Achieving high field-effect mobility in CuSCN thin-film transistors with thiocyanate-functionalized polymers as fluorine-free dielectrics

Abstract

Interface engineering is crucial for optimizing interactions occurring upon contact of two different materials to produce electronic devices with high performance. Over the past few decades, copper(I) thiocyanate (CuSCN) has emerged as a promising hole transport material for a wide range of applications. Advances in understanding CuSCN interfaces have yielded significant improvement in photovoltaic properties and stability, being comparable to the state-of-the-art systems, through deeper insight into interfacial phenomena. While thin-film transistors (TFTs) were recognized as early applications of CuSCN in emerging technologies, CuSCN–dielectric interfaces – where charge transport occurs – have only been sparsely investigated. Herein, a series of functional copolymers with tunable dielectric properties were synthesized and employed as polymer dielectrics for CuSCN-based TFTs. A systematic increase of the dielectric constant and field-effect mobility was observed with an increased proportion of the thiocyanato (–SCN) functional group. Interestingly, TFTs with a purely SCN-functionalized polymer serving as a dielectric exhibited a promising mobility of 0.03 cm² V⁻¹ s⁻¹. This value is remarkably comparable to that achieved using the CuSCN-compatible high-k fluorinated terpolymer P(VDF–TrFE–CFE), despite the latter possessing a dielectric constant that is three fold higher. This work emphasizes that chemical compatibility at the interface is essential for further development of fluorine-free CuSCN-based devices.

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1. Introduction

Thin-film processing technology has emerged as a promising method for fabricating semiconductor devices, enabling miniaturization and minimizing cost while maintaining high performance of the devices.^1–4 Understanding the compatibility of each device component is key to success in developing thin-film-based electronics toward high efficiency and stability.^5,6 Copper(I) thiocyanate (CuSCN) is one of only a few solution-processable inorganic semiconductors showing the unique combination of a wide band gap and hole-transporting characteristics. CuSCN has been demonstrated as an excellent transparent p-type semiconductor for various thin-film devices due to less parasitic absorption than conventional hole-transport materials and its processability from exotic sulfur-based solvents, allowing for versatile device fabrication.^7–10 However, despite its extensive applications as a hole-transporting interlayer and semiconducting p-channel layer,^11–16 only a handful of articles have reported the interfacial properties of CuSCN. Arora et al. and Hang et al. reported a rapidly decaying photovoltaic efficiency in perovskite solar cells with a direct CuSCN–metal contact.^17,18 They unveiled photo-induced alloying or interfacial reactions as degradation mechanisms. Avoiding direct contact with metal electrodes, i.e., by inserting a spacer or replacing metal electrodes with carbon electrodes, successfully prevented degradation and extended the device lifetime. Additionally, damage caused by solvent during CuSCN deposition and structural mismatch between the perovskite layer and CuSCN also led to interfacial defects, resulting in a shortened lifetime of unpassivated devices, necessitating specific treatment and device engineering to yield more robust systems.^19–21 However, these studies were limited to CuSCN as a hole-transporting interlayer in solar cells only.

For thin-film transistors (TFTs), CuSCN offers the unique possibility of enabling the construction of solution-processable, transparent, p-channel devices, which is a rare combination. In the past few decades, continuous developments have resulted in performance improvements of CuSCN TFTs, particularly by doping and defect healing.^22–25 One component that has been largely unexplored in CuSCN TFTs is the dielectric layer, which is essential for the field-effect operation and has a direct impact on the carrier transport taking place at the dielectric/semiconductor interface. The choice of a suitable dielectric material depends not only on its dielectric constant (permittivity, k) but also on the interfacial properties between semiconductor and dielectric layers, including surface roughness, interfacial energy, and dipolar disorder. These factors greatly impact the operating voltage, carrier accumulation, and transport across the channel.^26,27 Accordingly, controlling the semiconductor–dielectric interface is pivotal for advancing device performance simultaneously with semiconductor engineering. For instance, although CuSCN-based TFTs with high-k oxide dielectrics exhibited poor performance, the hole mobility (µ_h) could be systematically enhanced by suppressing trapping states through optimization of annealing temperature and surface treatment with self-assembled monolayers.²⁸

Organic dielectrics offer exceptional tunability through chemical synthesis or composite formation and have received great attention for use as gate dielectrics. One of the key challenges is the intrinsically low dielectric constant of organic materials. As such, fluoropolymers, especially poly(vinylidene fluorine) or PVDF, have become interesting candidates in energy storage applications owing to strong polarization of the isotactic configuration (β-phase) and hydrophobicity of the fluorine groups. Conversely, the highly crystalline nature leads to the formation of permanent dipole moments and generates ferroelectricity, which hampers capacitive behavior due to hysteresis loss. To this end, organic dielectrics with high-k properties and suppressed ferroelectricity have been achieved via copolymerization. Incorporating a small proportion of chlorofluoroethylene (CFE) or chlorotrifluoroethylene (CTFE) into poly(vinylidene fluorine-co-trifluoroethylene) [P(VDF–TrFE)] to obtain terpolymers P(VDF–TrFE–CFE) or P(VDF–TrFE–CTFE) effectively disrupts crystallization of the copolymer, enabling its applications as a relaxor ferroelectric. These terpolymers exhibited high dielectric constants (k > 40) with reduced hysteresis loss, facilitating their reliable use as gate dielectrics.^29–32 For CuSCN-based TFTs, P(VDF–TrFE–CFE) is widely regarded as a very suitable high-k dielectric (k = 48–60),^33,34 reliably yielding high-performance CuSCN devices with excellent µ_h in the range of 0.01 – 0.02 cm² V⁻¹ s⁻¹.^{7,22,24,25,35} In contrast, devices incorporating Cytop, an amorphous fluorinated polymer (k ≈ 2), operated at high voltages with a low output current level. The compatibility of P(VDF–TrFE–CFE) with CuSCN was evidenced by a significantly greater mobility, which was 1–2 orders of magnitude higher than that in Cytop-based devices.³⁶ Nevertheless, the specific reasons for the mobility improvement in the former remain unclear, especially when both dielectrics are fluorinated polymers. The limited understanding prevents further rational design for improving the dielectric/CuSCN interface. Despite their excellent dielectric properties, fluorinated polymers suffer from poor degradability due to the high strength of C–F bonds, environmental persistence, and bioaccumulation, which have them labelled as “forever chemicals”. Therefore, it has become urgent to find alternatives to fluorinated polymers with comparable dielectric performance but which are more sustainable.³⁷ It is generally known that chemical and structural discontinuity at the interface between two different materials can lead to interfacial defects or dangling bonds, which serve as scattering or recombination centers.^38–40 These imperfections significantly hinder charge transport, particularly with TFTs where charge transport predominantly occurs at the interface. CuSCN is a coordination polymer, in which both the Cu⁺ center and ambidentate SCN⁻ ligand are tetrahedrally coordinated. When deposited from diethyl sulfide (DES) solvent, CuSCN films could form with the Cu-terminated high-energy surfaces,³⁵ which could be detrimental to carrier transport if left unpassivated due to the carrier trapping states.^41,42

Herein, to systematically investigate the effects of the polymer dielectric on CuSCN-based TFTs, we synthesized polystyrene (PS) derivatives with chloro (Cl) and thiocyanato (SCN) pendant groups and employed them as the dielectric layers in CuSCN TFTs. Styrenics are versatile polymers that do not contain polar or charge trapping groups (compared to methacrylate-based dielectrics containing ester groups), whereas the benzene ring allows for functionalization. Similar to PVDF-based polymers, functionalization and copolymerization with polar moieties have been shown to increase the k of styrenics to >3.4 and improve the device characteristics of TFTs based on organic semiconductors.^43–46 In our case, the two pendant groups (Cl and SCN) were selected based on their ability to coordinate with Cu(I) in CuSCN as shown in recent reports.^22,23,25 A relatively large and tunable k in the range of 8.9 to 18.2 was obtained depending on the Cl : SCN ratio. Even with a significantly lower k compared to P(VDF–TrFE–CFE), our simple fluorine-free SCN-incorporated polymer provided a promising µ_h of up to 0.03 cm² V⁻¹ s⁻¹, emphasizing the importance of chemical compatibility at the interface for performance improvements.

2. Experimental section

2.1. Materials

4-Vinylbenzyl chloride (VBC, 90%, Acros Organics), 1,1′-azobis(cyclohexanecarbonitrile) (ABCN, 98%, Sigma-Aldrich), potassium thiocyanate (KSCN, 98%, Alfa Aesar), copper(I) thiocyanate (CuSCN, 99%, Sigma Aldrich), polystyrene (PS, 35 kDa, Sigma-Aldrich), anhydrous toluene (99.85%, Acros Organics), diethylsulfide (DES, 98%, TCI), methanol (99.9%, Fisher Scientific), tetrahydrofuran (THF, 99.99%, Honeywell for polymer synthesis; and 99.9% inhibitor-free, Sigma-Aldrich for device fabrication), N,N-dimethylformamide (DMF, 99.8%, Acros Organics), hydrochloric acid (HCl, 37%, Carlo Erba) and nitric acid (HNO₃, 65%, Merck) were used as-received. Ultrapure water (18 MΩ cm) was used throughout all experiments.

2.2. Synthesis of poly(vinyl benzyl chloride) (PVBC)

VBC (1.53 g, 10.00 mmol) and ABCN (48.9 mg, 0.20 mmol) were dissolved in 3.60 mL of anhydrous toluene. The mixture was bubbled with nitrogen, and the reaction mixture was stirred at 80 °C for 48 h under a nitrogen atmosphere. After polymerization, the resulting mixture was precipitated with methanol. The product was then dissolved in THF and reprecipitated two more times with methanol. Finally, the product was dried in a vacuum oven to obtain a polymer with an apparent number average molecular weight (M_n) of ∼35 000 g mol⁻¹, a weight average molecular weight (M_w) of ∼ 70 100 g mol⁻¹, a molar weight distribution (MWD) of 1.84, and a conversion of ∼99%. ¹H NMR spectroscopy (DMSO-d₆, 600 MHz) δ [ppm]: 1.07–2.07 (m, 6H), 4.30–4.50 (s, 2H, –CH₂–Cl), 6.30–7.31 (m, 9H).

2.3. Synthesis of thiocyanate-containing polymers

PVBC obtained using the procedure described above (152.6 mg, 1.00 mmol) and KSCN (113.7 mg, 1.17 mmol; 48.6 mg, 0.50 mmol; or 19.4 mg, 0.20 mmol) were dissolved in 2.23 mL of DMF and stirred at 70 °C for 24 h. Finally, the solutions were dialyzed with ultrapure water (18 MΩ cm) to remove impurities and then freeze-dried. ¹H NMR spectroscopy (DMSO-d₆, 600 MHz) of PVBT δ [ppm]: 1.07–2.07 (m, 6H), 4.10–4.43 (s, 2H, –CH₂–SCN), 6.30–7.31 (m, 9H). The compositions of the copolymers were calculated from their ¹H NMR spectra. The apparent M_n values of PVBT, P(VBC_0.49-co-VBT_0.51) and P(VBC_0.80-co-VBT_0.20) were 29 100 g mol⁻¹, 28 300 g mol⁻¹ and 28 700 g mol⁻¹. Their apparent M_w values were 55 300 g mol⁻¹, 48 900 g mol⁻¹ and 46 200 g mol⁻¹, respectively, with a MWD of 1.90, 1.73 and 1.61, respectively. The compositions for the synthesis and molecular weights of the resultant polymers are listed in Table S1 (SI).

2.4. Analytical tools for characterization of polymerization and polymers

¹H NMR spectra were recorded with a Bruker 600 MHz Avance III HD spectrometer at room temperature in DMSO-d₆. The apparent molecular weights of PVBC, PVBT, and copolymers were determined by gel permeation chromatography (GPC, Malvern PANalytical, Viscotek TDAmax) with a refractive index detector. Polystyrene standards were selected as standards for the calibration (PSS Mainz). The sample was diluted with THF and filtered through 0.22 µm pore size poly(tetrafluoroethylene) (PTFE) syringe filters (Vertical). Three single-pore (6, 7, and 10 µm particle size, linear M) GPC/SEC columns with a flow rate of 1 mL min⁻¹ were used and run at 35 °C. Dried polymers were investigated by Fourier transform infrared spectroscopy (FT-IR, PerkinElmer, Frontier FT-IR equipped with a Universal-ATR accessory). The potassium (K) content in the samples was measured by inductively coupled plasma – optical emission spectroscopy (ICP-OES, Agilent Technologies, 710 Series) with Ar gas as a plasma source. Polymers were digested in a mixed solution of 7.5 mL of HNO₃ and 4.5 mL of H₂O₂ under microwave irradiation at 180 °C.

2.5. Preparation of CuSCN and polymer solutions

CuSCN solution was prepared by dissolving 20 mg mL⁻¹ of CuSCN powder in DES. The solution was stirred under a N₂-filled atmosphere at room temperature and then filtered with a 0.22 µm nylon syringe filter. Solutions of PS, PVBC, PVBT, and copolymers were prepared by dissolving the dried polymer (30 mg) in 0.5 mL of THF. The solutions were stirred overnight (>10 h) under ambient conditions and then filtered using a 0.45 µm PTFE syringe filter before use.

2.6. Characterization of dielectrics

Metal–insulator–metal (MIM) capacitors were fabricated using the following procedure. First, 1 in × 1 in borosilicate glass substrates were sequentially cleaned with detergent, deionized water, acetone, and isopropanol (15 min each) before being treated with UV/O₃ for 30 min. Next, 40 nm Al (99.99%, Kurt J. Lesker) was thermally evaporated under high vacuum through a shadow mask as a bottom electrode. Subsequently, the polymer solution was spun at 1500 rpm for 60 s onto the bottom Al electrodes (without any treatment to avoid oxidation of the underlying Al layer) to deposit the dielectric layer and annealed at 80 °C for 2 h. Finally, the top Al electrodes were also thermally evaporated through a shadow mask to complete the parallel plate capacitors with a device area of 0.01 cm². To characterize the dielectric properties, impedance spectra of the MIM devices were recorded using a Solartron Analytical 1260A impedance analyzer with a 4-wire configuration. The fabrication of MIM devices and dielectric characterization were carried out in a N₂-filled glove box. The dielectric constant (k) was determined from the equation of the standard parallel plate capacitor:where C is the capacitance value in the low frequency range (<1 kHz), ε₀ is the permittivity of vacuum, d is the thickness of the dielectric film, and A is the area of the parallel plate.

Dielectric breakdown strength was measured also with MIM devices in DC mode by recording a cumulative failure probability (P) that the device breaks at a particular applied electric field (E), which is based on the Weibull distribution:⁴⁷

P(E) = 1 − exp[−(E/E_b)^β] (2)

where E_b is the characteristic breakdown strength and β is the shape parameter evaluating the data scattering.

2.7. Fabrication of thin-film transistors

Staggered top-gate bottom-contact (TG-BC) TFTs were fabricated with the Cr/Au/CuSCN/dielectric/Al configuration via the following steps. First, 1 in × 1 in borosilicate glass substrates were cleaned as described above. Next, a Cr/Au layer was deposited by thermal evaporation under high vacuum through a precision shadow mask to produce 10 nm/25 nm source–drain contacts (S–D) (geometric dimensions: channel width W = 30 µm and channel length L = 1000 µm). The substrates with patterned S–D contacts were again treated with UV/O₃ for 20 min to improve wettability. Afterwards, a CuSCN layer with a thickness of 30–40 nm was prepared by spin-casting the CuSCN solution in DES at 2000 rpm for 60 s and annealed at 100 °C for 20 min under a N₂-filled atmosphere. After that, the dielectric layer was prepared by spin-casting the polymer solution in THF at 1500 rpm for 60 s and annealed at 80 °C for 2 h under a N₂-filled atmosphere. Finally, 40 nm Al was thermally evaporated through a shadow mask as a top gate electrode to complete the devices. The current–voltage (I–V) characteristics of the TFTs were measured with a probe station connected to a Keysight B2912A 2-channel source-measurement unit. TFT parameters were determined using the model based on standard gradual channel approximation.⁷

2.8. Analysis of elemental depth profiles

A blend of PVBC and PVBT (PVBC:PVBT) was prepared by mixing both homopolymer solutions (PVBC and PVBT solutions, 60 mg mL⁻¹ in THF) at a 0.87:1 volume ratio (see Table S2, SI). Film stacks were prepared on ITO-coated glass substrates by sequentially spin-coating CuSCN and P(VBC_0.49-co-VBT_0.51) or the PVBC:PVBT blend using the same procedure for device fabrication. The depth profiles of the stacks were recorded using an IONTOF ToF-SIMS V instrument. A 25 keV Bi³⁺ ion beam in the High Current Bunched Mode (HCBM) was used to analyze the sample, over a 150 µm × 150 µm area with a 128 × 128 primary beam raster. Sputter depth profiles were acquired using a 10 keV Ar_15 000⁺ gas cluster ion beam (GCIB) over an area of 500 µm × 500 µm. Final sputter crater depths were measured using the Zygo NexView optical interferometer.

3. Results and discussion

3.1. Synthesis and characterization of thiocyanate-functionalized dielectric polymers

PVBC was synthesized by free-radical polymerization of vinyl benzyl chloride in solution (see Fig. 1a), followed by purification to remove unreacted monomers. The thiocyanate polymer was synthesized by the reaction of PVBC with KSCN in solution to yield PVBT and P(VBC-co-VBT) (Fig. 1b) by varying the ratios of KSCN and VBC in the polymer.

Fig. 1 Scheme showing the preparation of poly(vinyl benzyl thiocyanate) PVBT and poly(vinyl benzyl chloride-co-vinyl benzyl thiocyanate) P(VBC-co-VBT). (a) Vinyl benzyl chloride was first polymerized, followed by (b) post-functionalization with potassium thiocyanate.

PVBT and P(VBC-co-VBT) copolymers were characterized by FT-IR spectroscopy (Fig. 2a). The sharp band at 2155 cm⁻¹ was associated with the C≡N stretching vibration of the thiocyanate groups.⁴⁸ In the case of the homopolymer (PVBT), the IR spectrum showed very strong signals with the bands at 2155 and 1510 cm⁻¹, belonging to stretching vibrations of the –SCN group and C=C–C of the aromatic ring of the benzyl group. The strength of the SCN signal at 2155 cm⁻¹ was intensified with the increasing proportion of VBT in the comonomer. ¹H NMR spectroscopy measurements indicated the successful syntheses of PVBT and P(VBC-co-VBT) (Fig. 2b). The signal of methylene protons adjacent to chlorine appeared at 4.66 ppm,⁴⁹ whereas the signal of methylene protons adjacent to thiocyanate appeared at 4.26 ppm.^50,51 The signal at 7.25–6.52 ppm was associated with protons in the phenyl ring, while the signal at 2.60–1.20 ppm corresponded to aliphatic protons of the polymer backbone, as shown in Fig. 2b.⁵⁰ The molar ratio of the monomer units in the copolymers was calculated by comparing the integrals of the –CH₂ proton in VBC (δ = 4.6 ppm)⁴⁹ with the –CH₂ proton of VBT (δ = 4.2 ppm).^50,51 The results showed that P(VBC_0.49-co-VBT_0.51) and P(VBC_0.80-co-VBT_0.20) were obtained from reactions performed with molar ratios of VBC units and KSCN of 0.5:0.5 and 0.8:0.2. The obtained molar fractions of the different comonomer units in the polymers were hence similar to the feed molar ratios of reagents. The polymers were purified by dialysis against ultrapure water for 9 days to remove ions.

Fig. 2 (a) FTIR spectra and (b) ¹H NMR spectra of PVBC, PVBT, P(VBC_0.49-co-VBT_0.51) and P(VBC_0.80-co-VBT_0.20).

For device applications, the synthesized polymers were prepared as thin solid films by spin-coating. UV-vis spectroscopy was used to characterize their optical properties (Fig. S1). The PS film (as a reference dielectric) exhibited a characteristic absorption peak at 262 nm. A shift of the characteristic peak to higher wavelengths, associated with a para-substitution,⁵² was observed in the synthesized polymers. All films showed a high transparency of >97% in the 400–700 nm range. Next, the dielectric constant (k) of the polymeric films was determined by solid-state impedance spectroscopy on MIM capacitors, with the results reported in Fig. 3a–f and Table 1. The measured k of PS was 3.2, close to literature values.^53–55 The para substitution on the phenyl rings led to an enhancement in electronic polarization, increasing k to 8.9 for PVBC. Converting the chloro to thiocyanato group further raised k to 10.1, 14.0 and 18.2 for P(VBC_0.80-co-VBT_0.20), P(VBC_0.49-co-VBT_0.51) and PVBT, respectively. All polymer samples exhibited excellent capacitive behavior with the phase angle θ ≈ −90° for frequencies up to 100 kHz. For PVBT, a noticeable drop in θ can be observed at frequencies below 10 Hz. Because the polymers were purified by dialysis (K⁺ concentration <0.02 wt%, see further discussions below) and the magnitude of the impedance remained high, the contribution of ion diffusion was ruled out. We therefore attribute this to changes in the space charge relaxation process instigated by the substitution of the –SCN group.^56,57

Fig. 3 (a) Schematic structure of the metal–insulator–metal (MIM) capacitor with a device area of 0.01 cm² illustrated in red rectangles and chemical structures of polymer dielectrics. Bode plots showing the magnitude (|Z|) and phase angle (θ) of the complex impedance vs. frequency (f) for (b) reference PS and (c–f) synthesized polymers. (g) Weibull distribution of the dielectric breakdown field of the polymers.

Table 1 Film thickness (d), dielectric constant (k), dielectric breakdown strength (E_b), and breakdown probability distribution shape parameter (β) of PS, PVBC, P(VBC-co-VBT), and PVBT. All dielectric films were fabricated on a glass substrate by spin-coating of 60 mg mL⁻¹ solutions in THF

Polymer	d (nm)	k	E b (MV m^−1)	β
PS	530	3.2 ± 0.7	109	3.0
PVBC	550	8.9 ± 0.1	133	3.5
P(VBC0.80-co-VBT0.20)	600	10.1 ± 0.1	267	5.0
P(VBC0.49-co-VBT0.51)	600	14.0 ± 1.2	185	5.3
PVBT	575	18.2 ± 0.3	155	3.1

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In addition to favorable polarizability, dielectric breakdown strength (E_b) is a critical parameter in determining the robustness of dielectric films under an applied electric field. A higher E_b could allow a thinner dielectric layer to be used while maintaining a low current leakage level and high stability under bias stress.⁵⁸ Then, the MIM devices were subsequently subjected to breakdown strength measurements. Fig. 3g presents the cumulative breakdown probability as a function of the applied electric field based on the Weibull distribution [eqn (2)]. The dielectric breakdown strength E_b is defined as the characteristic breakdown field corresponding to a cumulative device failure of 63.2% (x-intercept in the figure) whereas the distribution shape parameter β corresponds to the slope of the plot. Our reference PS exhibited an E_b of 109 MV m⁻¹, and improved breakdown strengths of 133 and 155 MV m⁻¹ were achieved with the functionalized homopolymers PVBC and PVBT, respectively. Notably, the copolymers revealed a substantial enhancement in the dielectric strength, reaching 267 MV m⁻¹ and 185 MV m⁻¹ for P(VBC_0.80-co-VBT_0.20) and P(VBC_0.49-co-VBT_0.51), respectively, with a narrow failure distribution (β ∼ 5). Possible explanations for enhanced E_b can be either intrinsic properties or thickness contribution. The latter factor can be ruled out as generally E_b tends to increase with decreasing thickness and vice versa, for sub-micron polymer dielectrics.⁵⁹ Therefore, this evolution in E_b could be attributed to lower free volume in the copolymer compared to the homopolymers, as sometimes observed in other systems.⁶⁰

3.2. Application as gate dielectrics in thin-film transistors

To evaluate the influence of the side-groups on device performance, CuSCN-based TFTs were fabricated using the synthesized polymers as the gate dielectrics. Devices based on the PS dielectric were employed as reference. TG-BC architecture was chosen in this study to avoid damage resulting from DES solvent on the polymeric dielectric. While we controlled the thickness of the dielectric layer to be comparatively similar (530–600 nm), the geometric capacitance (C_i) still varied significantly due to the wide range of the dielectric constant. To ensure comparison was made under similar channel conditions, the transfer curves were instead plotted against gate-induced charge density (n_ind), which can be calculated from:where q is the elementary charge constant, C_i is the areal capacitance of the dielectric layer, and V_GS is the gate-source bias voltage.

The results of PS- and PVBC-based devices, shown in Fig. S2, showed poor on/off drain current ratios (I_on/I_off) of less than 10, suggesting incompatibility with the CuSCN channel. Interestingly, PVBC yielded devices with high conductivity that could not be turned off. We speculate that the chloro pendants may lead to p-doping as the device characteristics were similar to CuSCN TFTs p-doped by metal chlorides reported in our previous work.²³ In contrast, TFT devices based on thiocyanate-containing polymers clearly exhibited the field effect with I_on/I_off on the order of ∼1000 and current saturation, as shown in Fig. 4. Surprisingly, the operating voltage for TFTs with thiocyanate polymers was found to be lower (reduced to −16 V for PVBT) than using high-k P(VDF–TrFE–CFE) at a similar thickness (biased to −30 V).²² As significant hysteresis was observed, device parameters, including saturation field-effect mobility (µ_h,sat), threshold voltage (V_th), and subthreshold swing (S_th), were determined separately for the forward and reverse scans. With the increasing SCN proportion, the hysteresis tended to decrease while the channel was turned on more easily with µ_h,sat progressively increasing. The latter reached ∼0.03 cm² V⁻¹ s⁻¹ in PVBT-based devices. This value is comparable to that of pristine CuSCN TFTs based on the exotic, high-k P(VDF–TrFE–CFE).^22,24,35 Most importantly, if we compare devices based on PVBC and PVBT, substituting the Cl pendant with the SCN pendant completely changed the device behavior: from an always-on channel to a switchable channel with current saturation. This demonstrates specific chemical compatibility between SCN and CuSCN. For example, the ambidentate SCN could potentially coordinate to Cu(I) sites on the surface of CuSCN.^47,61,62

Fig. 4 (a) Transfer characteristics of CuSCN-based transistors plotted with the gate voltage converted to the induced charge density (n_ind) in order to compare dielectrics with a large difference in the dielectric constant. Devices were fabricated in the top-gate bottom-contact architecture (a, inset) using a gate dielectric made of P(VBC_0.80-co-VBT_0.20), P(VBC_0.49-co-VBT_0.51), or PVBT. The scan directions are indicated by black arrows. Corresponding output curves of the devices with (b) P(VBC_0.80-co-VBT_0.20), (c) P(VBC_0.49-co-VBT_0.51), and (d) PVBT dielectrics.

To account for the large difference in k, V_th and S_th were converted into threshold charge density (n_th) and interfacial trap density (D_it) from:

where k_B is the Boltzmann constant, and T is the temperature (298 K). The data are included in Table 2 (TFT parameters without conversion are summarized in Table S3, SI). Interestingly, n_th which represents the threshold amount of gate-induced charge was reduced from 18.2 × 10¹¹ to 4.9 × 10¹¹ cm⁻² (forward scans) with increasing SCN content. For TFTs based on P(VBC_0.80-co-VBT_0.20), the large hysteresis resulted in V_th > 0 for the reverse scan. As the SCN proportion increased from 20% to 51% and 100%, the hysteresis was significantly reduced, with V_th < 0 and devices operating in the p-type enhancement mode. At first glance, the large hysteresis and large shifts in V_th, especially in the cases of co-polymer dielectrics, may suggest problems with interfacial traps at the semiconductor/dielectric interface; however, D_it values of all devices were in the same range (Table 2), indicating that this was likely not the case. As discussed below, this may instead be due to charge traps in the bulk of the dielectric layer.

Table 2 Average maximum transconductance (g_m) and TFT device parameters determined separately for forward (fwd) and reverse (rvs) scans. The errors in the average value are given by standard deviation. The statistics were analyzed from at least 8 devices for each dielectric

Dielectric	g m (×10^−7 S)	µ h,sat (×10^−2 cm^2 V^−1 s^−1)		n th (×10^11 cm^−2)		D it (×10^12 eV^−1 cm^−2)
		fwd	rvs	fwd	rvs	fwd	rvs
Note: Device parameters for CuSCN TFTs based on PS and PVBC were not determined due to the poor field-effect characteristics.
P(VBC0.80-co-VBT0.20)	0.73 ± 0.06	1.66 ± 0.12	0.14 ± 0.02	18.2 ± 0.3 (Vth < 0)	14.4 ± 1.3 (Vth > 0)	6.5 ± 0.1	11.0 ± 0.9
P(VBC0.49-co-VBT0.51)	1.16 ± 0.74	1.53 ± 0.14	1.25 ± 0.10	9.7 ± 0.3 (Vth < 0)	5.4 ± 0.3 (Vth < 0)	5.3 ± 0.4	10.3 ± 0.3
PVBT	3.05 ± 0.22	2.63 ± 0.36	3.29 ± 0.12	4.9 ± 0.8 (Vth < 0)	4.3 ± 0.3 (Vth < 0)	6.4 ± 0.6	11.1 ± 1.5

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Considering the apparent hysteresis loops, TFTs based on SCN-containing dielectrics exhibited clockwise or higher back sweep current (BSC) hysteresis behavior. After the forward sweep, holes remained strongly induced in the channel; more positive bias was needed to remove them and turn the channel off. In general, BSC hysteresis can be attributed to the polarization of the associated dielectric layer with three possible origins:⁶³ (i) mobile ions embedded in the dielectric, (ii) ferro- or quasi-ferroelectric nature, and (iii) charge injection from the gate electrode into the dielectric layer. Egginger et al. reported higher BSC hysteresis due to Na⁺ contamination in the poly(vinyl alcohol) (PVA) dielectric.⁶⁴ The commercially available “electronic-grade” PVA, which contained sodium acetate <0.09 wt%, exhibited a noticeable hysteresis when used as a gate dielectric for C₆₀-based TFTs. The authors showed that dialysis could significantly lower the ion impurities and effectively minimize hysteresis. In our case, ICP-OES was employed to determine the amounts of remaining cations. The results showed that the concentration of K⁺ was lower than 0.02 wt% in all synthesized polymers, which had been purified also by dialysis. Based on this information, we surmise that (i) ionic impurities were not likely a predominant factor to explain the observed hysteresis in our devices. Secondly, in ferroelectric-gated transistors, I_on occasionally starts to saturate as the V_GS sweeping window increases due to the polarization limit.^65–67 To study this behavior, we characterized our TFTs using different V_GS scan windows of ±30, ±45, and ±60 V. As shown in Fig. 5a–c, increasing the scan window led to an increase of I_on with an enlarged hysteresis loop instead of reaching a limit. Therefore, the hysteresis was also not likely due to (ii) ferroelectricity either. Next, when we considered the gate leakage current (I_GS) and the size of the hysteresis loop, it could be observed that both became larger as the V_GS scan range increased. This is consistent with the hysteresis based on (iii) charge injection from the gate electrode to the dielectric, as reported in the literature.^63,68 In this mechanism, applying a negative V_GS (for a p-channel) leads to injection of electrons into the dielectric layer. These negative charges are trapped even when the bias is removed, hence sustaining the electric field that induces holes in the channel at V_GS = 0 V and leading to higher BSC hysteresis. When the V_GS window is increased, more charges are injected into the dielectric, leading to higher I_GS and a wider hysteresis loop.

Fig. 5 Transfer curves at three different V_GS windows: ± 30, ± 45, ± 60 V of TFTs based on (a) P(VBC_0.80-co-VBT_0.20), (b) P(VBC_0.49-co-VBT_0.51), and (c) PVBT dielectrics, shown with leakage current (I_GS). (d) Retention characteristics of P(VBC_0.49-co-VBT_0.51)-based TFTs read at V_GS = 0 V and V_DS = −30 V, after programming (PRG, V_GS = −60 V for 1 s) and after erasing (ERS, V_GS = 60 V for 1 s), as shown by blue and red symbols, respectively.

Owing to the presence of a large hysteresis, TFTs based on the thiocyanate-containing polymers were investigated as nonvolatile memory devices. P(VBC_0.49-co-VBT_0.51) was selected because this dielectric yielded TFTs with a high drain-source current (I_DS) and large hysteresis loop. The memory characteristics were studied by a retention test as shown in Fig. 5d. A virgin device showed an initial I_DS (I_DS,0) of 2 × 10⁻⁸ A. When a programming pulse (V_GS,prg = −60 V, V_DS,prg = 0 V for 1 s) was applied, the device was set to a programmed current (I_{DS, t=1s}) of 5 × 10⁻⁶ A. During the retention phase, I_DS dropped by half within 90 s and then fluctuated between 40 and 50% of I_DS,0 over 1000 s (reading was done at V_GS,read = 0 V, V_DS,read = −30 V). After that, an erasing pulse (V_GS,ers = 60 V, V_DS,ers = 0 V for 1 s) was applied. Subsequently, the read current was restored close to the initial value of 3 × 10⁻⁸ A. Although the performance is not yet optimized, these polymers open up a new application for CuSCN-based memory devices.

3.3. Comparison between the copolymer and the polymer blend

To compare against the P(VBC_0.49-co-VBT_0.51) copolymer, we investigated whether films of a polymer blend containing the same amount of thiocyanate units, made by blending two homopolymers, would display similar properties. PVBC and PVBT homopolymers were blended to yield a 1 : 1 molar ratio of monomer units (see calculation in the SI and Table S2). The elemental profile in Fig. S3a characterized by secondary ion mass spectroscopy (SIMS) revealed a clear phase separation of the blend film. The Cl-rich PVBC was segregated on the top layer while PVBT was concentrated on the bottom part close to the semiconductor–dielectric interface. In contrast, such segregation was not observed in the copolymer P(VBC_0.49-co-VBT_0.5) as shown in Fig. S3b. A similar phenomenon of vertical separation in polymer blends is well known in the literature.⁶⁹ Segregation between poly(3-hexylthiophene) and its fluorinated derivative was observed and attributed to the difference in surface energies.⁷⁰ Phase separation was also observed in the blend of poly(vinylidene fluoride-co-trifluoroethylene) and poly(methyl methacrylate), which were segregated to top and bottom layers, respectively.⁷¹ In terms of device operation, the homopolymer blend led to inoperable devices with poor field-effect characteristics (Fig. S4), dramatically different from the copolymers. This study shows that the tunable dielectric properties are specific to P(VBC-co-VBT) and cannot be obtained by simply blending the two homopolymers together.

4. Conclusions

In summary, polymer dielectrics, namely PVBC (Cl-based homopolymer), PVBT (SCN-based homopolymer), and two P(VBC-co-VBT) copolymers were synthesized and characterized. Importantly, our simple polymer design and facile synthetic method can systematically tune the dielectric properties of the polymers, with the dielectric constant systematically increasing with the proportion of the thiocyanate group. A remarkably high k of 18 was obtained using PVBT. CuSCN-based TFTs exhibited a notably high hole mobility although the dielectric constant was lower than one-third of that for the widely employed fluorinated dielectric, P(VDF–TrFE–CFE), suggesting good interfacial compatibility between the SCN-containing dielectric and CuSCN. Achieving the high k value also facilitated device operation at a lower voltage range (V_G ≥ −16 V) compared to PS (V_G ≥ −60 V). However, a large hysteresis and gate voltage-dependent transfer characteristics provided evidence of gate-injected and trapped electrons in the dielectric layer. This point suggests that further optimization is expected, for example, by adding a charge blocking layer. Last but not least, preliminary use of combination of CuSCN and SCN-polymers in transistor-based memory devices can open up a new application for coordination polymer-based electronics. This work also paves the way toward the generation of polymer dielectrics for CuSCN-based devices that are fluorine-free, thereby circumventing the crucial issue of environmental persistence of fluorinated polymers.

Author contributions

Chitsanucha Chattakoonpaisarn: methodology, investigation, validation, formal analysis, data curation, visualization, and writing – original draft; Vatita Leamkaew: methodology, investigation, validation, formal analysis, data curation, visualization, and writing – original draft; Matilde Brunetta: methodology and investigation; Sarah Fearn: methodology and resources; Patipan Sukpoonprom: methodology and supervision; Taweesak Sudyoadsuk: methodology and supervision; Vinich Promarak: resources and funding acquisition; Nicola Gasparini: resources and supervision; Daniel Crespy: conceptualization, resources, project administration, supervision, and writing – review and editing; Pichaya Pattanasattayavong: conceptualization, funding acquisition, resources, project administration, supervision, visualization, and writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5tc03581c.

Acknowledgements

C. C. and V. L. acknowledge PhD scholarships from the Vidyasirimedhi Institute of Science and Technology (VISTEC). This research has received funding support from the National Science, Research and Innovation Fund (NSRF) via the Program Management Unit for Human Resources & Institutional Development, Research and Innovation [grant number B49G680107].

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Footnote

† These authors contributed equally to the work and are co-first authors.

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