V). The transfer and output characteristics were virtually hysteresis-free and the leakage currents were at least one order of magnitude below "on" currents; clear "off" and "on" operating states could be distinguished. The average field-effect mobility for the 1 V OFETs was calculated to be 0.14 cm2 V-1 s-1 for the PDPPTT devices and 0.06 cm2 V-1 s-1 for those with TIPS-Pentacene/PaMS as the active layer. Transistors with the semiconductor blend as the active layer showed a lower subthreshold slope (S = 169mVdec-1) and one order of magnitude lower leakage current density than those fabricated with PDPPTT (c.f. Table 1). It appears

Table 1

Figures-of-merit for OFETs with P(VDF-HFP)-based nanocomposite gate dielectric.

Nanocomposite Semiconductor l (cm2V-1s-1) VT (V) SS (mV dec"1) On/off (ratio)

BST-P(VDF-HFP) PDPPTT 0.14 -0.5 221 103

TIPS-Pentacene/PaMS 0.06 -0.55 169 103

BZ-P(VDF-HFP) TIPS-Pentacene/PaMS 0.08 -0.85 153 103

10-7 10-8 10 10-10 10-11 10-12

10 10-8

^ 10-9 _p

J 10-10 10 10-12

-0.8 -0.6 -0.4 -0.2 Gate Voltage, VG [V] (a)

-1.0 -0.8 -0.6 -0.4 -0.2 Gate Voltage, VG [V]

200 ¿2

240 200 160 120 80 40

50 40 30 20

2 10 Q

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Dr 5 - {/-0.7 V -

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0.0 -0.2 -0.4 -0.6 -0.8 Drain Voltage, VSD [V] (d)

Fig. 3. Transfer and output characteristics of (a) and (b) PDPPTT and (c) and (d) TIPS-Pentacene/PaMS OFETs using PVP-capped BST-P(VDF-HFP) dielectric layer. VSD = -1 V, channel length (L) = 2000 цш and channel width (W) = 40 цш/50 цш, respectively.

that the vertical phase separation occurring within the semiconductor blend [25], in which the TIPS-Pentacene layer is sandwiched between two layers of PaMS, contributes to a more robust and trap-free interface at the gate dielectric.

4. Conclusion

In summary, solution-processed dielectric layers suitable for 1 V operation of OFETs with polymeric and small molecule organic semiconductors are reported. The dielectric layer consists of a bilayer film composed of a high-k nanocomposite film that is partially capped by a low-k polymer. The OFET devices are prepared in high yield (>90%), operate with minimal hysteresis, display very low subthreshold slope (<200 mV dec"1) and possess mobilities reaching 0.1 cm2 V"1 s"1 at 1 V. These materials are

promising candidates for fabrication of OFETs suitable for use in aqueous based biosensors and low power electronics.

Acknowledgement

This work was funded by EPSRC. References

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