Organic Charge‐Transfer Complex based Microstructure Interfaces for Solution‐Processable Organic Thin‐Film Transistors toward Multifunctional Sensing

The development of organic field‐effect transistor (OFET) based sensors is in high demand for flexible electronics. Herein, a new and feasible procedure to grow donor‐acceptor complex microstructures sandwiched between the semiconducting and insulating layers is reported. To realize a well‐distributed, uniform‐sized structure through coating on the gold patterned substrate, the dopping ratio of the organic donor‐acceptor complex in the host polymer is optimized. This method leads to one‐step fabrication of high‐performance semiconductor thin‐films and a microstructured binary system attached to it via successful phase separation. The OFET sensors prepared by this technique demonstrate ideal hole‐transport properties and good pressure response. In addition, the thermal‐sensitivity is also revealed, which enables the device architectures to be multifunctional. This work opens a new avenue to manufacture functional microstructure and flexible OFET based pressure sensors.


Introduction
As a large class of smart electronics, functional flexible sensors are currently attracting much attention owing to their great potential in healthcare monitoring, artificial e-skin, and wearable electronic devices. [1] Various sensors that transduce external stimuli (i.e., heat, pressure, light, humidity, and gas) into electric or other signals are developed for high sensitivity realization. In particular, pressure sensors mainly originate from the detecting mechanism of piezoelectricity, [2] triboelectricity, [3] piezoresistivity, [4] or capacitance change [5] to the tactile, depending on the active layer. A two-terminal or single-electrode nanogenerator usually generates repeated on-time charge flow and potential changes induced by externally applied forces on the basis DOI: 10.1002/aelm.202300205 of a piezoresistive or triboelectric response; herein, the electric signals can be quickly evaluated using the green self-powered mode. [6] In resistance upon pressure, sensitive materials, such as graphene, carbon nanotubes, metal nanoparticles, conductive nanowires, and organic polymers, can be easily attached to or mixed into flexible substrates for electronic sensor fabrication combined with the structure design. [7] Besides, flexible organic field-effect transistors (OFETs) working as charge transport and amplifying devices under the dielectric layer modulation principle are also good research candidates. In the three-terminal architecture, microstructured dielectric or suspended gates exhibiting capacitance reactions are designed, benefiting from the flexible and solution-processable nature. [8] The flexible OFET-detecters working as e-skin can function as intelligent elements in wearable electronics, including continuous health-monitoring devices and robotics. [1c] Recently, hybrid energy transmission systems have been considered for constructing new and efficient electric architectures. [9] For example, tribotronic organic thin-film transistor -based pressure sensors with a high sensitivity of up to 210 kPa −1 were realized by incorporating a triboelectric layer. [10] On the other hand, the distinctive feature of most organic semiconductors in OFETs supplies thermal sensing as a result of thermally-activated charge tranporting feature. [11] Rational cooperation of suitable organic semiconductor selection and the attachment of a pressure-sensitive material/structure support multifunctional applications. However, the factor of complicated procedures during device manufacturing remains a huge issue, owing to the introduction of functional layers. In the meanwhile, from two or more-step growth to one-step composite material preparation with heterogeneous structure, functional resistance sensors were reported to present efficient proprioception and exteroception for intelligent prostheses and soft robots. [12] Therefore, aiming for simple-fabricated sensing transistors without any damage to the semiconducting molecular packing and thinfilm morphology is highly required to efficiently transmit external stimuli.
The organic charge-transfer complex is a new class of organic semiconductor containing two or more kinds of components in a highly-ordered molecular-level-heterojunction packing state. [13]  Intermolecular interactions, such as -interations, chargetransfer interactions, and hydrogen/halogen bonding facilitate the formation of well-ordered binary frameworks. [14] Some polar binary systems with noncentrosymmetric space groups and spontaneous charge polarization have been reported to reveal an interesting ferroelectricity phenomenon, indicating their piezoelectric appeal. [15] The good binding ability of the charge transfer complexes allows them to be easily grown via the solution process, even from blending solutions with macromolecular polymers. [16] Moreover, the superior compatibility of the charge-transfer complexes with other organic materials makes them suitable in the flexible organic electronics field. [17] Hence, a solution-processed multifunctional semiconductor/complex microstructure layer can be predicted using a one-step solution strategy.
In this work, we fabricated multifunctional flexible organic thin-film transistors by inserting a charge-transfer complex microstructure layer, through a facile blend solution spin-coating method. Undergoing a clear phase separation, the buffer layer structure can be well controlled by doping proportion tuning. Nearly no negative effect was observed on the device performance, compared to pristine polymer OFETs. Due to the polar nature of the co-crystal, the press stimulus-induced compression will generate an external surface potential that influences the overall dielectric layer capacitance. As a result, the as-prepared sensors presented significant pressure-stimuli responses and even thermo-sensitive behaviors. Figure 1a illustrates the layer-by-layer spin-coating procedure associated with low-temperature annealing, that is developed to manufacture the organic semiconducting/piezosensitive/ insulating layers. The sequential film deposition of solutionprocessable organic materials offered a facile and effective pathway to prepare OTFT-based pressure sensors. Figure   film and the complex microstructure was the successful phase separation of the two different elements. After spin-coating the blend chloroform solution onto the S/D electrode-patterned PET substrate, 80°C vacuum annealing was conducted for approximately 25 min to eliminate the residual solvents and ensure an adequate donor-acceptor supramolecular system gathering. Bearing DBCz/TCNQ microstructures on the polymer film, a n-butyl acetate solution of PMMA was spin-coated onto it and annealed under a 110°C vacuum condition for 60 min to build a dense insulating structure. The organic dielectric layer enabled the OTFT to work at relatively low voltages, accompanied with well film thickness modulation. Finally, the top Al gate electrode was thermally deposited on the PMMA surface to finish the device construction with an additional microstructured layer, which will be very useful for sensitive pressure sensing.

Electrical Properties of Flexible OTFTs
For the micro-co-crystal structure monitoring, various DBCz/TCNQ doping ratios (i.e., 1, 5, 10, 15, and 20 wt.%) in DPP-DTT were selected to explore and optimize the effect of the microstructure interface on the device performance. The as-prepared films were further characterized by optical microscopy (OM), atomic force microscopy (AFM), and powder X-ray diffraction (pXRD) analysis. As we can see in Figure 2 and Figure S1 (Supporting Information), microsized DBCz/TCNQ grains grew on the DPP-DTT polymer film, as expected. [18] No obvious binary domain was found when a very small amount of the donor-acceptor complex was introduced (1 wt.%). However, as the dopant ratio increased to 5 wt.%, ≈200-nm-diameter nanoparticles with 70-nm thickness appeared along with some gathered ribbon architectures. The spin-coating process was believed to endow the supramolecular system nucleation. Moreover, the quick growth incorporated with the surrounding molecules would facilitate a ribbon-shaped co-crystal formation. The extra binary sources diffused to the nuclei and aggregated into a ribbon-like structure with a clear shape and edges at a high doping content. The 10 wt.% doping ratio led to a uniformly sized nanoribbon morphology with approximately 500−1000 nm length, 100−200 nm width, and ≈80 nm thickness. However, an uneven distribution of large complex domains and nanoparticles caused by the excessive number of dopants at higher doping ratios (15 and 20 wt.%) was observed, which was supposed to damage the good contact for the semiconductor film and subsequent dielectric layer. The pXRD patterns of these dopped films revealed that the DPP-DTT aggregation signals at the (100) peak showed comparable strengths, revealing that the donor-acceptor system had a negligible influence on the polymer edging-on orientation ( Figure S2, Supporting Information). [19] The PMMA insulating films with the thickness of ≈800 nm would fill the spare space and exhibit almost the same smooth surface www.advancedsciencenews.com www.advelectronicmat.de morphology on different microstructure-based substrates ( Figure S3, Supporting Information). These results illustrated the doping effects on the morphology control and implied that the 10 wt.% doping ratio is the best choice for the prime microstructure construct. Figure 3 displays the typical transfer and output characteristics of the various OTFTs measured under ambient conditions. Table S1 (Supporting Information) summarizes the detailed extracted electrical performances of these devices. When a high source-drain voltage V D (>V G − V T ) was considered with respect to the OTFTs, the field-effect mobility (μ) was calculated as fol-lows in the saturation regime from the linear fit of I D 0.5 versus V G through Equation 1 where I D is the source-drain current, C i is the capacitance per unit area of the dielectric (here, C i = 2.52 nF cm −2 ), V G is the gate voltage, V T is the threshold voltage, W and L are the channel width and length, respectively. The devices with 1% dopant exhibited the best hole mobility of 2.2 cm 2 V −1 s −1 with the threshold voltage of ≈3 V. Less traps and the chargetransfer complex-macromolecular polymer interactions caused by the DBCz/TCNQ clusters were regarded as the main factors for the performance improvement of charge transport. [20] Meanwhile, OFETs with 10% doping exhibited similar mobilities, compared to the undoped device (undopped: 1.6 cm 2 V −1 s −1 , 5%: 1.3 cm 2 V −1 s −1 , 10%: 1.5 cm 2 V −1 s −1 ), confirming that the microstructured interface did not have negative influence on the charge transmission along/between the -conjugated backbones. These results suggested that micro/nanoscale interface passivation combined with flexible polymer dielectrics is an effective and promising strategy for achieving hysteresis-free, operational stable, and low-voltage OFETs with good electrical performance. The successive solution-deposited approach of PMMA layer resulted in good dielectric properties upon the operated gate voltage. Consequently, both the good charge transport properties and the functional microstructure by this one-step separation approach guaranteed the possibility of the sensing ability of these OTFT devices.

Pressure-Sensing Behaviors of OFETs
Very recently, we reported that the DBCz/TCNQ co-crystal belonged to a polar Pc space group with a mixed-stacking packing mode. [21] The noncentrosymmetric alignment generated polar ± (102) facets with accumulated charges. Meanwhile, the electrostatic repulsion caused the polarization-induced twisting of freestanding nanobelts. When an external pressure was applied to the active layer, the DBCz/TCNQ microstructure was deformed toward the PMMA dielectric layer and resulted in a polarized state change with a charge flow. Several parameters, including sensitivity (S), detection limit, and response time, were fully investigated to evaluate the sensing properties of these OTFT pressure sensors. The key parameter S was determined as follows by plotting the relative changes in the output current ΔI and applied pressure ΔP (Equation 2) where I 0 is the initial source-drain current when the sensor is not loaded by an external pressure. All the doped devices displayed obvious pressure-sensing behaviors ( Figure S4, Supporting Information). Among them, the 10% doping-based OTFT showed the best performance due to its well-distributed, uniform sized microstructure. Figure 4a shows the transfer characteristics of the 10% DBCz/TCNQ doping flexible OTFT sensor in response to 0.01 to 10 kPa of external pressures. The response current of the OTFT-based pressure sensor operated at V G of −15 V and V D of −15 V steadily increased as the applied pressure increased.
The sensitivity values were estimated and plotted to be a two approximately linear region response mode with different pressures (Figure 4b). The sensor under the low pressure region of 0.01-0.1 kPa gave a high sensitivity (S 1 ) up to 2.73 kPa −1 , while that in the high-pressure region (0.1-10 kPa) showed a relatively lower sensitivity (S 2 ) of 0.16 kPa −1 . The pressure sensitivity transition between these two regions was caused by the DBCz/TCNQ microstructured interfaces and the pressure-induced capacitance changes of the gate dielectrics affected by the deformation. The increasing current trend would not change until the pressure was up to a threshold pressure of 0.1 kPa, affording a good sensitivity at very low pressure range. Meanwhile, the capacitance and the current moderately increased at higher pressures, leading to a subsequent low-sensitivity region. It is concluded that our DBCz/TCNQ interface-inserted OTFT pressure sensors can distinguish different loading levels of external pressures with a 10 Pa detection limit. In addition to sensitivity, the response time is another essential parameter of sensing devices. The dynamic response measurements were performed to measure the response time of the DBCz/TCNQ doping OTFT sensor at V G = −15 V and V D = −15 V. The time-resolved response of the OTFT pressure sensor was measured by periodically applying 4 kPa of external pressure. Figure 4c shows that the dynamic signal of the OTFT pressure sensor quickly responds to the external stimulus with a rise time (t r ) of ≈0.3 s and a fall time (t f ) of 0.13 s. Air stability is also important for the practical applications of this kind of OTFT. Here, unencapsulated flexible OTFTs with DBCz/TCNQ interfaces were exposed to ambient conditions and measured every day to investigate their stability. Figure 4d presents the transfer characteristics of these devices with air exposure of over 2 months. The OTFTs with the DBCz/TCNQ interface showed nearly no decay in mobility and on-current with a slightly increased off-current. Hence, the flexible device could still work well after a long period of air exposure (i.e., 2 months). The comparison experiments displayed no detectable pressure response of the pristine DPP-DTT OTFTs, illustrating that the DBCz/TCNQ-doped microstructure acted as the piezosensitive element in this architecture. Overall, our work of this functional device presented considerable sensing behavior (Table S2, Supporting Information), and the further optimization is still ongoing. Figure 4e illustrates the possible sensing mechanism of these devices. When no external pressure was applied, the nanoribbonshaped donor-acceptor complex remained in its initial state, and the devices worked in their original state. When an external pressure was applied to the sensor, the DBCz/TCNQ framework underwent compression with spontaneous displacement and polarization. It generated the current flow and the electrical potential, leading to a gradual change in capacitance. Perhaps, there was friction electrification from the microstructure and the PMMA layer. In this case, the polarization of DBCz/TCNQ co-crystal microstructures changed the relative capacitance of the dielectric upon pressures and further the output current, because of their noncentrosymmetric lattice structure and the mismatch between the co-crystal and the insulator.

OTFT Sensor Array for Pressure Mapping
A proof-of-concept OTFT pressure sensor array with 3 × 3 pixels was further fabricated on a flexible PET substrate to verify the practicability of this microstructure-based OTFT sensors for pressure sensing (Figure 5a). This pressure sensor array enabled the integrated OTFT devices to realize a spatially mapped image upon the external pressures. Three weights (i.e., 2, 5, and 10 g) were placed on the sensor array, which corresponded to the pressures of 400, 1000, and 2000 Pa, respectively (Figure 5b). Through the measurement of transfer characteristics of the sensor array, the pressure-induced change in the output current (ΔI) was used to produce two-(2D) and 3D space maps (Figure 5c,d). The uniformity and the pressure transmission properties of the sensor array made it a 2D mapped image with different color degrees of greenness at different applied pressures. A higher pressure was denoted by a darker color of the green pixel, while a lower pressure was depicted by a brighter color pixel on the map. The distribution of various pressures was effectively identified by the color degree, indicating that the OTFT pressure sensors and arrays had a good potential for recognizing different weights/forces and detecting the pressure distributions.

Temperature Dependence of the Flexible Sensor
We also studied the temperature dependence of the DPP-DTTbased OTFT sensor in the 323−358 K temperature range. The transfer and output curves at the variable temperature (Figure 6; Figure S5, Supporting Information) displayed good field effect and heat-dependent characteristics, suggesting that the DPP-DTT polymer film kept a good semiconducting feature, despite of the doping step in the beginning polymer solution. The measured current increased with the temperature increase because of the thermally activated dominating charge transport of the DPP-DTT-based OFETs. The statistical relationship between the temperature and the relative current change [(I T − I T 0 )∕I T 0 ] was defined as sensitivity in this work. [22] The electrical transport is described as follows (Equation 3) where A*, E A , and denote the Richardson coefficient, effective activation energy, and ideality factor, respectively. According to the variable temperature output curves, the barrier for the pristine device is 71.9 meV, 102.6 meV for 1% doping, 110.1 meV for 5% doping, and 152.6 meV for the 10% doping sensor. The sensors possess a higher barrier and lower thermal sensitivities, but the values are still acceptable. Performing as a bifunctional sensor, the DPP-DTT OTFT device with a binary complex microstructure can deliver both good pressure sense and acceptable thermosensitivity.

Conclusions
In summary, we have demonstrated a new design strategy of piezo-sensitive microstructures modified on the semiconducting layer through one-step spin-coating DBCz/TCNQ doped polymer solutions. Via dopping ratio modulation, the phase separation induced uniform supramolecular nanoribbons were observed to distribute well on the semiconducting thin-film surface. The optimal interface (10% doping) favored the formation of a smooth insulating film with good compatibility and good contact with the semiconductor. Consequently, the as-prepared flexible DBCz/TCNQ dopped OTFTs showed good charge transport performance and air stability, with a mobility of 1.5 cm 2 V −1 s −1 , comparable to that of pristine devices. For pressure sensing, a good sensitivity of 2.73 kPa −1 was achieved with a very low pressure detection limit and a fast response time of ≈0.13 s. Moreover, the sensor array based on these OTFTs units demonstrated highresolution 2D spatial pressure mapping. The thermosensitive behavior was also observed from the DBCz/TCNQ-doped OTFTs.
Our work provides a simple and effective approach to fabricate functional OTFTs with respect to pressure and thermal sensing, which has great application potential in advanced electronics and sensing.
Solution Preparation: To prepare the modified semiconductor layer, different amounts of DBCz and TCNQ mixtures (molar ratio of 1:1) were dissolved in DPP-DTT chloroform solution of ≈7.3 mg mL −1 concentration, according to the doping ratios. Sonicate them for 10 min and then heat at 60°C for 15 min until the full dissolution. Polymethyl methacrylate Figure 6. The characteristics curves and temperature resolution curves of the DPP-DTT sensors with different doping ratios: a,e) 0%, b,f) 1%, c,g) 5%, and d,h) 10% measured at 50−85°C with a precision of 5°C. was dissolved in n-butyl acetate and stirred at 110°C for 6 h to obtain the dielectric solution with a concentration of 55 mg mL −1 .
Fabrication of Flexible OTFTs: A top-gate/bottom-contact structure was chosen to fabricate flexible OTFTs. First, patterned Au with a thickness of 50 nm was deposited as the source-drain electrodes on a plastic PET substrate by using a metal mask with a channel W of 4.3 mm and L of 0.3 mm. Then, the prepared DBCz/TCNQ doped DPP-DTT solution was spin-coated on the Au patterned substrates (1200 rpm, 60 s) and annealed at 80°C for 25 min to form the organic semiconductor layer and binary microstructure on it. After that, the PMMA solution was spin-coated to produce an insulating layer (1200 rpm, 60 s), followed by thermal annealing at 110°C for 1 h. Finally, 50 nm of Al film as the gate electrode was thermally evaporated on the dielectrics with the use of another shadow mask.
Fabrication of Sensor Array: Tailor-made shadow masks were used to prepare the OTFT pressure sensor array for 3 × 3 pixels, in which channels W and L were 4.3 mm and 0.3 mm, respectively, and the area of each pixel was ≈23 mm 2 . The technological procedure of this OTFT sensor array was the same as those of the OTFT pressure sensors.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.