Solution‐Processed PTCDI‐C8 Crystalline Wire: Dual Role as Mask and Active Layer for Organic Transistors with Increased Switching Speeds

Organic field‐effect transistors (OFETs) have been extensively studied over the past decades because of their suitability for low‐cost, large‐area, and flexible electronics. However, improvements are needed to satisfy the demands of high‐speed applications. The switching speed of a logic device is affected by the charge‐carrier mobility (µ) and the square of the channel length (L) at a given gate–source bias. Therefore, increasing µ and/or reducing L are crucial for achieving high‐speed OFET‐based digital circuits. In this study, an n‐type OFET is fabricated with increased switching speed via a dual‐role approach involving solution‐grown, highly ordered single‐crystalline N,N'‐dioctyl‐3,4,9,10‐perylenetetracarboxylic diimide (PTCDI‐C8) wires, which serve as a mask for short‐channel formation up to the microscale and an active layer with enhanced charge mobility. Additionally, the performance of the n‐type short‐channel OFET and resistive‐load‐type inverters is evaluated. For comparative purposes, long‐channel (50 µm) devices with PTCDI‐C8 wires and short‐channel devices with a PTCDI‐C8 film are fabricated and the device performance is analyzed. The short‐channel device with the PTCDI‐C8 wires exhibits a significantly higher switching speed. Thus, the dual‐role approach is a simple and straightforward method for fabricating short‐channel devices, paving the way for further advancements in OFET technology requiring high switching speeds.


Introduction
3] OSCs have numerous advantageous properties, such as room-temperature processability, [4] mechanical flexibility, [5,6] DOI: 10.1002/aelm.202300548[11] However, for the commercialization of organic electronic devices, further enhancements in electrical performance are required-particularly with regard to the charge-carrier mobility and ON/OFF switching frequency.The maximum rate at which a circuit can be switched between the ON and OFF states, which is a critical factor for high-density integrated circuits, can be described by the cutoff frequency (f c ).14] At a given V GS, f c is proportional to /L 2 .Thus, increasing μ and/or reducing L are crucial for increasing f c .
][17] By increasing charge-carrier mobility of the semiconductors, the f c of organic devices has been increased.However, increasing μ involves complex synthesis procedures to develop new high-performance semiconductors.Another approach for achieving high speed is reducing L.Although this typically requires complex and expensive fabrication processes, OFETs with reduced channel lengths have been fabricated through methods such as nanoimprint lithography (NIL), atomic force microscope (AFM) lithography, electron-beam lithography (EBL), and liftoff photolithography.However, they suffer from drawbacks such as complexity, high costs, limited substrate compatibility, and process optimization challenges.[20] AFM lithography is a simple mechanical method; however, it does not ensure a constant channel length and leaves residue around the edge of the formed channel as the AFM tip moves through the surface and scratches the electrode. [21,22]EBL provides high-resolution manufacturing but has a low pixel rate and high cost. [23,24][27] Although non-lithographic methods such as Si etching and direct writing have also been developed, they have limitations.Si etching is cost-effective but only applicable to Si substrates. [28,29]Direct writing for short-channel fabrication employs inkjet printing, [30][31][32] which is advantageous for flexible devices but requires the optimization of the interface between the substrate and printed materials. [33]Thus, the development of a short-channel fabrication method that is simple and cost-effective remains challenging.
We previously reported a unique fabrication method for the formation of short-channel OFETs using a solution-processed, highly crystalline N,N′-dioctyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C 8 ) crystal wire, which was used as a mask to form microscale channel length of devices. [10]In addition, we found that the formed PTCDI-C 8 wire had a single-crystalline nature. [34]Polycrystalline films of OSCs are widely used in both research and practical applications.A major issue with these active layers however is that the grain boundaries between the crystallites act as energy barriers that hinder the transport of mobile charge carriers.Additionally, these grain boundaries also contribute to the formation of localized states within the band gap that trap charge carriers.The trapping and release process can further reduce the mobility, and long-term trapped charges can increase the threshold voltage.On the other hand, single crystalline OSCs can provide a semiconductor channel with no grain boundaries and a much lower trap site density compared to polycrystalline films. [35]As a result, transistors with superior electrical performance can be achieved with the single crystalline OSCs. [36]The single-crystalline PTCDI-C 8 wire, used as a mask for the short-channel formation, also emerges as a promising candidate for a high-performance n-type OSC.
In this study, we investigated the formation and performance of short-channel OFETs prepared via dual-role approach utilizing a PTCDI-C 8 wire as both a mask for the formation of channel in microscale and an active semiconducting layer for high charge-carrier mobility.Additionally, we assembled the OFETs with an appropriate load resistor to fabricate load-type inverters and analyzed their performance.The dynamic electrical responses of load-type inverters with the single-crystalline PTCDI-C 8 wire were compared with those of long-channel devices with a PTCDI-C 8 wire and short-channel devices with a polycrystalline PTCDI-C 8 film, which were fabricated separately for comparative studies in dynamic response characteristics.

Results and Discussion
Bottom-gate bottom-contact geometry was employed for the preparation of n-type short-channel device based on crystal wires via dual role approach utilizing solution-processed PTCDI-C 8 wire.The whole procedure for the formation of the short-channel devices is shown in Figure 1 and in Figure S1, Supporting Information.A PTCDI-C 8 solution in 1,2-dichlorobenzene (o-DCB) (0.020 wt%) filtered by a polytetrafluoroethylene (PTFE) syringe filter was dropped onto the capillary, which was positioned at the edge of the crosslinked poly(4-vinylphenol) (CL-PVP) gate dielectric-coated substrate perpendicular to the long axis of the patterned ITO (Figure 1a).Prior to the placement of the capillary tube, two layers of commercial polyimide tape were adhered to the edge of a glass substrate to give gap (≈100 μm) between the tube and the glass substrate.The substrates were placed on a hotplate at 80°C for 100 min, leading to the formation of single-crystalline PTCDI-C 8 wires from the solution drop edge (Figures 1b,c). [10]A 25 nm-thick Au film was deposited over the wire via thermal vacuum evaporation (< 9.9 × 10 −6 Torr) at a rate of 0.08 nm s −1 (Figure 1d).Then, the crystal wire and overlying Au film were lifted off by spraying acetone with a wash bottle (Figure 1e) to form source and drain electrodes with a (sub)micrometer channel length (Figure 1f).The fabrication of the bottom-contact n-type short-channel device was completed by forming the PTCDI-C 8 wire as an active layer on top of the formed short-channel through the aforementioned solution process.The same PTCDI-C 8 solution was dropped on a capillary tube placed on top of the Au electrode (Figure 1g) (note that the capillary tube was parallel to the long axis of the underlying gate electrode).The substrate was baked at the same temperature (Figure 1h), resulting in the completion of the short-channel device with PTCDI-C 8 semiconducting wires (Figure 1i).
The PTCDI-C 8 crystal prepared via a solution process was used as both a mask for channel formation and an active layer because of its well-defined needle shape and single-crystalline nature.The shape and structure of the PTCDI-C 8 crystal were analyzed through polarized optical microscopy (POM) and fluorescence microscopy; the results are shown in Figure 2. The POM images (obtained with polarizer and analyzer fixed at 90°) of the PTCDI-C 8 crystals showed a uniform color change of the crystals from dark to bright red when the polarizer was rotated relative to the substrate (Figure 2a), indicating that the crystals had a single-crystalline nature, which enhanced the charge-transport properties. [37,38]The molecular orientation of the PTCDI-C 8 crystal was previously investigated by our group through twodimensional grazing-incidence X-ray diffraction (2D-GIXD). [34]he 2D-GIXD patterns, measured perpendicular and parallel to the long axis of the crystals, revealed that the PTCDI-C 8 crystals had a directionally grown single-crystalline nature.The - conjugation of the neighboring PTCDI-C 8 molecules along a specific direction led to the formation of the needle-shaped crystals in which molecules predominantly self-assembled along the - stacking direction.The strong intermolecular coupling between the packed molecules resulted in high charge-carrier mobility along the long axis of the needle-type crystal. [39]Additionally, the POM images indicated that the crystal had a welldefined micrometer-wide needle shape (Figure 2a).The strict directional growth of the crystals makes them ideal as a mask for uniform (sub)micrometer-long channel formation.The top panel of Figure 2b presents an optical microscopy image of the PTCDI-C 8 crystals grown over the short-channel area, which was prepared using the PTCDI-C 8 crystal wire as a mask.The identity of the crystals was confirmed through fluorescence microscopy.An emission peak of PTCDI-C 8 (590 nm) matched the range of the Rhodamine Red-X filter (575-640 nm) (Figure 2c), resulting in the red color of PTCDI-C 8 in the fluorescent microscopy image (bottom panel of Figure 2b). [40]Importantly, the channel area (indicated by yellow arrow) between the two electrodes appeared bright gray with no red tint, indicating that PTCDI-C 8 used for channel patterning was completely removed from the channel area.
The channel area was further investigated using an AFM; the image is shown in Figure 3a.The image shows a well-defined gap between the source and drain electrodes, which originated from the width of the PTCDI-C 8 crystal.The channel length is solely affected by the width and shape of the PTCDI-C 8 crystal wire, which can be controlled by solution concentration and solvent evaporation temperature.A low solution concentration (0.001 wt.%) leads to the failure of crystal formation, while a high concentration (0.050 wt.%) results in the formation of high-density crystal wires, disturbing their use as a mask.At low evaporation temperatures (50 °C), PTCDI-C 8 tends to form a short and bent-shaped crystal wire.In contrast, the rapid solvent evaporation at high temperatures (110 °C) leads to the disconnection of the PTCDI-C 8 crystal wire and reduced crystallinity, making it unsuitable for the use as both an active layer and a mask.Through process optimization, a solution concentration of 0.020 wt.% and the temperature of 80 °C were used in this study.The inset of Figure 3a clearly shows an approximately 1 μm-wide and 25 μm-deep U-shaped channel region with no residual PTCDI-C 8 crystal mask or damage.The AFM image (Figure 3b) of the PTCDI-C 8 crystal deposited as an active layer, directly prepared via the solution process over the ≈1 μm channel region, indicates that the crystal was well-bridged over the channel in contact with the surface of the CL-PVP gate dielectric (inset of Figure 3b).The POM, fluorescence microscopy, and AFM images support the dual role of the PTCDI-C 8 crystal wire as both a mask and an active layer.
The output characteristic curves of the short-channel OFET with a PTCDI-C 8 crystal are shown in Figure 4a.At low V GS (10 and 30 V), the drain current became saturated beyond the pinchoff voltage.At a high V GS (40 V), the current became unstable around the pinch-off point, and ideal current saturation was not observed.In the low-V DS region, at all V GS values, the drain current increased nonlinearly with an increase in V DS .This nonlinearity often appears in short-channel devices and is caused by the contact resistance between Au electrodes and the OSC. [41]n field-effect transistors with long channels, the contact resistance is negligible in relation to the resistance of the semiconductor channel.However, in short-channel OFETs, the contact resistance is comparable to the channel resistance and is affected by both V GS and V DS .The variability of the contact resistance affects the current response, resulting in non-ideal characteristics  of short-channel field-effect transistors.At low V DS values, the contact resistance increases with V DS , and charge injection from the source to the OSC is impeded.This reduces I DS below the current levels expected from the gradual channel approximation, which manifests as the relatively flat part of the output curve near 0 V. [42,43] The transfer characteristic curves with a small degree of hysteresis of the short-channel OFET with a PTCDI-C 8 crystal active layer were obtained at V DS = 40 V, swept in both directions (Figure 4b).When compared to the hysteresis of the shortchannel OFET with a PTCDI-C 8 polycrystalline film (shown in Figure S2, Supporting Information), it exhibited much narrower hysteresis.This suggests that a closely packed single crystalline active layer with fewer trap sites [44] is better candidate for high-performance electronic device.The field-effect mobility (μ), threshold voltage (V th ), subthreshold swing (ss), and ON/OFF current ratio (I ON /I OFF ) in the saturation region were extracted from the transfer characteristic curve.The field-effect mobility in the saturation regime was calculated as follows [5] : where W represents the channel width and C i represents the capacitance per unit area of the gate dielectric (C i = 57.35pF mm −2 ).The V th was determined from the x-intercept of the √ |I DS | versus V GS plot.The subthreshold swing (inverse of the subthreshold slope) is a measure of how fast |I DS | increases with the gate-source voltage below the threshold voltage and is calculated as follows: The highest electron mobility (μ) of PTCDI-C 8 crystal-based short-channel OFET was 3.00 × 10 −2 cm 2 V −1 s −1 and V th , ss and I ON /I OFF were 28.15 V, 13.17 V dec −1 , 1.13 × 10 5 , respectively, whereas the mean electron mobility was 9.58 × 10 −3 cm 2 V −1 s −1 .It was previously demonstrated that bottom-contact transistors exhibit lower performance than top-contact transistors, [45,46] because the gate electric field is partially shielded by the source and drain electrodes and the channel is formed only in the OSC region between these electrodes, leading to a limited charge injection between the electrodes and the OSC.The calculated  charge-carrier mobility of our short-channel device was close to the values measured for other PTCDI-based compounds with long channels in the top-contact configuration, [47][48][49] indicating the effectiveness of our approach.
To evaluate the switching speed and corresponding applicability of the fabricated short-channel device in digital circuitry, an NMOS-like resistive-load inverter circuit was developed, and the transient electrical responses of the inverters were investigated at different input pulse frequencies.The resistive-load inverter circuit, which is the optimal choice for directly evaluating the switching characteristics of a transistor, was fabricated by connecting an n-type transistor to a load resistor, as shown in Figure 5a.The resistance of the load was 60 MΩ, and a V DD of 60 V was applied via an external voltage source.For comparison, an NMOS-like resistive-load inverter was prepared using i) shortchannel device with a PTCDI-C 8 thin film, where the short channel was prepared via our method and the PTCDI-C 8 thin film was deposited via thermal vacuum deposition (bottom-contact configuration), and ii) long-channel (L = 50 μm) device with a PTCDI-C 8 crystalline wire, where the long channel (top-contact configuration) was prepared via thermal vacuum deposition and the wire was prepared via our solution method.
The transient electrical responses of the inverters are shown in Figure 5b-d.The inverter with the long-channel (L = 50 μm) device with the PTCDI-C 8 crystal exhibited a clear inverter-like response to the input voltage pulse only at a frequency below 5 Hz (Figure 5b).However, the inverters based on the shortchannel FET with the PTCDI-C 8 film (Figure 5c) and crystal wire (Figure 5d) exhibited fast switching responses to the input voltage even at frequencies up to 100 Hz, with some voltage spikes.These spikes are often observed in inverter circuits based on short-channel devices due to parasitic capacitance and can be minimized via device structure optimization. [40,50,51]In addition to the cutoff frequency, the dynamic response of the inverter device can be characterized in detail according to the rise/fall times and propagation delays.The rise time ( r ) is the time required for the output voltage to increase from the 10% to 90% level, and the fall time ( f ) is the time required for the voltage to decrease from the 90% to 10% level.Both indicate how rapidly V OUT switches from "logic 0′ to 'logic 1."Meanwhile, propagation delay indicates how rapidly the inverter responds to a change in V IN and is the time interval between the change in V IN and a 50% change in V OUT .A low-to-high propagation delay ( PLH ) is observed when V OUT increases, and a high-to-low propagation delay ( PHL ) is observed in the opposite case.For the inverter based on the shortchannel FET with the PTCDI-C 8 film (Figure 5c),  r ,  f ,  PLH , and  PHL were 8.22, 7.80, 2.73, 1.90 ms, according to the 50-Hz measurement.For the inverter based on the short-channel FET with the PTCDI-C 8 crystal wire (Figure 5d),  r ,  f ,  PLH , and  PHL were 7.02, 3.52, 0.99, and 1.84 ms, respectively, according to the 50-Hz measurement.The inverters with short-channel FETs exhibited faster switching responses than those with long-channel FETs, and the inverter based on the short-channel FET with the highly crystalline wire exhibited a faster response than that with the polycrystalline film.This indicates that the switching speed of an FET device is directly affected by μ and L. According to the results of this study, increasing μ and reducing L are crucial for achieving high-speed digital circuits.

Conclusion
A dual-role approach utilizing a single-crystalline PTCDI-C 8 crystal wire as both a mask for micrometer-long channel formation and an active layer was used to fabricate a high-speed organic logic circuit.Experimental results indicated that the PTCDI-C 8 crystal wire effectively served as a precise mask for the formation of a uniform and approximately micrometer-long channel and as an active layer with enhanced charge-carrier mobility owing to its single-crystalline nature.The fabricated short-channel OFET exhibited n-type charge-transport behavior with a field-effect mobility of up to 3.00 × 10 −2 cm 2 V −1 s −1 .Additionally, resistive-load inverters based on the short-channel devices with the PTCDI-C 8 crystal exhibited switching responses even at frequencies up to 100 Hz, outperforming long-channel devices with the PTCDI-C 8 crystal and short-channel devices with a PTCDI-C 8 thin film.This dual-role approach using the PTCDI-C 8 crystal wire is a simple and cost-effective method for fabricating short-channel OFETs, introducing new possibilities for high-speed organic electronic devices and paving the way for commercial applications in lowcost, large-area, and flexible electronics.Further exploration and optimization of this approach can lead to more efficient organic devices, driving the advancement of organic electronics.

Experimental Section
The n-type semiconductor, i.e., PTCDI-C 8 , was purchased from Sigma-Aldrich and used without further purification.To form the gate dielectric, 1,2-dichlorobenzene (o-DCB); poly(4vinylphenol) (PVP), Mw ≈ 25 000 g mol −1 ; propylene glycol monomethyl ether acetate (PGMEA); and poly(melamine-coformaldehyde) methylated (PMF) ( Mn ≈ 432 g mol −1 , 84 wt.% solution in butan-1-ol) were purchased from Sigma-Aldrich and used as is.Indium tin oxide (ITO) glass substrates were purchased from DASOM RMS and patterned via photolithography for use as gate electrodes.The patterned ITO glass substrates were cleaned with a detergent solution (30 min), deionized water (15 min), acetone (15 min), and propan-2-ol (15 min) using an ultrasonic cleaner, in that order, followed by an 80-s UV/ozone cleaning process.A gate-dielectric precursor solution filtered through a polytetrafluoroethylene (PTFE) syringe filter (pore size of 0.22 μm) was spin-coated onto the cleaned substrates at 3000 rpm for 30 s.The gate-dielectric precursor solution was prepared by dissolving PVP and PMF in PGMEA with a mass ratio of 0.10:0.05:0.85,respectively, followed by 2 days of stirring at 25 °C to achieve full homogeneity.The substrates were then softbaked on a hotplate at 90 °C for 10 min and then baked further in a vacuum oven at 175 °C for 1 h for thermal crosslinking to obtain dense polymer thin films. [52]The thickness (Ambios Technology XP-100) and capacitance (Ivium Technologies CompactStat.e) of the crosslinked PVP (CL-PVP) were measured as 450 nm and 57.35 pF mm −2 , respectively.The 25-nm thick Au source and drain electrodes were prepared by thermal vacuum evaporation using solution-processed crystal wire mask followed by washing out the wire and overlying Au film with acetone for microscale short channel over the top of the gate dielectric layer.The bottomgate bottom-contact n-type OFETs were completed by the deposition of PTCDI-C 8 as an active layer via the solution process on top of the electrodes.For comparison, additional PTCDI-C 8 transistors were prepared: a short-channel device with a PTCDI-C 8 thin film and a 50 μm-long channel device with PTCDI-C 8 crystalline wire.The PTCDI-C 8 thin film was prepared via thermal vacuum deposition (thickness of 40 nm with average deposition rate of 0.06 nms −1 at < 9.9 × 10 −6 Torr), and the long channel was prepared via thermal vacuum deposition (25-nm thick gold film with deposition rate of 0.08 nms −1 ), using a metal shadow mask.The output and transfer characteristics of the devices were measured with HP 4145B and HP 4156A precision semiconductor parameter analyzers under ambient conditions (40-50 % RH, 20-25°C).The output curves, i.e., drain current (I DS ) versus drain voltage (V DS ), were measured at fixed gate voltages (V GS ) of 0, 10, 20, 30, and 40 V, while V DS was swept from 0 to 40 V.The transfer curves, i.e., I DS versus V GS , were measured at a fixed V DS of 40 V, while V GS was swept from −10 to 40 V. Load-type inverters were prepared by connecting an appropriate load resistor to the prepared n-type transistors to evaluate the switching speeds of the devices.The dynamic switching behaviors of the load-type inverters were analyzed using a digital oscilloscope (Rigol DS2102), an arbitrary wavefunction generator (Rigol DG4062), and a high-voltage amplifier (Pintek HA-405).The high-voltage amplifier was used to generate an alternating input voltage (V IN ) in the range of 0-60 V.The output voltage (V OUT ) was monitored using the Rigol DS2102 digital oscilloscope.A supply voltage (V DD ) of 60 V was applied using a Keithley 6517A instrument.

Figure 1 .
Figure 1.Schematic of the fabrication processes for the n-type short-channel device using the dual-role approach involving the PTCDI-C 8 wire.Shortchannel mask formation: a) dropping PTCDI-C 8 solution onto a capillary tube placed on top of the gate dielectric (CL-PVP)-coated ITO/glass substrate; b) formation of the PTCDI-C 8 wire via heating of the substrate; c) formation of the PTCDI-C 8 wire along the gate electrode as a mask for the formation of the short channel; d) deposition of the Au thin film over the wire; e) removal of the crystal-wire mask and overlying Au film with acetone; f) formed source and drain electrodes with (sub)micrometer channel length.Active-layer formation: g) dropping the same PTCDI-C 8 solution onto a capillary tube placed on top of the formed electrodes; h) formation of the PTCDI-C 8 wire via heating of the substrate; i) complete n-type short-channel device with PTCDI-C 8 crystal wires.

Figure 2 .
Figure 2. a) POM images of a PTCDI-C 8 crystal wire, with the crosshair showing the angle of the analyzer and polarizer relative to the substrate.b) (top) Optical microscopy and (bottom) fluorescence microscopy images of the PTCDI-C 8 crystal grown around the channel area (indicated by the yellow arrow) of the completed bottom-contact device.c) Absorbance and fluorescence spectra of the PTCDI-C 8 solution in o-DCB.

Figure 3 .
Figure 3. AFM images of the a) channel region formed via the solution process and b) PTCDI-C 8 crystal bridged over the channel (inset: 3-D image of the grown PTCDI-C 8 crystal).

Figure 4 .
Figure 4. a) Output and b) transfer characteristic curves of the n-type short-channel OFET with a PTCDI-C 8 crystal active layer and CL-PVP polymeric gate dielectric.

Figure 5 .
Figure 5. Dynamic voltage response of resistor-load-type inverters based on the n-type OFET.a) Circuit diagram of the dynamic voltage response measurement setup at V DD = 60 V. b) Dynamic voltage response of the inverter based on an n-type OFET (L = 50 μm) with the PTCDI-C 8 crystal, c) based on an n-type OFET (L = 1 μm) with a PTCDI-C 8 film prepared via thermal deposition, and d) based on an n-type OFET (L = 1 μm) with the PTCDI-C 8 crystal.