Organic Thin Film Transistors (OTFTs) offer a promising technology for low-cost large-area electronic applications such as active-matrix displays, electronic papers, flexible microelectronics and physical or chemical sensor arrays -. It has been known that temperature is an important physical parameter that is often measured and thermal sensors have a lot of application potentials. The need for reliable, inexpensive and harmless temperature sensors that can be operated simply is also a concern for electronic skins, electronic health monitoring, and detecting patients’ body temperatures. However, OTFT-based temperature sensing devices are very sensitive towards a small change in temperature and usually work at highly operating voltages . It is natural that employment of high-k dielectric is one of the most important strategies to reduce operating voltage of OTFT devices .
On the other hand, we have reported a kind of organic pn heterojunctions, in which high density of carriers formed a conduction channel at interface -. Ambipolar transport is dependence of the first active layer thickness (F16CuPc, fluorinated copper phthalocyanine) in an F16CuPc/CuPc (copper phthalocyanine) hetero- junction, and only n-channel operation is observed when F16CuPc films are over a critical thickness of ~12 nm. We have also investigated the temperature dependence of electrical properties of ambipolar OTFTs based on F16CuPc/α6T (sexithiophene) pn heterojunction, which hints that the pn heterojunction device has a potential use as a temperature sensor working at low operating voltages without dielectric engineering  . In this study, we report on temperature dependence of electrical properties of OTFTs based on F16CuPc/α6T pn heterojuction only working at n-channel and their applications in temperature sensors.
2. Experimental Details
The device configurations are shown in Figure 1. Frist, a top-contact F16CuPc TFT was fabricated. A heavily n-doped Si substrate acts as the gate electrode with a 300 nm thermally grown SiO2 layer (Ci ~ 10 nF/cm2) as the gate dielectric. F16CuPc thin films of 20 nm were vacuum-deposited and the substrate temperature was set at 120˚C. Au source and drain electrodes of approximately 50 nm were vacuum-deposited through a shadow mask with a channel width of 5 mm and a length of 70 µm. Then, α6T thin films were vacuum-deposited on the above-mentioned F16CuPc device at room temperature. The characteristics of the device were in-situ measured with a two-channel voltage current source/monitor system (R6245, ADVANTEST) at α6T film thicknesses of 0, 1, 2, 3, 5, 7 10, 15 and 20 nm, respectively, controlled by a shutter. All films were deposited under a base pressure of less than 1 × 10−3 Pa, and thicknesses and growth rates were monitored by a thickness and rate monitor (CRTM-6000, ULVAC). Temperature dependence of electrical characteristics of these devices were measured with R6245 in a cryostat (E202C5L, DAIKIN), which was temperature-controlled from 300 K to 100 K through a cryocooler using a He-gas flowing method.
3. Results and Discussion
Figure 2(a) shows output characteristics of a sandwich device with α6T thin films of 0 nm (namely, the top- contact F16CuPc TFT), which typically works in an n-channel operational mode. The linear and saturation regions can be observed with the increases of drain voltage (VD) and gate voltage (VG). Figure 2(b) shows the same for the sandwich device when α6T thin films of 20 nm were deposited. We observe a dramatic difference in the shape and magnitude of the drain current (ID). In the sandwich device ID is up to 15.2 μA at VG and VD of 60 V and does not saturate. Furthermore, a large bulk current in the sandwich device at VG = 0 V could also be observed, which originates from the charge carriers at the interface of F16CuPc/α6T -. Figure 2(c) shows characteristics of the sandwich device with various thicknesses of α6T thin films. Mobility and threshold voltage were extracted in the linear region as followed :
where W, L, and Ci are the channel width, channel length, and gate dielectric capacitance per unit area, respectively.
Figure 1. Schematic cross sections of (a) a single layer and (b) a sand- wich device (the dimensions are not scaled).
The dependence of and VT shift () on the thickness of α6T are shown in Figure 2(d). Obviously, undergoes a significant shift from +27.13 to +18.10 V with increasing the thickness of α6T. When the thickness of α6T is more than a critical thickness of 5 - 7 nm, undergoes hardly any shift. On the other hand, the values of is lightly decreased with increasing the thickness of α6T due to the effect of the bulk current. Thus the attributes to the increase of ID in the sandwich device. It has been known that the bulk current results the organic pn heterojunction interface dipolar, and the electrons and holes are free charges, which are significantly distinguished from ionized impurities, localized in the space charge region for convention inorganic pn junctions, and is a tan−1 function of film thickness (t) and expressed as following :
where and are the total charges density and the full width at half maximum, respectively. The blue dotted line in Figure 4(d) is fitted to Equation (2), yielding and of 3.2 nm and 6.84 × 10−8 C/cm2 (4.28 × 1011 electrons or holes/cm2), respectively. The value of is in complete agreement with half of the critical thickness of 5 - 7 nm.
Figure 3(a) and Figure 3(b) show transfer characteristics of another F16CuPc/α6T TFT in the linear regions at different temperatures. ID decrease as the temperature is lowered. Using Equation (1), and were evaluated from the local slopes of the transfer characteristics in the linear regions. As shown in Figure 3(c), the temperature dependence of mobility can be divided into two regions, and the mobility are clearly thermally activated with thermal activation energy, which was calculated by Arrhenius behavior as . At temperatures over 200 K, the value of is 40.1 meV, similar to that of the single layer device . At temperatures ranging from 100 to 200 K, we have a second regime with a much lower of 16.3 meV, where the charge transport is dominated by shallow traps. The temperature dependence of for the pn heterojunction device is given in Figure 3(d). At temperatures above 200 K, increases linearly with decreasing temperature. The variation of of 0.185 V/K is larger than the variation of (~0.02 V/K) in the single
Figure 2. Output characteristics of a sandwich F16CuPc/α6T TFT with α6T thin films of (a) 0 nm and (b) 20 nm, (c) transfer characteristics of the sandwich device with various thicknesses of α6T thin films in the linear regions, and (d) dependence of and on the thicknesses of α6T thin films, respectively.
Figure 3. (a) (b) Transfer characteristics of another F16CuPc/α6T TFT in the linear regions at different temperatures, temperature dependence of (c) mobility and (d) threshold voltage of the sandwich F16CuPc/α6T TFT, respectively.
layer device . This result is due to the interface dipolar charges . At temperatures ranging from 100 K to 200 K, we have a second regime with much lower variation of 0.090 V/K.
where k is a VG-dependent temperature variation factor,. Using Equation (3), k can be calculated for each VG, and the result is summarized and shown in Figure 4(b). Increasing VG, k increases, and then k decreases when VG > 5 V. The maximum value of k (~0.11 dec/K) could be obtained at VG = 5 V. On the other hand, ID at low VD and VG is comparable to that at high VD and VG unlike a single layer device. Thus the pn heterojunction device could be characterized as a temperature sensor working at low VD and VG.
Figure 5(a) shows electrical response of a sandwich F16CuPc/α6T TFT biased at VD = VG = 5 V to temperature cycles, and the measured output current shows over three times of magnitude increase from 8.91 × 10−8 to 2.83 ×10−7 A when the temperature increases from 250 to 300 K. As shown in Figure 5(b), the log(Is) linearly depends on temperatures, and k = 9.26 × 10−3 dec/K could be obtained when being fitted with Equation (3); this well corresponds to the above-mentioned data. This result shows that the pn heterojunction device may successfully be used as temperature sensing components.
In this paper, we report on temperature dependence of electrical properties of OTFTs based on an F16CuPc/α6T pn heterojunction and their applications in temperature sensors. The mobility follows a thermally activated hopping process. At temperatures over 200 K, the value of EA is 40. 1 meV, similar to that of the single layer device. At temperatures ranging from 100 to 200 K, we have a second regime with a much lower EA of 16.3 meV, where the charge transport is dominated by shallow traps. Similarly, at temperatures above 200 K, VT increases linearly
Figure 4. (a) VG dependence of log10 (ID) on the temperatures, and (b) dependence of k on VG.
Figure 5. (a) Electrical response of source current (IS) of a sandwich F16CuPc/α6T TFT to temperature cycles, and (b) log10(IS) temperature dependence of the sandwich device. The inset of Figure 5(a) shows the circuit diagrams of temperature sensor working at VD = VG = 5 V.
with decreasing temperature, and the variation of VT of 0.185 V/K is larger than the variation of VT (~0.02 V/K) in the single layer device. This result is due to the interface dipolar charges. At temperatures ranging from 100 K to 200 K, we have a second regime with much lower variation of 0.090 V/K. By studying VG dependence of log10(ID) on the temperatures, the maximum value of k (~0.11 dec/K) could be obtained at VG = 5 V. Furthermore, the pn heterojunction device could be characterized as a temperature sensor well working at low VD and VG.
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