Control of Threshold Voltage for Top-Gated Ambipolar Field-Effect

Research Article www.acsami.org

Control of Threshold Voltage for Top-Gated Ambipolar Field-Effect Transistor by Gate Buffer Layer Dongyoon Khim,†,‡ Eul-Yong Shin,† Yong Xu,† Won-Tae Park,† Sung-Ho Jin,§ and Yong-Young Noh*,† †

Department of Energy and Materials Engineering, Dongguk University, 30, Pildong-ro 1-gil, Jung-gu, Seoul 100-715, Republic of Korea ‡ Center for Plastic Electronics, Department of Physics, Blackett Laboratory, Imperial College London, London SW7 2AZ, United Kingdom § Department of Chemistry Education, Graduate Department of Chemical Materials, and Institute for Plastic Information and Energy Materials, Pusan National University, Busan 609-735, Republic of Korea S Supporting Information *

ABSTRACT: The threshold voltage and onset voltage for pchannel and n-channel regimes of solution-processed ambipolar organic transistors with top-gate/bottom-contact (TG/BC) geometry were effectively tuned by gate buffer layers in between the gate electrode and the dielectric. The work function of a pristine Al gate electrode (−4.1 eV) was modified by cesium carbonate and vanadium oxide to −2.1 and −5.1 eV, respectively, which could control the flat-band voltage, leading to a remarkable shift of transfer curves in both negative and positive gate voltage directions without any side effects. One important feature is that the mobility of transistors is not very sensitive to the gate buffer layer. This method is simple but useful for electronic devices where the threshold voltage should be precisely controlled, such as ambipolar circuits, memory devices, and light-emitting device applications. KEYWORDS: organic-field effect transistors, ambipolar transport, threshold voltages, flat-band voltages, buffer layers

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respectively.15 In a metal−insulator−semiconductor (MIS) structure, the Vfb in the absence of a fixed charge in the insulator or at the insulator/silicon interface can be generally expressed by

INTRODUCTION Solution-processed organic field-effect transistors (OFETs) are promising candidates for use in the next generation of electronics because of their unique qualities, including flexible and large area application by cost-effective manufacturing system.1−3 Impressive progress has been made regarding the increase of the mobility more than 10 cm2/(V s), but a clear understanding of organic semiconducting materials and device physics is still required.4−6 The electrical characteristics of OFETs are determined not only by the intrinsic properties of the semiconductor but also by extrinsic factors coming from the contacts, dielectric, doping, and device structure.7−13 One of the most important parameters for OFETs is the threshold voltage (Vth). Vth is the minimum gate-to-source voltage (Vgs) needed to create a conducting path in the channel. In Si metal oxide semiconductor field-effect transistors (MOSFETs), Vth separates the weak and strong inversion, whose value is mainly determined by the doping dose and the substrate-source voltage.14 However, in OFETs, its conventional meaning is lost since OFETs are thin-film transistors using intrinsic organic semiconductors. So, only two regions, the hole and electron accumulation regions, would be ideally considered at both sides of the flat-band voltage (Vfb) corresponding to the gate voltage yielding a flat energy band in the semiconductor, as hole accumulation (Vgs > Vfb) and electron accumulation (Vgs < Vfb), © XXXX American Chemical Society

Vfb =

Wm − Ws q

where Wm and Ws are work functions of the gate metal and semiconductor, respectively, and q is the elementary charge. Meanwhile, recently high mobility conjugated polymers with low band gap have been intensively synthesized based on push−pull structures for photovoltaic devices and transistors.16−18 Thanks to the low injection barrier between the work function of the source/drain (S/D) electrode and transport energy level such as highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO), these are fascinating materials for application to ambipolar electronics.12,19 Compared to the charge transport behavior of unipolar OFETs, such as Vth, onset voltage (Von), and hole and electron mobility, ambipolar OFETs are sensitive to extrinsic effects, such as electrode metals, selection of gate dielectrics, Received: March 26, 2016 Accepted: June 20, 2016

A

DOI: 10.1021/acsami.6b03671 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

performed under nitrogen using a Keithley 4200-SCS semiconductor parameter analyzer. The work function of Al and modified Al electrodes was measured by the ultraviolet photoelectron spectroscopy (UPS) which was conducted in a PHI-5000 ultrahigh vacuum surface analysis system using a monochromatized Al Kα and He I (hν = 21.2 eV) excitation source at a pressure of 1 × 10−6 Pa. The surface of V2O5 and Cs2CO3 on top of the PMMA dielectric was also monitored using atomic force microscopy (AFM) (Nanoscope, Veeco Instrument Inc.).

doping of organic semiconductors, and gate electrode.9,11,13,20,21 The well-controlled electrical parameters of ambipolar devices are essential for application in light-emitting devices and logic circuits.15,19 In this paper, we report a facile way to control Vth and onset voltage (Von) for solution-processed ambipolar top-gate/ bottom-contact (TG/BC) OFETs by insertion of a gate buffer layer in between the gate electrode and the gate dielectric. This is an approach by control of Vfb, which is different from charge injection layer or doping, where Vth is mainly controlled by charge injection properties from electrode to semiconductor.11−13 The Von and Vth of the OFETs using poly(3hexylthiophene) (P3HT), poly(3,8-naphthalenedicarboximide2,6-diyl]-alt-5,5′-(2,2′-bithiophene)) (P(NDI2OD-T2)), and diketopyrrolopyrole-thieno[3,2-b]thiophene (DPPT-TT) are effectively tuned by vanadium oxide (V2O5) for the p-channel and cesium carbonate (Cs2CO3) for the n-channel.

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RESULTS AND DISCUSSION V2O5 and Cs2CO3 were used as the p-type and n-type buffer layers, respectively, due to widely noted materials for effectively tuning the work function of metals.13,22,23 To clarify this inference, the work functions of the pristine Al and the modified Al were measured by ultraviolet photoelectron spectroscopy (UPS). Figure 2a shows the UPS results of Al

EXPERIMENTAL SECTION

TG/BC organic transistors were made using Corning glass substrate with a prepatterned Au S/D electrodes by lift-off methods (Figure 1a).

Figure 2. (a) Ultraviolet photoelectron spectroscopy spectra showing the evolution of work function of Al modified by gate buffer layers. (b) Energy level of P3HT, P(NDI2OD-T2), and DPPT-TT thin films and work function of Al and modified Al by gate buffer layers. (c) AFM images of V2O5 and Cs2CO3 on top of PMMA dielectrics.

Figure 1. (a) Schematic of top-gate/bottom-contact OFETs with gate buffer layer. (b) Chemical structure of DPPT-TT, P(NDI2OD-T2), and P3HT as semiconducting polymers. P3HT as the p-channel semiconductor and P(NDI2OD-T2) as the nchannel semiconductor were purchased from Rieke Metal and Polyera Inc. and used as received. DPPT-TT is a low bandgap ambipolar semiconductor [bandgap (Eg) = 1.2−1.3 eV] and was synthesized in our laboratory. The P3HT, P(NDI2OD-T2), and DPPT-TT inks were spin-coated from chlorobenzene (5 mg/mL) on a prepatterned substrate, and the films were annealed at 150 °C (P3HT, P(NDI2ODT2) and 200 °C (DPPT-TT) for 20 min. Subsequently, a 500 nm thick poly(methyl methacrylate) (PMMA) (Sigma-Aldrich) layer was deposited by spin-coating and then annealed at 80 °C for 60 min. To form the gate buffer layer, V2O5 and Cs2CO3 were thermally evaporated at 0.2−0.3 Å/s (final thickness of 10 nm) on top of the gate dielectric, and finally, Al gate electrode was formed by thermal evaporation using a metal shadow mask. All device measurements were

(40 nm), V2O5 (10 nm)/Al (40 nm), and Cs2CO3 (10 nm)/Al (40 nm). It was found that the work function increased from 4.1 eV (pristine Al) to 5.1 eV (with V2O5) or decreased to 2.1 eV (with Cs2CO3). We believe that this variation of work function (ΔEf ∼ 3.0 eV) is enough to tune Vfb and, in turn, to adjust the threshold voltage. The surface of V2O5 and Cs2CO3 on top of the PMMA dielectric was also investigated using atomic force microscopy (AFM). Figure 2c displays the height mode AFM images of V2O5 and Cs2CO3 on top of the PMMA dielectric layer. The V2O5 on PMMA shows a very smooth surface (rms roughness 0.4 nm), and the thermally deposited B

DOI: 10.1021/acsami.6b03671 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 3. (a) The p-type transfer characteristics of P3HT OFETs with and without a V2O5 buffer layer, corresponding (b) linear fit for threshold voltage, and (c) gate leakage current. (d) The n-type transfer characteristics of P(NDI2OD-T2) OFETs with and without V2O5 buffer layer, corresponding (e) linear fit for threshold voltage, and (f) gate leakage current.

Table 1. Electrical Parameters of Top-Gate/Bottom-Contact OFETsa semiconductor P3HT P(NDI2OD-T2) DPPT-TT

a

carrier type

buffer layer

hole hole electron electron hole electron hole electron hole electron

W/O V2O5 W/O V2O5 W/O W/O V2O5 V2O5 Cs2CO3 Cs2CO3

mobility [cm2/(V s)] 0.11 0.21 0.21 0.20 1.48 0.10 1.62 0.087 1.42 0.13

(±0.021) (±0.03) (±0.12) (±0.011) (±0.32) (±0.09) (±0.11) (±0.08) (±0.09) (±0.087)

Vth [V] −26.2 −19.7 12.6 30.7 −28.5 41.9 −22.5 45.9 −35.8 40.0

(±2.4) (±3.0) (±2.1) (±1.8) (±1.5) (±3.5) (±2.5) (±2.8) (±1.4) (±2.1)

Von [V] 3 8 0 15 −17.1 40 −9.6 45.7 −22.5 34.1

Ci = 6.0 nF/cm2, W/L is 1.0 mm/20 μm.

Cs2CO3 layer is randomly formed on PMMA as nanoclusters, which is consistent with previously reported results.20 We first checked OFETs using P3HT and P(NDI2OD-T2) as representative unipolar p- and n-channel materials, whose gate buffer layer would work properly in our TG device. Figure 3 shows the transfer characteristics of TG/BC P3HT and P(NDI2OD-T2) OFETs with Al and V2O5/Al gate electrodes and the corresponding linear fit for Vth extraction. The mobility and Vth were calculated in saturation regime (at Vd = ±60 V) using the gradual channel approximations equations. The key device parameters including μFET, Vth, and Von are summarized in Table 1. As shown in Figures 3a and 3d, P3HT and P(NDI2OD-T2) OFETs exhibit general p- and n-type transfer characteristics with high mobility up to 0.1 and 0.2 cm2/(V s), respectively. After insertion of the V2O5 buffer layer, the most important observation of P3HT OFETs is the shift of the Vth (ΔVth) from −26.2 to −19.7 V. Likewise, with the V2O5 buffer layer, P(NDI2OD-T2) OFETs show an opposite trend, shifting Vth

from 12.6 to 30.7 V. One important feature is that the mobility is not so sensitive to the gate buffer layer. For example, the mobility of P3HT OFETs is increased a little from 0.11 cm2/(V s) (pristine) to 0.21 cm2/(V s) (including the V2O5 buffer layer), and the P(NDI2OD-T2) OFETs maintain a mobility of 0.2 cm2/(V s) no matter whether a V2O5 buffer layer is inserted or not (the hole mobility of P(NDI2OD-T2) OFETs is increased a bit from 0.0072 to 0.0095 cm2/(V s); see Figure S1 in the Supporting Information). The gate leakage current (Ig) using the V2O5 buffer layer is comparable to the counterparts using only the Al gate electrode (see Figure 3c,f). It means that the gate buffer layer on top of the PMMA as a dielectric does not influence the insulating properties of the gate dielectrics. Next, we applied this method to ambipolar OFETs for tuning the Vth of both the hole and the electron. Figure 4 displays the transfer characteristics of ambipolar DPPT-TT OFETs with various gate buffer layers. As reference, OFETs with only an Al gate electrode exhibit high hole mobility (1.48 cm2/(V s)) at Vd = −60 V and electron mobility (0.1 cm2/(V s)) at Vd = 60 V C

DOI: 10.1021/acsami.6b03671 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) The p-type and (b) n-type transfer characteristics of DPPT-TT OFETs depending on bias condition without and with gate buffer layer as Cs2CO3 for n-type buffer and V2O5 for p-type buffer. Linear fits for (c) p-type and (d) n-type DPPT-TT OFETs. Threshold voltage and onset voltage evolution of (e) p-type and (f) n-type DPPT-TT OFETs.

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CONCLUSIONS In conclusion, we have demonstrated that gate buffer layers can modulate the ambipolarity of TG/BC polymer OFETs, including Von and Vth. It was obvious that the modified Al electrodes with V2O5 and Cs2CO3 could control Vfb, leading to positive and negative shifts of Von and Vth. All devices showed hysteresis-free behavior with low leakage current. The degree of this change could be tuned by minutely adjusting the Ef of Al by varying the thickness of the gate buffer layer. This method is simple but useful for electronics devices where the threshold voltage should be precisely controlled, such as ambipolar circuits, memory devices, and light-emitting device applications.

without any hysteresis, with a large Vth (−28.5 V for p-channel and 41.9 V for n-channel operation) and onset voltage (Von) (−17.1 V for hole and 40.0 V for electron). With gate buffer layers on top of the PMMA dielectric, ambipolarity, including the Vth and Von, of DPPT-TT OFETs, is properly tuned toward negative and positive voltages by Cs2 CO3 and V 2O 5, respectively. The Von of DPPT-TT OFETs with V2O5 is −9.6 V (ΔVon,h = 7.5 V) in p-channel operation, and that with Cs2CO3 is 34.1 V (ΔVon,e = 5.9 V) in n-channel operation. In ambipolar transistors, Von could be explained as the voltage to change the transport carrier types (holes or electrons) in the channel depending on the gate voltages.11,15 Therefore, the ΔVon directly refers to the ratio change upon hole and electron accumulations in the channel of ambipolar FETs. Figure 4c,d shows the linear fit for extracting Vth and mobility. As expected, Vth was also shifted to the same direction as ΔVon (see Figure 4c−f). It should be noted that this result is different from those of the unipolar device. In OFETs, two regions would be ideally considered as hole accumulation (Vgs > Vfb) and electron accumulation (Vgs < Vfb) on both sides of the Vfb. However, charge transport of both regimes would be limited by the presence of a charge injection barrier and the intrinsic properties of a disordered semiconductor.12,24 For example, P3HT as a p-channel material is able to form conducting channel upon hole accumulation when Vgs > Vfb, but it will change to an insulating state for electron accumulation when Vgs < Vfb.24 While, in ambipolar transistor, hole accumulation (Vgs > Vfb) and electron accumulation (Vgs < Vfb) are concurrently presented, and our results clearly show that the accumulation characteristics of both regimes would be controllable by adjusting Vfb. Evolution of Vfb is attributed to the work function difference (Al: −4.0 eV; V2O5/Al: −5.1 eV; Cs2CO3: −2.1 eV) of gate electrode by buffer layers (see Figure 2b).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b03671. More details on ambipolar transport of P(NDI2OD-T2) OFETs, typical ambipolar transfer, and output curves of P(NDI2OD-T2) OFETs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (Y.-Y.N.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2014M3A7B4051749) and Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & D

DOI: 10.1021/acsami.6b03671 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Electron Injection Enhancement by a Cs-salt Interlayer in Ambipolar Organic Field-Effect Transistors and Complementary Circuits. J. Mater. Chem. 2012, 22, 16979−16985. (21) Chung, Y.; Johnson, O.; Deal, M.; Nishi, Y.; Murmann, B.; Bao, Z. Engineering the Metal Gate Electrode for Controlling the Threshold Voltage of Organic Transistors. Appl. Phys. Lett. 2012, 101, 063304. (22) Chu, C. W.; Li, S. H.; Chen, C. W.; Shrotriya, V.; Yang, Y. Highperformance Organic Thin-film Transistors with Metal Oxide/Metal Bilayer Electrode. Appl. Phys. Lett. 2005, 87, 193508. (23) Huang, J.; Xu, Z.; Yang, Y. Low-Work-Function Surface Formed by Solution-Processed and Thermally Deposited Nanoscale Layers of Cesium Carbonate. Adv. Funct. Mater. 2007, 17, 1966−1973. (24) Kergoat, L.; Herlogsson, L.; Piro, B.; Pham, M. C.; Horowitz, G.; Crispin, X.; Berggren, M. Tuning the Threshold Voltage in Electrolyte-Gated Organic Field-Effect Transistors. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 8394−8399.

Future Planning (NRF-2013M3C1A3065528), and the NRF (2011-0028320) by the Ministry of Science, ICT & Future Planning.

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DOI: 10.1021/acsami.6b03671 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX