Low-temperature crosslinked soluble polyimide as a dielectric for organic thin-film transistors: enhanced electrical stability and performance

We have prepared a low-temperature cross-linked soluble polyimide (SPI) as a dielectric material for organic thin-film transistors (OTFTs) to improve their electrical stability. Two types of SPIs (DOCDA/6FHAB and 6FDA/6FHAB) were synthesized by a one-step polymerization process using 5-(2,5-dioxytetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride (DOCDA) and hexafluoroisopropylidene diphthalic anhydride (6FDA) as the dianhydrides and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FHAB) as a diamine. To further enhance the electrical performance, the SPI thin films were crosslinked with methylated/ethylated (hydroxymethyl)benzoguanamine (HMBG) through a low temperature process at 160 °C. Crosslinking considerably improved the insulating properties, resulting in a substantial reduction in leakage current from 10−7 A cm−2 to 10−9 A cm−2 at 2.0 MV cm−1. When crosslinked SPIs were used as gate dielectrics in OTFTs, device stability and reliability, as measured by the off-current, threshold voltage, and hysteresis, improved significantly. Our results demonstrate the potential of crosslinked SPIs as effective gate dielectric materials for advanced organic thin-film transistors.

Among various solution-processable polymer dielectrics, polyimide (PI) has garnered attention as a promising solution-processable polymer dielectric for electronic device applications.This interest is driven by its advantageous properties, such as elevated thermal and chemical resistance, high glass transition temperature, and robust mechanical properties [11,12].However, despite their potential as a gate dielectric, PI faces some obstacles when implemented in electronic devices.The conventional approach to fabricating thin PI films involves high-temperature imidization [12,13].In this process, the poly(amic acid) precursor of PI is subjected to thermal treatment at temperatures above 350 • C to facilitate its conversion into PI.However, the elevated thermal demand associated with this method presents a practical challenge for specific applications.In addition, fully aromatic PIs have limited solubility in organic solvents due to their rigid backbone structures and strong inter-chain interactions.The formation of charge transfer complex interactions, both intra-and inter-chain, contributes to the poor solubility and makes solution processing difficult [14][15][16].Consequently, these limitations have led to extensive research efforts to develop strategies to overcome them.
To address these issues, researchers have explored one-pot synthesis methods using base catalysts at lower temperatures to produce aromatic PIs.This method does not require additional thermal treatment at temperatures exceeding 350 • C.This simplifies the synthesis and processing necessary for adapting electronic device fabrication.With this strategy, the solubility of aromatic PIs in conventional organic solvents can be significantly increased by introducing fluorinated or bulky groups into their rigid backbones [14,17].These modifications facilitate the dissolution of PIs in organic solvents, especially in soluble PI (SPI), which simplifies the solution processing for device fabrication.This creates new potential for their application in flexible electronic devices.
Moreover, certain functional groups on the PI backbone have the ability to provide additional properties such as high-or low-k properties and crosslinking capabilities and serve as reaction sites for extra agents [18][19][20].In particular, crosslinking systems have received considerable attention from various research groups as an effective means to improve the dielectric performance of organic thin-film transistors (OTFTs).Yi et al developed photosensitive polyimide (PSPI) gate dielectrics for OTFTs using a one-step condensation polymerization process.PSPI exhibited advantageous properties, including a low processing temperature of 160 • C and a leakage current density of 1.7 × 10 −10 A cm −2 at 3.0 MV cm −1 [21].Lee et al synthesized crosslinked fluorine-containing SPI gate dielectrics with a perfluorocyclobutane structure, which exhibited excellent chemical and thermal stability [22].Vicca et al prepared crosslinked PVP gate dielectrics that achieved a reduced crosslinking temperature of 130 • C while maintaining an electrical field effect mobility of 0.5 cm 2 V −1 s −1 in pentacene TFTs [23].
During curing, the hydroxyl groups of DOCDA/6FHAB and 6FDA/6FHAB were consumed, resulting in a decrease in the dielectric constant from 4.09 and 3.46-3.53and 3.10, respectively.However, the crosslinking process significantly improved the insulating properties, resulting in a remarkable reduction in leakage current from ∼10 −7 A cm −2 to ∼10 −9 A cm −2 at 2 MV cm −1 .When used as a gate dielectric in pentacene TFTs, the electrical stability of the devices was significantly improved, especially when two crosslinked SPIs were used, resulting in the disappearance of hysteresis in both devices.In addition, the fluorinated SPI of 6FDA/6FHAB exhibited superior electrical stability and performance.Pentacene TFT with crosslinked 6FDA/6FHAB as the gate dielectric exhibited field-effect mobility, threshold voltage, subthreshold swing, and on/off ratio of 0.298 cm 2 V −1 s −1 , −5.7 V, 5.26 V dec −1 and 1.34 × 10 5 , respectively.As a consequence, the improved electrical stability of OTFTs with crosslinked SPIs as gate dielectrics was confirmed.

Synthesis of SPIs
For the synthesis of SPIs, a solution of DOCDA (1.0 g, 3.8 mmol), 6FHAB (1.4 g, 3.8 mmol), and isoquinoline (0.73 g) as a base catalyst in 10.0 ml of DMAc was prepared in a 50 ml two-necked flask.In a similar manner, a solution of 6FDA (1.0 g, 2.3 mmol), 6FHAB (0.82 g, 2.3 mmol), and isoquinoline (0.43 g) in 10.0 ml of DMAc was prepared in a 50 ml two-necked flask.The monomers were completely dissolved in the reactor under mechanical stirring at a total solid concentration of 20 wt %.The reactor was progressively heated from 25 • C to 70 • C and maintained at this temperature for 2 h.Then the reaction temperature was further increased and the mixture was heated at 160 • C overnight until the solution reached viscosity saturation.SPI was precipitated by adding the saturated solution to excess methanol, washed several times with methanol, and filtered to remove the solvent and catalyst.The obtained SPI was then dried in a vacuum oven at 80 • C for 20 h.The final polymer yield for DOCDA/6FHAB was 60.4% (1.37 g) and 6FDA/6FHAB was 69.2% (1.26 g), respectively.

Fabrication of the crosslinked SPI film
To prepare the SPI thin film, a mixture of 0.1 g DOCDA/6FHAB and 1.15 g CHO was prepared at 50 • C. The solution was then spin-coated onto a glass substrate with aluminum deposition at 3000 rpm for 30 s.The coated thin films were subjected to thermally annealing on a hot plate at 90 • C for 10 min and 160 • C for 40 min.The same process was repeated to obtain the 6FDA/6FHAB thin film.To prepare the polyimide film crosslinked at low temperature, a solution of 0.1 g DOCDA/6FHAB dissolved in 1.15 g CHO was prepared at 50 • C. The crosslinker HMBG was added to the SPI solution at a concentration of 0.02 g.The SPI solution containing HMBG was spin-coated onto a glass substrate with patterned aluminum electrodes at 500 rpm for 30 s and 4000 rpm for 30 s.The spin-coated thin films were subjected to soft baking on a hot plate at 80 • C for 5 min, followed by hard baking in a vacuum oven at 160 • C for 3 h.The same process was repeated to obtain the 6FDA/6FHAB thin film.The total thickness of the SPI and crosslinked SPI films was controlled to be 300 nm.

Device fabrication
To fabricate the metal-insulator-metal (MIM) device, patterned aluminum electrodes were prepared on glass substrates as the bottom and top electrodes.A 30 nm thick aluminum electrode was deposited on the substrate by thermal evaporation under a pressure of 1 × 10 −6 torr.The SPI and crosslinked SPI films were deposited between the bottom and top aluminum electrodes.The fabricated MIM devices were then measured to evaluate the dielectric and insulating properties of the SPI and crosslinked SPI films.Bottom-gate and top-contact structures were used to fabricate pentacene OTFTs with SPI and crosslinked SPI films as gate dielectrics.Patterned aluminum was used as the gate electrode on the glass substrate.A 50 nm thick pentacene semiconductor layer was deposited on the gate dielectric-coated substrate.A 50 nm thick gold electrode was then formed on the pentacene semiconductor as source and drain channels.All electrodes and pentacene semiconductors were deposited by thermal evaporation using shadow masks.The deposition rate for the 50 nm thick pentacene was 0.3 Å s −1 , and the device temperature was maintained at 80 • C during deposition.The OTFTs had a channel length (L) of 50 µm and a width (W) of 1000 µm.

Characterization
1 H NMR spectra were recorded using a Bruker Ascend 400 MHz instrument in DMSO-d 6 .FT-IR spectra were recorded using a Bruker VERTEX 80 FT-IR spectrometer.To measure the molecular weight of SPIs, high temperature gel permeation chromatography (GPC) analysis was performed using a Waters Model 150 C ALC/PL-GPC 220 equipped with Styragel columns in N, N-dimethylformamide (DMF) solution at 80 • C. The gate-insulator thicknesses were determined using an alpha-step surface profiler (a-step DC50, KLA-Tencor).Atomic force microscopy (AFM) images were acquired using a tapping-mode microscope (Nanoscope IV; Digital Instruments).The images of the films were captured using an optical microscope (OM, Nikon Eclipse LV50-POL).Surface energy was measured using a contact angle analyzer (Phoenix 450, SEO) with deionized (DI) water and diiodomethane.Capacitance measurements were performed using a Precision LCR meter (E4980A, Agilent).The TFT characteristics were measured using a semiconductor parameter analyzer (4200-SCS, Keithley).All electrical measurements were performed in ambient air.

Results and discussion
In this study, two types of SPIs containing hydroxyl groups were synthesized as crosslinking systems.Two SPIs were prepared from dianhydride monomers (DOCDA and 6FDA) and a diamine monomer (6FHAB).These specific monomers were selected to examine the effects of fluorine content on the properties of electronic device.The SPIs (DOCDA/6FHAB and 6FDA/6FHAB) were synthesized in a one-step condensation polymerization method in dimethylacetamide (DMAc) with isoquinoline as the base catalyst, according to a previously described procedure [22,24].To prepare the crosslinked SPI films, an etherification reaction was carried out between the SPIs with hydroxyl groups from 6FHAB and HMBG with hydroxy or hydroxymethyl groups.Figure 1 shows the chemical structures of SPIs and SPIs that have been crosslinked.To verify the structures of the SPIs, 1 H NMR characterization in DMSO-d 6 (ppm) was performed, as shown in figure S1.These chemical shifts are consistent with the structures of DOCDA/6FHAB and 6FDA/6FHAB, as described in the Experimental Section.Notably in both synthesized SPIs, we clearly observed the characteristic peak of the hydroxyl group of 6FHAB at 10.4-10.5 ppm (1H, -OH of 6FHAB) in both synthesized SPIs.In addition, the absence of amide proton peaks at 10.0-11.0ppm and carboxylic acid peaks at 11.0-13.0ppm for both DOCDA/6FHAB and 6FDA/6FHAB indicates that the synthesis of our SPIs using one-step polymerization resulted in a fully imidized and completely polymerized main chain structure [22,24].The number-average molecular weight (M n ), weight-average molecular weight (M w ), and polydispersity index (PDI) of SPIs were determined by GPC using polystyrene standards and DMF as the eluent.The M n , M w and PDI of DOCDA/6FHAB were 26.5 kg mol −1 , 40.8 kg mo1 −1 , and 1.54, respectively.These values were 13.7 kg mol −1 , 33.5 kg mol −1 , and 2.44 for 6FDA/6FHAB.
Figure S2 shows the thermal properties of the two SPI samples.DOCDA/6FHAB and 6FDA/6FHAB SPI had decomposition temperatures (T d ) of 418.64 • C and 425.39 • C, respectively, for 5 wt% weight loss.The heat resistance of SPIs (DOCDA/6FHAB and 6FDA/6FHAB) was confirmed to be above 400 • C.
Figure 2 illustrates the FTIR results for the two types of SPIs, (a) DOCDA/6FHAB and (b) 6FDA/6FHAB, both prior to and following the crosslinking reaction with HMBG.
The crosslinking density of the insulating film can be controlled based on the HMBG content and curing temperature [25].To obtain the most electrically stable insulating film, we fixed the ratio of SPI resin to HMBG at 5:1 (w/w) based on previous studies employing HMBG or PMF as a crosslinking agent [25][26][27].The curing reaction was carried out at a temperature of 160 • C, which was determined as the optimal condition to achieve a sufficiently high crosslinking density [23].After HMBG crosslinking, we were able to confirm the reduction of -OH groups in the SPIs backbone indicated at 3600 cm −1 .Additionally, we directly observed a typical peak related to ether (C-O-C) stretching at 1080 cm −1 , indicating bonding between SPIs and HMBG.This is shown by the black asterisks on the red line in figure 2.More detailed peak analysis results can be found in figure S3.In addition, the crosslinked SPIs (DOCDA/6FHAB-HMBG and 6FDA/6FHAB-HMBG) exhibited characteristic HMBG peaks, including alkane (-CH 2 -) bending at 1480 cm −1 and imine (C=N) stretching at 1588 cm −1 [28].
The FT-IR results confirmed that SPIs were successfully crosslinked with HMBG.The SPIs formed network structures which was identified as DOCDA/6FHAB-HMBG and 6FDA/6FHAB-HMBG, respectively.An immersion test was conducted to validate the formation of these crosslinked thin films by submerging them in various organic solvents for 1 min, followed by drying.The crosslinking process was observed by analyzing optical microscopy images, as illustrated in figure S4.It is important to note that both SPIs dissolved easily in NMP.Nonetheless, the crosslinked SPIs incorporating HMBG were clearly insoluble in NMP.In addition, the crosslinked SPIs exhibited substantial resistance not only to NMP but also to other commonly used polar solvents in polymer dissolution such as DMAc, GBL, and CHO.Even after one minute of immersion in each solvent, the surface morphologies and swelling properties of the crosslinked films remained unchanged.
Figure 3 shows the AFM images of the spin-coated SPIs and crosslinked SPIs thin films on a silicon wafer substrate.The root-mean-square (RMS) roughness of the DOCDA/6FHAB and 6FDA/6FHAB films were measured to be 0.296 nm and 0.345 nm, respectively.Following the thermal crosslinking reaction, a slight reduction in the surface RMS roughness was observed, with crosslinked DOCDA/6FHAB and 6FDA/6FHAB films measuring 0.285 and 0.288 nm, respectively.The crosslinked SPI films exhibited smoother and denser surfaces than their pre-crosslinked states, as shown in table 1. Water contact angle of crosslinked SPI  (DOCDA/6FHAB) increased from 58.4 • to 65.4 • upon the formation of crosslinked network.Similarly, the water contact angle of the crosslinked SPI (6FDA/6FHAB) film increased from 68.7 • to 70.9 • , as shown in table 1.
The water contact angles and surface energies of the SPI thin films decreased slightly after crosslinking with HMBG.The decrease in water contact angles and surface energies of the SPI thin films after crosslinking with HMBG can be attributed to several factors.These factors collectively contribute to the observed decrease in water contact angles and surface energies.
One factor is the reduction in polar groups such as -OH from main chain of SPIs, which leads to a decrease in surface energy.Additionally, the presence of alkyl and methyl end groups within HMBG contributes to the hydrophobic nature of the surface.Another factor contributing to the decrease in surface energy is the increased smoothness of the insulating film surface after crosslinking.Figure 3 shows a significant reduction in surface roughness after crosslinking, as observed through AFM analysis.In general, the incorporation of crosslinking agents restricts chain mobility and can result in densification of the polymer network, leading to smoother and more uniform surfaces.In addition, the changes in the surface properties (water contact angle and surface energy) before and after curing were more pronounced for the DOCDA-based SPI than for the 6FDA-based SPI.Because 6FDA/6FHAB already contains a substantial number of fluorine groups, the introduction of HMBG curing groups results in a smaller change in the surface energy.The reduced surface roughness and surface energy after crosslinking contribute significantly to the growth and morphology of organic semiconductors such as pentacene [17,[29][30][31][32][33][34].
To investigate this further, a 50 nm layer of pentacene was deposited on both types of SPI before and after curing and domain sizes and surface morphologies were observed using AFM (figure S6).The largest pentacene domains were observed on the surface of the cured 6FDA/6FHAB-HMBG film, which had the lowest surface energy.However, the differences in domain sizes were not substantial.While there weare  differences in surface energy depending on the molecular composition and curing state, these differences did not appear to have a significant effect on the pentacene domain size or morphology.
To evaluate the capacitance and leakage current density, a MIM structure was fabricated using both SPI and crosslinked SPI films (figure 4).With the addition of HMBG, the crosslinked SPI films were prepared using the same process as the SPI solution.During the curing process, water was removed, and the film became denser, resulting in a reduction of approximately 10% in the film thickness after curing.To account for these changes, all the SPI film thicknesses used in the measurements, regardless of the curing state, were adjusted to fall within the range of 280-340 nm.Then, a thinner insulating layer typically increases capacitance, which improves TFT mobility.However, regardless of the cross-linking process, we observed an increase in leakage current when the insulating layer thickness was reduced to 150 nm or less.To eliminate thickness-related effects, we used an insulating film of approximately 300 nm thickness for our study.The measured capacitance and dielectric constant values for DOCDA/6FHAB and cross-linked SPI films were 11.58 nF cm -2 , 11.07 nF cm -2 , and 4.09, 3.53, respectively.Similar values were observed for the 6FDA/6FHAB and crosslinked SPI films at 8.84 nF cm -2 , 8.72 nF nF cm -2 , and 3.46, 3.10 at 10 kHz, respectively.At  2 MV cm -1 , the leakage current density of the crosslinked SPI (DOCDA/6FHAB) film improved from 4.28 × 10 -7 to 8.43 × 10 -9 A cm −2 , and for the cross-linked SPI (6FDA/6FHAB) film, it improved from 1.92 × 10 -7 to 2.89 × 10 -8 A cm −2 .The dielectrical properties of SPIs and cross-linked SPIs thin films are summarized in Table 2.
After the crosslinking reaction, the dielectric properties of the SPI film decreased due to the reduction in the number of polar hydroxyl groups in the main chain and the incorporation of the comparatively less polar HMBG crosslinking agent, which formed a crosslinking network.In contrast, the electrical properties associated with the leakage current within the MIM device improved due to the creation of a crosslinked network structure within the SPI thin film.These results are analogous to those obtained in a previous investigation of crosslinked poly(4-vinylphenol) using HMBG as the gate dielectric [23].
To evaluate the electrical performance, pentacene TFTs using SPI and crosslinked SPI films as gate dielectrics were fabricated.Pentacene TFTs were constructed with bottom-gate and top-contact configurations, figure 5(a).A 40 nm thick layer of pentacene was deposited onto the gate dielectric using a thermal evaporator at a deposition rate of 0.  5(c).The transfer curve was obtained by sweeping V GS from 10 to −30 V while maintaining V DS set at −30 V.The field-effect carrier mobility (µ) was derived from a plot of I DS 0.5 versus V GS in the saturation regime using the following equation: where C i and V T represent the capacitance per unit area of the SPI gate insulator and threshold voltage, respectively.The output characteristics of the OTFTs with DOCDA/6FHAB-based SPIs and 6FDA/6FHAB-based SPIs before and after crosslinking are shown in figure S5.
The electrical characteristics of the pentacene TFTs using SPI and crosslinked SPI dielectrics are summarized in table 3.For pentacene TFTs using crosslinked DOCDA/6FHAB gate dielectric, the field-effect carrier mobility, threshold voltage, and on/off current ratio were 0.147 cm 2 V -1 s -1 , −12.5 V and 3.18 × 10 5 , respectively.For pentacene TFTs with crosslinked 6FDA/6FHAB gate dielectric, these values were 0.298 cm 2 V -1 s -1 ,-5.7 V and 1.34 × 10 5 , respectively.Pentacene TFTs with crosslinked SPI gate dielectrics exhibited lower field-effect mobility, threshold voltage, and subthreshold swing, whereas the on/off current ratio increased.The decrease in capacitance resulting from the crosslinking reaction, which leads to a decrease in hydroxyl groups in the backbone, contributes to the decrease in field-effect mobility.However, the stability of the device improved, including the off-current, hysteresis, threshold voltage, and subthreshold swing.Interestingly, it can be observed that regardless of the crosslinking state, the I GS values for both types of SPI are measured to be 10 −9 A or lower (figures 5(b) and (c)).In assessing the electrical properties of TFTs, the minimal I GS difference serves as additional evidence that our developed SPIs are stable insulating materials.As shown in figure 4(b), the uncured SPI insulator exhibits low leakage current characteristics of 10 −7 A cm −2 or less at 1 MV.The level of leakage current exhibited by two SPIs was found to be comparable to other reported polymer insulators, such as PS and PVA.The excellent leakage characteristics demonstrated by I GS , when TFTs were fabricated with SPIs, were attributed to the low leakage current, which remained unaffected by the curing state.Nevertheless, the uncured SPIs introduced hysteresis issues due to the presence of several polar -OH groups within the polymer chains.To solve this issue, we disclose the use of the HMBG crosslinking agent to decrease the leakage current in SPI and eliminate hysteresis characteristics, as discussed.In addition, consistent with previous studies [17,35,36], the excellent surface properties of the fluorinated PI were reported, and the performance of the devices with the 6FDA/6FHAB gate dielectric was superior to that with DOCDA/6FHAB, as observed in this study.

Conclusion
We have successfully synthesized two SPIs with hydroxyl groups, DOCDA/6FHAB and 6FDA/6FHAB, using a one-step polymerization method.We further demonstrated the fabrication of crosslinked SPI thin films by incorporating the crosslinker HMBG at a low temperature of 160 • C. The insulating properties of the SPIs were improved by crosslinking with HMBG, resulting in at least one order of magnitude reduction in the leakage current density measured at 2 MV cm −1 .Both SPIs exhibited this improvement resulting in a leakage current density of 10 −9 MV cm −1 .After crosslinking, improvements were observed not only in the insulation properties but also in the thermal stability (T d5% > 400 • C), solvent resistance, and surface roughness.However, the capacitance and dielectric constant decreased due to the reduction in the number of hydroxyl groups.Two types of SPI, one with crosslinking and the other without, were used as gate dielectric layers for pentacene TFT to validate their electrical properties.Following crosslinking, the results indicated a decrease in hysteresis and an increase in the on-off ratio, indicating improved electrical stability of the device.The 6FDA-based SPIs, especially those containing highly electronegative fluorine groups, performed better performance as gate dielectrics than DOCDA-based SPIs.Our research has resulted in the development low-temperature-curable SPIs that exhibit excellent insulating properties and processability, and is expected to be suitable for a wide range of organic and flexible electronic devices.

Figure 4 .
Figure 4. (a) Dielectric constant, ε (as a function of frequency, f ) and (b) leakage current density, Ileak (as a function of electric field, Efield) of SPIs and cross-linked SPIs with HMBG.

Table 1 .
RMS roughness, water contact angle and surface energy of SPIs and cross-linked SPIs.

Table 2 .
Dielectric properties of SPIs and cross-linked SPIs thin-films measured with 30 MIM devices.
a Measured at 1 kHz.b Measured at 2 MV cm −1 .

Table 3 .
Electrical properties of pentacene TFTs with SPIs and cross-linked SPI gate dielectrics measured with 30 devices.