All-Aerosol-Jet-Printed Carbon Nanotube Transistor with Cross-Linked Polymer Dielectrics

The printability of reliable gate dielectrics and their influence on the stability of the device are some of the primary concerns regarding the practical application of printed transistors. Major ongoing research is focusing on the structural properties of dielectric materials and deposition parameters to reduce interface charge traps and hysteresis caused by the dielectric–semiconductor interface and dielectric bulk. This research focuses on improving the dielectric properties of a printed polymer material, cross-linked polyvinyl phenol (crPVP), by optimizing the cross-linking parameters as well as the aerosol jet printability. These improvements were then applied to the fabrication of completely printed carbon nanotube (CNT)-based thin-film transistors (TFT) to reduce the gate threshold voltage (Vth) and hysteresis in Vth during device operation. Finally, a fully aerosol-jet-printed CNT device was demonstrated using a 2:1 weight ratio of PVP with the cross-linker poly(melamine-co-formaldehyde) methylated (PMF) in crPVP as the dielectric material. This device shows significantly less hysteresis and can be operated at a gate threshold voltage as low as −4.8 V with an on/off ratio of more than 104.


Introduction
Printed electronics have emerged as a low-cost alternative to silicon electronics and enable the manufacturing of electronic devices without high-temperature, -pressure, and -vacuum systems [1][2][3][4][5][6]. For flexible and large-area applications such as sensors, radiofrequency identification (RFID), and displays, printed electronics offer compatibility with various flexible substrates [7][8][9][10][11]. Moreover, most printed devices made of organic materials are biocompatible and have numerous applications in drug delivery, invasive sensors, skin electronics, wearables, etc. [12][13][14][15]. The main roadblock for printed electronics is their lower performance because of the resolution of the printing technique and the poor printability, availability, and compatibility of materials for solution processing or ink formation [16][17][18][19][20][21]. Yet, some materials, such as carbon nanotubes (CNTs), have shown extraordinary qualities in terms of electrical, mechanical, and thermal properties [22][23][24][25]. CNTs can be solutionprocessed for printed applications and are compatible with various other inks and flexible substrates. In recent years, tremendous progress has been made for printed CNT thinfilm transistors (TFTs) to achieve a high on/off ratio of up to 10 6 and mobility up to 25 cm 2 V −1 s −1 . However, the devices face challenges such as large hysteresis in gate threshold voltages, sub-threshold swings, and the availability of reliable printed dielectric materials [6,26,27]. Most previously printed CNT-TFTs have been fabricated by using atomic layer deposition (ALD) to deposit a thin layer of a dielectric (mainly metal-oxidebased) for high performance [28][29][30]. The hydroxyl group (-OH) present in these dielectrics acts as an interface trap at the junctions of CNTs and dielectric layers [31,32]. Amorphous dielectrics also suffer from bulk charge traps due to broken bonds that result in hysteresis angle measurements, dielectric materials with 10 wt.% concentration were spin-coated on the glass substrate and cured at 170 • C. FTIR spectroscopy was conducted using a Bruker ALPHA II FTIR Spectrometer (Bruker Co., Billerica, MA, USA), and C-V measurement of the dielectric film was performed using an 802-150 MDC Mercury Probe station (MDC, Chatsworth, CA, USA). All Scanning Electron Microscope (SEM) analyses were carried out using an FEI Helios Nanolab 400 system (FEI, Hillsboro, OR, USA). For SEM analysis, all materials were printed on a silicon substrate. Nanomaterials 2022, 12, x FOR PEER REVIEW 4 of 13 measured using a RheoSense m-VROC viscometer (RheoSense, San Ramon, CA, USA) , and the contact angle was measured using a Kruss tensiometer (Kruss, Matthews, NC, USA). For contact angle measurements, dielectric materials with 10 wt.% concentration were spin-coated on the glass substrate and cured at 170 °C. FTIR spectroscopy was conducted using a Bruker ALPHA II FTIR Spectrometer (Bruker Co., Billerica, MA, USA), and C-V measurement of the dielectric film was performed using an 802-150 MDC Mercury Probe station (MDC, Chatsworth, CA, USA). All Scanning Electron Microscope (SEM) analyses were carried out using an FEI Helios Nanolab 400 system (FEI, Hillsboro, OR, USA). For SEM analysis, all materials were printed on a silicon substrate.

PVP-PMF Weight Ratio Optimization (Effect of PVP-PMF Weight Ratio on Dielectric Properties and Printability)
Before printing the transistor, the formulation and printability of the crPVP ink were optimized. To investigate the optimal weight ratio of PVP to PMF, crPVP ink was formulated at different weight ratios of PVA to PMF (1:1, 2:1, 3:1, and 5:1). Previous research has shown that hydroxyl group species with various hydrogen-bonding interactions are responsible for the absorption of water molecules into the PVP dielectric, and that leads to Nanomaterials 2022, 12, x FOR PEER REVIEW measured using a RheoSense m-VROC viscometer (RheoSense, San Ramon, CA, and the contact angle was measured using a Kruss tensiometer (Kruss, Matthew USA). For contact angle measurements, dielectric materials with 10 wt.% concen were spin-coated on the glass substrate and cured at 170 °C. FTIR spectroscopy wa ducted using a Bruker ALPHA II FTIR Spectrometer (Bruker Co., Billerica, MA, USA C-V measurement of the dielectric film was performed using an 802-150 MDC M Probe station (MDC, Chatsworth, CA, USA). All Scanning Electron Microscope analyses were carried out using an FEI Helios Nanolab 400 system (FEI, Hillsbor USA). For SEM analysis, all materials were printed on a silicon substrate.

PVP-PMF Weight Ratio Optimization (Effect of PVP-PMF Weight Ratio on Dielectri Properties and Printability)
Before printing the transistor, the formulation and printability of the crPVP in optimized. To investigate the optimal weight ratio of PVP to PMF, crPVP ink was f lated at different weight ratios of PVA to PMF (1:1, 2:1, 3:1, and 5:1). Previous resear shown that hydroxyl group species with various hydrogen-bonding interactions sponsible for the absorption of water molecules into the PVP dielectric, and that le

PVP-PMF Weight Ratio Optimization (Effect of PVP-PMF Weight Ratio on Dielectric Properties and Printability)
Before printing the transistor, the formulation and printability of the crPVP ink were optimized. To investigate the optimal weight ratio of PVP to PMF, crPVP ink was formulated at different weight ratios of PVA to PMF (1:1, 2:1, 3:1, and 5:1). Previous research has shown that hydroxyl group species with various hydrogen-bonding interactions are responsible for the absorption of water molecules into the PVP dielectric, and that leads to increased leakage and hysteresis in transistors. The weight ratio and annealing temperature are essential factors in cross-linking to reduce the threshold voltage variation in TFT operation. It has been demonstrated previously that an annealing temperature of 170 • C or more forms a highly cross-linked film with lower hysteresis than in the other temperature range. So, all samples were annealed at 170 • C unless otherwise stated.
FTIR spectroscopy was conducted to measure the IR absorbance of the hydroxyl group present in PVA after cross-linking with PMF at various weight ratios. The wavenumbers associated with the hydrogen-bonded hydroxyl group, associated hydroxyl group, and nonhydrogen-bonded (free) hydroxyl group are ≈3340 cm −1 , ≈3410 cm −1 , and ≈3530 cm −1 , respectively. From Figure 3a, it is evident that the intensity of the IR absorbance decreases with an increase in PMF content in the solution. The 1:1 weight ratio has the lowest absorbance peak for all of the wavelengths of different hydroxy groups and hence shows the highest cross-linking. increased leakage and hysteresis in transistors. The weight ratio and annealing temperature are essential factors in cross-linking to reduce the threshold voltage variation in TFT operation. It has been demonstrated previously that an annealing temperature of 170 °C or more forms a highly cross-linked film with lower hysteresis than in the other temperature range. So, all samples were annealed at 170 °C unless otherwise stated. FTIR spectroscopy was conducted to measure the IR absorbance of the hydroxyl group present in PVA after cross-linking with PMF at various weight ratios. The wavenumbers associated with the hydrogen-bonded hydroxyl group, associated hydroxyl group, and non-hydrogen-bonded (free) hydroxyl group are ≈3340 cm −1 , ≈3410 cm −1 , and ≈3530 cm −1 , respectively. From Figure 3a, it is evident that the intensity of the IR absorbance decreases with an increase in PMF content in the solution. The 1:1 weight ratio has the lowest absorbance peak for all of the wavelengths of different hydroxy groups and hence shows the highest cross-linking.  Then, to investigate the dielectric properties of PVP-PMF, metal-insulator-semiconductor (MIS) capacitors were fabricated using silver electrodes and the PVP-PMF dielectric with different weight ratios on a silicon wafer. The capacitance vs. voltage (C-V) curve of the dielectric with respect to their weight ratios at 100 KHz was measured. From the C-V curve, the dielectric constants were calculated depending on the thickness of the dielectric, and the graph is shown in Figure 3b. For this type of measurement, the instrumental uncertainty in the values of the dielectric constant is less than 1% for a single-layer specimen. It is observed that the 5:1 weight ratio of the PVP-PMF film has the highest dielectric constant for a 170 °C annealing temperature. For other films, the dielectric constant is slightly lower, with a variation of 18% from the 5:1 to 1:1 weight ratio. Molecular dipoles and mobile ions present in the dielectric bulk are responsible for the change in the dielectric constant, so a higher frequency of 100 KHz was chosen to minimize their effects and ensure that most charge carriers are electrons.
The printability of PVP-PMF is very crucial for fabricating fully printed TFTs. As the devices will be printed using the aerosol jet printing technique, various parameters, such as sheath gas flow (SG), atomization gas flow (UA), platen temperature, and printing speed (PS), need to be optimized to achieve a pinhole-free dielectric layer. Apart from printing parameters, ink formulation, curing conditions, substrates, the surface quality of the previously printed layer, and atmospheric moisture content also influence the quality of the dielectric film. In the AJ300 aerosol jet system, an ultrasonic atomizer is used to print the PVP-PMF ink, as it requires a much lower amount of ink and prevents ink wastage. The viscosity range for the ultrasonic atomizer is 1-5 cP. To achieve the viscosity range, first, the PVP-PMF inks were diluted to a 5 wt.%-2 wt.% concentration and checked for consistent jettability. For the initial experiments, the printing parameters used for all Then, to investigate the dielectric properties of PVP-PMF, metal-insulator-semiconductor (MIS) capacitors were fabricated using silver electrodes and the PVP-PMF dielectric with different weight ratios on a silicon wafer. The capacitance vs. voltage (C-V) curve of the dielectric with respect to their weight ratios at 100 KHz was measured. From the C-V curve, the dielectric constants were calculated depending on the thickness of the dielectric, and the graph is shown in Figure 3b. For this type of measurement, the instrumental uncertainty in the values of the dielectric constant is less than 1% for a single-layer specimen. It is observed that the 5:1 weight ratio of the PVP-PMF film has the highest dielectric constant for a 170 • C annealing temperature. For other films, the dielectric constant is slightly lower, with a variation of 18% from the 5:1 to 1:1 weight ratio. Molecular dipoles and mobile ions present in the dielectric bulk are responsible for the change in the dielectric constant, so a higher frequency of 100 KHz was chosen to minimize their effects and ensure that most charge carriers are electrons.
The printability of PVP-PMF is very crucial for fabricating fully printed TFTs. As the devices will be printed using the aerosol jet printing technique, various parameters, such as sheath gas flow (SG), atomization gas flow (UA), platen temperature, and printing speed (PS), need to be optimized to achieve a pinhole-free dielectric layer. Apart from printing parameters, ink formulation, curing conditions, substrates, the surface quality of the previously printed layer, and atmospheric moisture content also influence the quality of the dielectric film. In the AJ300 aerosol jet system, an ultrasonic atomizer is used to print the PVP-PMF ink, as it requires a much lower amount of ink and prevents ink wastage. The viscosity range for the ultrasonic atomizer is 1-5 cP. To achieve the viscosity range, first, the PVP-PMF inks were diluted to a 5 wt.%-2 wt.% concentration and checked for consistent jettability. For the initial experiments, the printing parameters used for all inks are a 20-24 sccm UA flow rate, a~50 sccm SG flow rate, and 5-6 mm/s PS. For a low UA flow rate, it was hard to obtain a continuous ink stream; rather, the ink stream behaved like a spray of aerosols, as shown in Figure 4a. If the UA flow rate was too high, the volume of the ink increased and led to overspilling, as shown in Figure 4c. During printing, the UA flow rate was given more importance, while the SG flow was kept nearly constant because it is easier to control the thickness of the printed film by controlling the UA flow rate. For fixed UA and SG flow rates, the PS can be varied depending on the line thickness of the ink stream and the pitch of the design pattern. The PS should be under the machine acceleration limit for a certain pitch in the design pattern. Hence, 5-6 mm/sec PS was selected to provide the minimum needed overlap for continuity between subsequent printed lines and to obtain the minimum thickness at the same time for fixed UA and SG. As the 4 wt.% and 5 wt.% inks were more viscous, it was found that the ink needed a very high UA, above 30 sccm, to jet out at the highest atomization setting. The viscosity increases with an increase in PVP wt.; among the 5:1 PVP-PMF inks, the 4 wt.% and 5 wt.% inks did not form continuous lines, as shown in Figure 5b.
Nanomaterials 2022, 12, x FOR PEER REVIEW 6 inks are a 20-24 sccm UA flow rate, a ~50 sccm SG flow rate, and 5-6 mm/s PS. For UA flow rate, it was hard to obtain a continuous ink stream; rather, the ink stream haved like a spray of aerosols, as shown in Figure 4a. If the UA flow rate was too hig volume of the ink increased and led to overspilling, as shown in Figure 4c. During p ing, the UA flow rate was given more importance, while the SG flow was kept n constant because it is easier to control the thickness of the printed film by controllin UA flow rate. For fixed UA and SG flow rates, the PS can be varied depending on th thickness of the ink stream and the pitch of the design pattern. The PS should be u the machine acceleration limit for a certain pitch in the design pattern. Hence, 5-6 mm PS was selected to provide the minimum needed overlap for continuity between s quent printed lines and to obtain the minimum thickness at the same time for fixe and SG. As the 4 wt.% and 5 wt.% inks were more viscous, it was found that th needed a very high UA, above 30 sccm, to jet out at the highest atomization setting viscosity increases with an increase in PVP wt.; among the 5:1 PVP-PMF inks, the 4 and 5 wt.% inks did not form continuous lines, as shown in Figure 5b. However, for the 2.5 wt.% concentration, PVP-PMF inks at all wt. ratios (2:1, 3:1 5:1) formed steady ink streams for 22 sccm UA and 50 sccm SG, as shown in Figure 5 the 2:1 wt. ratio dielectric. Then, the PS was checked for the 2.5 wt.% ink to ensure ov coverage without voids or pinholes. One of the advantages of aerosol jet printing i the UA, SG, and PS can be modified during printing to improve film quality. Final PVP-PMF inks with various weight ratios (2:1, 3:1, and 5:1) at a concentration of 2.5 in PGMEA were printed at 22 UA, 50 SG, and 6mm/sec PS. These parameters were to print the dielectric layer of fully printed CNT devices. In the initial experiment, to the best weight ratios of PVP-PMF, all of the dielectric inks were printed with the parameter settings for a fair comparison.  quent printed lines and to obtain the minimum thickness at the same time for fixed UA and SG. As the 4 wt.% and 5 wt.% inks were more viscous, it was found that the ink needed a very high UA, above 30 sccm, to jet out at the highest atomization setting. The viscosity increases with an increase in PVP wt.; among the 5:1 PVP-PMF inks, the 4 wt.% and 5 wt.% inks did not form continuous lines, as shown in Figure 5b. However, for the 2.5 wt.% concentration, PVP-PMF inks at all wt. ratios (2:1, 3:1, and 5:1) formed steady ink streams for 22 sccm UA and 50 sccm SG, as shown in Figure 5a for the 2:1 wt. ratio dielectric. Then, the PS was checked for the 2.5 wt.% ink to ensure overall coverage without voids or pinholes. One of the advantages of aerosol jet printing is that the UA, SG, and PS can be modified during printing to improve film quality. Finally, all PVP-PMF inks with various weight ratios (2:1, 3:1, and 5:1) at a concentration of 2.5 wt.% in PGMEA were printed at 22 UA, 50 SG, and 6mm/sec PS. These parameters were used to print the dielectric layer of fully printed CNT devices. In the initial experiment, to find the best weight ratios of PVP-PMF, all of the dielectric inks were printed with the same parameter settings for a fair comparison. However, for the 2.5 wt.% concentration, PVP-PMF inks at all wt. ratios (2:1, 3:1, and 5:1) formed steady ink streams for 22 sccm UA and 50 sccm SG, as shown in Figure 5a for the 2:1 wt. ratio dielectric. Then, the PS was checked for the 2.5 wt.% ink to ensure overall coverage without voids or pinholes. One of the advantages of aerosol jet printing is that the UA, SG, and PS can be modified during printing to improve film quality. Finally, all PVP-PMF inks with various weight ratios (2:1, 3:1, and 5:1) at a concentration of 2.5 wt.% in PGMEA were printed at 22 UA, 50 SG, and 6 mm/s PS. These parameters were used to print the dielectric layer of fully printed CNT devices. In the initial experiment, to find the best weight ratios of PVP-PMF, all of the dielectric inks were printed with the same parameter settings for a fair comparison.

Performance of Printed TFTs Based on Different Weight Ratios
CNT-based TFTs were printed with various weight ratios of PVP to PMF (2:1, 3:1, and 5:1) as the dielectric layer using the same parameter settings and the same device structure. In this work, all of the devices were printed as top-gate TFTs to reduce hysteresis, and the CNT layer was printed on top of the silver source and drain to avoid the direct contact of carbon nanotubes with the environment and prevent the rapid degradation of the devices. The DC transfer characteristics, that is, the plot between Id (drain current) and Vgs (gate to source voltage), of devices with various weight ratios are shown in Figures 6 and 7. From the transfer curves, it can be observed that it takes more positive gate voltage (Vth) to turn off the transistors with 3:1 and 5:1 weight ratios than the 2:1 wt. ratio TFTs. This is the result of a very thick dielectric layer, as 3:1 and 5:1 wt. ratio inks are more viscous and print thicker layers when printed using similar parameters. More diluted inks of 3:1 and 5:1 wt. ratios, such as those with concentrations less than 2.5 wt.%, will result in thinner dielectric layers. Additionally, the on/off ratios of TFTs printed using 3:1 and 5:1 have an on/off ratio of 10 3 , whereas most TFTs printed using a 2:1 weight ratio have an on/off ratio of 10 4 . This is because of the high off-current in 3:1 and 5:1 PVP-PMF dielectrics. For the TFTs with a 2:1 PVP-PMF dielectric, the threshold voltage is around 20 V for a 1-1.5-micron-thick dielectric.
CNT-based TFTs were printed with various weight ratios of PVP to PMF ( and 5:1) as the dielectric layer using the same parameter settings and the same structure. In this work, all of the devices were printed as top-gate TFTs to reduce h sis, and the CNT layer was printed on top of the silver source and drain to avoid th contact of carbon nanotubes with the environment and prevent the rapid degrad the devices. The DC transfer characteristics, that is, the plot between Id (drain c and Vgs (gate to source voltage), of devices with various weight ratios are shown ures 6 and 7. From the transfer curves, it can be observed that it takes more positi voltage (Vth) to turn off the transistors with 3:1 and 5:1 weight ratios than the 2:1 w TFTs. This is the result of a very thick dielectric layer, as 3:1 and 5:1 wt. ratio inks a viscous and print thicker layers when printed using similar parameters. More dilut of 3:1 and 5:1 wt. ratios, such as those with concentrations less than 2.5 wt.%, wi in thinner dielectric layers. Additionally, the on/off ratios of TFTs printed using 5:1 have an on/off ratio of 10 3 , whereas most TFTs printed using a 2:1 weight rat an on/off ratio of 10 4 . This is because of the high off-current in 3:1 and 5:1 PVP-PM lectrics. For the TFTs with a 2:1 PVP-PMF dielectric, the threshold voltage is arou for a 1-1.5-micron-thick dielectric.
However, we were more interested in hysteresis in the gate voltage during f (on → off) and backward (off → on) sweeps. The hysteresis curves of TFTs in F with various weight ratios clearly show that the 5:1 wt. ratio has the highest hyste the gate threshold voltage, and it decreases with an increase in PMF content. So, PVP-PMF dielectric has the lowest hysteresis in Vth compared to that of 3:1 and 5 this agrees with the conclusion of previous research. In this case, there can be two main causes of hysteresis in the transistor perfor interface charge traps and bulk dielectric defects. Water molecules present in the phere can be absorbed by the surfaces of dielectric materials having hydroxyl Then, these molecules will diffuse into the bulk of the dielectric and create a pola effect. Because of this, during the forward and reverse sweeps, we experience a ch the threshold voltage, which keeps increasing with the increase in the number of droxyl bonds. Other researchers have also concluded that a higher PVP concentr However, we were more interested in hysteresis in the gate voltage during forward (on → off) and backward (off → on) sweeps. The hysteresis curves of TFTs in Figure 6 with various weight ratios clearly show that the 5:1 wt. ratio has the highest hysteresis in the gate threshold voltage, and it decreases with an increase in PMF content. So, the 2:1 PVP-PMF dielectric has the lowest hysteresis in Vth compared to that of 3:1 and 5:1, and this agrees with the conclusion of previous research.
In this case, there can be two main causes of hysteresis in the transistor performance: interface charge traps and bulk dielectric defects. Water molecules present in the atmosphere can be absorbed by the surfaces of dielectric materials having hydroxyl groups. Then, these molecules will diffuse into the bulk of the dielectric and create a polarization effect. Because of this, during the forward and reverse sweeps, we experience a change in the threshold voltage, which keeps increasing with the increase in the number of free hydroxyl bonds. Other researchers have also concluded that a higher PVP concentration in PVP-PMF mixtures leads to a higher free hydroxyl group concentration, which results in increased interface charge trapping between the semiconductor and dielectric layers. Figure 7 shows the variability analysis of the transfer characteristics of devices with different weight ratios. For each wt. ratio, three working devices were characterized for the Id-Vg curve with the same voltage and current settings. The variability in the gate threshold voltage and the on/off ratio is very low in all of the devices, irrespective of the weight ratio. ure 7 shows the variability analysis of the transfer characteristics of devices with different weight ratios. For each wt. ratio, three working devices were characterized for the Id-Vg curve with the same voltage and current settings. The variability in the gate threshold voltage and the on/off ratio is very low in all of the devices, irrespective of the weight ratio.

Demonstration of Optimized Device Characteristics
Finally, a 2.5 wt.% concentration of the PVP-PMF dielectric with a 2:1 weight ratio was chosen to fabricate a fully aerosol-jet-printed device on the flexible Kapton substrate with optimized printing parameters to lower the threshold voltage. The device threshold voltage was improved by controlling the thickness of the printed dielectric film. The greater the thickness of the dielectric layer, the higher the gate voltage required to operate the device. This is because of the dependence of the gate capacitance on the dielectric thickness, and the gate capacitance is related to transconductance. Figure 8 shows the positive correlation between the threshold voltage and dielectric thickness. Establishing a one-to-one correlation between the threshold voltage and dielectric thickness is difficult, as the threshold voltage also depends on the charge in the dielectric and operating temperature, and discussing these factors is outside the scope of this work. So, here, the

Demonstration of Optimized Device Characteristics
Finally, a 2.5 wt.% concentration of the PVP-PMF dielectric with a 2:1 weight ratio was chosen to fabricate a fully aerosol-jet-printed device on the flexible Kapton substrate with optimized printing parameters to lower the threshold voltage. The device threshold voltage was improved by controlling the thickness of the printed dielectric film. The greater the thickness of the dielectric layer, the higher the gate voltage required to operate the device. This is because of the dependence of the gate capacitance on the dielectric thickness, and the gate capacitance is related to transconductance. Figure 8 shows the positive correlation between the threshold voltage and dielectric thickness. Establishing a one-toone correlation between the threshold voltage and dielectric thickness is difficult, as the threshold voltage also depends on the charge in the dielectric and operating temperature, and discussing these factors is outside the scope of this work. So, here, the thickness of the dielectric was controlled to reduce the gate threshold voltage. The high threshold voltage was obtained with a thick dielectric layer of over 1 µm. The thickness of 2:1 PVP-PMF was decreased by decreasing the UA flow rate and keeping the rest of the parameters the same. The thickness optimization comes from the tradeoff between the UA flow rate and continuous line coverage by the ink stream. The UA flow rate was decreased to a minimum of 20 sccm to deposit a pinhole-free dielectric layer. Below this rate, the film was too thin to leak between S/D and gate contacts. The morphology of the dielectric film was good for this UA setting, with uniform and continuous printing. of 2:1 PVP-PMF was decreased by decreasing the UA flow rate and keeping the rest of the parameters the same. The thickness optimization comes from the tradeoff between the UA flow rate and continuous line coverage by the ink stream. The UA flow rate was decreased to a minimum of 20 sccm to deposit a pinhole-free dielectric layer. Below this rate, the film was too thin to leak between S/D and gate contacts. The morphology of the dielectric film was good for this UA setting, with uniform and continuous printing. The devices with thin dielectric layers were characterized, and Figure 9a,b show the transfer and output curves of one transistor, respectively. It can be observed that the device current drops below 1nA at around a 4V gate voltage. The on/off ratio of the device was found to be 1.5 × 10 4 , and the device mobility was calculated as 6.1 cm 2 V −1 s −1 . This performance is a promising result for the low-voltage operation of CNT-based transistors. The low threshold voltage of −4.8 V can be extracted from the linear transfer curve of the device, where the highest slope intersects the gate voltage axis, as shown in Figure 9c. The devices with thin dielectric layers were characterized, and Figure 9a,b show the transfer and output curves of one transistor, respectively. It can be observed that the device current drops below 1 nA at around a 4 V gate voltage. The on/off ratio of the device was found to be 1.5 × 10 4 , and the device mobility was calculated as 6.1 cm 2 V −1 s −1 . This performance is a promising result for the low-voltage operation of CNT-based transistors. The low threshold voltage of −4.8 V can be extracted from the linear transfer curve of the device, where the highest slope intersects the gate voltage axis, as shown in Figure 9c.  Figure 10 shows the transfer curve of the same device after 100 days, and it compares it with curves 2 days after printing. The on and off current increased substantially in 100 days, and performance degraded. This could be caused by the disruption of CNT networks and the penetration of ambient species inside the dielectric film. The lifetime of these CNT-/PVP-PMF-based TFTs can be enhanced by encapsulating them with some in-  Figure 10 shows the transfer curve of the same device after 100 days, and it compares it with curves 2 days after printing. The on and off current increased substantially in 100 days, and performance degraded. This could be caused by the disruption of CNT networks and the penetration of ambient species inside the dielectric film. The lifetime of these CNT-/PVP-PMF-based TFTs can be enhanced by encapsulating them with some inert/nonreactive material. Figure 9. (a) Transfer (at Vds = −10 V), (b) output characteristics, and (c) linear transfer char tics showing threshold voltage extraction of printed transistor with 2:1 wt. ratio of PVP to P 80 µm, W = 500 µm). Figure 10 shows the transfer curve of the same device after 100 days, and it com it with curves 2 days after printing. The on and off current increased substantially days, and performance degraded. This could be caused by the disruption of CN works and the penetration of ambient species inside the dielectric film. The lifet these CNT-/PVP-PMF-based TFTs can be enhanced by encapsulating them with so ert/nonreactive material.

Discussion
In this work, the printability of cross-linked PVP ink was explored for its use in based flexible transistors fabricated by AJP. The optimal weight ratio of PVP to P the crPVP solution was found to be 2:1, and the annealing temperature was kept at for compatibility with the flexible substrate. Other wt. ratios showed large hysteres ing forward and backward sweeps in transfer curves of fully printed CNT TFTs. F tinuous printing and ink stability, it was found that 2.5 wt.% PVP-PMF in PGMEA the best-printed film without pinholes. The thickness of the dielectric film was e control by changing the UA flow rate when the concentration was 2.5 wt.%. Hen thickness of the polymer dielectric was optimized by changing AJP parameters to a thickness below 1 µm, which enables the low-voltage operation of printed dev Figure 10. Transfer characteristics of printed device with 2:1 wt. ratio of PVP to PMF after 2 days and 100 days of device printing without encapsulation.

Discussion
In this work, the printability of cross-linked PVP ink was explored for its use in CNTbased flexible transistors fabricated by AJP. The optimal weight ratio of PVP to PMF in the crPVP solution was found to be 2:1, and the annealing temperature was kept at 170 • C for compatibility with the flexible substrate. Other wt. ratios showed large hysteresis during forward and backward sweeps in transfer curves of fully printed CNT TFTs. For continuous printing and ink stability, it was found that 2.5 wt.% PVP-PMF in PGMEA offers the best-printed film without pinholes. The thickness of the dielectric film was easy to control by changing the UA flow rate when the concentration was 2.5 wt.%. Hence, the thickness of the polymer dielectric was optimized by changing AJP parameters to obtain a thickness below 1 µm, which enables the low-voltage operation of printed devices. It was found that the TFT has a threshold voltage as low as −4.8 V with an on/off ratio of more than 10 4 . Finally, the environmental stability of the device was analyzed over 100 days to show the performance degradation over time. This work investigated the use of polymer dielectrics in fully printed transistors with low hysteresis and a low threshold voltage.

Conflicts of Interest:
The authors declare no conflict of interest.