Self-assembly of poly (ionic liquid) block copolymer based dielectrics on semiconductor formation and performance

Abstract

Wearable, disposable, implantable and lightweight electronic devices all require low power consumption, typically less than 3 V. Polymerized ionic liquids represent an emerging class of dielectric materials which can enable low voltage organic thin film transistor operation (OTFT) while in the solid state, reducing the potential for toxic or flammable electrolyte leaks. Here we show that bottom gate bottom contact OTFTs were successfully fabricated using a library of 12 poly-(styrene)-b-poly(1-(4-vinylbenzyl)-3-butylimidazolium-r-poly(ethylene glycol) methyl ether methacrylate) block copolymers with varying VBBI⁺ and PEGMA ratios, and can achieve OTFTs that turn on between 0 and 1 V. We demonstrate that the poly(ionic liquid) microstructure dictates the self-assembly which correlates to OTFT performance. Notably, we indicate that changing the surface chemistry of the substrate the poly(ionic liquid) is cast on is less significant in driving block copolymer self-assembly compared to block ratios and the counter ion. We also report that the semiconductor crystalline domains are similar when deposited on the poly(ionic liquids), however the films differ in terms of semiconductor molecular order in the amorphous domains. Overall, we demonstrate that device performance is primarily linked to the poly(ionic liquid) self-assembly, while also highlighting the importance of both the crystalline and amorphous domains in the semiconductor structure of thin films.

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Introduction

Wearable devices have been present in modern technology for centuries, with some of the most notable examples being eyeglasses and wrist watches.¹ As technology evolves, so do the potential applications from smart textiles to wearable biological sensors.^2–4 Therefore, there is a growing market seeking flexible, lightweight, and comfortable electronic devices that can be integrated directly into these applications.³ An ideal candidate for these application are organic thin film transistors (OTFTs), which are three electrode logic operators based on organic semiconductor materials. Their ability to be processed at low-temperature and solution deposition enables their integration onto light weight, flexible and stretchable substrates.^3,5–8 However, these technologies still face several challenges, such as low electronic performance through high threshold voltages, poor stability, and reduced device lifespan.¹ Given the nature of these devices, they will be required to be powered by both printed and common batteries, which supply potential energy in the range of 1 to 3 V.^9,10 As such, the electronic devices must operate within this range, which for the application of OTFTs, means that the operating (threshold) voltage must be less than 3 V.

The OTFT threshold voltage (V_T) is primarily a function of the dielectric and semiconductor interface and can be reduced by reducing the thickness and increasing the permittivity of the gate dielectric.¹¹ Typical dielectrics such as silicon dioxide (SiO₂) and aluminum oxide (Al₂O₃) are brittle and have relatively low permittivity, making them less ideal for flexible electronics. In contrast, electrolyte gating solutions have demonstrated large charge carrier densities at the semiconductor interface with a fraction of the voltage required by typical solid state gate dielectrics due to the formation of an electrical double-layer (EDL). This EDL conducts mobile ions that support a large electric field gradient such that only the thin interfacial layer is polarized. The resulting geometric capacitance is therefore much larger than that of typical dielectric, rendering the electrolyte gate more energy efficient.⁹ However, electrolytes are often composed of corrosive liquids and may require cryogenic temperatures for operation, which make them less ideal for wearable applications.⁹

Polymerized ionic liquids (PILs) are ionic polymers which display similar ionic transport to electrolytes while being solid at the room temperature like a polymer. PILs have also been shown to improve OTFT performance and reduce the turn-on voltage through increased capacitance.^10,12–15 Similar to electrolyte gated devices, PILs are able to form an EDL when a bias is applied, however in this case, the mobile anion diffuses through a stationary charged polymer matrix in response to the electric field.^{10,12,13,16–18} High performance of the gating material has been shown to be directly related to the mechanical stability and ionic conductivity, which we have previously shown can be done by using block copolymers by incorporating a mechanically stable and a highly conductive block.^10,12,13 The PIL conductivity is a function of the polymer structure and choice of counter ions, but questions remain on the influence of PIL block copolymer self-assembly on the orthogonal fabrication of OTFT and the resulting device performance.

We demonstrated that the PIL block copolymer morphology in a top gate OTFT led to optimal device performance when that block copolymer self-assembly was in a mixed morphology.¹⁰ In these devices, the PILs were deposited on the semiconductor P(NDI2OD-T2), and the corresponding morphology of the PIL was a result of being deposited on top of the semiconductor.

In our previous work, we used P(NDI2OD-T2) as the semiconductor in top gate OTFT configuration to investigate the thin-film self-assembly of PILs on top of this semiconductor as a function of device performance. The PILs used are from a library of 12 poly-(styrene)-b-poly(1-(4-vinylbenzyl)-3-butylimidazolium-r-poly(ethylene glycol) methyl ether methacrylate) block copolymers, which will henceforth be referred to as S-b-(VBBI⁺ [X⁻]-r-PEGMA) where X is the counterion, or S-b-(VY-r-PZ) where the Y and Z are the VBBI⁺ and PEGMA loading percentage, respectively. Here, 4 VBBI⁺/PEGMA ratios were synthesized, each paired with 3 different anions (Fig. 1c, where X = TFSI⁻, PF₆⁻, or BF₄⁻). From this study, we demonstrated that the PIL block copolymer morphology in a top gate configuration led to optimal device performance when the self-assembly was mixed morphology.¹⁰

Fig. 1 (a) Bottom gate top contact device architecture employed in this study. (b) Grazing incidence wide-angle X-ray scattering (GIWAXS), grazing incidence small-angle X-ray scattering (GISAXS), and Raman spectroscopy film morphology analysis. (c) PIL and the mobile anions and (d) F₁₆CuPc chemical structure.

However, this study left unclear whether block copolymer self-assembly was a function of the semiconductor thin film properties or PIL composition, and if the semiconductor morphology would be a function of PIL self-assembly if deposited in bottom gate bottom contact (BGTC) (Fig. 1a) configuration. Therefore, this study employs bottom gate bottom contact OTFTs successfully fabricated using this same library of PILs. Here, the PILs were deposited by solution deposition on silicon (Si) substrates followed by the evaporation of semiconducting molecule 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadecafluoropthalocyanine copper(II) (F₁₆CuPc, Fig. 1d). Morphology analysis of both the PIL and F₁₆CuPc layers are conducted to determine how their self-assembly drives device performance, isolating what drives block copolymer self-assembly, and demonstrating the universal self-assembly behaviour of the PIL layer and its effect on semiconductor formation and resulting low V_T OTFT operation.

Methods

Materials

F₁₆CuPc was obtained from TCI America. All solvents were obtained from Sigma-Aldrich, and gold (99.999%) was obtained from Kurt J. Lessker. Polymerized ionic liquids (PILs) used in this study are represented in Fig. 1c, and were synthesized following the procedure previously reported.¹² As the same material is used in this study as reported by Peltekoff et al., ionic conductivity is reported in Table S1 (ESI†), while NMR data can be found in our previous work.¹²

Thin-film and device fabrication

Thin films were fabricated on silicon substrates (15 mm × 20 mm) obtained from nanoFab at the University of Alberta, which were cleaned in sequence by sonication baths in soapy water, DI water, acetone, and methanol. These substrates were then plasma treated for 10 minutes to remove any residual solvents. The PILs (60 mg mL⁻¹ in 1-butanone) were deposited by spin coating at 2000 rpm for 60 s and annealed at 130 °C for 1 hour. The spin coating and annealing procedure was then repeated to reduce to effect of pinholes in the film. Following the second annealing, F₁₆CuPc was deposited by physical vapor deposition at a rate of 0.2 Å s⁻¹ to create a film that is 40 nm thick. The gate electrodes were deposited by physical vapor deposition using a shadow mask with a channel width of 1000 μm and length of 30 μm to create 20 individual bottom gate top-contact transistors per substrate. Gold was deposited at a rate of 1 Å s⁻¹ until a thickness of 50 nm was reached.

Device characterization

Transistor devices were characterized by using a custom-built automatic multitester. The tester consists of 48 gold-plated (20 nm) nickel probe tips, which contacts the source–drain electrodes of the individual transistors and the gate electrode.¹⁹ The multitester providing high throughput testing capability but introduces a resistance of ∼750 mOhm to testing. A Keithley 2614B and an MCC USB DAQ were used to control the source–drain voltage (V_SD) and gate voltage (V_GS) to obtain source–drain current (I_SD) measurements. The multitester was kept at room temperature in air at atmospheric pressure for the duration of characterization. To obtain the transfer curves, the V_SD was fixed to a constant 2 V (with the exception of S-b-(V100-r-P0) of the BF₄⁻ anion, which had to be obtained at 3 V to operate devices in the saturation regime). The V_GS was swept from −1 to 3 V at a rate of 50 mV steps every 80 ms, with a full forward and backward sweep being performed in 9.6 s, resulting in a frequency of approximately 0.1 Hz. The measurements were performed at this frequency to ensure EDL formation could occur in all the materials, based on the frequencies of EDL formation observed in our previous work.¹² Output curves were obtained by sweeping the V_SD from 0 to 3 V while holding the V_GS constant in steps of 0.75 V from 0–3 V.

To determine the saturation regime electron field-effect mobility (μ_e), threshold voltage (V_T), and on/off current ratio (I_on/off) from the resulting transfer curves, the modified MOSFET equation (eqn (1)) was to relate the saturation regime field-effect mobility and threshold voltage to the V_GS and I_SD measurements.

Note here that C_i is the capacitance (F cm⁻²), while W (μm) and L (μm) are the channel width and length, respectively.

Thin-film characterization

Polarized Raman microscopy characterization maps were obtained following the procedure outlines by Cranston et al.²⁰ Two types of polarized Raman spectra (Z(X,X)Z′ and Z(X,Y)Z′) were recorded using a Renishaw inVia Qontor confocal Raman microscope. The Raman microscope uses a Leica Microsystems bright-field microscope with a DM2700 light source. A 500 mW 532 nm wavelength laser with a 2400 L mm⁻¹ grating was used to obtain measurements in the spectral range of 900–2000 cm⁻¹, focused on the sample by an X50L objective. Each polarized Raman spectra was taken at the same location on the sample. Raman maps (20 × 20 μm) were generated from 400 individual spectra using a 1.0 μm step size with a 5% laser power (25 mW) and a 2 s exposure time. To ensure no degradation from laser exposure, microscope images of the film before and after the scans are present in Fig. S7 (ESI†).

Each spectrum was fitted to the theoretical Lorentz curve using Wire 5.6 inVia software to obtain integral intensity (I_xx and I_XY) of the B_1g pyrrole stretch Raman mode (1535 cm⁻¹). As per Cranston et al. F₁₆CuPc can use eqn (2) to estimate the angle of the molecules with respect to the substrate normal (β).

The thickness of the films was measured and averaged for 6 different using a Dektak XT profilometer (Bruker). Deionized (DI) water and diiodomethane contact angle (θ) was measured on an Si and P(NDI2OD-T2) on quartz substrates by dispensing 0.5 μL and 1 μL drops of each reference liquid respectively. Images of each drop were taken using a VCA Optima goniometer camera (AST Products Inc.) and processed using ImageJ and a drop-shape analysis package.²¹

Atomic force microscopy (AFM) 2.5 μm × 2.5 μm images were obtained using a Park NX10 AFM, with ScanAsyst-Air probes in PeakForce Tapping mode. Images were obtained using a Tap300Al-G silicon tip with a radius of 10nm, a force of 40 N m⁻¹, a scan rate of 0.25 Hz. Basic image processing was performed in Gwyddion to flatten and remove noise and scan lines.

Grazing-incidence wide-angle X-ray scattering (GIWAXS)

Thin films of F₁₆CuPc on the various PILs were prepared as described in the thin film fabrication methodology. GIWAXS experiments were performed at the Canadian Light Source in Saskatoon, Canada, using the Brockhouse Diffraction Sector Undulator (BXDS IVU) beamline with a photon energy of 15.1 keV and a Rayonix MX300 CCD detector (73 × 73 μm pixel size), placed approximately 418 mm from the sample with an angle of incidence of θ = 0.3°. All GIWAXS data was calibrated against a silver behenate standard and a P3HT standard and analyzed using the GIXSGUI software package in MATLAB, where both polarization and solid angle corrections were applied.²² A second derivative baseline was subtracted from all GIWAXS diffraction patterns to reduce background scattering and to better visualize the data.

Grazing-incidence small-angle X-ray scattering (GISAXS)

GISAXS samples of the PILs on Si were prepared following the thin film fabrication methodology, however, for these experiments no F₁₆CuPc is deposited on the PIL. GISAXS experiments were performed at the Canadian light source (CLS) using the Brockhouse X-ray diffraction sector-low energy wiggler (BXDS-WLE) beamline. A photon energy of 15.1 keV was selected using a Si(111) monochromator. The beam size was defined by slits having a 0.2 mm vertical gap and a 0.3 mm horizontal gap, and the angle of incidence was set to 0.01°. GISAXS patterns were collected with a Rayonix MX300 CCD detector (73.242 μm pixel size; 30 cm diameter), which was placed 2506 mm from the sample center. The GISAXS data were calibrated against a silver behenate standard and analyzed using the GIXSGUI software package.²² Both polarization and solid-angle corrections were applied. The Yoneda peak was used for linecuts, wherein the cut was taken at approximately q = 0.012 Å⁻¹ for each sample.

Results and discussion

Bottom gate top contact OTFTs were fabricated using the poly(S)-b-poly(VBBI⁺[X⁻]-r-PEGMA) polymers as the gating material (Table S1, ESI†), with F₁₆CuPc deposited on top as the semiconducting layer. The transfer curves of the resulting devices can be found in Fig. 2a–c for PF₆⁻, bis(trifluoromethanesulfonyl)azanide (TFSI⁻), and BF₄⁻, respectively. The extracted electron mobility (μ_e) and threshold voltage (V_T) values are compared in Fig. 2d–f. Sample output curves can be found in Fig. S1 (ESI†) and the thickness of the PIL layers can be found in Table S3 (ESI†). μ_e ranged from 0.014 × 10⁻³ to 1.2 × 10⁻² cm² V⁻¹ s⁻¹, while threshold voltage ranged from −0.4 V to 1 V.

Fig. 2 (a)–(c) Sample transfer curves for OTFT devices fabricated with the semiconductor F₁₆CuPc and dielectric poly(S)-b-poly(VBBI⁺[X⁻]-r-PEGMA) block copolymers, with varying PEGMA/VBBI⁺ ratios for (a) TFSI⁻, (b) PF₆⁻, and (c) BF₄⁻ anions. Average (d) mobility, (e) threshold voltage, and (f) I_on/I_off. Values presented are the average of the last four of five measurements each from 40 devices, with error bars representing the standard deviation. Each device was operated at 2 V_SD, except for the devices for S-b-(V100-r-P0) with BF₄⁻, which was operated at 3 V_SD as the devices did not function at the lower voltage. Devices with a PEGMA loading of 75% did not turn on due the PIL not forming a sufficient EDL, illustrated as by *.

F₁₆CuPc in a BGTC with SiO₂ dielectric has been reported to have a threshold voltage between 5 V to 20 V, with a source drain voltage (V_DS) of 50 V and a gate voltage (V_GS) range from 0 V to 60 V.^23–25 In this study, by using the PIL dielectric, low V_T ranges (−0.4 V to 1 V) are achieved while maintaining low potential operation (V_DS = 1–2 V). Similar drops in V_T were observed when pairing PIL dielectrics with other semiconductors such as P(NDI2OD-T2).¹⁰

In the case of devices fabricated using the PF₆⁻ and BF₄⁻ anion-based PILs, increasing the PEGMA loading from 25 mol% to 50 mol% resulted in an overall improvement in device performance, as characterized by the increase in μ_e and decrease in V_T.

The highest μ_e observed for each anion was 3.76 × 10⁻⁴ cm² V⁻¹ s⁻¹, 1.30 × 10⁻² cm² V⁻¹ s⁻¹, and 2.34 × 10⁻² cm² V⁻¹ s⁻¹, for S-b-(V75-r-P25)-TFSI⁻, S-b-(V50-r-P50)-PF₆⁻, and S-b-(V50-r-P50)-BF₄⁻ respectively. The highest electron mobility occurred in devices assembled with BF₄⁻ with a PEGMA loading of 50 mol%, which agrees with the findings of Brixi et al.¹⁰ This could suggest that this particular block copolymer has the ideal combination of thin film morphology in the device balanced with high ionic conductivity, leading to a more pronounced gating effect with a greater number of charge carriers and less traps at the semiconductor/dielectric interface. Furthermore, this electron mobility is within the same order of magnitude to devices fabricated with F₁₆CuPc on SiO₂ while once again reducing the threshold voltage, emphasizing the efficiency this PIL has as the gating material.²⁶

In all cases, the device hysteresis is very low, which differs from Brixi et al., where relatively high hysteresis was observed.¹⁰ This could suggest that the BGTC configuration improves the contact resistance between the electrodes and the semiconducting material compared to the top gate top contact (TGTC) configuration. The molecular ordering of the semiconductor or dielectric may also be higher in this study, decreasing the overall hysteresis.

Leakage current is observed across all devices, as is typically observed with PIL gated OTFT devices as a result of the migration of ions.^27–29 Most notably, however, are devices fabricated with TFSI⁻ (Fig. S1(g and h), ESI†), which show more leakage current than those fabricated with BF₄⁻ and PF₆⁻. Given that the PILs fabricated with TFSI⁻ have higher conductivity relative to the other anions (Table S1, ESI†), this likely results in the larger migration of these ions, resulting in higher perceived leakage current.

The F₁₆CuPc based devices fabricated with 0% PEGMA containing PILs are close to non-functional, given by their low I_on/I_off ratios, suggesting that the MOSFET model may not be the appropriate fit due to their poor performance. However, devices fabricated with 75 mol% PEGMA containing PILs were completely non-functional, indicating that there is both a minimum and maximum PEGMA loading percentage required to appropriately gate the device.

Fig. 6b demonstrates that the use of PILs with TFSI⁻ anion results in relatively consistent electrical performance, despite significant changes in conductivity of the PIL with increasing PEGMA loading (Table S1, ESI†).¹³ The performance reported in Fig. 6d–f shows poor performance relative to the other devices for same VBBI⁺/PEGMA ratios, where change in μ_e, V_T, and I_on/I_off are all within one standard deviation of each other. This indicates that either the morphology of the PILs or the assembly of F₁₆CuPc on the PIL surface could be less influential for charge transport. It may also be a result of film thickness given that devices fabricated with the TFSI⁻ anion results in thinner films (Table S1, ESI†) compared to those fabricated on PF₆⁻ and BF₄⁻. The reduced thickness may result in consistently poor device performance and decrease the influence of the PIL conductivity or morphology.

However, in order to conclusively explain the differences in electrical performances for all anions and VBBI⁺/PEGMA ratios, the block copolymer self-assembly on Si at the semiconductor/dielectric interface, as well as morphology of F₁₆CuPc on the PIL must be investigated.

Block copolymer self-assembly analysis

In order to characterize the block copolymer self-assembly of the PILs on Si, GISAXS scattering patterns were analyzed by taking linecuts of the Yoneda peak along q_r axis. Expected q-spacing of typical block copolymer self-assembly phases were plotted with the linecuts to indicate where peaks should appear and highlight the observed correlations. Due to weak ordering, many of the features are broad and appear as weak shoulders. Symbols used on linecuts represent the expected q-spacing of lamellar (LAM, red squares), cylinder (HEX, green inverted triangles), and bicontinuous gyroidal (BCG, black triangles) phases. The alignment of a shoulder with an indicated symbol suggests the film morphology displays characteristics of the corresponding ordered self-assembly mode.¹⁰

From Fig. 3, the morphologies when the anion is substituted for BF₄⁻ are presented. Relatively weak ordering is observed across all signals, similar to Brixi et al., and the PIL paired with this the anion has a preference for mixed BCG and LAM packing modes when the PEGMA content is less than 50 mol%.¹⁰ As the PEGMA loading increases, the morphology tends towards HEX, where 75 mol% PEGMA loading results in predominantly HEX packing and agrees once again with the findings of Brixi et al.¹⁰

Fig. 3 (a)–(d) Grazing-incidence small-angle X-ray scattering patterns and (e)–(h) line cuts at the Yoneda peak for thin films of poly(S)-b-poly(VBBI⁺[BF₄⁻]-r-PEGMA) block copolymers, with varying PEGMA/VBBI⁺ ratios: (a) and (e) S-b-(V100-r-P0), (b) and (f) S-b-(V75-r-P25), (c) and (g) S-b-(V50-r-P50) on Si, and (d) and (h) S-b-(V25-r-P75) with the BF₄⁻ counterion. For (e)–(h), red squares = LAM, black triangles = BCG, green inverted triangles = HEX and q* is assigned as the first peak to make the morphology assignment. Simplified visual examples of a unit cell for the predominant morphologies identified from GISAXS are overlaid.

Fig. 4 shows the self-assembly when the anion is substituted for TFSI⁻, with the GISAXS linecuts indicating a preference towards LAM packing at low PEGMA loading content, which agrees with Brixi et al.¹⁰ As the PEGMA loading increases, the morphology tends towards a mixed LAM and HEX in S-b-(V50-r-P50) packing before becoming HEX dominated in S-b-V(V25-r-P75).

Fig. 4 (a)–(d) Grazing-incidence small-angle X-ray scattering patterns and (e)–(h) line cuts at the Yoneda peak for thin films of poly(S)-b-poly(VBBI⁺[TFSI⁻]-r-PEGMA) block copolymers on Si, with varying PEGMA/VBBI⁺ ratios: (a) and (e) S-b-(V100-r-P0), (b) and (f) S-b-(V75-r-P25), (c) and (g) S-b-(V50-r-P50), and (d) and (h) S-b-(V25-r-P75) with the TFSI⁻ counterion. For (e)–(h), red squares = LAM, black triangles = BCG, green inverted triangles = HEX and q* is assigned as the first peak to make the morphology assignment. Simplified visual examples of a unit cell for the predominant morphologies identified from GISAXS are overlaid.

Finally, the morphology of the PILs when PF₆⁻ is substituted as the anion is described in Fig. 5. HEX and BCG morphology are favoured with 0% PEGMA loading, with LAM and BCG morphology for 25% loading and LAM and HEX morphology for 50% loading. However, similar to the previous anions, when the loading reaches 75% the HEX morphology is favoured, suggesting that at 75% PEGMA loading, HEX is the predominant packing mode.

Fig. 5 (a)–(d) Grazing-incidence small-angle X-ray scattering patterns and (e)–(h) line cuts at the Yoneda peak for thin films of poly(S)-b-poly(VBBI⁺[PF₆⁻]-r-PEGMA) block copolymers on Si, with varying PEGMA/VBBI⁺ ratios: (a) and (e) c (b) and (f) S-b-(V75-r-P25), (c) and (g) S-b-(V50-r-P50), and (d) and (h) S-b-(V25-r-P75) with the PF₆⁻ counterion. For (e)–(h), red squares = LAM, black triangles = BCG, green inverted triangles = HEX and q* is assigned as the first peak to make the morphology assignment. Simplified visual examples of a unit cell for the predominant morphologies identified from GISAXS are overlaid.

Given that the GISAXS diffraction signals for each anion and VBBI⁺/PEGMA ratio are similar to those reported by Brixi et al., this indicates that differences in surface chemistry given by the different contact angles (Table S2 and Fig. S9, ESI†) between P(NDI2OD-T2) and Si plays a less significant role in the self-assembly compared to the copolymer composition and choice of counterion.¹⁰ When relating these results to the OTFT performance, it can be concluded that despite the counterion present, when the PEGMA loading reaches 75%, the self-assembly tends towards HEX packing that leads to either a lack of EDL formation or formation of EDL only at localized points that are too small to allow for functional devices. Overall, this packing and PEGMA loading content is not favourable for these OTFT devices.

Meanwhile, the best devices appear to have a mix packing modes, suggesting that the mixed morphology facilitates mobile anion transport by creating more pathways for ions to travel through the polymer film. This is further supported by the fact that TFSI⁻ shows preferences for one packing mode (LAM) which resulted in poor OTFT devices. Therefore, we can say the morphology of the PIL influences the EDL formation, leading to higher effectiveness gating in mixed-morphology PILs whereas low- to no-gating in devices strongly exhibiting a single morphology.

F₁₆CuPc self-assembly analysis

It is well known that the surface chemistry of the dielectric material will influence the self-assembly and morphology of the semiconductor when it is deposited on the dielectric, therefore influence the corresponding OTFT performance. GIWAXS analysis was performed to characterize the molecular orientation of the F₁₆CuPc semiconductor on the series of PILs. Fig. 5a–d show the corresponding 2D scattering patterns for F₁₆CuPc on the PIL with the counterion BF₄⁻ for varying PEGMA/VBBI⁺ ratios (Fig. S2 and S3 show the similar plots for the counterion PF₆⁻ and TFSI⁻, respectively, ESI†). The corresponding azimuthally integrated diffraction patterns for the counterion PF₆⁻, TFSI⁻, and BF₄⁻, are presented in Fig. 5e–g, respectively, with the corresponding d₁₀₀-spacings for the q values reported in Table S4 (ESI†). The GIWAXS scattering patterns were characterized by a low intensity signal for all films cast on PIL compared to films cast on Si (Fig. S5, ESI†), suggesting the F₁₆CuPc on the PIL exhibits lower crystallinity.³⁰ This low crystallinity is typical for films of phthalocyanines deposited on polymer layers.^31,32 The highest intensity single is observed at the (100) reflection out-of-plane q_z at q = 0.50 Å⁻¹, and a weaker in-plane (010) reflection observed at ∼q = 2 Å⁻¹, which agrees with the single crystal data.³³ There is a preference for out-of-plane scattering along q_z, indicative of F₁₆CuPc oriented toward the substrate surface in an edge-on packing motif (Figure fh). This is further confirmed by the pole figures (Fig. S4, ESI†) where the (100) diffraction peak (0.25 Å⁻¹ < q < 0.65 Å⁻¹) is integrated for −83° < χ < −3° to determine the molecular orientation.^34–37 Edge-on orientation is characterized by a peak intensity of |χ| < 45° while face-on grains are characterized by |χ| > 45°.³⁴ Given that the single Gaussian peak occurs between −45° and −83° for each of the films, this indicates edge-on orientation is dominant for all samples.

It is notable that no matter the VBBI⁺/PEGMA ratio or counterion present, the GIWAXS pattern for the F₁₆CuPc remains similar, suggesting that the crystalline domains of the F₁₆CuPc are not influenced by the choice of PIL block copolymer and their corresponding self-assembly that may lead to changes in surface chemistry. The GIWAXS scattering patterns do not indicate large differences in the crystalline domain with respect to degree of crystallinity and molecular orientation (Fig. S4, ESI†), nor do the full width half maxima (FWHM) of the (100) diffraction peak (Table S5, ESI†) differ significantly between samples. Furthermore, the pole figures suggest that the crystalline domains of the films are largely edge-on, which is favourable for charge transport.^38,39 Therefore, these results indicate that the self-assembly of the crystalline domains of F₁₆CuPc is not responsible for the large differences observed in OTFT performance.

However, given that GIWAXS does not provide insight into the amorphous domains, Raman polarized microscopy was selected to probe the molecular orientation of both the amorphous and crystalline domains of the films.^20,40–43 This technique has been previously shown to characterize the molecular orientation of CuPc through the internal molecular vibration approach, where the intensity of the Raman band depends on the orientation of a materials’ axes of symmetry with respect to the direction of the excited laser and scattered light polarizations.²⁰ Therefore, 20 × 20 μm polarized Raman microscopy maps were collected for each of the F₁₆CuPc films on ranging PILs with varying polymer compositions (Fig. 6). The mapping of the molecular angle to the normal of the surface (β) is plotted for each film (Fig. 6(a–c), (e–g) and (i–k)), while the distribution of β is shown through a histogram to determine differences between the films (Fig. 6d, h and l).

Fig. 6 (a)−(d) Grazing-incidence wide-angle X-ray scattering patterns of F₁₆CuPc on PEGMA loading of (a) 0%, (b) 25%, (c) 50%, and (d) 75% for the BF₄⁻. (e)–(g) Azimuthally integrated linecuts for thin films of F₁₆CuPc on poly(S)-b-poly(VBBI⁺[X⁻]-r-PEGMA) block copolymers, with varying PEGMA/VBBI⁺ ratios, where X is the counterion (e) PF₆⁻, (f) TFSI⁻, or (g) BF₄⁻. (h) F₁₆CuPc β phase packing. Integration occurs from −83° < χ < −1° and 0.12 Å⁻¹ < q < 2.3 Å⁻¹ to account for the Yoneda peak.

From Fig. 7, we see that there are more significant variations in molecular order in the amorphous regions across each film even if GIWAXS indicated that the crystalline domains are consistent across films. Here, we see that F₁₆CuPc films deposited on PILs containing TFSI⁻ have a narrower distribution of molecular orientation that tends towards more face-on orientation and a more uniform film. In contrast, F₁₆CuPc films deposited on PILs containing BF₄⁻ show a broader and less uniform distribution of molecular angles, while F₁₆CuPc films deposited on PILs containing PF₆⁻ demonstrate very similar films despite changes in the PEGMA loading. These results suggest that while the F₁₆CuPc crystalline domains are similar regardless of PIL composition, the amorphous regions are highly dependent on the choice of counter ion and block copolymer composition.

Fig. 7 Raman polarized maps showing the distribution of angles (β) of F₁₆CuPc on poly(S)-b-poly(VBBI⁺[X]-r-PEGMA) block copolymer with varying VBBI⁺/PEGMA ratios for the counterion for (a)–(c) TFSI⁻, (e)–(g) BF₄⁻, and (i)–(k) PF₆⁻ anions. Note the legend here is in degrees (°). (d), (h) and (l) Histogram of the distribution of angles (β) for each film shown in (a)–(c), (e)–(g), and (i)–(k) for (d) TFSI⁻, (h) BF₄⁻, and (l) PF₆⁻.

Atomic force microscopy (AFM) imaging was performed on F₁₆CuPc films deposited on S-b-(V50-r-P50) with the anion BF₄⁻, TFSI⁻, and PF₆⁻ (Fig. 8(a–c) respectively). It appears that the F₁₆CuPc crystal sizes are roughly the same size no matter the sublayer, however, differences in the PIL layer thickness and morphology may result in variations in the height profiles (Fig. S11, ESI†). In contrast, F₁₆CuPc film roughness is relatively consistent regardless of choice of PIL (Fig. S11 and Table S3, ESI†). Therefore, these results suggest that choice of PIL does not modify the crystalline domains of the F₁₆CuPc as observed with GIWAXS but could influence the amorphous domains by breaking the film uniformity.

Fig. 8 Atomic force microscopy images (2.5 μm × 2.5 μm) for S-b-(V50-r-P50) with the anion (a) BF₄⁻, (b) TFSI⁻, and (c) PF₆⁻.

Conclusion

Overall, we show that by using PILs, low power consumption transistor devices can be achieved, with threshold voltages ranging from −0.4 V to 1 V. Furthermore, while we show that device performance is primarily linked to the PIL morphology and self-assembly as a function of the VBBI⁺/PEGMA ratio and anion, where higher effectiveness gating is observed in mixed-morphology PILs whereas low- to no-gating in devices strongly exhibiting a single morphology. Additionally, similar packing modes were observed for the PIL devices when cast on both Si and P(NDI2OD-T2), indicating that surface chemistry is less influential on block copolymer self-assembly than the PEGMA loading and counterion and showing the universal self-assembly behaviour of these PIL materials. We also highlight the importance of characterizing all domains of a semiconducting film, illustrating that while the crystalline domains can remain consistent across samples, the amorphous regions can vary drastically depending on the deposition surface, potentially influencing overall device performance.

Finally, we show that the highest performing device agrees with the results from Brixi et al., who indicated that S-b-(V50-r-P50) paired with the BF₄⁻ anion is the highest-performing dielectric from this library.¹⁰ Therefore, while there are differences arise between these two studies in how the semiconductor assembles, we can conclude that for both the example of a polymer and a small molecule semiconductor, S-b-(V50-r-P50) with BF₄⁻ anion is the best PIL material, indicating in the case of this library of materials, this self-assembly has the most significant influence on device performance.

Author contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. L. E. D. conducted the experimental work and data analysis and wrote the initial draft of the manuscript. S. B. conducted experiments and assisted in data analysis and editing of the manuscript. C. L. R. and T. L. K. performed the GISAXS analysis and assisted in editing the manuscript. J. G. M. (CGFigures) provided assets by design graphics used in the figures and processing the AFM images. B. H. L. acquired funding, managed supervision, directed the study, and assisted in editing the manuscript.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

The authors would like to thank the Canadian Light Source for providing beamtime, and Dr. Chang-Yong Kim and Dr. Adam Leontowich for their expertise and technical support. The Canadian Light Source is supported by CFI, NSERC, the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research. We thank Dr. Spyridon at nanoFab at the Advanced Research Complex (University of Ottawa) for performing AFM imaging on the samples. This work was supported by NSERC Discovery grant RGPIN2020-04079 (B. H. L.), the Canada Research Chairs Program 950-230724 (B. H. L.), and NSERC PGS-D (L. E. D.).

References

1. E. Sazonov and W. A. Daoud, Grand Challenges in Wearable Electronics, Front. Electron., 2021, 2,  DOI:10.3389/felec.2021.668619.
2. H. Peng, Wearable Electronics, Natl. Sci. Rev., 2022, 10(1), nwac193,  DOI:10.1093/nsr/nwac193.
3. X. Sun, Organic Thin Film Transistors in Wearable Electronics: Prospects in Sensor Technology and Healthcare, Appl. Comput. Eng., 2024, 66(1), 71–78.
4. H. C. Ates, P. Q. Nguyen, L. Gonzalez-Macia, E. Morales-Narváez, F. Güder, J. J. Collins and C. Dincer, End-to-End Design of Wearable Sensors, Nat. Rev. Mater., 2022, 7(11), 887–907,  DOI:10.1038/s41578-022-00460-x.
5. Y. Wen and Y. Liu, Recent Progress in N-Channel Organic Thin-Film Transistors, Adv. Mater., 2010, 22(12), 1331–1345,  DOI:10.1002/adma.200901454.
6. O. A. Melville, B. H. Lessard and T. P. Bender, Phthalocyanine-Based Organic Thin-Film Transistors: A Review of Recent Advances, ACS Appl. Mater. Interfaces, 2015, 7(24), 13105–13118,  DOI:10.1021/acsami.5b01718.
7. S. R. Forrest, Organic Thin Film Transistors, Organic Electronics, Oxford University Press, Oxford, 2020; pp. 803–917 DOI:10.1093/oso/9780198529729.003.0008.
8. J. Ajayan, S. Sreejith, M. Manikandan, V. Bharath Sreenivasulu, N. Aruna Kumari and A. Ravindran, An Intensive Study on Organic Thin Film Transistors (OTFTs) for Future Flexible/Wearable Electronics Applications, Micro Nanostruct., 2024, 187, 207766,  DOI:10.1016/j.micrna.2024.207766.
9. M. Velický, Electrolyte versus Dielectric Gating of Two-Dimensional Materials, J. Phys. Chem. C, 2021, 125(40), 21803–21809,  DOI:10.1021/acs.jpcc.1c04795.
10. S. Brixi, C. L. Radford, M. N. Tousignant, A. J. Peltekoff, J. G. Manion, T. L. Kelly and B. H. Lessard, Poly(Ionic Liquid) Gating Materials for High-Performance Organic Thin-Film Transistors: The Role of Block Copolymer Self-Assembly at the Semiconductor Interface, ACS Appl. Mater. Interfaces, 2022, 14(35), 40361–40370,  DOI:10.1021/acsami.2c07912.
11. J. F. Martínez Hardigree and H. E. Katz, Through Thick and Thin: Tuning the Threshold Voltage in Organic Field-Effect Transistors, Acc. Chem. Res., 2014, 47(4), 1369–1377,  DOI:10.1021/ar5000049.
12. A. J. Peltekoff, V. E. Hiller, G. P. Lopinski, O. A. Melville and B. H. Lessard, Unipolar Polymerized Ionic Liquid Copolymers as High-Capacitance Electrolyte Gates for n-Type Transistors, ACS Appl. Polym. Mater., 2019, 1(11), 3210–3221,  DOI:10.1021/acsapm.9b00959.
13. A. J. Peltekoff, S. Brixi, J. Niskanen and B. H. Lessard, Ionic Liquid Containing Block Copolymer Dielectrics: Designing for High-Frequency Capacitance, Low-Voltage Operation, and Fast Switching Speeds, JACS Au, 2021, 1(7), 1044–1056,  DOI:10.1021/jacsau.1c00133.
14. S. A. Algarni, T. M. Althagafi, P. J. Smith and M. Grell, An Ionic Liquid-Gated Polymer Thin Film Transistor with Exceptionally Low “on” Resistance, Appl. Phys. Lett., 2014, 104(18), 182107,  DOI:10.1063/1.4875746.
15. D. R. Nosov, B. Ronnasi, E. I. Lozinskaya, D. O. Ponkratov, L. Puchot, P. Grysan, D. F. Schmidt, B. H. Lessard and A. S. Shaplov, Mechanically Robust Poly(Ionic Liquid) Block Copolymers as Self-Assembling Gating Materials for Single-Walled Carbon-Nanotube-Based Thin-Film Transistors, ACS Appl. Polym. Mater., 2023, 5(4), 2639–2653,  DOI:10.1021/acsapm.2c02223.
16. S. Chen, F. Frenzel, B. Cui, F. Gao, A. Campanella, A. Funtan, F. Kremer, S. S. P. Parkin and W. H. Binder, Gating Effects of Conductive Polymeric Ionic Liquids, J. Mater. Chem. C, 2018, 6(30), 8242–8250,  10.1039/C8TC01936C.
17. J.-H. Choi, W. Xie, Y. Gu, C. D. Frisbie and T. P. Lodge, Single Ion Conducting, Polymerized Ionic Liquid Triblock Copolymer Films: High Capacitance Electrolyte Gates for n-Type Transistors, ACS Appl. Mater. Interfaces, 2015, 7(13), 7294–7302,  DOI:10.1021/acsami.5b00495.
18. T. Fujimoto and K. Awaga, Electric-Double-Layer Field-Effect Transistors with Ionic Liquids, Phys. Chem. Chem. Phys., 2013, 15(23), 8983,  10.1039/c3cp50755f.
19. J. Manion and B. H. Lessard, High-Throughput Characterization Is Key to Report Reliable Organic Thin-Film Transistor Performance, Nat. Rev. Mater., 2024, 9, 377–378,  DOI:10.1038/s41578-024-00689-8.
20. R. R. Cranston, T. D. Lanosky, R. Ewenike, S. Mckillop, B. King and B. H. Lessard, Polarized Raman Microscopy to Image Microstructure Changes in Silicon Phthalocyanine Thin-Films, Small Sci., 2024, 4, 2300350,  DOI:10.1002/smsc.202300350.
21. A. F. Stalder, G. Kulik, D. Sage, L. Barbieri and P. Hoffmann, A Snake-Based Approach to Accurate Determination of Both Contact Points and Contact Angles, Colloids Surf., A, 2006, 286(1–3), 92–103,  DOI:10.1016/j.colsurfa.2006.03.008.
22. Z. Jiang, GIXSGUI: A MATLAB Toolbox for Grazing-Incidence X-Ray Scattering Data Visualization and Reduction, and Indexing of Buried Three-Dimensional Periodic Nanostructured Films, J. Appl. Crystallogr., 2015, 48(3), 917–926,  DOI:10.1107/S1600576715004434.
23. X. Yan, J. Wang, H. Wang, H. Wang and D. Yan, Improved N-Type Organic Transistors by Introducing Organic Heterojunction Buffer Layer under Source/Drain Electrodes, Appl. Phys. Lett., 2006, 89(5), 053510,  DOI:10.1063/1.2227714.
24. R. Ye, M. Baba, K. Suzuki and K. Mori, Fabrication of Highly Air-Stable Ambipolar Thin-Film Transistors with Organic Heterostructure of F16CuPc and DH-A6T, Solid State Electron, 2008, 52(1), 60–62,  DOI:10.1016/j.sse.2007.07.010.
25. S. Yadav, P. Kumar and S. Ghosh, Optimization of Surface Morphology to Reduce the Effect of Grain Boundaries and Contact Resistance in Small Molecule Based Thin Film Transistors, Appl. Phys. Lett., 2012, 101(19), 193307,  DOI:10.1063/1.4766913.
26. N. T. Boileau, O. A. Melville, B. Mirka, R. Cranston and B. H. Lessard, Type Copper Phthalocyanines as Effective Semiconductors in Organic Thin-Film Transistor Based DNA Biosensors at Elevated Temperatures, RSC Adv., 2019, 9(4), 2133–2142,  10.1039/C8RA08829B.
27. L. Han, J. Li, S. Ogier, Z. Liu, L. Deng, Y. Cao, T. Shan, D. Sharkey, L. Feng, A. Guo, X. Li, J. Zhang and X. Guo, Eliminating Leakage Current in Thin-Film Transistor of Solution-Processed Organic Material Stack for Large-Scale Low-Power Integration, Adv. Electron. Mater., 2022, 8(9), 2200014,  DOI:10.1002/aelm.202200014.
28. Y. Xia and C. D. Frisbie, Low-Voltage Electrolyte-Gated OTFTs and Their Applications, Organic Electronics II, Wiley, 2012, pp. 197–233 DOI:10.1002/9783527640218.ch6.
29. M. N. Tousignant, M. Ourabi, J. Niskanen, B. Mirka, W. J. Bodnaryk, A. Adronov and B. H. Lessard, Poly(Ionic Liquid) Dielectric for High Performing P- and N-Type Single Walled Carbon Nanotube Transistors, Flexible Printed Electron., 2022, 7(3), 034004,  DOI:10.1088/2058-8585/ac928f.
30. R. B. Ewenike, B. King, A. M. Battaglia, J. D. Quezada Borja, Z. S. Lin, J. G. Manion, J. L. Brusso, T. L. Kelly, D. S. Seferos and B. H. Lessard, Toward Weak Epitaxial Growth of Silicon Phthalocyanines: How the Choice of the Optimal Templating Layer Differs from Traditional Phthalocyanines, ACS Appl. Electron. Mater., 2023, 5(12), 7023–7033,  DOI:10.1021/acsaelm.3c01389.
31. B. Ronnasi, B. King, S. Brixi, S. Swaraj, J. Niskanen and B. H. Lessard, Electron Donating Functional Polymer Dielectrics to Reduce the Threshold Voltage of N-Type Organic Thin-Film Transistors, Adv. Electron. Mater., 2024, 2300810,  DOI:10.1002/aelm.202300810.
32. A. J. Peltekoff, M. N. Tousignant, V. E. Hiller, O. A. Melville and B. H. Lessard, Controlled Synthesis of Poly(Pentafluorostyrene-Ran-Methyl Methacrylate) Copolymers by Nitroxide Mediated Polymerization and Their Use as Dielectric Layers in Organic Thin-Film Transistors, Polymers, 2020, 12(6), 1231,  DOI:10.3390/polym12061231.
33. D. G. de Oteyza, E. Barrena, J. O. Ossó, S. Sellner and H. Dosch, Thickness-Dependent Structural Transitions in Fluorinated Copper-Phthalocyanine (F 16 CuPc) Films, J. Am. Chem. Soc., 2006, 128(47), 15052–15053,  DOI:10.1021/ja064641r.
34. H. Lee, D. Lee, D. H. Sin, S. W. Kim, M. S. Jeong and K. Cho, Effect of Donor–Acceptor Molecular Orientation on Charge Photogeneration in Organic Solar Cells, NPG Asia Mater., 2018, 10(6), 469–481,  DOI:10.1038/s41427-018-0054-1.
35. J. E. Allen, K. G. Yager, H. Hlaing, C.-Y. Nam, B. M. Ocko and C. T. Black, Enhanced Charge Collection in Confined Bulk Heterojunction Organic Solar Cells, Appl. Phys. Lett., 2011, 99(16), 163301,  DOI:10.1063/1.3651509.
36. M. A. Reus, L. K. Reb, A. F. Weinzierl, C. L. Weindl, R. Guo, T. Xiao, M. Schwartzkopf, A. Chumakov, S. V. Roth and P. Müller-Buschbaum, Time-Resolved Orientation and Phase Analysis of Lead Halide Perovskite Film Annealing Probed by In Situ GIWAXS, Adv. Opt. Mater., 2022, 10(14), 2102722,  DOI:10.1002/adom.202102722.
37. J. L. Baker, L. H. Jimison, S. Mannsfeld, S. Volkman, S. Yin, V. Subramanian, A. Salleo, A. P. Alivisatos and M. F. Toney, Quantification of Thin Film Crystallographic Orientation Using X-Ray Diffraction with an Area Detector, Langmuir, 2010, 26(11), 9146–9151,  DOI:10.1021/la904840q.
38. L. E. Dickson, R. R. Cranston, H. Xu, S. Swaraj, D. S. Seferos and B. H. Lessard, Blade Coating Poly(3-Hexylthiophene): The Importance of Molecular Weight on Thin-Film Microstructures, ACS Appl. Mater. Interfaces, 2023, 15(47), 55109–55118,  DOI:10.1021/acsami.3c12335.
39. H. Lee, S. Oh, C. E. Song, H. K. Lee, S. K. Lee, W. S. Shin, W.-W. So, S.-J. Moon and J.-C. Lee, Stable P3HT: Amorphous Non-Fullerene Solar Cells with a High Open-Circuit Voltage of 1 V and Efficiency of 4, RSC Adv., 2019, 9(36), 20733–20741,  10.1039/C9RA03188J.
40. T. V. Basova and B. A. Kolesov, Raman Polarization Studies of the Orientation of Molecular Thin Films, Thin Solid Films, 1998, 325(1–2), 140–144,  DOI:10.1016/S0040-6090(98)00485-4.
41. M. Szybowicz, T. Runka, M. Drozdowski, W. Bała, A. Grodzicki, P. Piszczek and A. Bratkowski, High Temperature Study of FT-IR and Raman Scattering Spectra of Vacuum Deposited CuPc Thin Films, J. Mol. Struct., 2004, 704(1–3), 107–113,  DOI:10.1016/j.molstruc.2004.01.053.
42. B. A. Kolesov, T. V. Basova and I. K. Igumenov, Determination of the Orientation of CuPc Film by Raman Spectroscopy, Thin Solid Films, 1997, 304(1–2), 166–169,  DOI:10.1016/S0040-6090(97)00174-0.
43. C.-W. Huang, X. You, P. J. Diemer, A. J. Petty, J. E. Anthony, O. D. Jurchescu and J. M. Atkin, Micro-Raman Imaging of Isomeric Segregation in Small-Molecule Organic Semiconductors, Commun. Chem., 2019, 2(1), 22,  DOI:10.1038/s42004-019-0122-7.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc03157a

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