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 [1]-[3]. 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 [4]. It is natural that employment of high-k dielectric is one of the most important strategies to reduce operating voltage of OTFT devices [3].

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 [5]-[10]. 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 [9] [10]. 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.

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 (C_i ~ 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.

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 (V_D) and gate voltage (V_G). 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 (I_D). In the sandwich device I_D is up to 15.2 μA at V_G and V_D of 60 V and does not saturate. Furthermore, a large bulk current in the sandwich device at V_G = 0 V could also be observed, which originates from the charge carriers at the interface of F₁₆CuPc/α6T [5]-[10]. 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 [11]:

where W, L, and C_i are the channel width, channel length, and gate dielectric capacitance per unit area, respectively.

The dependence of and V_T 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 I_D 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 [12]:

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. I_D 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 [9]. At temperatures over 200 K, the value of is 40.1 meV, similar to that of the single layer device [13]. 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

layer device [13]. This result is due to the interface dipolar charges [12]. At temperatures ranging from 100 K to 200 K, we have a second regime with much lower variation of 0.090 V/K.

Figure 4(a) shows plots of log (I_D) as a function of temperature at different V_G from Figure 3(b), which can be analyzed by Equation (3) as following:

where k is a V_G-dependent temperature variation factor,. Using Equation (3), k can be calculated for each V_G, and the result is summarized and shown in Figure 4(b). Increasing V_G, k increases, and then k decreases when V_G > 5 V. The maximum value of k (~0.11 dec/K) could be obtained at V_G = 5 V. On the other hand, I_D at low V_D and V_G is comparable to that at high V_D and V_G unlike a single layer device. Thus the pn heterojunction device could be characterized as a temperature sensor working at low V_D and V_G.

Figure 5(a) shows electrical response of a sandwich F₁₆CuPc/α6T TFT biased at V_D = V_G = 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(I_s) 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 E_A 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 E_A of 16.3 meV, where the charge transport is dominated by shallow traps. Similarly, at temperatures above 200 K, V_T increases linearly

with decreasing temperature, and the variation of V_T of 0.185 V/K is larger than the variation of V_T (~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 V_G dependence of log₁₀(I_D) on the temperatures, the maximum value of k (~0.11 dec/K) could be obtained at V_G = 5 V. Furthermore, the pn heterojunction device could be characterized as a temperature sensor well working at low V_D and V_G.

Rongbin Ye,Koji Ohta,Mamoru Baba, (2016) Temperature Dependence of Electrical Properties of Organic Thin Film Transistors Based on pn Heterojuction and Their Applications in Temperature Sensors. Journal of Computer and Communications,04,10-15. doi: 10.4236/jcc.2016.45002