Organic Field-Effect Transistors with a Bilayer Gate Dielectric

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Letter

Organic field-effect transistors with a bilayer gate dielectric comprising an oxide nanolaminate grown by atomic layer deposition Cheng-Yin Wang, Canek Fuentes-Hernandez, Minseong Yun, Ankit K. Singh, Amir Dindar, Sangmoo Choi, Samuel Graham, and Bernard Kippelen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10603 • Publication Date (Web): 19 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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ACS Applied Materials & Interfaces

Organic Field-Effect Transistors with a Bilayer Gate Dielectric Comprising an Oxide Nanolaminate Grown by Atomic Layer Deposition Cheng-Yin Wang1, Canek Fuentes-Hernandez1, Minseong Yun1, Ankit Singh2, Amir Dindar1, Sangmoo Choi1, Samuel Graham2, Bernard Kippelen1,* 1. Center for Organic Photonics and Electronics (COPE), School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States 2. Center for Organic Photonics and Electronics (COPE), Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States KEYWORDS: organic field-effect transistors, top-gate geometry, nanolaminate, operational stability, environmental stability, CYTOP.

ABSTRACT: We report on top-gate OFETs with a bilayer gate dielectric comprising an Al2O3 /HfO2 nanolaminate layer grown by atomic layer deposition and an amorphous fluoropolymer layer (CYTOP). Top-gate OFETs display average carrier mobility values of 0.9 ± 0.2 cm2/Vs and threshold voltage values of -1.9 ± 0.5 V and high operational and environmental stability under different environmental conditions such as damp air at 50 °C (80% relative humidity) and prolonged immersion in water at a temperature up to 95 °C.

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Organic field-effect transistors (OFETs) enable flexible displays,1-2 wearable electronics,3 and sensors.4-5 OFETs have reached a level of performance that is comparable to that of thin-film amorphous silicon transistors.6 As the research on OFETs continues to mature, studies that focus on the operational and environmental stability of high performance OFETs7-11 become critical to assess more potential applications. To date, OFETs with a top-gate geometry have been shown to generally display superior environmental stability compared to OFETs with a bottom gate geometry. This is because chemical or physical interactions between reactant molecules in the environment, such as oxygen and water, and the organic semiconductor channel layer lead to changes (reversible or irreversible) of the electrical properties of an OFET. Environmental barriers have been used to encapsulate bottom-gate OFETs using a bilayer of parylene and gold.9 However, OFETs with a top-gate geometry have been shown to display improved environmental stability over bottom-gate OFETs7 even in the absence of encapsulation layers because the topgate gate dielectric also acts as an environmental barrier. Indeed, we and others have demonstrated that properly engineered gate dielectric layers can yield n- or p-channel top-gate OFETs,11-12 OFET-based sensors,13 and circuits14 with unprecedented long-term operational and environmental stability; even under immersion over extended periods of time in aqueous environments. The bilayer gate dielectric in such OFETs is comprised of an amorphous fluoropolymer layer (CYTOP) and a layer of Al2O3 grown by atomic layer deposition (ALD). However, single layers of Al2O3 by ALD have been found to be easily corroded in humid environments.15 In contrast, single layers of other metal-oxides such as TiO215 and ZrO216-17 fabricated by ALD display improved resistance to corrosion in humid environments while displaying good environmental barrier properties. Furthermore, the barrier properties of single metal-oxide layers by ALD have been shown to be significantly inferior to those displayed by


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ALD nanolaminates (NLs).15-18 In a NL structure, two different metal-oxides are grown successively using ALD to form a layer with a total thickness ranging from of a few nanometers, up to tens of nanometers. The formation of NL structures can allow for the tailoring of film stress and decreases the porosity and density of defects, resulting in denser layers that yield lower water vapor transmission rates than those displayed by a single metal-oxide layer.19 The use of NLs by ALD as gate dielectric has been proposed before in bottom-gate OFETs,20 but not in the context of top-gate OFETs with a bilayer dielectric comprising also a an amorphous fluoropolymer layer. Here, we report on the properties top-gate OFETs with a bilayer gate dielectric comprising a NL layer (Al2O3 and HfO2 by ALD) and an amorphous fluoropolymer layer (CYTOP). As a dielectric, HfO2 has many attractive properties since it can be processed by ALD using tetrakis dimethylamido hafnium (IV) (TDMAH), using water vapor (H2O) as the oxidizer agent, to yield layers with a high dielectric constant (k~ 15), a wide bandgap of 5.8 eV, and which in the past have been used as gate dielectric layer to demonstrate OFETs with low operating voltages.21 We show through operational and environmental stability studies under various conditions that OFETs with a CYTOP/NL bilayer gate dielectric exhibit superior stability compared to devices with a conventional CYTOP/Al2O3 bilayer gate dielectric. In particular, the replacement of a single layer of Al2O3 with a Al2O3/HfO2 NL structure yields devices that can sustain immersion in hot water at 95 °C for up to 1 h. Top-gate OFETs were fabricated with a bottom-contact geometry on glass substrates (Corning 1737) as shown in Figure 1 (a). Source and drain electrodes comprised of Ti/Au 6/70 nm were deposited in a Denton Explorer E-beam system through a shadow mask at a deposition rate of 1.0 Å/s and at an initial pressure of 1.0 × 10-5 Torr. On top of the source and drain


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electrodes a layer of pentafluorobenzenethiol (PFBT) (Sigma Aldrich) was formed by immersion in a 10 mmol PFBT solution for 15 min in a N2-filled glove box, followed by rinsing in pure ethanol for a few seconds and annealing on a hot plate for 5 min at 60 °C to dry it. A 6,13Bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) (Sigma Aldrich) and Poly[bis(4phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) (Sigma Aldrich) blend (1:1 weight ratio) was used as the organic semiconducting layer. TIPS-pentacene and PTAA blend were dissolved in 1, 2, 3, 4-Tetrahydronaphthalene anhydrous, 99%, (Sigma Aldrich) at a concentration of 30 mg/ml. A 70 nm-thick layer of TIPS-pentacene and PTAA was spin-coated onto source and drain electrodes at 500 rpm for 10 s (acceleration of 500 rpm/s) and ramped to 2000 rpm for 20 s (acceleration of 1000 rpm/s).14 Spin-coated films were then annealed at 100 °C for 15 min on a hot plate in a N2-filled glove box. A bilayer gate dielectric was fabricated by diluting CYTOP (Asahi Glass, CTL-809M) with solvent (Asahi Glass, CTL-SOLV180) to concentration of 2% by volume. A layer of CYTOP was spin-coated on top of the organic semiconducting layer at 3000 rpm for 60 s (acceleration of 10000 rpm/s) to yield a c.a. 35 nm-thick layer.14 Samples were then annealed at 100 °C for 10 min on hot plate in a N2-filled glove box. After that, an Al2O3 and HfO2 NL was deposited in a Savannah 100 ALD system at 100 °C. The NL comprised 200 cycles of Al2O3 on CYTOP to serve as a nucleation layer, followed by 5 cycles of Al2O3 and 5 cycles of HfO2 repeated 20 times yield a c.a. 38 nm-thick layer. 100 nm-thick Ag or Au gate electrodes were then deposited through a shadow mask in a Kurt J. Lesker SPECTROS thermal evaporator with initial pressures below 1.0 × 10-6 Torr and at an average deposition rate of 1.0 Å/s. Reference OFETs with CYTOP/Al2O3 bilayer gate dielectric were also fabricated as previously reported.14 We note that refractive index values of materials and layer-thicknesses in


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the OFETs were extracted from modeling of spectroscopic ellipsometry data acquired using a J.A. Woollam M-2000UI ellipsometer as described in reference 14. OFET devices were characterized in a N2-filled glove box (O2 and H2O < 0.1 ppm, 25°C) and at ambient conditions (25°C and 20 to 40% RH) using an Agilent E5272A source/monitor unit. To extract carrier mobility, capacitance density values of CYTOP/Al2O3 and CYTOP/NL were measured in capacitors with bottom Au electrodes and top Ag electrodes. Sixteen capacitors were fabricated with area ranging from 10.7 × 10-3 cm2 to 41.8 × 10-3 cm2. From the measurements of the capacitance versus area and using layer-thickness values extracted from modeling of spectroscopic ellipsometry data, capacitance density values of 35.3 nF/cm2 and 39.1 nF/cm2 were obtained for CYTOP/Al2O3 and CYTOP/NL, respectively consistent with previous reports.11 The environmental stability was investigated by storing the OFETs in an environmental chamber under different conditions as will be described next and then transferred to a N2-filled glove box for characterization. OFETs were also immersed in 95 °C warm water and their properties characterized under ambient conditions after different immersion times. The barrier properties of the bilayer gate dielectrics were also characterized by immersing in 95 °C water samples comprising Ca sensors deposited on glass and encapsulated using CYTOP/Al2O3 or CYTOP/NL bilayers fabricated as previously described. The degradation during immersion in hot water of these Ca sensors with two types of barrier layers was investigated through visual inspection tests. The transfer characteristic of top-gate OFETs with CYTOP/NL bilayer gate dielectrics and Au gate electrodes are shown in Figure 1 (b). Average hole mobility values of 0.9 ± 0.2 cm2/Vs and average threshold voltage values of -1.9 ± 0.5 V where found for these OFETs. These values are comparable to those displayed by top-gate reference OFETs with


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CYTOP/Al2O3 bilayer gate dielectric. To test the devices, we subjected top-gate OFETs through the following successive conditions of environmental exposure: ambient air for 90 days; vacuum annealing at 100 °C for 16 h; damp air (50 °C and 80% RH for 24 h); vacuum annealing at 100 °C for 16 h; ambient air for 24 h; immersion in distilled water for 16 h at room temperature; vacuum annealing at 100 °C for 16 h; ambient air for 24 h; vacuum annealing at 100 °C for 16 h; immersion in distilled water at 50 °C for 16 h; and vacuum annealing at 100 °C for 16 h. Figure 1 (c) displays the evolution of the charge mobility, threshold voltage, and the number of operating OFETs after different conditions of environmental exposure. These conditions were chosen to be consistent with the ones used in previous environmental stability characterization experiments performed on top-gate OFETs with a CYTOP/Al2O3 bilayer gate dielectric.11 As shown in Fig. 1 (c), for the OFETs that are still operational after water immersion the changes observed in their properties are consistent with previous findings, namely that the charge mobility values remain nearly constant and that the threshold voltage values decrease after vacuum annealing and increase with exposure to humid atmospheres. Next, we fabricated a separate batch of NL-OFETs and Ref-OFETs. Figure 2 (a) displays a comparison of transfer characteristics measured in devices exposed to air. NL-OFETs displayed average hole mobility values of 0.7 ± 0.1 cm2/Vs and average threshold voltage values of 0.1 ± 0.4 V. Ref-OFETs yielded an average hole mobility value of 0.6 ± 0.1 cm2/Vs and average threshold voltage value of 0.0 ± 0.4 V. The performance of both types of devices is comparable and within the range of batch-to-batch variations. Figure 2 (b) displays DC bias stress characteristics over a period of 1 h for top-gate OFETs with Ag gate electrodes and bilayer gate dielectrics composed of CYTOP/Al2O3 layer or a CYTOP/NL structure. In this measurement, the temporal step was 10 s and the gate-to-source voltage and drain-to-source


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voltage were both set at -10 V. Negligible on-current variation was observed on both Ref-OFETs and NL-OFETs in these DC bias stress measurements for 1 h demonstrating that the use of a CYTOP/NL bilayer gate dielectric yields OFETs with an operational stability at room temperature comparable to that of devices with a CYTOP/Al2O3 gate dielectric. Then, we immersed both types of OFETs into 95 °C water, and measured their transfer characteristics in air after successive immersions times. Figure 2 (c) displays the charge mobility and threshold voltage values as well as the number of functional OFETs as a function of total immersion time revealing major differences in device stability. First, the electrical properties of Ref-OFETs remain stable in water at 95 °C for 2 min and then are irreversibly damaged after 3 min of immersion. In contrast, NL-OFETs display stable characteristics for over 16 min in water at 95 °C, after which, the threshold voltage values decrease monotonically at least up to 1 h of immersion time. Remarkably, throughout the entire 1 h of total immersion time, the average hole mobility values remain stable and in the range between 0.5 to 0.7 cm2/Vs; with 50% of the NLOFETs remaining functional after these harsh conditions of environmental exposure. To further investigate the barrier properties and to provide insight into the failure mechanism, we conducted similar immersion experiments on samples with Ca sensors encapsulated by either CYTOP/Al2O3 or CYTOP/NL bilayers. The degradation of Ca is easily recognized because it becomes transparent as it is oxidized in contact with water. Figure 3 (a) to (b) and (c) to (d) display photographs of the Ca sensors after 30 s, and 3 min of immersion time in water at 95 °C which reveal the sudden disappearance of the Ca sensors encapsulated with a CYTOP/Al2O3 bilayer after 3 min of immersion. In contrast, the degradation of the Ca sensors encapsulated with a CYTOP/NL bilayer is much slower and appears to be arising from point defects in these particular samples in contrast to the general barrier failure in samples with the


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CYTOP/Al2O3 bilayer. These observations are consistent with recently published studies on the barrier properties of CYTOP/NL layers in hot damp air conditions (50 oC and 85% relative humidity).22 In these studies, we found barrier failure to be primarily determined by residual stress on the ALD layers leading to mechanical failure due to the propagation of cracks starting around areas of concentrated stress (i.e. around particles and defects). According to these studies, a possible path towards improving the mechanical reliability of CYTOP/NL bilayer gate dielectrics could be the introduction of a stress relief layer between CYTOP and the NL; at the cost of adding complexity to the gate dielectric structure and potentially decrease its capacitance density. Although the barrier properties of a CYTOP/NL barrier with identical structure than the one used in this paper were found to be inferior to those of barriers with thicker CYTOP layers (> 200 nm, which from an OFET perspective is not desirable due to a reduced capacitance density), the present study clearly demonstrates that in the context of top-gate OFETs, the use of a NL layer represents a superior solution when compared with the use of an Al2O3 layer in a bilayer gate dielectric structure having a 40 nm CYTOP layer. Hence, we believe the results presented in this paper represent an important step towards developing OFETs with superior operational and environmental stability that withstand conditions as extreme as being submerged in near-boiling water. We have fabricated top-gate OFETs using CYTOP/NL bilayer gate dielectrics and shown that these devices exhibit superior environmental reliability when immerse in water at higher temperature (95 °C) compared with reference OFETs using a CYTOP/Al2O3 bilayer gate dielectric. The electrical characteristics of the original (CYTOP/Al2O3) and the modified (CYTOP/NL) structure are comparable in terms of mobility, threshold voltage, on/off current ratio, and stability in different environments. OFETs with CYTOP/NL gate dielectric show better


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stability when immersed in water at a temperature up to 95 °C over 1 h. In contrast, OFETs with CYTOP/Al2O3 gate dielectric which can only sustain in 95 °C water for less than 3 min. These results can be rationalized by the fact that nanolaminate structures display a better environmental barrier properties compared to a single metal-oxide layer. We believe that these results illustrate against conventional wisdom that OFETs can be robust to harsh environments providing that the structure of the device and the choice of materials are judicious.


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Exposure conditions

Figure 1. (a) The device structure of top-gate OFETs. (b) Transfer characteristics of top-gate OFETs in N2. (c) Performance characteristics of top-gate OFETs exposed to various environments.


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Time (s) Figure 2. (a) Transfer characteristics of Ref- and NL-OFET with Ag gate. (b) DC bias stress in air for 1 h on OFETs with CYTOP/Al2O3 layer (Ref-OFETs) and OFETs with CYTOP/NL layer (NL-OFETs). (c) Performance parameters of Ref-OFETs and NL-OFETs immersed in 95 °C water.


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30 s

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Figure 3. Photographs of encapsulated 100 nm calcium samples. (a) Encapsulated by CYTOP/Al2O3 in water at 95 °C for 30 s. (b) Encapsulated by CYTOP/Al2O3 in water at 95 °C for 3 min. (c) Encapsulated by CYTOP/NL in water at 95 °C for 30 s. (d) Encapsulated by CYTOP/NL in water at 95 °C for 3 min. AUTHOR INFORMATION Corresponding Author *B. Kippelen. Email: [email protected] ACKNOWLEDGMENTS This research was supported in part by the Department of the Navy, Office of Naval Research Award No. N00014-14-1-0580 and N00014-16-1-2520, through the MURI Center CAOP, Office of Naval Research Award N00014-04-1-0313, by the US Department of Energy through the Bay


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Area Photovoltaic Consortium under Award Number DE-EE0004946, by the Renewable Bioproducts Institute at Georgia Tech, and by USDA. S.C. acknowledges support from the Academic Training Program of Samsung Display and B.K. acknowledges a Visiting Professorship from the University of Cologne, Germany.

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12. Hwang, D. K.; Fuentes-Hernandez, C.; Kim, J.; Potscavage, W. J.; Kim, S.-J.; Kippelen, B., Top-Gate Organic Field-Effect Transistors with High Environmental and Operational Stability. Adv. Mater. 2011, 23, 1293-1298. 13. Yun, M.; Sharma, A.; Fuentes-Hernandez, C.; Hwang, D. K.; Dindar, A.; Singh, S.; Choi, S.; Kippelen, B., Stable Organic Field-effect Transistors for Continuous and Non-destructive Sensing of Chemical and Biologically Relevant Molecules in Aqueous Environment. ACS Appl. Mater. Interfaces 2014, 6, 1616-1622. 14. Choi, S.; Fuentes-Hernandez, C.; Yun, M.; Dindar, A.; Khan, T. M.; Wang, C.-Y.; Kippelen, B., Organic Field-Effect Transistor Circuits Using Atomic Layer Deposited Gate Dielectrics Patterned by Reverse Stamping. Org. Electron. 2014, 15, 3780-3786. 15. Kim, L. H.; Kim, K.; Park, S.; Jeong, Y. J.; Kim, H.; Chung, D. S.; Kim, S. H.; Park, C. E., Al2O3/TiO2 Nanolaminate Thin Film Encapsulation for Organic Thin Film Transistors via Plasma-Enhanced Atomic Layer Deposition. ACS Appl. Mater. Interfaces 2014, 6, 6731-6738. 16. Meyer, J.; Schneidenbach, D.; Winkler, T.; Hamwi, S.; Weimann, T.; Hinze, P.; Ammermann, S.; Johannes, H.-H.; Riedl, T.; Kowalsky, W., Reliable Thin Film Encapsulation for Organic Light Emitting Diodes Grown by Low-Temperature Atomic Layer Deposition. Appl. Phys. Lett. 2009, 94, 233305. 17. Seo, S.-W.; Jung, E.; Chae, H.; Cho, S. M., Optimization of Al2O3/ZrO2 Nanolaminate Structure for Thin-Film Encapsulation of OLEDs. Org. Electron. 2012, 13, 2436-2441. 18. Abdulagatov, A. I.; Yan, Y.; Cooper, J. R.; Zhang, Y.; Gibbs, Z. M.; Cavanagh, A. S.; Yang, R. G.; Lee, Y. C.; George, S. M., Al2O3 and TiO2 Atomic Layer Deposition on Copper for Water Corrosion Resistance. ACS Appl. Mater. Interfaces 2011, 3, 4593-4601. 19. Meyer, J.; Schmidt, H.; Kowalsky, W.; Riedl, T.; Kahn, A., The Origin of Low Water Vapor Transmission Rates Through Al2O3/ZrO2 Nanolaminate Gas-Diffusion Barriers Grown by Atomic Layer Deposition. Appl. Phys. Lett. 2010, 96, 243308. 20. Baek, Y.; Lim, S.; Kim, L. H.; Park, S.; Lee, S. W.; Oh, T. H.; Kim, S. H.; Park, C. E., Al2O3/TiO2 Nanolaminate Gate Dielectric Films with Enhanced Electrical Performances for Organic Field-Effect Transistors. Org. Electron. 2016, 28, 139-146. 21. Tiwari, S. P.; Zhang, X. H.; Potscavage, W. J.; Kippelen, B., Low-Voltage SolutionProcessed n-Channel Organic Field-Effect Transistors with High-k HfO2 Gate Dielectrics Grown by Atomic Layer Deposition. Appl. Phys. Lett. 2009, 95, 223303. 22. Bulusu, A.; Singh, A.; Wang, C. Y.; Dindar, A.; Fuentes-Hernandez, C.; Kim, H.; Cullen, D.; Kippelen, B.; Graham, S., Engineering the Mechanical Properties of Ultrabarrier Films Grown by Atomic Layer Deposition for the Encapsulation of Printed Electronics. J. Appl. Phys. 2015, 118, 085501.


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0

-2

-4

-6

-8

0 -10

VGS(V) ACS Paragon Plus Environment

7 6 5 Air Vacuum 50 oC 80% RH Vacuum Air Water Vacuum Air Vacuum 50 oC water Vacuum

-5

10

W/L = 2550 m/ 180m VDS = -10 V

5

|IDS|1/2 (A)1/2

-4

1.2 1.0 0.8 0.6 0.4 0 -2 -4 -6

# of working OFETs

(b) 10

Page 16 of 19

VTH (V)

Organic semiconductor D PFBT S Glass

|IDS| (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

(c)

 (cm2/Vs)


Exposure conditions

4

-6

3

-7

2

-8

1

10

10

10

VDS = -10 V

-9

10

2

0

-2

-4

-6

0 -8 -10

1.025

0.8 0.6 0.4 0.2 0

-2

VGS(V)

(b)

(c)

 (cm2/Vs)

Ref OFETs NL OFETs

# of working VTH (V) OFETs

5

|IDS|1/2 (A)1/2

-5

10

|IDS| (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


-4

(a) 10

IDS(t)/IDS(0)

Page 17 of 19

Ref OFETs NL OFETs

1.000

VGS= -10 V, VDS= -10 V

-4 8 6 4 2 0

Step= 10 s

0.975 0

1200

2400

3600

Time (s) ACS Paragon Plus Environment

Ref OFETs NL OFETs

0 1 2 3 5 8 16 32 64 Immersion time (min)


(b)

CYTOP/Al2O3

(a)

(c)

(d)

CYTOP/NL

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

30 s

3 min


Page 18 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


OFETs in water at 95 °C

4

Gate Nanolaminate Al2O3 CYTOP Organic Semiconductor Drain

3

Source

2

Glass

0 min 3 min 16 min 2

0

-2

-4

VGS (V)


-6

-8

1

0 -10

|IDS|1/2 (A)1/2

Page 19 of 19

Organic Field-Effect Transistors with a Bilayer Gate Dielectric

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