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. 2019 Dec 11;4(26):22082–22088. doi: 10.1021/acsomega.9b03195

Exploitation of Thermoresponsive Switching Organic Field-Effect Transistors

Yang-Hsun Cheng †, Ai-Nhan Au-Duong †,‡, Tsung-Yen Chiang †, Zi-Yuan Wei †, Kai-Lin Chen †, Juin-Yih Lai †,‡, Chien-Chieh Hu ‡,*, Chu-Chen Chueh §,∥,*, Yu-Cheng Chiu †,§,*
PMCID: PMC6933784  PMID: 31891088

Abstract

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In this work, a novel thermoresponsive switching transistor is developed through the rational design of active materials based on the typical field-effect transistor (FET) device configuration, where the active material is composed of a blend of a thermal expansion polymer and a polymeric semiconductor. Herein, polyethylene (PE) is employed as the thermal expansion polymer because of its high volume expansion coefficient near its melting point (90–130 °C), which similarly corresponds to the overheating point that would cause damage or cause fire in the devices. It is revealed that owing to the thermistor property of PE, the FET characteristics of the derived device will be largely decreased at high temperatures (100–120 °C). It is because the high volume expansion of PE at such high temperature (near its Tm) effectively increases the distance of the crystalline domains of poly(3-hexylthiophene-2,5-diyl) to result in a great inhibition of current. Besides, the performance of this device will recover back to its original value after cooling from 120 to 30 °C owing to the volume contraction of PE. The reversible FET characteristics with temperature manifest the good thermal sensitivity of the PE-based device. Our results demonstrate a facile and promising approach for the development of next-generation overheating shutdown switches for electrical circuits.

1. Introduction

Over the past decade, electronic devices have become important elementary units for developing diverse consuming electronics in our society. During the operation of electronic devices, heat loss will inevitably occur and heat will be the main byproduct. In some cases, the electrical devices in the circuits, especially for the integrated electronics such as cell phones, vehicles, and so forth could be overheated because of waste heat generated from the short circuit, spark gap, or poor heat dissipation.1 Such overheated devices have aroused a serious concern because they might associate with major damage such as fire, explosion, and injury. For this reason, the commercial devices are usually equipped with two elements combining the functions of a temperature sensor and circuit breaker; however, it may cause obstacles in working response or complicate the device design. Thus, research attention has been recently increased to seek more simple and effective methods to limit these shortages.1

Based on this requirement, we are particularly interested in developing a transistor switch, the most basic element in an electronic circuit that can enable rapid shutdown at abnormal temperature. Among the developed transistors, organic field-effect transistors (OFETs) have gained considerable research interest because of the advantages of low cost, light weight, low power consumption, facile integration capability, and simple structure design. Thus, OFETs have been employed for widespread applications, for example, signal switching and amplification in modern electronic devices.2−9 More recently, numerous efforts have been even devoted to the development of stimuli-responsive OFETs that are responsive to physical,10−12 chemical,13,14 or biological stimuli.15 Generally, the strategy to realize the sensing functions of OFETs relies on the rational designs of active channel materials, electrodes, and gate dielectric layers. For example, a high-performance pressure sensor device was fabricated with high sensitivity and response times by directly incorporating microstructured plasma desorption mass spectrometry films into the dielectric layer of an OFET device.11 Another study of phototransistors consisting of the blend of poly(3-hexylthiophene-2,5-diyl) (P3HT) and TiO2 nanoparticles as the active layer has been demonstrated to show stable electrical performance and fast response to light illumination.12 However, to the best of our knowledge, thermoresponsive OFETs for applications in overheating-circuit protection have been rarely investigated.

In this study, we describe a new approach to realize a thermoresponsive switching OFET through the rational design of active material, for which the active channel consists of a blend of a thermal expansion polymer and a polymeric semiconductor. Herein, polyethylene (PE) is chosen as the thermal expansion polymer because of its significant volume expansion property near the melting point (90–130 °C), which similarly corresponds to the overheating point that would cause damage in the devices or would cause fire arising from the exposure of integrated electronic components to the high-temperature internal fluids (such as oil and coolant). Meanwhile, P3HT is selected as the model semiconducting material. We interestingly manifest that the blended PE can effectively increase the distance of packing motif between the conducting domains as temperature increases, thereby restricting the output current at a relatively high temperature.16,17 Therefore, the direct performance-temperature dependence of our fabricated OFET devices is examined upon the heating and cooling of external temperature (ranging from 30 to 120 °C). We have also investigated the morphology of the active layers upon temperature variation by using atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering to correlate the structure–performance relationship. The results revealed in this work present a proof-of-concept device that can be easily integrated into the electronic circuits to serve as an overheating controller.

2. Results and Discussion

2.1. Safety Circuit Device Design Using a Thermal Expansion Polymer

An ideal thermoresponsive switching transistor device should be based on the typical transistor device because it is capable to be easily integrated into the circuit as the switched-mode power supply unit. When a large amount of heat (>100 °C) is accumulated in the connected circuits, the output current of the thermoresponsive transistor will be largely decreased to serve as an overheating controller. Basically, the current flow into an OFET device can be modulated by the semiconductor/dielectric interface, thereby controlling the ON/OFF states of the device. Based on this principle, we herein design a thermoresponsive transistor device through a rational design of the active channel materials. The typical P3HT is herein employed as the model semiconducting material and is further blended with PE that possesses a high thermal expansion coefficient, as illustrated in Figure 1.

Figure 1.

Figure 1

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Device configuration of the studied thermoresponsive switching OFET and the structures of the studied polymers for the active channel.

As summarized in Table 1, PE possesses a unique feature in response to the temperature change, which is suitable to realize the thermoresponsive function. PE exhibits a large thermal expansion coefficient of 3.8 to 8.6 × 10–4 m/(m·K), which is ∼10 times greater than the value of P3HT and polystyrene (PS). This makes PE possess a significant volume expansion near its melting point (∼90 °C). The differential scanning calorimetry (DSC) curves for the pristine P3HT, PE, and the P3HT/PE blends are presented in Figure S1. As seen, the P3HT/PE blends show a Tm of ∼90 °C and a Tc of ∼77 °C, which is similar to the neat PE, indicating that the phase transition of the P3HT/PE blends mainly depends on the thermal expansion of PE. Such large volume expansion upon increasing temperature thus could inhibit the current flow between the source and drain electrodes owing to the insulating nature of PE. Besides, because the volume expansion or contraction of PE is highly dependent on temperature, the working condition of the derived OFET based on conjugated polymer/PE blends thus can be switched or controlled by the external temperature.

Table 1. Thermal Properties of P3HT, PE, and PS.

characteristics P3HT PS PE refs
melting temperature, Tm (°C) 238 212 90–130 (16, 25)
thermal expansion coefficient (m/(m·K)) 7.6 × 10–5 6–8 × 10–5 3.8–8.6 × 10–4 (16, 25)
percentage change in volume per cubic meter per 100 °C (%) 2.28 1.8–2.4 11.4–25.8 (16, 26)
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2.2. Fabrication and Characterization of the Studied OFET Devices

We first fabricate the OFET devices based on the P3HT/PE blends with different blending ratios of 2:1; 1:1, and 1:2 (defined by the weight percentage between P3HT and PE; i.e., 67%:33%, 50%:50%, and 33%:67%, respectively) and investigate their FET characteristics. Figure 2 presents their transfer characteristics and square root of the drain current measured at room temperature, and the relevant device parameters are summarized in Table 2. All devices exhibit a typical p-type behavior; however, the saturated hole mobility (μ) of the device is decreased as the PE content in the active channel increases. The μ of the pristine P3HT device is 7.69 × 10–2 cm2 V–1 s–1 and it is gradually decreased to 0.35 × 10–2 cm2 V–1 s–1 as the blending weight ratio of PE increases to 70%. Such a decrease in mobility is apparently related to the insulating nature of amorphous PE, which impedes the charge transport of the P3HT matrix to a certain degree.

Figure 2.

Figure 2

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(a) Transfer characteristics and (b) square root of drain current of the studied OFETs based on the P3HT/PE blends with different blending ratios at room temperature.

Table 2. Detailed OFET Characteristics of the Studied Devices Measured at Room Temperature (30 °C).

samples average μ (cm2/(V s)) average ON/OFF current ratio average ON current at Vg = −60 V (A) average threshold voltage Vth (V)
P3HT (7.69 ± 0.70) × 10–2 2.10 × 105 2.41 × 10–5 –1.17 ± 2.78
P3HT/PE (2:1) (6.84 ± 0.66) × 10–3 7.80 × 103 2.08 × 10–6 –2.11 ± 2.12
P3HT/PE (1:1) (3.81 ± 0.67) × 10–3 1.10 × 103 3.86 × 10–6 1.21 ± 1.49
P3HT/PE (1:2) (3.50 ± 0.38) × 10–3 6.10 × 102 1.46 × 10–6 16.74 ± 3.33
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To better understand the influence of PE addition, the surface morphology of the studied blends after annealing at 120 °C is characterized by AFM. As shown in Figure 3, the morphology of the pristine P3HT film consists of fibril-like structures. After adding 30 wt % PE, the fibril-like structures of P3HT could be still preserved as evidenced by the uniform distribution of self-assembled nanoaggregates. Notably, slightly different to the pristine P3HT film, adding 30 wt % PE seems to even better induce the formation of nanofibers that possess a finer diameter of 10–20 nm, whereas, as the blending amount of PE goes over 50 wt %, large-scale phase separation between P3HT and PE starts to occur, which can be attributed to their distinct surface energy. P3HT with a relatively lower surface energy (19.8 mJ·m–2)18 prefers to exist near the n-octadecyltrimethoxysilane (OTS) interface owing to its similar surface energy, while PE with higher surface energy (30–31 mJ·m–2)19 tends to accumulate near the surface of the film.20,21 The UV–vis absorption spectra of the P3HT/PE blends (after annealing at 120 °C, Figure S2) were measured to verify this observed behavior. The studied blends exhibit similar absorption bands because of the strong interchain π–π stacking of P3HT.22 Note that the λmax (518, 544, and 593 nm) of the pristine P3HT film was slightly red-shifted to 524, 550, and 600 nm as the PE content continuously increases to 70 wt %. It is probably due to the induced phase separation and the promoted aggregates of polymer chains in the film state, which is consistent with the AFM results. Besides, the low loading amount of semiconducting P3HT will also reduce the overall charge transport property. These abovementioned reasons clearly clarify the largely decreased mobility of the OFET devices based on the P3HT/PE blend films with a ratio of 1:1 and 1:2. To recognize the requirement of adequate mobility for practical applications, the OFET device based on the P3HT/PE blend film with a ratio of 2:1 (denoted as the P3HT/PE device hereafter) that delivers a mobility of 0.68 × 10–2 cm2 V–1 s–1 with a threshold voltage (Vth) of −2.11 V and an ON/OFF current ratio (ION/IOFF) of 7.80 × 103 is thus preferably selected for further study.

Figure 3.

Figure 3

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AFM phase images of the annealed films based on the P3HT/PE blend: (a) P3HT, (b) P3HT/PE (2:1), (c) P3HT/PE (1:1), and (d) P3HT/PE (1:2).

2.3. Switching Behaviors of the OFET Devices in Response to Temperature Change

2.3.1. Performance of the Studied OFET Devices upon Temperature Change

To examine the thermoresponsive function of the P3HT/PE (2:1) device, its FET characteristics are measured under different temperatures. Because unstable and unreliable performance of the P3HT/PE device might be acquired when measured under a temperature beyond the melting point (Tm) of PE (∼130 °C, Table 1), the device is set to be tested in the range between 30 and 120 °C (Figure 4), and the detailed OFET characteristics of both P3HT and P3HT/PE (2:1) devices upon temperature changes are summarized in Tables S1 and S2, respectively. For the P3HT device, the entire transfer curve is shifted toward the positive direction as the measured temperature increases, which leads to a higher drain current (ON state) and enhanced mobility as the measured temperature increases (as shown in Figure 5a,b). This phenomenon could be attributed to thermally activated charge transport that is a common effect in the typical OFETs. Besides, the varying behavior observed herein is similar to the case reported in the literature.23,24

Figure 4.

Figure 4

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Transfer characteristics of the studied OFETs based on (a,b) a pristine P3HT film and (c,d) a P3HT/PE (2:1) blend film measured under different temperatures between 30 and 120 °C, wherein the temperature is gradually (a,c) increased from 30 to 120 °C or (b,d) cooled from 120 to 30 °C. The entire transfer curve of the P3HT device is shifted toward the positive direction as the measured temperature increases (see the red arrow in (a)). However, (e,f) the extracted transfer curve of the P3HT/PE device has clearly shown a very special behavior that its transfer curve initially shifted toward the positive direction under a measured temperature below 100 °C and subsequently moved to the negative direction as the measured temperature increases from 100 to 120 °C (red arrow in (e)). Both devices could return back to their respective original position (blue arrow in (b,f)) when the measured temperature is cooled down to room temperature (30 °C).

Figure 5.

Figure 5

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(a) ON current and (b) mobility of both the P3HT device and P3HT/PE device as a function of the measured temperature with a gradual increase from 30 to 120 °C (under a heating process). (c) ON current and (d) mobility of both the P3HT device and P3HT/PE device as a function of the measured temperature with a gradual decrease from 120 to 30 °C (under a cooling process).

Interestingly, the P3HT/PE (2:1) device shows a different varying behavior as compared to the P3HT device. As shown in Figure 4c, the transfer curve of the P3HT/PE device is initially shifted toward the positive direction under a measured temperature below 90 °C. As the measured temperature increases from 90 to 120 °C (close to the Tm of PE), the shift of the resulting transfer curve reverses to the negative direction, leading to a significant decrease in current of the transfer curve. Clearly, this suppress in the electrical properties of the blending device results from the phase transition of the P3HT/PE blends (at ∼90 °C), as shown in Figure S1. Accordingly, at the measured temperature of 120 °C, the P3HT/PE device delivers the significantly decreased mobility of 0.48 × 10–2 cm2 V–1 s–1 with a reduced ON current of 1.81 × 10–6 A. Such largely decreased performance of the P3HT/PE device at 120 °C is very different to the case of the P3HT device at 120 °C. Besides, it represents a 13% decrease in ON current and a 30% decrease in mobility compared to its performance measured at 30 °C, revealing the capability of “current-flow inhibition” under an overheating condition (120 °C herein). In addition, PS which possesses the similar coefficient thermal expansion with P3HT was also employed as a reference dielectric to compare the current inhibition capability with the PE polymer (as seen from Figure S3). The P3HT/PS device shows a similar trend to the P3HT/PE device upon heating (30–120 °C), for which the mobility starts to decrease at ∼ 80 °C. However, for the P3HT/PS device, the effect of thermal inhibition of mobility is relatively smaller than that of the P3HT/PE device because of the different thermal expansion coefficients between PS and PE.

Moreover, the performance of both devices could recover back to their respective original states (almost similar values of mobility) when the measured temperature is cooled down to room temperature (Figure 5c,d). To recognize their recovery ability, a series of heating–cooling cycles (five cycles) was run on P3HT and P3HT/PE devices to evaluate recovery performance (Figure 6). Both of these showed comparable performance (focusing on values of mobility) in the cycling test that further reveals the possibility in reusability of the P3HT/PE device after inhibition at high external temperature. Such reversible behavior observed in the resulting performance manifests the good thermal sensitivity of the P3HT/PE device. Besides, it validates the concept of using a thermal expansion polymer to enrich the thermoresponsivity of an OFET device because the volume expansion or contraction of PE can effectively modulate the charge transport of the P3HT matrix on temperature changes.

Figure 6.

Figure 6

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Average mobility of (a) P3HT and (b) P3HT/PE device after serial heating–cooling cycles with an initial increase from 30 to 120 °C (under a heating process) and then decrease from 120 to 30 °C (under a cooling process).

2.3.2. Proposed Mechanism for the Thermoresponsive Behavior

Considering the influence of the volume expansion or contraction of PE in relation to the resulting morphology of the derived active channel, we next further investigate the variation of morphology of the studied films upon temperature by synchrotron grazing incidence wide-angle X-ray scattering (GIWAXS, Figure S4). The crystalline lattice constants and orientation information by (h00) and (0k0) scatterings of each film are collected after it reaches the desired temperature. As seen, the GIWAXS patterns of both films present a similar (100) peak because of the lamellar structure of P3HT and a similar (010) scattering peak because of the π–π interchain stacking of P3HT, which confirms the intermolecular self-assembly due to the π–π interactions. However, in the P3HT/PE blend film, the diffraction peak of P3HT is weakened (Figure S4c), which indicates that the amorphous PE surrounds the lamellar structure of P3HT.

Table 3 summarizes the d-spacing values of the studied films as determined by the relation of d = 2π/q*. For the pristine P3HT film, the d-spacing value shows a slight decrease of ∼0.01 nm in the π–π stacking peak and no apparent change in the lamellar spacing after heating to 120 °C, whereas, the P3HT/PE blend film shows an opposite trend, for which no change in the π–π stacking peak and an increase of ∼0.6 nm in the lamellar spacing are observed after heating to 120 °C. The distinctly different temperature-dependent variation of nanostructures of the P3HT and P3HT/PE films explains their different performance-temperature dependency as observed. The high thermal expansion of PE clearly endows the derived blend with certain thermoresponsivity because it expands as temperature increases, which will further separate the crystalline domains of P3HT. Without using such amorphous and thermal expansion polymer, there are no changes among the crystalline regions of P3HT. Instead, the polymer chains will become closer as temperature increases, enabling more efficient charge transport between chains as observed.

Table 3. Crystallographic Parameters of the Pristine P3HT Film and the P3HT/PE Blend Film Measured Under Different Temperatures.
sample temperature (°C) π–π stacking (nm–1) lamellar spacing (nm–1)
P3HT 30 0.389 1.716
  120 0.379 1.715
P3HT/PE (2:1) 30 0.393 1.656
  120 0.393 1.747
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3. Conclusions

In summary, we report a simple method to develop a thermoresponsive switching OFET by using an active material consisting of a semiconducting polymer, P3HT, and a thermal expansion polymer, PE. We manifest that, owing to the thermistor matrix of PE, the FET characteristics of P3HT are largely changed at high temperatures (100–120 °C). The high volume expansion of PE at such high temperature (near its Tm) effectively increases the distance of the crystalline domains of P3HT to result in a great inhibition of current. Compared to the performance measured at room temperature (∼30 °C), our optimized P3HT/PE (with a weight ratio of 2:1) device exhibits a 13% decreased ON current and a 30% decreased mobility at a high temperature of 120 °C, in contrast to the pristine P3HT device. Moreover, the performance of this device could recover back to its original value after cooling from 120 to 30 °C owing to the volume contraction of PE. This reversible behavior of performance with temperature manifests the good thermal sensitivity of the P3HT/PE device. We thus successfully demonstrate a proof-of-concept device that can facilitate the future development of thermoresponsive OFETs for overheating protection applications.

4. Experimental Section

4.1. Materials

P3HT (Mw of 54 000–75 000 g/mol), PE (Mw of 35 000 g/mol), and OTS were purchased from Sigma-Aldrich. Anhydrous toluene (99.8%) was obtained from Acros Organics. Other reagents were reagent grade and used without further purification.

4.2. Fabrication and Characterization of OFET Devices

The transistor switch was fabricated on a substrate of highly doped n-type Si wafer with 300 nm thick SiO2. Later, the surface of the substrate was functionalized with an OTS self-assembled monolayer. Before spin-coating the active layers, the OTS-treated substrate was washed with toluene, acetone, and isopropyl alcohol and then blow-dried with nitrogen gas before use. The studied P3HT and P3HT/PE blend (70%:30%, 50%:50%, and 30%:70%) with a concentration of 8 mg/mL in toluene were spin-coated onto the OTS-treated substrates to obtain thin film with a thickness of ∼40 nm in a glovebox. The thermal annealing process was continuously carried out at 120 °C for 1 h inside the glovebox. Finally, the top-contact gold electrode (70 nm) was thermally deposited by evaporation through a regular shadow mask with a channel length (L) and width (W) of 50 and 1000 μm, respectively. All the device performance of the fabricated transistors were conducted inside a glovebox by using a Keithley 4200-SCS semiconductor parameter analyzer (Keithley Instruments Inc., Cleveland, OH, USA) which equipped a hot plate for controlling desired temperature.

4.3. Characterization

The surface morphologies of the polymer films were characterized using a Nanoscope 3D controller atomic force microscope (AFM, Digital Instruments) operated in the tapping mode. Additionally, the change in the nanostructure of the polymer film with temperature was measured on an in situ heating stage at beamline BL13A1 with a wavelength of 0.827 Å (15 keV) at an incident angle of 0.15° in the NSRRC.

Acknowledgments

The authors thank the financial support from the Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (108L9006) and the Ministry of Science and Technology in Taiwan (MOST 108-3017-F-002-002). C.-C.C and Y.-C.C. also acknowledge the financial support from the Ministry of Science and Technology in Taiwan (MOST 108-2221-E-002-026-MY3 and 108-2221-E-011-047).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b03195.

  • Detailed OFET characteristics of the studied devices measured under different temperatures between 30 and 120 °C; DSC curves of pristine P3HT, PE, and the P3HT/PE blends at a heating rate of 10 °C/min; UV–vis absorption spectra of the studied polymers in the film state; and GIWAXS images of the studied films annealed at different temperatures (PDF)

Author Contributions

⊥ Y.-H.C. and A.-N.A.-D. contributed equally to this work.

The authors declare no competing financial interest.

Supplementary Material

ao9b03195_si_001.pdf (407.9KB, pdf)

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