Temperature Sensors Based on Organic Field-Effect Transistors

: The rapid growth of wearable electronics, Internet of Things, smart packaging, and advanced healthcare technologies demand a large number of ﬂexible, thin, lightweight, and ultralow-cost sensors. The accurate and precise determination of temperature in a narrow range (~0–50 ◦ C) around ambient temperatures and near-body temperatures is critical for most of these applications. Temperature sensors based on organic ﬁeld-effect transistors (OFETs) have the advantages of low manufacturing cost, excellent mechanical ﬂexibility, easy integration with other devices, low cross-sensitivity, and multi-stimuli detectability and, therefore, are very suitable for the above applications. This article provides a timely overview of research progress in the development of OFET-based temperature sensors. First, the working mechanism of OFETs, the fundamental theories of charge transport in organic semiconductors, and common types of OFET temperature sensors based on the sensing element are brieﬂy introduced. Next, notable advances in the development of OFET temperature sensors using small-molecule and polymer semiconductors are discussed separately. Finally, the progress of OFET temperature sensors is summarized, and the challenges associated with OFET temperature sensors and the perspectives of research directions in this ﬁeld are presented. meV compared with pure pentacene (157 meV). This results in a stronger dependence of the conductivity on the temperature of the thermistor ﬁlm.


Introduction
Organic field-effect transistors (OFETs) or organic thin-film transistors (OTFTs) use organic semiconductors (OSCs) including covalent organic frameworks (COFs) and metalorganic frameworks (MOFs) as active channel materials, which enables the high-speed fabrication of electronic products on flexible plastic substrates under mild conditions at an ultralow cost [1][2][3][4][5][6]. OFETs have a wide range of applications, such as flexible displays, smart labels, intelligent packaging, various types of sensors, wearable electronics, implantable electronics, etc. OFET-based sensors, in particular, are ultralow in cost and can be integrated with other electronics as on-chip sensors, making them an excellent fit for the rapidly growing Internet of Things (IoT) technology, wearable electronics, and healthcare applications [7][8][9][10][11].
Temperature determination is one of the most common applications for modern-day sensors. The field of medicine has a demand for high functioning temperature sensors to be used in vital sign monitoring [12][13][14]. Furthermore, recent developments towards designing artificial skin, or electronic skin (e-skin), require accurate temperature monitoring as well as flexible systems with spatial resolution for practical application [15,16]. Temperature determination is also imperative for monitoring other physical parameters such as pressure, humidity, and gas concentration, which can have a temperature dependence and cannot be accurately determined without adjusting for temperature fluctuation [17].
Similar to a conventional metal oxide semiconductor field-effect transistor (MOSFET), an OFET device consists of three electrodes (source, drain, and gate), a dielectric layer, and an organic semiconductor layer. Figure 1 shows the four common structures of OFETs, bottom-gate top-contact (BGTC), bottom-gate bottom-contact (BGBC), top-gate top-contact (TGTC), and top-gate bottom-contact (TGBC). The OFET current-voltage characteristics are schematically shown in Figure 1e. The output characteristics illustrate the changes of the drain current (I DS ) with the drain voltage (V DS ) at different gate voltages (V GS ). At a very low V DS , the I DS -V DS relationship is ohmic (linear), and this region is called the linear regime (or region). As V DS increases, I DS increases at a slower rate, and the I-V curve becomes non-linear. After V DS reaches a certain point, where V DS = V GS − V T , I DS becomes independent of V DS , and the channel is called pinched off. V T here is the threshold voltage, which is the minimum gate voltage required to fill the charge traps at the channel. The region where V DS > V GS − V T is called the saturation regime. The changes of I DS with V GS at a fixed V DS are the transfer characteristics. In the linear regime (V DS << V GS ), I DS and V GS have the following relationship [18,19]: where W is the channel width, L is the channel length, µ lin is the field effect mobility in the linear regime, and C i is the capacitance of the dielectric layer. Organic materials are composed of molecules as the basic building blocks, which are made of carbon, oxygen, hydrogen, nitrogen, and other elements. The constituent atoms in an organic molecule are strongly held with covalent bonds between atoms by sharing the valence electrons. Therefore, most organic materials are insulators because valence electrons are highly localized. A characteristic of organic semiconductors is that their skeleton has an extended π-conjugated system, usually composed of alternating C-C and C=C bonds. The overlapping (or hybridization) of the molecular orbitals forms an extended highest occupied molecular orbital (HOMO) and a lowest unoccupied molecular orbital (LUMO), where the injected holes (in HOMO) and electrons (in LUMO) can move relatively easily. As the molecular size increases, such as in the case of high molecular weight polymers, long-range π conjugation results in a band-like structure with a small band gap (<3 eV) between HOMO and LUMO, similar to typical inorganic semiconductors. However, organic molecules are held together by weak van der Waals forces or London dispersion forces to form a solid, so the intermolecular distance is quite large, usually 0.3-0.4 nm. This makes intermolecular charge transport more difficult than intramolecular charge transport. Organic molecules are much larger than the spherical constituent atoms and ions in inorganic materials and have more complex geometric shapes. Therefore, most From the I DS -V GS curve, µ lin can be extracted: In the saturation regime (V DS > V GS − V T ), I DS and V GS have the following relationship: where I DS,sat and µ sat are the drain current and mobility in the saturation regime, respectively. µ sat can be calculated from the saturation transfer curves using the following equation: Similar to the conventional resistor-type temperature sensor, or thermistor, the sensitivity of the OFET sensor can be described using the thermal coefficient of resistance (TCR), which can be calculated by where R is the resistance of the channel at a given temperature, R 0 is the resistance of the channel at a known reference temperature, and ∆T is the difference between the sensing and reference temperatures [20]. For OFET devices, the ratio of R−R 0 R 0 can be easily calculated using the variation of I DS with the temperature at constant V GS and V DS , The conventional two-electrode resistor-type temperature sensor can only give one independent output parameter, such as current or voltage. The change in current or voltage may be induced by interfering stimuli such as pressure and humidity, causing cross-sensitivity issues. On the other hand, the OFET temperature sensor can provide numerous parameters, such as I DS and V DS (in different regimes), onset voltage (V 0 ), threshold voltage (V T ), and mobility (µ) [18,19]. This multi-parameter sensing capability enables OFET temperature sensors to distinguish temperature-induced parameter changes from pressure, light, chemical substances, and other stimuli-induced changes, providing high selectivity and reliability of multi-functional sensing [21,22].
Organic materials are composed of molecules as the basic building blocks, which are made of carbon, oxygen, hydrogen, nitrogen, and other elements. The constituent atoms in an organic molecule are strongly held with covalent bonds between atoms by sharing the valence electrons. Therefore, most organic materials are insulators because valence electrons are highly localized. A characteristic of organic semiconductors is that their skeleton has an extended π-conjugated system, usually composed of alternating C-C and C=C bonds. The overlapping (or hybridization) of the molecular orbitals forms an extended highest occupied molecular orbital (HOMO) and a lowest unoccupied molecular orbital (LUMO), where the injected holes (in HOMO) and electrons (in LUMO) can move relatively easily. As the molecular size increases, such as in the case of high molecular weight polymers, long-range π conjugation results in a band-like structure with a small band gap (<3 eV) between HOMO and LUMO, similar to typical inorganic semiconductors. However, organic molecules are held together by weak van der Waals forces or London dispersion forces to form a solid, so the intermolecular distance is quite large, usually 0.3-0.4 nm. This makes intermolecular charge transport more difficult than intramolecular charge transport. Organic molecules are much larger than the spherical constituent atoms and ions in inorganic materials and have more complex geometric shapes. Therefore, most organic semiconductors are disordered (polycrystalline or amorphous), which makes charge transport in organic semiconductors even more difficult and highly dependent on the processing conditions of the material. As a result, the charge transport mechanism in organic semiconductors is extremely complicated and has been a subject of extensive study since its discovery.
As shown in Figure 2, the charge carrier mobility of organic semiconductors may increase or decrease with increasing temperature, depending on the model, which results in a decrease in resistance (a negative temperature coefficient, NTC) or an increase in resistance (a positive temperature coefficient, PTC), respectively. Since I DS in the linear (Equation (1)) or saturated (Equation (2)) regime is a function of the carrier mobility of the organic semiconductor channel, it can be used as an output signal to detect the temperature change. MTR and VRH models, in which mobility increases with temperature, are most widely used in polycrystalline or amorphous organic semiconductors. Some highly crystalline small molecules [28][29][30][31] and polymers [32][33][34][35] have been found to follow a "band-like" Chemosensors 2022, 10, 12 4 of 20 transport model around room temperature, in which mobility decreases with temperature. The mobility of organic semiconductors is also field dependent [23,24], and the field-effect mobility is often strongly dependent on the gate voltage [27]. Consequently, the sensitivity of an OFET temperature sensor may also be influenced by the voltage applied.
(Equation (1)) or saturated (Equation (2)) regime is a function of the carrier mobility of the organic semiconductor channel, it can be used as an output signal to detect the temperature change. MTR and VRH models, in which mobility increases with temperature, are most widely used in polycrystalline or amorphous organic semiconductors. Some highly crystalline small molecules [28][29][30][31] and polymers [32][33][34][35] have been found to follow a "band-like" transport model around room temperature, in which mobility decreases with temperature. The mobility of organic semiconductors is also field dependent [23,24], and the field-effect mobility is often strongly dependent on the gate voltage [27]. Consequently, the sensitivity of an OFET temperature sensor may also be influenced by the voltage applied. Deep charge carrier traps in the organic semiconductor or the dielectric layer near their interface must be filled before a significant current flows in the channel; therefore, a larger number of deep traps leads to an increase in the threshold voltage (VT) and the onset voltage (V0) [18,19,37]. As temperature increases, some deep traps become shallow traps, resulting in a decrease in VT and V0 [36,37]. Therefore, VT or V0 can be directly used as a parameter to detect the temperature change. As shown in Equations (1) and (2) and Figure  1, IDS in the subthreshold region is also significantly dependent on the threshold voltage VT and hence on the temperature. Deep charge carrier traps in the organic semiconductor or the dielectric layer near their interface must be filled before a significant current flows in the channel; therefore, a larger number of deep traps leads to an increase in the threshold voltage (V T ) and the onset voltage (V 0 ) [18,19,37]. As temperature increases, some deep traps become shallow traps, resulting in a decrease in V T and V 0 [36,37]. Therefore, V T or V 0 can be directly used as a parameter to detect the temperature change. As shown in Equations (1) and (2) and Figure 1, I DS in the subthreshold region is also significantly dependent on the threshold voltage V T and hence on the temperature.
I DS is proportional to the capacitance (C i ) of the dielectric layer (Equations (1) and (2)), while C i is proportional to the relative permittivity or dielectric constant (k) of the dielectric material: C i = kC 0 , where C 0 is the vacuum capacitance. The properties of the dielectric layer material, such as dielectric constant and surface trap density, also have a profound impact on I DS , V T , and even mobility [38]. Therefore, a dielectric material whose properties change dramatically with temperature can induce significant changes in OFET parameters and thus can be used as a sensor element.
Instead of using an OFET component as the sensing element, a separate temperaturesensing element can be integrated with OFET to form an OFET temperature sensor system. For instance, a thermistor is often connected to the gate electrode of the OFET to modulate the OFET I-V characteristics and achieve more sensitive and reliable temperature sensing. Similarly, a capacitor or diode unit can be connected to the OFET as the temperature sensing element to overcome the low sensitivity and complicated temperature response of the OFET active layer. Various representative OFET-based temperature sensors with different sensing elements are schematically shown in Figure 3.
IDS is proportional to the capacitance (Ci) of the dielectric layer (Equations (1) and (2)), while Ci is proportional to the relative permittivity or dielectric constant (k) of the dielectric material: Ci = kC0, where C0 is the vacuum capacitance. The properties of the dielectric layer material, such as dielectric constant and surface trap density, also have a profound impact on IDS, VT, and even mobility [38]. Therefore, a dielectric material whose properties change dramatically with temperature can induce significant changes in OFET parameters and thus can be used as a sensor element.
Instead of using an OFET component as the sensing element, a separate temperaturesensing element can be integrated with OFET to form an OFET temperature sensor system. For instance, a thermistor is often connected to the gate electrode of the OFET to modulate the OFET I-V characteristics and achieve more sensitive and reliable temperature sensing. Similarly, a capacitor or diode unit can be connected to the OFET as the temperature sensing element to overcome the low sensitivity and complicated temperature response of the OFET active layer. Various representative OFET-based temperature sensors with different sensing elements are schematically shown in Figure 3.  [40]; (c) separate diode (shows the diode direction of the p-channel OFET) [41]; (d) separate thermistor [42]; (e) separate capacitor [43].
This review aims to introduce and analyze representative studies on OFET temperature sensing devices in terms of materials, device design, and sensing performance. We discuss small-molecule and polymer semiconductors separately, the structures of which are shown in Figure 4. We have classified the device structures by the sensing elements, OFET active layer, dielectric layer, separate thermistor, and other types of sensing elements. Finally, we summarize our findings and provide future prospects on OFET-based temperature sensors. The purpose of this article is to provide readers with a better understanding of the status quo and the challenges that must be overcome before OFET-based temperature sensors can become a viable sensor technology. (b) dielectric layer [40]; (c) separate diode (shows the diode direction of the p-channel OFET) [41]; (d) separate thermistor [42]; (e) separate capacitor [43].
This review aims to introduce and analyze representative studies on OFET temperature sensing devices in terms of materials, device design, and sensing performance. We discuss small-molecule and polymer semiconductors separately, the structures of which are shown in Figure 4. We have classified the device structures by the sensing elements, OFET active layer, dielectric layer, separate thermistor, and other types of sensing elements. Finally, we summarize our findings and provide future prospects on OFET-based temperature sensors. The purpose of this article is to provide readers with a better understanding of the status quo and the challenges that must be overcome before OFET-based temperature sensors can become a viable sensor technology.

Small-Molecule Semiconductor-Based OFET Temperature Sensors
Organic small-molecule semiconductors have the advantages of straightforward synthesis, higher purity, and higher charge carrier mobility compared to their polymer counterparts [44][45][46][47]. OFETs based on small-molecule semiconductors have been extensively studied as sensors for various applications such as gas sensor, chemical sensors, biosensors, pressure sensors, etc. [48][49][50]. However, there is a scarcity of research publications on small-molecule-based OFET temperature sensors, presumably due to some of the chal-

Small-Molecule Semiconductor-Based OFET Temperature Sensors
Organic small-molecule semiconductors have the advantages of straightforward synthesis, higher purity, and higher charge carrier mobility compared to their polymer counterparts [44][45][46][47]. OFETs based on small-molecule semiconductors have been extensively studied as sensors for various applications such as gas sensor, chemical sensors, biosensors, pressure sensors, etc. [48][49][50]. However, there is a scarcity of research publications on smallmolecule-based OFET temperature sensors, presumably due to some of the challenges that will be discussed in this review. Nonetheless, promising results have been obtained through efforts made in the improved materials processing and device design. This section will introduce and discuss small-molecule OFET temperature sensors based on the classification of temperature sensing elements: organic semiconductor channel layer, dielectric layer, separate thermistor, and other types.

OFET Semiconductor Channel as Sensing Element
Pentacene is one of the most commonly used semiconducting layers in OFET devices for various applications [51][52][53][54]. Jung et al. report an OTFT temperature sensor using pentacene as the semiconducting layer on a flexible polyethylene naphthalate (PEN) film for administering point-of-care diagnoses [39]. A 50 nm pentacene film was deposited on top of a polyvinylphenol (PVP) gate dielectric. When the device is measured between 0 and 80 • C, the I DS increases with increasing temperature. This I DS modulation is mainly due to the positive shift of the V T , so the most significant increase in current is observed in the subthreshold region with a linear increase from~10 −10 A at 0 The same group observed a similar linear I DS -T dependence in the subthreshold region within the range of 0 to 180 • C for a temperature sensor based on pentacene OTFT on a SiO 2 /Si substrate protected by a polyvinyl alcohol (PVA) mask [55]. I DS in the subthreshold region increases by almost four orders of magnitude with an increase in temperature from 0 to 180 • C. However, a lower V T is required to achieve better sensitivity at the expense of I DS -T linearity. It was found that µ sat in the saturation region increases with temperature in the relatively low temperature range of 0 to 60 • C due to the thermally activated hopping transport, which agrees with the trend in the similar temperature range (−13 to 67 • C) observed by Kawakami et al. [56]. The activation energy (E a ) obtained with an Arrhenius fit is 66 meV, indicating a weak dependence of µ on temperature. In the higher temperature range of 60 to 180 • C, µ decreases due to the dominance of carrier scattering at elevated temperatures. As a result, I DS,sat increases slightly with temperature in the range of 0 to 180 • C. The dramatic increase in subthreshold current (I DS,sub ) with increasing temperature is mainly due the positive shift of V T . However, V T is affected by the charge traps at the dielectric/semiconductor interface, bias stress, moisture, oxygen, morphology of the semiconductor layer, etc., so it is difficult to use the I DS,sub as a sensor output in practical applications. Therefore, using the more reliable I DS in the saturation region is preferred. However, as previously mentioned, the I DS in the saturation region has a weak dependence on temperature, making it challenging to use as the sensor output.
Copper phthalocyamine (CuPc) and fluorinated copper phthalocyanine (F 16 CuPc) are widely studied p-type and n-type small-molecule semiconductors for OFETs because of their good carrier mobility and environmental stability. CuPc and F 16 CuPc exhibit a weak dependence of µ sat on temperature around room temperature, each having an E a of 73 and 39 meV [57,58], respectively, which are similar to that of pentacene [55,56]. Therefore, I DS in the saturation region of OFETs based on these materials would also be weakly dependent on temperature and not suitable as the signal output of temperature sensors around room temperature. Consequently, Boileau et al. used the V T response to the temperature of OFETs based on CuPc and F 16 -CuPC for DNA sensing [59]. The goal was to design an accurate temperature sensor working in the optimal temperature range (40 to 70 • C) where DNA binding occurs. The authors observed that the hole and electron mobilities (µ h and µ e ) of CuPc and F 16 CuPc increase with temperature with coefficients of 1%/ • C and 0.1%/ • C, respectively, in the temperature range of 25 to 90 • C. On the other hand, the V T changes Chemosensors 2022, 10, 12 7 of 20 significantly from −7.6 V to −1.8 V with a change rate of 0.11 V/ • C for CuPc. For the n-type F 16 CuPc-based OFET, the V T changes to the negative direction from −9.9 V to −26.4 V with a charge rate of −0.25 V/ • C, indicating its better sensitivity than the CuPc device. The opposite V T shift directions and different sensitivities observed for the two semiconductors may be due to their different carriers (holes and electrons) associated with different carrier traps at the interface between the semiconductor and the dielectric.
To improve the temperature sensitivity of F 16 CuPc-based OFETs, Ye et al. developed an OFET device using a pn heterojunction layer of α-sexithiophene (α6T)/F 16 CuPC [60]. Specifically, a layer of p-type semiconductor α6T was deposited on top of the F 16 CuPC semiconducting layer and the source/drain electrodes ( Figure 5). It has been reported that the use of a pn heterojunction active channel layer in OFETs can realize ambipolar charge transport characteristics and/or improve charge carrier mobility [61][62][63][64][65]. In particular, the V T is a function of the thickness of the top layer in the pn heterojunction [66], which may allow the use of V T as the sensitivity parameter of the OFET temperature sensor in a more controllable manner. The authors found that when the thickness of the α6T layer increased from 0 nm to~5-7 nm, V T changed significantly from 23.13 V to 18.10 V, and then stabilized. In the linear regime (|V DS | < |V GS − V T |), the OFET with α6T (20 nm)/F 16 CuPC (50 nm) displayed a linear log(I DS )-versus-T relationship: log(I D ) = kT + c 0 in the temperature range of 100 to 300 K (Figure 5a), which is gate voltage dependent. The slope k increases with V GS first, reaching the maximum value of 0.11 dec/ • C at V GS = 5 V (most sensitive), and then drops rapidly with further increasing V GS ( Figure 5b). As shown in Figure 5c, µ follows a thermally activated Arrhenius behaviour (µ ∝ exp(−E a /k B T)) with two distinct temperature ranges. The activation energy E a is 40.1 meV above 200 K and drops to 16.3 meV in the lower temperature range of 100 to 200 K. The device also showed linear temperature dependence of V T with different slopes in the two temperature ranges (Figure 5d). In the range of 100 to 200 K, the sensitivity (∆V T /∆T) is −0.90 V/ • C, and between 200 K and 300 K, the sensitivity is −0.185 V/ • C. These two regions are attributed to two different mechanisms dominating in each region. The charge transport in the lower temperature range is dominated by the MTR mechanism, while the charge transport in the higher temperature range is dominated by the thermal enhancement of the hopping transport or VRH. These sensitivities are higher than those of a previously reported OFET with a similar structure with only F 16 CuPC (−0.02 V/ • C) [58], indicating the beneficial effect of using a pn heterojunction structure to improve the sensitivity of the sensor.
An organic-inorganic hybrid perovskite-based ambipolar FET temperature sensor was reported by Haque et al. recently [67]. The active MAPbI 3 layer is fabricated using a stoichiometric mixture of powdered methylammonium iodide (MAI) and lead iodide (PbI 2 ). The OTFT exhibited excellent air stability without the need for an encapsulation layer. The charge transport obeys the MTR mechanism where traps, mostly localized in the grain boundaries, have energy levels close to the conduction band, and the trapped electrons can be easily released into the conduction band by thermal energy. The temperature dependence of V T was used as the sensitivity parameter for the device. The V T shows a linear relationship with temperature in the range of 29 to 45 • C with a temperature coefficient value of 200 mV/ • C, which is higher than the values reported for inorganic semiconductor-based FET temperature sensors. It was found that maintaining a stoichiometric ratio of MAI and PbI 2 is critical to achieving a high sensitivity. The devices with the PbI 2 -rich and MAI-rich films exhibited much lower temperature coefficients of 12 and 100 mV/ • C, respectively, since unreacted PbI 2 and MAI can reduce carrier traps. ature dependence of VT was used as the sensitivity parameter for the device. The VT shows a linear relationship with temperature in the range of 29 to 45 °C with a temperature coefficient value of 200 mV/°C, which is higher than the values reported for inorganic semiconductor-based FET temperature sensors. It was found that maintaining a stoichiometric ratio of MAI and PbI2 is critical to achieving a high sensitivity. The devices with the PbI2-rich and MAI-rich films exhibited much lower temperature coefficients of 12 and 100 mV/°C, respectively, since unreacted PbI2 and MAI can reduce carrier traps.

OFET Dielectric Layer as Sensing Element
Insulating polymers are often used as dielectric materials in OFETs due to their compatibility with flexible substrates and solution processibility via spin coating, casting, and printing under ambient conditions [68]. Moreover, as with the polymer semiconductors, the structure and functionality of the polymer dielectric can be tuned to meet the requirements of the device. In particular, the properties of some polymer dielectrics are more temperaturedependent than organic semiconductors, so they can be used as sensing elements for OFET temperature sensors.
The 2015 paper by Wu et al. [40] aims to solve the issue with OFET sensor sensitivity at higher temperatures. A common issue with OFET temperature sensors is that above~300 K the devices show limited thermal sensitivity. This design incorporates a thermoresponsive polar polymer into the dielectric layer of the DNTT-based OFET, which induces interfacial interactions to facilitate charge transport at high temperatures. Polylactide (PLA) was used as the polar material that serves both as a flexible substrate and as a gate dielectric for the device with both n-and p-type small-molecule semiconducting active layers. Specifically, three-arm stereocomplex PLA (tascPLA) was synthesized from the ring-opening polymerization of D-lactide and used for these devices. The MTR mechanism can describe the role that the dielectric plays in temperature sensing. At the interface, PLA induces multiple charge carrier traps with varying energy levels, which decreases the carrier concentration. As the temperature increases, the thermal energy can release carriers from these traps and thus modulate the I DS response. The DNTT-tascPLA-OFET shows a much stronger response in I DS with temperature in the range of 25 to 150 • C compared to the device using SiO 2 as the dielectric (Figure 6a,b), indicating the high thermal sensitivity of PLA. The device showed excellent cycling stability at 100 • C (Figure 6c,d). Mobility and V T also show high sensitivities to temperature (Figure 6e,f). Subbarao et al. used CuPc as the semiconducting layer in conjunction with a tri-layer dielectric system where the polar PVA layer is deposited between a layer of poly(methyl methacrylate) (PMMA) and a layer of Al2O3 [69]. The sensor results displayed two distinct linear trends in the ranges of 240 to 300 K and 300 to 370 K with sensitivities of 0.45 nA/°C and 8 nA/°C, respectively. Below 300 K it can be assumed that the VRH mechanism dominates over the MTR mechanism, while above 300 K the opposite occurs. The device showed quite fast response and recovery times of 25 and 15 s, respectively. The response and recovery times included the time to mechanically switch between temperatures, and the actual sensor response and recovery times are expected to be much faster [69]. Subbarao et al. used CuPc as the semiconducting layer in conjunction with a tri-layer dielectric system where the polar PVA layer is deposited between a layer of poly(methyl methacrylate) (PMMA) and a layer of Al 2 O 3 [69]. The sensor results displayed two distinct linear trends in the ranges of 240 to 300 K and 300 to 370 K with sensitivities of 0.45 nA/ • C and 8 nA/ • C, respectively. Below 300 K it can be assumed that the VRH mechanism dominates over the MTR mechanism, while above 300 K the opposite occurs. The device showed quite fast response and recovery times of 25 and 15 s, respectively. The response and recovery times included the time to mechanically switch between temperatures, and the actual sensor response and recovery times are expected to be much faster [69].
Mandal et al. incorporated hexagonal barium titanate nanocrystals (h-BTNCs) as a dielectric film and pentacene as the channel layer to fabricate ultrafast, flexible OFET temperature sensor devices [70]. The perovskite BaTiO 3 is a ferroelectric material with its ferroelectric property originated from the crystal structure distortion upon temperature change. The BaTiO 3 film synthesized by the traditional high-temperature process is very rough, which will have a negative impact on the current at the semiconductor/dielectric interface. Furthermore, the perovskite BaTiO 3 only shows ferroelectric properties at low temperatures. To circumvent these issues, the authors developed a low-temperature (~60 • C) process to prepare h-BTNCs. Thin films of h-BTNCs with very smooth surfaces can be obtained simply by spin coating. A series of OFETs with a bilayer dielectric of h-BTNC (55 nm) on Al 2 O 3 (15 nm), a pentacene semiconductor layer, Cu source, and drain electrodes were fabricated on a thin Al-coated poly(ethylene terephthalate) (PET) substrate (10 µm), where the Al layer is the gate electrode. The devices were encapsulated with a PDMS layer. The OFETs had a low operating voltage of 1.5 V and achieved a high mobility of 1.46 cm 2 /(V·s). When the device was used as a temperature sensor, it showed a linear dependence of I DS on temperature with a slope (sensitivity) of 20 nA/ • C between 27 and 45 • C. The greatly enhanced sensitivity is mainly due to the use of the h-BTNC layer since the contributions of pentacene, and other layers are very small. The flexibility of the device was also evaluated, and the sensor maintained a stable response with a minimal level of hysteresis in the temperature range when subjected to bending with a radius of 4 mm. The device was used to monitor the temperature changes of the air inhaled and exhaled through the nose a few cm from the device. Rapid response and recovery times of 24 and 51 ms, respectively, were achieved.
Rullyani et al. reported the use of poly(N-isopropylacrylamide) (PNIPAM) as a thermoresponsive polymer dielectric for OFET sensors [71]. The PNIPAM layer undergoes reversible phase transitions above its lower critical solution temperature of 32 • C. These transitions enhance the charge transport in the semiconducting layer (pentacene in this case) due to an improved semiconductor/dielectric interface with increasing temperature. The sensor was evaluated between 30 and 45 • C with a sensitivity of 2.58 µA/ • C with the I DS being on the order of microamperes. The device is suitable for sensing the human body temperatures.

Thermistor as Sensing Element
As aforementioned, I DS is a preferred parameter as an output signal of OFET temperature sensors because of its high reliability, but its temperature dependence is weak. On the other hand, V T has strong temperature dependence, but it is affected by numerous factors in the fabrication process and the device operating environment. To solve this issue, thermistors, whose resistance changes notably with temperature, have been integrated with OFETs to make temperature sensors with a high sensitivity.
Ren et al. [42] integrated a thermistor with an OFET to improve the dependence of I DS on temperature (Figure 7), and used dynamic range (DR) to present the sensitivity of the device. The thermistor consists of pentacene film embedded with Ag nanoparticles and two electrodes. One electrode of the thermistor is connected to the gate of the pentacene-based OTFT, so the gate bias of the OFET can be modulated by the resistance of the thermistor. Ag nanoparticles act as charge traps, so the conductivity activation energy of the thermistor film is increased to 361 meV compared with pure pentacene (157 meV). This results in a stronger dependence of the conductivity on the temperature of the thermistor film. two electrodes. One electrode of the thermistor is connected to the gate of the pentacenebased OTFT, so the gate bias of the OFET can be modulated by the resistance of the thermistor. Ag nanoparticles act as charge traps, so the conductivity activation energy of the thermistor film is increased to 361 meV compared with pure pentacene (157 meV). This results in a stronger dependence of the conductivity on the temperature of the thermistor film. The integrated temperature sensor device shows almost three orders of magnitude increase in I DS with temperature when measured between 15 and 70 • C with a measurement step of 5 • C. The sensitivity of the temperature sensor is represented by the DR, which is the ratio between the maximum and minimum changes of I DS at a specific temperature (T) with reference to the lowest temperature (15 • C): DR = ∆I T,max /∆I T,min , where ∆I T = I T − I 15 . A higher DR implies a more sensitive device. This device had a DR of 10 bits (10 2 ), which is the highest value reported among organic temperature sensors. The sensitivity of the device deteriorates rapidly above 70 • C, which may be due to the evaporation of water absorbed by the polyvinylpyrrolidone (PVPy) dielectric layer at high temperatures, resulting in a decrease in capacitance.
The same research group reported a flexible low operating power 16 × 16 activematrix OFET temperature sensor array by adopting a similar device design [72]. A highperformance p-type semiconductor dinaphtho[2,3-b:2 ,3 -f ]thieno[3,2-b]thiophene (DNTT) was used as the active layer in the OFET device. An Al 2 O 3 dielectric layer was used to decrease leakage current down to~50 pA. A thermistor consisting of an Ag nanoparticlesdoped pentacene film active layer was connected to the DNTT-OFET. The array fabricated on an ultrathin (12 µm) PET substrate can be operated at low voltages below 4 V. In the temperature range between 20 and 100 • C, the array achieved a temperature resolution of 0.4 • C and a TCR of −4.4%/ • C. The device maintained its integrity after 10,000 bending cycles, and the array was able to provide excellent 2D spatial resolution. The superb flexibility and durability of the sensor array make it suitable for conformal temperature measurement of objects with irregular and varied surface geometry such as flexible electronic products and human bodies.
Temperature and pressure sensor arrays are of great interest because of their potential for artificial skin applications with spatial resolution. Yokota et al. copolymerized butyl acrylate with octadecyl acrylate P (BA-OA) and added 25% graphite filler particles to fabricate positive temperature coefficient (PTC) thermistor sensors [73]. The PTC sensors were integrated with OFETs to create a 12 × 12 active matrix sensor array on a polyimide substrate, where an anodized aluminum oxide layer modified by a phosphonic acid selfassembled monolayer (SAM) was used as the gate dielectric, and a 30 nm DNTT layer was used as the semiconductor later in OFETs. This ultra-flexible sensor array was able to accurately measure temperature shifts after being placed between two lobes of a live rat lung, where small temperature fluctuations occur due to respiration. The array performed well between 29.8 and 37.0 • C with minimal hysteresis (1 • C) and a spatial resolution of 5 mm. Furthermore, the device had a rapid response time of 100 ms and remained stable for 1800 thermal cycles within the operating temperature range.
A recent work by Ishaku and Gleskova reported a temperature sensor with a DNTT-OFET connected to a commercially available thermistor with a negative temperature coefficient (NTC) [74]. The OFET included a dielectric of aluminum oxide (AlO x ) with an octadecylphosphonic acid (C 18 PA) SAM for low voltage operation. Previous works have reported using C 18 PA to improve the µ sat of charge carriers in OTFTs by inducing edge-on orientation of the semiconducting layer [75]. The integrated OFET-sensor was evaluated between 20 and 50 • C with the thermistor modulating the gate voltage. A low operational voltage of −7 V and a TCR of −2.44%/ • C were achieved. Nakayama et al. reported two types of organic temperature sensing devices using a combination of p-and n-type semiconductors to fabricate analog-to-digital converter (ADC) OFETs [76]. The organic semiconductor layers of 3,11-didecyldinaphtho[2,3-d:2 ,3d']benzo[1,2-b:4,5-b']dithiophene (C10-DNBDT) and GSID 104031-1 (BASF SE) for p-and n-type OFETs, respectively, were solution processed. A unit consisting of a complementary metal oxide semiconductor (CMOS) comparator and an inverter made of the p-and nchannel OFET devices is connected to a temperature sensor unit composed of two PEDOT: PSS resistors (TCR of −0.87%/ • C) and two Cr/Au resistors (TCR: 0.064%/ • C) to form a temperature detector. The temperature detector can produce a binary digital (1-bit) readout depending on whether the temperature was above a designated threshold value (T th ). The authors further fabricated a 2-bit parallel ADC composed of three comparators and one temperature sensor unit for more detailed temperature detection. The 2-bit ADC can have four output combinations, (0,0,0), (0,0,1), (0,1,1), (1,1,1), corresponding to the temperature ranges of <33 • C, 33-50 • C, 50-67 • C, and >67 • C, respectively. Such digital signals can be easily read with common radio-frequency identification (RFID) readers.

Other Types of Sensing Elements
Someya et al. reported a large-area network of temperature and pressure sensors on a flexible plastic substrate using organic semiconductors [41]. The temperature sensor cell consists of an organic diode as the temperature sensing element and an OFET. The OFET device is composed of a pentacene semiconductor layer, a polyimide dielectric layer, and Au source, drain, and gate electrodes. The organic diode has a structure of PET/ITO/CuPc (30 nm)/PTCDI (50 nm)/Au, where PTCDI (3,4,9,10-perylenetetracarboxylic diimide) is an n-type semiconductor, forming a pn heterojunction with the p-type CuPc. The pn heterojunction layer has a much stronger temperature dependence than CuPc or PTCDI alone. Therefore, this organic diode functions as a sensing element. The magnitude of I DS of the temperature sensor cell increases markedly from~0.4 µA to~1.4 µA in the range of 30 to 80 • C or~20 nA/ • C.
Özgün et al. reported a temperature-responsive array made by integrating pairs of organic semiconductor-based switching diodes and OFETs on a silicon-on-insulator (SOI) wafer, where pentacene was used as the semiconductor in both components [77]. When two OFETs with different sizes are driven, the voltage drop on their diodes is measured at different temperatures. The difference in voltage drop between two diodes is plotted against temperature between 27.2 and 59.0 • C. Below 35 • C, the data shows large deviations, while the rest of the data from 35.0 to 59.0 • C shows a linear trend with a sensitivity of 16.3 mV/ • C. The device fabrication is relatively simple and compatible with common microfabrication processes.
Developing polymer composites is a method to improve electrical properties of polymer materials. The interactions between the polymer and the organic/inorganic material in the polymer matrix can lead to higher sensitivity and better overall performance in sensors [78]. Polymer materials can also be used as a matrix to embed active materials to take advantage of polymer properties, such as processability and stability, for organic electronic applications. In 2009, Graz et al. [79] incorporated lead titanate (PbTiO 3 ) nanoparticles into the matrix of ferroelectric P (VDF-TrFE). A two-step polarization process is used to polarize the foil made of this composite material, so that some areas of the PbTO 3 nanoparticles and the polymer matrix have parallel polarization directions, while other areas have anti-parallel polarization directions. Areas with parallel polarization directions are pyroelectric, while areas with anti-parallel polarization directions are piezoelectric. The composite foil was laminated to amorphous silicon (a-Si) TFTs on their polyimide gate dielectric substrate in the adjacent parallel and anti-parallel polarization directions, respectively. The I DS of a TFT is modulated pyro-or piezoelectrically depending on the polarization directions of the composite foil area. The device significantly reduced cross-sensitivity between the piezoelectric and pyroelectric effects and displayed a linear relationship correlated to temperature.
Sensor devices can be connected to one of the electrodes of an OFET and their response to the target analyte can be used to modulate a characteristic of the OFET device that changes the output signal of the OFET. In 2014, Cosseddu et al. introduced a temperature sensor based on an organic charge modulated field effect transistor (OCMFET) using the pyroelectric poly(vinylidene difluoride) (PVDF) [43]. The polymer film forms the dielectric layer of a parallel plate capacitor, which is connected to the floating gate of an OFET device. Temperature modulations affect the charge separation in the PVDF layer, which modulates the carrier concentration that directly correlates the I DS response of the sensor. Calibration curves of ∆I DS -T for this temperature sensor were established within a working range of 10 to 42 • C. Table 1 summarizes the OFET-based temperature sensors using the small-molecule materials discussed in this section organized by sensing the elements and reporting the OFET channel material, working range, output signal, and the reported sensitivity of each device.
Polythiophenes are among the earliest and most-studied polymer semiconductors for OFETs [86][87][88][89]. In 1993, Ohmori et al. fabricated Schottky gated OFET devices using poly(3-alkylthiophene)s with different alkyl side chains as the semiconducting layer for temperature sensors [90]. Poly(3-ethylthiophene) (P3ET) demonstrated the best response to temperature changes in the range of 14 to 75 • C when measured in the saturation region. The charge carrier mobilities were evaluated to be 0.0004 cm 2 /V and 0.006 cm 2 /V at the high and low temperatures of 14 • C and 75 • C, respectively. This confirmed that the charge transport of P3ET obeys the VRH theory, where a higher temperature augments the probability of hopping transport events, leading to higher carrier mobility. Song et al. reported the use of a poly(3-hexylthiophene) (P3HT)-based OFET as a temperature sensor [91]. The P3HT film was deposited on top of a PMMA dielectric layer with a bottom ITO gate electrode. The I DS increases linearly with temperature in the range between 25 and 100 • C. The authors used the hole mobility temperature coefficient ∆µ h /∆T = 2.34 × 10 −5 cm 2 /V·s· • C to assess the thermal sensitivity of the device. In a 2019 study, Zhu et al. used crosslinked polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) as the dielectric material and PDPPFT4 or poly(isoindigo-bithiophene) (PII2T) as the semiconductor channel layer in OFETs [92]. The devices showed a linear relationship in the response of I DS to temperature changes between 25 and 55 • C. Devices with PDPPFT4 and PII2T showed TCRs of −2.89%/ • C and −4.23%/ • C, respectively. The hysteresis effect was negligible for both sensors due to the use of the non-polar gate dielectric. The sensors remained stable after 50 cycles under a 30% uniaxial strain, indicating their excellent stretchability.
The 2017 work from Zhao et al. developed a thermoelectric-gated transistor using P3HT as the active layer for temperature sensing [93]. The architecture of the electrolytegated transistor (EGT) is shown in Figure 8a. Poly(vinylphosphonic acid-co-acrylic acid) P(VPA-AA) was used as a polyanionic electrolyte insulating layer. An applied negative gate bias induces charge separation, forming two electrical double layers (EDLs) (or Helmholtz layers), where positive protons accumulated at the electrolyte/gate interface, while the negative anions accumulated at the semiconductor/electrolyte interface. The two EDLs are separated by only a few angstroms, resulting in a large capacitance. This device is then connected to the working electrode of an ionic thermoelectric supercapacitor (ITESC) containing an NaOH-containing poly(ethyleneoxide) (PEO-NaOH) electrolyte (Figure 8b). A temperature gradient (∆T) between the two electrodes of the ITESC causes ions to diffuse to the opposite poles, creating an open voltage (V thermal ) between the two electrodes. The V thermal is used to bias the gate of the transistor, and thus the I DS response is directly dependant on ∆T. The thermal transconductance (g thermal ) is defined for the ionic thermoelectric-gated transistor as g thermal = ∆I DS /S∆T, where ∆I DS is the change in I DS caused by the temperature change ∆T, and S is the ionic Seebeck coefficient of PEO-NaOH. Therefore, g thermal can represent the thermal sensitivity of the device. The ionic Seebeck coefficient (S) of PEO-NaOH is found to be as high as 11 mV/ • C, which is much higher than that of traditional thermoelectric materials (~100 µV/ • C). Hence, higher sensitivity can be achieved for this thermoelectric-gated organic transistor compared to the devices using traditional thermoelectric materials. Interestingly, the authors found that the type of gate material used affects the transfer characteristics of the transistor. When a high work function metal Cu (4.3 eV) is used as the gate, it is easier to turn on the transistor than the device using a low-work function metal Ti (3.8 eV) as the gate. Consequently, the Cu-gated transistor can detect a small ∆T as low as 1 • C, while the Ti-gated transistor requires a minimum ∆T of 3.8 • C to obtain an appreciable I DS . This device displayed negligible hysteresis and fast response time (25-50 s) in the working range of 15 to 45 • C. It should be noted that the response time is rather limited by the response of the thermoelectric component, and not by the ionic thermoelectric-gated transistor. A simple resistor-load heat-gated invertor was fabricated, which can be switched around ∆T = 0 with only 20 • C variation. A maximum gain of eight was achieved. This is the first demonstration that heat signal can be used as the input for logic circuits.
Chemosensors 2022, 10, x FOR PEER REVIEW 16 of 21 Table 2 summarizes the polymer semiconductor-based OFET temperature sensors discussed in this section.

Conclusions and Outlook
Due to the high demand for a large amount of flexible, thin, lightweight, and ultralow-cost sensors for applications in wearable electronics, Internet of Things, smart  Table 2 summarizes the polymer semiconductor-based OFET temperature sensors discussed in this section.

Conclusions and Outlook
Due to the high demand for a large amount of flexible, thin, lightweight, and ultralowcost sensors for applications in wearable electronics, Internet of Things, smart packaging, advanced healthcare, etc., there is growing interest in the development of sensors using organic semiconductor materials. OFET-based sensors have attracted special attention due to their ability to process on flexible substrates, for seamless integration with other components of the electronic device, and to reduce cross-sensitivity and achieve multidetection. This review presented the progress made in the development of OFET-based temperature sensors, which have the broadest and most important applications among all types of sensors. We divided the OFET-based sensors by the organic semiconductor channel materials into two groups: small molecules and polymers. The devices are further classified by the sensing element: OFET semiconductor channel, OFET dielectric layer, separate thermistor, and other types. Notable advances have been made in the development of OFET-based temperature sensors, which include the demonstration of high sensitivity (e.g., TCR > 2%/ • C), excellent mechanical flexibility and stability, elimination of crosssensitivity, and successful integration of OFET temperature sensors with other electronic devices. These developments show the potential of OFET sensors in the above-mentioned emerging applications.
Nevertheless, the authors found that compared to the abundance of organic semiconductor materials developed for other organic electronics and the large number of studies on temperature-variable charge transports of organic semiconductors, research on OFETs as temperature sensors is significantly less. This may be because the applications of these sensors have only emerged recently, but the complicated and largely material-and devicedependent charge transport mechanism for organic semiconductors appears to be a more important factor. Specifically, the use of V T , I DS , or other parameters in the linear regime of the OFET as the sensor readout can provide high sensitivity. However, these parameters are extremely sensitive to the trap density in the channel material and dielectric and at their interface, which changes significantly with tiny changes in processing conditions and the environment. The I DS in the saturation regime is a more robust output signal, but its temperature dependence is usually too weak for most organic semiconductors.
Since few organic semiconductors have been studied for OFET temperature sensors, further efforts are required to investigate more organic semiconductors in order to gain a better understanding of the relationship between the chemical structure and the temperature dependency of OFET parameters. Furthermore, material processing and device fabrication for OFET temperature sensors need to be more rigorously controlled and optimized compared to other types of applications. In addition, it is necessary to specifically design new materials with the required characteristics of OFET temperature sensors, such as high sensitivity, easy control of film quality, and less susceptibility to other stimuli in the environment. In particular, the above-mentioned temperature sensors are typically used around the room temperature or near-body temperature (0 to 50 • C). However, most of the known organic semiconductors undergo drastic changes in the charge transport mechanism, causing severe attenuation of the temperature dependence of the conductivity in this temperature range. Innovative design of organic semiconductor materials is needed to solve this problem.
Using the dielectric layer as the sensing element is an effective way to increase the sensitivity and overcome some of the issues with the devices using the channel-sensing layer. However, a high-quality and stable semiconductor channel layer is still needed to make full use of this approach. Many studies have used a separate sensing element, such as a thermistor or a capacitor, to couple to the OFET unit, which is probably the easiest and fastest way to develop OFET-based temperature sensor products. However, the temperature sensing of this type of devices is based on the two terminal sensing elements and, therefore, the devices are less intelligent than the three-terminal OFET sensors using the channel or the dielectric layer as the sensor element. For instance, they may have the cross-sensitivity issue.
Overall, there have been remarkable advances in the development of OFET temperature sensors, which gives an optimistic outlook on the manufacture of flexible and inexpensive OFET temperature sensors for practical use.

Conflicts of Interest:
The authors declare no conflict of interest.