Electrolyte-Gated Vertical Organic Transistor and Circuit - The Journal

Jun 13, 2018 - (1) Several vertical architectures are explored including space .... of P3HT and the illustration of working principle under VGS < 0 V...
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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Electrolyte-Gated Vertical Organic Transistor and Circuit Xinning Luan, Jiang Liu, and Huaping Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03600 • Publication Date (Web): 13 Jun 2018 Downloaded from http://pubs.acs.org on June 18, 2018

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Electrolyte-Gated Vertical Organic Transistor and Circuit Xinning Luan, Jiang Liu, Huaping Li* Atom Nanoelectronics Inc. 440 Hindry Avenue, Unit E, Inglewood California 90301, USA

ABSTRACT An electrolyte-gated vertical organic transistor (VOT) was reported of high transconductance (1.0 mS) and 104 ION/IOFF ratio. Under the gating potential, the filtrated ions through porous Ag electrode stabilized the electrochemically p-doped poly(3- hexylthiophene) (P3HT) to form ohmic contact with Ag electrode for enhanced charge injection. The gate modulation involving in mobile ions was reflected in their slow temporal responses and long retention time. The VOT circuit was demonstrated by loading VOT with a 10 KΩ resistor to exhibit the voltage gain of 7.4.

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1. Introduction Vertical organic transistors have attracted researchers’ interests to overcome the limitations of lateral organic field effect transistors structures.1 Several vertical architectures are explored including space charge limit transistors,2,3 permeable base transistors,4,5charge-blocking layer transistors,6,7 and charge injection transistors.8-25 Among these, charge injection transistors are widely studied using a porous electrode such as thin metal film,8-11 carbon nanotubes (CNT),12-14 silver nanowires,15 graphene,16-21 or patterned metal22-24 and ITO.25,26 In these charge injection transistors, the gate induced electrostatic effect is possibly affected by conductive electrodes and cannot effectively modulate the active channels,7,19 though different opinion was reported.27 While mobile ions can freely infiltrate the porous electrode to stabilize electrochemically doped semiconductors under the driving force of applied gate potential.9 Solid state inorganic salts like LiF were proposed to drift and dope the organic semiconductor through the discontinuous aluminum electrodes in air.8 When LiF exposes to the air, HF from the hydroxylation could be reactive to electrodes, organic materials, even substrates. These would cause significant suspicions on the fidelity of the reported device performances. To avoid this possible issue, electrolyte was used in this work to replace LiF dielectrics. Electrolyte-gated lateral organic transistors were well documented by Frisbie’s research group.28 Electrostatic charging (field effect) was suggested for electrolyte-gated organic transistors in vacuum and bulk electrochemical doping in air.29 Using conjugated polyelectronics as active thin film, Berggren’s group showed the electrochemical doping in the interfacial layers.30 More recently, Rivnay’s research group pointed out that the gate-induced formation of an electrical double layer electrochemically dopes the organic semiconductor at the interface without electrolytes penetrated into channel materials.31 For organic semiconductors30-32 with facile

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penetration of electrolytes, the electrochemical doping occurs with the whole channel. In such condition, the performances of electrolyte-gated organic transistors are highly dependent on thin film thickness. 32 In this contribution, we reported the vertical organic transistor (VOT) in which P3HT (regioregular poly(3-hexyl)thiophene, purchased from Alfa Aesar) was used as channel material, polymer electrolyte (PE) as dielectrics, and 30 nm Ag thin film as porous electrode. The Ag was chosen because its workfunction (4.6 eV) is close to the highest occupied molecular orbit (HOMO, 4.9 eV) of P3HT, in sharply contrast to Al or V2O5 modified Al electrodes.8 The characteristics of VOTs were measured and their circuits were also investigated. This reported VOT structure with simplified fabrication and low-cost materials confer the industrial applications to drive OLED for vertical organic light-emitting transistors, to build in the integrated circuits with 3-D stacks. Moreover, the gate modulated ion distribution through porous silver electrode can act as the model for investigating voltage controlled ion-channel existed in biological system. The reversible redox reactions of active materials could be a new type of resistance switcher. 2. Experimental 2.1 Materials Aluminum slug (99.99%, 0.25 in × 0.25 in) and PEO (poly (ethylene oxide), average molecular weight, 6,000 g mol-1) were purchased from Alfa-Aesar. PMMA (poly(methyl methacrylate), average molecular weight, ~120,000 g mol-1) and LiCF3SO3 (lithium triflate, 99.995%) were purchased from Aldrich. Chlorobenzene (anhydrous, 99.8%) was purchased from SigmaAldrich. Ethyl acetate (residue free, 99.9%) was purchased from Acros. ITO (indium tin oxide)

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glass substrates (Indium-tin-oxide, sheet resistance, 10 Ω sq-1) were purchased from Foshan Meijin Yuan glass technology CO., LTD. All materials were used as received.

2.2 Ag|PE|Ag Fabrication On a clean ITO glass substrate, a layer of 100 nm Ag was evaporated. Polymer electrolyte comprising of PMMA, PEO, and LiCF3SO3 with a weight ratio of 2/1/0.18 was dissolved in ethyl acetate with a concentration of 79.5 mg/mL. The solution was sonicated for 30 min and spin-coated on as-evaporated Ag at 1000 r.p.m. for 60 s. The films were annealed at 120 °C for 10 min in nitrogen filled glovebox. The samples were then transferred into the vacuum deposition system (EcoVap, Mbruan, Inc.) for electrode deposition. Finally, a thin layer of Ag with the thickness of 30 nm were deposited on PE-coated Ag substrates via thermally evaporation through a shadow mask under a high vacuum of ~4×10-6 torr. 2.3 VOT Fabrication ITO glass substrates were cleaned by using sequential ultra-sonication in detergent water, de-ionized water, acetone, and isopropanol. Each ultrasonic bath lasted for 20 minutes. After drying under a nitrogen flow, the ITO glass substrates were subjected to UVozone treatments for 30 minutes in a UVO (ultra-violet oven) cleaner (Model No. 42, Jelight Company. lnc). A solution of poly(3- hexylthiophene) (P3HT) was prepared in chlorobenzene at a concentration of 2 wt% and spin-coated onto ITO glass substrates at 800 r.p.m. for 60 s. The P3HT-coated substrates were transferred into a nitrogen-filled glove box with O2 and H2O levels below 0.5 p.p.m. and annealed at 120 °C for 30 min. The samples were then transferred into the vacuum deposition system (EcoVap, Mbruan, Inc.) for electrode deposition. Finally, a thin layer of silver source electrode with a thickness of 30 nm was deposited on P3HT-coated substrates via thermally evaporation through a shadow mask under a high vacuum of ~4×10-6 torr. PE

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comprising of PMMA, PEO, and LiCF3SO3 with a weight ratio of 2/1/0.18 was dissolved in ethyl acetate with a concentration of 79.5 mg/ml. The solution was sonicated for 30 min and spin-coated on Ag deposited P3HT substrates at 1000 r.p.m. for 60 s, the films were annealed at 120 °C for 10 min in nitrogen atmosphere glovebox. On top of polymer electrolyte, 100 nm-thick aluminum film was deposited by thermal evaporation as gate electrode. The active area of VOFET is 0.0525 cm2. 2.4 Device Characterization C-V measurements were conducted on Al|PE|Al devices with Keithley 4200 SCS (semiconductor characterization system), which is controlled by a LabVIEW program. The VOT devices were electrically contacted using a clamp (3M) with spring loaded Au-coated probes for drain, source, and gate contact. All measurements took place inside a nitrogen atmosphere glovebox. The current density-voltage, and VIN-VOUT characterizations, temporal responses were measured with a Keithley 4200 SCS, which is controlled by a LabVIEW program. In the measurement of all devices, the source electrodes of VOT were held at ground potential. The drain voltage (VDS) and gate voltage (VG) were applied to the ITO and gate Al electrode, respectively. 3. Results and discussion The schematic structure of VOT is illustrated in Figure 1A. The active material, P3HT (300 nm, measured using Dektak 6 Surface Profilometer), was sandwiched between indium tin oxide (ITO) drain electrode and a thermally evaporated porous Ag source electrode (2 Angstrom per second) with a thin thickness of 30 nm. The Ag thin film was imaged with scanning electron microscope (SEM) showing connected grains forming percolation paths (Figure 1B). The

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porosity and the electric conductivity of Ag thin film need to be balanced. Generally, the thinner films (40 nm in thickness) will have better electric conductivity but poorer gate modulation indicating lower porosity.9 A layer of polymer electrolyte (PE) was deposited on top of Ag source electrode by spin coating PE solution in ethyl acetate. The PE solution consists of poly(methyl methacrylate) (PMMA):poly(ethylene oxide) (PEO):lithium triflate as illustrated in Figure 1C. On top of PE, a 100 nm-thick Al gate electrode was evaporated to complete P3HT VOT. The work function of electrodes and energy level of P3HT are shown in Figure 1D. As depicted in Figure 1D, when the negative potential is applied to Al gate (VGS < 0 V) in referral to grounded Ag source, the ions in PE will be redistributed with Li+ and CF3SO3- moving towards Al gate and Ag source electrodes, respectively. The accumulated CF3SO3- ions then infiltrate the pores of Ag thin film to stabilize electrochemically p-doped P3HT at interfacial regime which forms ohmic contact with Ag electrode.33,34

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Figure 1. A) Schematic structure of VOT comprising of PEDOT coated ITO, P3HT, porous Ag, PE, and Al from bottom to top. B) SEM image of Ag thin film showing sporadic worm-like network. C) Chemical structures of materials used in VOT. D) The energy diagram of the work function of electrodes and energy level of P3HT, and the illustration of working principle under VGS < 0 V.

In PE, PMMA was employed on purpose to eliminate the gate leakage. PE gated VOT devices rapidly degraded if only using PEO/ Li+CF3SO3-. PMMA also provides thermal resistance and mechanic strength to support metal via thermal vapor deposition. Even with 1/1 weight ratio PMMA/PEO, the heat generated from significant gate leakage melted polymer electrolyte.

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Additionally, PMMA assists the solubility of PEO/CF3SO3-Li+ in organic solvent for orthogonal solution processes on PE gated VOT without damaging active organic layer. The capacitance of PE was characterized using Keithley 4200 semiconductor characterization system with 4225 RPM remote amplifier. Figure 2A shows specific capacitance of the fabricated Ag/PE/Ag capacitor in the frequency range from 1 kHz to 10 MHz. As a reference, the specific capacitance of PEO/Li+CF3SO3- sandwiched between two Ag electrodes is compared in Figure 2A. The capacitance of PE (CPE) was measured to be 0.44 µF cm-2 with a 1 kHz AC signal (Figure 2B). This value is about one order of magnitude lower than that of PEO/Li+ salt (5 µF cm-2)

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higher than that of 100 nm PMMA (0.02 µF cm-2).35,36 The specific capacitance of PE in the range from 1 kHz to 10 MHz is about one order of magnitude smaller than that of PEO/Li+CF3SO3- likely due to the existence of PMMA.

Figure 2. A) Specific capacitance of PEO+Li+Tf- and PE sandwiched between two Ag electrodes as the function of frequency from 1 kHz to 1 MHz. B) Specific capacitance of PE sandwiched between two Ag electrodes as the function of applied voltage from -9 V to 9 V at 1 kHz.

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The device was characterized in nitrogen filled glovebox using Keithley 4200 SCS (semiconductor characterization system). Figure 3A shows output characteristics of VOT by sweeping VDS from 0 V to -1V applied at ITO drain electrode with reference to Ag source electrode at different gate potentials (0 V, -1 V, -2 V, -3 V, -4 V, -5 V and -6 V). The JD exponentially increased from 10-9 A to be saturated when VDS swept to -0.1 V, typical of p-type characteristics. The hole was injected from porous Ag electrode and drained out from ITO. The JD-VDS curve slightly move down and then dramatically move up from 10-6 A/cm2 to10-2 A/cm2, when VGS ramped from 0 V to -3 V, then to -6 V at a very small working voltage of VDS=-1V. This indicates the current density of VOT can be effectively modulated by the applied VGS potential. The transfer characteristics of VOT was measured by setting Ag as source electrode and applying VDS=-1V at ITO as shown in Figure 3B. Consistent with output characteristics, IDS slightly decreased and then remarkably increased from 10-6 A/cm2 to10-2 A/cm2 when VGS was swept from 0 V to -3 V, then -6 V. The slight decrease could be due to the reorganization of interfacial PE which had favorite dipole moment arrangement for charge injection37 or Li+ stabilized electrochemical n-doping under the diode potential of VDS= -1 V.22,38 The gate leakage is shown in Figure 3C. The noise signals observed in gate current could be due to the measurement environmental vibrations. These results imply that the increased IDS was not from gate leakage current, but arose from the injection carriers tunable by gate potential. When VGS < 3 V, the dominated CF3SO3- ions at interfacial layer played the major role to stabilize the electrochemically p-doped P3HT (P3HT took hole carriers injected from source electrode to be P3HT+). The p-doped P3HT formed ohmic contact with Ag electrode for enhanced hole injection, leading to about 10,000 times increase in JD until saturated. After VGS was released from -6 V to 0 V, JD remained on-state due to the stored potential (Figure 3B).9,39,40 A positive

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VGS was applied to Al gate electrode to redistribute Li+ and CF3SO3- ions (Figure 3B). The decrease of CF3SO3- ions and the increase of Li+ ions at interfacial layer made electrochemically p-doped P3HT unstable, resulting in the enlarged hole injection barrier until Li+ ions dominated at the interface layer. Therefore, a significant hysteresis was observed in the transfer curve of VOT (Figure 3B). Generally, the extreme hysteresis was caused by deeply trapped charge carriers. Here the large hysteresis could be due to slow ion motion and subsequently delayed response. Their transconductances (gm) were plotted in Figure 3D by taking derivative of drain current (IDS = JD × S, where S is the surface area) against VGS from Figures 3B. Their maximum gm under VDS=-1 V were 1.0 mS for these fabricated VOT, two orders of magnitude greater than P3HT VOT using graphene as a tunable contact.17

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VG (V) Figure 3. A) Output and B) transfer characteristic of VOT. C) The gate current density of VOT was plotted against VG. D) Transconductance was plotted against VG by taking the derivative drain current (IDS) against VG in I-V curve of B). The elapsed time between VG and VDS was set as 0.1 second. The scanning rate is 73 mV/second. The temporal responses of device current density were also recorded at VDS=-1 V by applying a rectangular pulse VGS potential between -6 V and 6 V. Figure 4 shows that JD remained at a low

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value of 10-5 A/cm2 under the potential of VGS=6 V. When the pulse potential was switched from VGS = 6 V to -6 V, JD sharply increased and then kept gradually increasing within the pulse (35 second). When VGS was switched from -6 V to 6 V, JD abruptly went down back to a low value. These gradual growths can be divided into a fast process that could be due to the barrier lowing27 and a slow accumulation process of ions toward electrodes (supporting information Figure S1). Such time scale in tens of seconds is consistent with other optoelectronics involving with ion motion.2,9,33,34,39,40 This slow process can be expedited by raising the device temperature or operating in liquid phase,29,31,32 leading to the elimination of hysteresis. Interestingly, the release of gathered ions from electrodes is prompt, evidenced as the swift drop-off.

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Time (s) Figure 4. Temporal response of current density of VOT by applying rectangular gate potential between 6 V and -6 V, showing the reversible on/off cycles.

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In order to test the applicability of VOT in simple circuits, a resistor loaded inverter was constructed by connecting the P3HT VOT in series with a 10 kΩ resistor. The circuit diagram is shown in Figure 5A. The terminal of resistor was set as the ground, the terminal of VOT as VDS, the gate of VOT as VIN, and the connection point of VOT and resistor as VOUT. The obtained input-output voltage characteristics (VIN - VOUT) of the device are plotted in Figure 5B. Under a voltage supply of VDS = 7 V, when VIN swept from 10 V to 0V, the VOT started to turn on when VIN = 2.5 V (the inverter’s threshold voltage), and completely turned on at VIN = 0 V with VOUT of 5.5 V. When VIN swept back from 0 V to 10V, the VOT remained on-status until VIN = 7 V, and completely turned off at VIN = 9 V with VOUT of 0.6V. The forward and reverse voltage gains were estimated to be 3.5 at 1.8 V and 7.4 at 8.2 V by deriving their VOUT/VIN. Analogous to the transfer characteristics of VOT, the detainment of VOT on-status might be arisen from the stored potential of PE capacitor.9,39,40 The dynamic responses of the resistor loaded inverter were tested by applying rectangular input voltage between 0 V and 10 V at a pulse of 15 s. As shown in Figure 5C, the output voltage of the inverter reversely switched from 4V to 0.5 V following the input pulse. Over 20 cycles, there was no obvious drop-off, indicating sufficient operating stability.

VDD=7V

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Time (s) Figure 5. A) The circuit diagram of the resistor-loaded VOT inverter. B) The voltage output curve of the resistor-loaded VOT inverter. The insert shows the gain of this inverter. C) Dynamic responses of the resistor-loaded VOT inverter by supplying a rectangular voltage input.

4. Conclusions

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In summary, the electrolyte-gated vertical organic transistor (VOT) was fabricated based on ptype P3HT polymer and porous Ag electrode. The characteristics of P3HT VOT were characterized to be the typical p-type transistor with 10 mA/cm2 on-current density and 1 µA/cm2 off-current density, about 104 ION/IOFF ratio. The slow temporal response and long retention time of P3HT VOT supports the operating mechanism that the redistribution of mobile ions stabilizes the electrochemically p-doped P3HT through porous electrode driving by applied gating potential. The P3HT VOT based circuit was demonstrated by connecting VOT with a 10 KΩ resistor to form a resistor loaded inverter with a voltage gain of 7.4. Supporting Information Available The division of temporal response of P3HT VOT into a fast process about 4 second and a slow ion accumulation about 20 second. ACKNOWLEDGMENT The financial support from Special Command Office (contract # H92222-17-P-0006) was greatly appreciated.

Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Abbreviations

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VOT, vertical organic transistor. PE, polymer electrolyte. PEO, poly (ethylene oxide). PMMA, poly(methyl methacrylate). ITO, indium tin oxide. SEM, scanning electron microscope. HOMO, highest occupied molecular orbit. UVO, ultra-violet oven. References 1. Lüssen, B.; Günther, A.; Fischer, A.; Kasemann, D.; Leo, K. Vertical Organic Transistors. J. Phys. Condens. Matter. 2015, 27, 443003. 2. Chao, Y.-C.; Meng, H.-F.; Horng, S.-F.; Hsu, C.-S. High-Performance Solution-Processed Polymer Space-Charge-Limited Transistor. Org. Electron. 2008, 9, 310-316. 3. Zan, H.-W.; Hsu, Y.-H.; Meng, H.-F.; Huang, C.-H.; Tao, Y.-T.; Tsai, W.-W. High Output Current in Vertical Polymer Space-Charge-Limited Transistor Induced by Self-Assembled Monolayer. Appl. Phys. Lett. 2012,101, 093307. 4. Bozler, C. O.; Alley, G. D. Fabrication and Numerical Simulation of the Permeable Base Transistor. IEEE Trans. Electron Devices 1980, 27, 1128-1141. 5. Serbena, J. P.; Hümmelgen, I. A.; Hadizad, T.; Wang, Z. Y. Hybrid Permeable ‐ Base Transistors Based on an Indenofluorene Derivative. Small 2006, 2, 372-374. 6. Kleemann, H.; Günther, A. A.; Leo, K; Lüssen, B. High-Performance Vertical Organic Transistors. Small 2013, 9, 3670-3677. 7. Kwon, H.; Kim, M.; Cho, H.; Moon, H.; Lee, J.; Yoo, S. Toward High‐Output Organic Vertical Field Effect Transistors: Key Design Parameters. Adv. Funct. Mater. 2016, 26, 68886895. 8. Li, S.-H.; Xu, Z.; Yang, G.; Ma, L.; Yang, Y. Solution-Processed Poly(3-Hexylthiophene) Vertical Organic Transistor. Appl. Phys. Lett., 2008, 93, 213301. 9. Luan, X.; Liu, J.; Pei, Q.; Bazan, G. C.; Li, H. Electrolyte Gated Polymer Light‐Emitting Transistor. Adv. Mater. Technol. 2016, 1, 1600103. 10. Ma, L.; Yang, Y. Unique Architecture and Concept for High-Performance Organic Transistors. Appl. Phys. Lett. 2004, 85, 5084-5086. 11. Xu, Z.; Li, S.-H.; Ma, L.; Li, G.; Yang, Y. Vertical Organic Light Emitting Transistor. Appl. Phys. Lett. 2007, 91, 092911. 12. Liu, B.; McCarthy, M. A.; Yoon, Y.; Kim, D. Y.; Wu, Z.; So, F.; Holloway, P. H.; Reynolds, J. R.; Guo, J.; Rinzler, A. G. Carbon‐Nanotube‐Enabled Vertical Field Effect and Light‐ Emitting Transistors. Adv. Mater. 2008, 20, 3605-3609.

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