Article pubs.acs.org/JPCC
Co-Actions of Ambient Pressure and Gas Molecular Adsorption on the Carriers’ Transport in Polycrystalline Pentacene Thin-Film Transistors Haoyan Zhao, Guifang Dong,* Lian Duan, Liduo Wang, and Yong Qiu* Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing, 100084, P. R. China S Supporting Information *
ABSTRACT: Organic transistors have proved to have potential applications in pressure sensors. However, few reports consider the coactions of pressure and ambient gas adsorption on the characteristics of the sensitive transistors. In this article, pentacene polycrystalline thin films were fabricated as the active layer of organic transistors, and the effects of ambient pressure and the gas adsorption on the carriers’ transport characteristics have been investigated. It was found that during the process from one atmosphere to vacuum (∼5 × 10−3 Pa) the device output, saturation source-drain currents (IDS), changed with pressure not monotonously but with an unexpected reversible minimum peak. Considering the variation of gas adsorption quantity and the distance between pentacene grains with pressure, we established models to understand the nature of the pressure sensitivity. We found that in low pressures the adsorption of gas molecules in grain boundaries was the main factor that affects device performance, whereas in high pressures, the shortening of the distance between pentacene grains was the main factor. Our research will benefit the understanding of charge-transport nature and, more importantly, give some instructions on using and designing highly sensitive pressure sensors.
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from one atmosphere down to high vacuum (10−3 Pa). The coactions of pressure and ambient molecules’ adsorption were considered, and it was found that the saturation source-currents changed with pressure not monotonously but with an unexpected minimum peak at a certain pressure, Pmin. The output current increased with pressure nearly linearly above Pmin and decreased with pressure below Pmin. This was a new phenomenon different from previous reports. We established models to give explanations and believed that the variations of gas adsorption quantity and the distance between pentacene grains were the main factors of pressure sensitivity.
INTRODUCTION For low-cost, flexible, and large-area process, organic field-effect transistors (OFETs) have been investigated widely in many fields of application, such as radio frequency identification tags, display drivers, and a variety of sensors.1−3 Pressure sensor is one of the most important applications of OFETs due to its potential use as electronic skin. In early 2004, Someya et al. reported an OFET pressure-sensor matrix mechanically based on a pressure-sensitive resistor connected to OFET in series.4 In recent years, Professor Bao’s group has reported excellent work on making artificial skins with a flexible OFET pressuresensor matrix that was mechanically based on sensitive gate dielectric structures.5,6 However, at present, the sensitivity of organic semiconductors to pressure has not been well considered. Although there were several reports on the effects of high pressure on the characteristics of OFETs, the pressure was too high to be suitable in skin application investigation.7,8 Yu-Wu Wang et al. have compared the properties of OFET in vacuum to those in air.9 However, their research results show that drain current increases with pressure, which is not consistent with the results of refs 7 and 8. Therefore, understanding how pressure influences the conductivity of organic semiconductors in the low-pressure range is expected. Moreover, the potential application of OFETs in outer space equipment also makes the research of OFETs properties in vacuum very important. In this article, we demonstrate the electric properties of pentacene crystalline thin-film FETs under different pressures © 2012 American Chemical Society
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EXPERIMENTAL METHODS Bottom-gate and top-contact OFET was fabricated on glass substrate with an indium−tin-oxide (ITO) gate electrode formed by photolithography. Polymethyl methacrylate (PMMA) films acted as the gate insulator with the film thickness of ∼280 nm and were prepared by spin-coating with 2000 rpm. PMMA was dissolved in 1,2-dichloroethylane with a concentration of 2.3%. Then, pentacene (Sigma-Aldrich, >99%, sublimed twice before use) polycrystalline thin film was deposited by thermal evaporation with a 0.04 nm/s deposition rate and a film thickness of 45 nm. Finally, the drain/source Received: August 14, 2012 Revised: December 4, 2012 Published: December 11, 2012 58
dx.doi.org/10.1021/jp308088b | J. Phys. Chem. C 2013, 117, 58−63
The Journal of Physical Chemistry C
Article
electrodes, 45-nm-thick gold films, were deposited through a shadow mask with a 0.02 nm/s deposition rate. The channel length was 50 μm and the width was 940 μm. The fabricated devices without encapsulation were put into a chamber. The air pressure in the chamber could be changed up and down through both adjusting the flapper valve connecting to the vacuum pump and putting air into the chamber through a needle valve. The thickness of polymer films and the morphologies of polymer film surfaces were measured under ambient air with a multimode atomic force microscope (AFM, Seiko instrument SPA 400) operated in tapping mode. The pressure was measured with a DL-4 vacuum gauge. The characteristics of the transistors were measured with a Keithley 4200 semiconductor characterization system. The source-drain currents in this article were measured under the conditions of VDS = −40 V and VGS = −40 V.
Figure 2. IDS versus gas pressure when the pressure increases from vacuum to one atmosphere.
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RESULTS AND DISCUSSION The AFM images of the pentacene films are shown in Figure 1a. The RMS of the thin film was 4.115 nm on average with a
the pressure decreased from one atmosphere to vacuum and in the curves when the ambient gas was changed into nitrogen and oxygen, although their minimum-current points (Pmin) were different from that in air due to the difference of gas species and condensed states of the pentacene thin films. Typical transfer IDS−VGS curves of OFETs under three pressure conditions had been presented in Figure 3. It is clear
Figure 3. Typical transfer IDS−VGS curves of OFETs: the green spots for high pressure, the blue spots for lowest pressure, the red spots for minimum current.
that Ion and Ioff of the minimum-current point are the lowest while VDS increased. Detailed comparison of their electrical properties had been given in Table 1. It is obvious that the mobility under highest pressure is the highest, and the mobility under minimum-current point pressure is the lowest, and the threshold voltage under the minimum-current point pressure is largest. To better understand what happened during the change of gas pressure, we looked more closely at the contact resistance between the electrodes and the pentacene film and whether it resulted in the change of IDS. We prepared a sample with the structure of glass substrate/ITO/pentacene/Au and measured the resistance under different gas pressures. The results are shown in Figure 4. Also, it can be found that the pentacene/Au contact was ohmic and the resistance, which consisted of the contact resistance and the pentacene film resistance, changed little. The maximum change, (Rmax − Rmin)/Rmax, was less than 2.8%, so the change of the contact resistance was not the main reason of the IDS variation, and instead, it is more likely due to
Figure 1. (a) AFM (3 μm × 3 μm) image of the pentacene based OFET. (b) Molecular structure of pentacene.
grain size of 2.962 × 104 nm2. It is shown in Figure 2 that the source-drain current (IDS) of the devices changes when the pressure rises from vacuum to one atmosphere. The curve has a minimum IDS at a pressure around 9 Pa which is denoted as Pmin. In the range of pressure smaller than Pmin, the current decreased with pressure quasi-exponentially, whereas in the range of pressure larger than Pmin, the current increased quasilinearly. IDS at 5.9 × 10−3 and 4.5 × 104 Pa was 2.8 and 18% higher than the minimum current, respectively. As shown in Figure s1 and Figure s2 in the Supporting Information, the minimum-current phenomenon also existed in the curves when 59
dx.doi.org/10.1021/jp308088b | J. Phys. Chem. C 2013, 117, 58−63
The Journal of Physical Chemistry C
Article
Table 1. Saturation Current Ion, off Current Ioff, Field-Effect Mobility μ, and Threshold Voltage VT of the OFET under the Highest Pressure, the Lowest Pressure, and the Minimum-Current Pressure highest pressure lowest pressure minimum-current
pressure (Pa)
Ion (μA)
Ioff (nA)
Ion/Ioff
mobility (cm2/V−1S−1)
threshold voltage (V)
4.5 × 10 5.9 × 10−3 9
9.50 8.26 8.03
17.8 9.48 7.92
890 871 1010
0.150 0.137 0.136
−15.1 −15.6 −16.3
4
situations result in the removal of a C p orbital from the πsystem, forming new gap states. Although the off current gets higher because of the rise of the gap states, when the device is turned on, these states may become traps for hole transport. As a result, the saturation current will become lower.10,16 Some groups consider that the effect of nitrogen is similar to oxygen, and both have the capability of forming new gap states,17,18 but in our opinion, for inert gas, molecules on the surface of pentacene will form dielectric barriers affecting the carrier hopping transport between grain boundaries in channel. The mechanism should be investigated deeply in the future. In summary, the adsorption of ambient gas molecules acts negatively on the carrier transport and decreases channel current. As far as we know, pressure is caused by the collision of gas particles on the surface of pentacene. Therefore, the top pentacene layer endures a force toward the inside of the thin film. It is reasonable to believe that the distance between the pentacene grains decreases with the pressure, as assumed in refs 9 and 10. The hopping rate of the carriers from one grain to another will increase in this compressed film, and the conductivity of pentacene film will be improved. Therefore, ambient gas pressure has a positive effect on carrier transport and will increase the channel current. Gas adsorption and gas pressure has reverse effects on the variation of channel current. We hypothesize that is the reason for minimum current at Pmin. Supposing the distance of the grain boundaries and the adsorption capacity correspond to the barrier width and height, respectively, as shown in Figure 5(a1, b1), the adsorption of gas molecules on the grain boundaries became stronger as the pressure increases from vacuum to Pmin. The equilibrium in eq 1 will move toward the positive
Figure 4. Resistance variation of ITO/pentacene/Au sample versus gas pressure and I−V characteristics of the sample under 0.2 Pa gas pressure.
the variation of pentacene conductivity in the channel of OFET device. Considering the environment of the device measurement, there were two factors that would contribute to the change of pentacene conductivity: ambient gas and pressure. In organic polycrystalline semiconductor thin film, it is reasonable to assume that the ambient gas molecules will come into the space between the semiconductor grains and be absorbed on the grain surfaces. It has been previously reported10 that adsorbed molecules such as water result in high trap density and high potential barrier height for carrier transport. Professor Bao’s group has also investigated the phenomenon that all devices show decreased current output and mobility as the relative humidity (RH) increase.11 They attribute the phenomenon to charge trapping at grain boundaries by polar water molecule. Several groups have investigated the O2 effect on the electrical characteristics of OFETs and find the saturation currents and mobilities of the transistors decrease as the ambient oxygen concentration increase.12,13 It is known that association of a molecule of low ionization potential (an electron donor, D) with a molecule of high electron affinity (an electron acceptor, A) can lead to the formation of a donor−acceptor complex or charge-transfer complex (eq 1),14 which acts negatively on the carrier transport and decreases saturation current. D + A ↔ [Dδ + • Aδ −]
(1)
The complex is unstable and there exists an equilibrium process. When the gas pressure increased, the volume concentration of oxygen will be relatively higher and the equilibrium will move toward the positive direction. Previous theoretical work has been established to investigate the O2− pentacene complexes in detail.15,16 In one situation, a doublebonded oxygen atom is substituted for the hydrogen at the sixposition (Figure 1b) of the pentacene molecule.15 Also, there is a possibility that two oxygen atoms dissociated from oxygen molecule are bonded to the C atoms in the 6- and 13-positions, forming the complex denoted as pentacene-2O.16 Both
Figure 5. Schematic diagrams of gas molecule adsorption and grain compressing process. (a1) molecular adsorption in high vacuum. The orange balls stand for pentacene molecules and the green rings stand for the gas molecules. (b1) molecular adsorption at the point of Pmin. (c1) molecular adsorption under the atmospheric pressure. (a2), (b2), and (c2) show the energy barriers in carriers’ transport process while VDS = 0 V. (a3), (b3) and (c3) show the energy barriers in carriers’ transport process while VDS < 0 V. 60
dx.doi.org/10.1021/jp308088b | J. Phys. Chem. C 2013, 117, 58−63
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direction, and more trap states will form as the number of complexes increases. Meanwhile, because of the relatively low pressure, the compression of the film is weak; as shown in Figure 5(a2, b2, a3, and b3), the energy barrier of charge hopping between two grains increases with pressure due to the enhancement of gas adsorption. Therefore, the negative action of gas adsorption will result in the decrease in the current. As shown in Figure 5(b1, c1), with the pressure increased from Pmin to one atmosphere, the adsorption is gradually saturated. This causes the distance between the grains to shorten and in turn narrows the energy barrier (shown in Figure 5(c2)). Under external electrical field (E), sourced from VDS
