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Investigation of the sensing mechanism of dual-gate low-voltage organic transistor based pressure sensor
Organic Electronics 75 (2019) 105431
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Investigation of the sensing mechanism of dual-gate low-voltage organic transistor based pressure sensor
T
Olamikunle Osinimu Ogunleyea,b, Heisuke Sakaia,1, Yuya Ishiic, Hideyuki Murataa,∗ a
School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa, 923-1211, Japan Department of Physics, Federal University Lokoja, Lokoja, P.M.B 1154, Nigeria c Faculty of Fiber Science and Engineering, Kyoto Institute of Technology, Kyoto, Kyoto, 606-8585, Japan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Dual-gate pressure sensor Dual-gate organic field-effect transistor Threshold voltage P(VDF-TrFE) Piezoelectric constant
Dual-gate pressure sensors consist of a piezoelectric polyvinylidene fluoride and trifluoroethylene P(VDF-TrFE) sensor and a low-voltage OFET read-out element. The pressure-induced voltage from the piezoelectric layer depletes charge carriers accumulated in the channel of the OFET, causing a shift in threshold voltage. By a quantitative analysis of results obtained from both the dual-gate organic pressure sensor and a conventional dual-gate OFET, we show that the piezoelectric constant of the sensing layer is estimated to be 72 pC/N. By comparing this value with that measured directly with piezoelectric measurement system, we concluded that the operation mechanism of the dual-gate pressure sensor was due to the piezoelectric behavior of the P(VDF-TrFE) layer.
1. Introduction The demand for wearable healthcare devices, electronic skin and flexible touch displays has given rise to an unprecedented interest in research and development of low-cost, large area, low-power electronic sensors [1–12]. Development of electronic sensors using organic materials has been explored over the years. These materials are compatible with high-throughput solution-processing methods; thus, could be used for low-cost production of sensors in large areas [13–15]. In the development of these organic electronic sensors, organic field effect transistors (OFETs) have been used for transduction and amplification of physical or chemical signals from sensing elements such as relative humidity, gas, and pressure [16]. The sensing mechanism could be resistive, capacitive or piezoelectric depending on the material properties, which would change in response to pressure applied. In connection with the active layer of an OFET, the change in such physical property induces the modulation of charge carriers in the semiconducting channel of the OFET leading to a change in electrical output of the OFET at different pressure magnitudes. In recent years, various architectures of pressure sensors using OFETs as the read-out element have been demonstrated; for example, a pressure sensor with PDMS film attached to a floating gate and a control gate to switch on a low-voltage OFET [1]. Under pressure, the capacitance of the film changes, leading to charge distribution in the floating
gate. This, in turn, modulates the field effect of the OFET's semiconducting channel. However, the change in output current (ΔI/I0) was approximately 0.04 at pressure magnitude of 78 kPa, where ΔI is the change in drain current (ID) when pressure is applied and I0 is the ID with minimum pressure. This was due to the small change in capacitance. To enhance the capacitance change, Schwartz et al. [2] used a PDMS microstructure in a top-gate, bottom-source and drain configuration. Although, this led to a significant on-current increase of factor 2.5 at pressure magnitude of 46 kPa from 6.8 kPa, the operation voltage of the OFET was quite high (−200 V). In addition, the elastic limit of PDMS hampers the magnitude of the capacitance change, therefore, limiting the sensitivity of the pressure-sensing device. Devices utilizing a suspended gate electrode configuration with an air gap to the dielectric has been reported to show an enhancement of on-current of more than 3 orders of magnitude at a low-pressure magnitude of 2 kPa exerted on the gate [3]. However, the OFET's high operation voltage (ranging from −60 V to −80 V), makes these pressure sensors unsuitable for practical application such as for low-power wearable devices. Despite progress in increasing capacitance changes by optimizing pressure-sensing device configurations, the OFET required high operation voltages to transduce these capacitance changes. To utilize OFETs with low operation voltages for pressure sensors, the sensor unit would be required to directly control charge carriers accumulated in the semiconducting channel in response to signal
∗
Corresponding author. E-mail address: [email protected] (H. Murata). 1 School of Science and Engineering, Kokushikan University, 4-28-1, Setagaya-ku, Tokyo 154-8515, Japan. https://doi.org/10.1016/j.orgel.2019.105431 Received 21 April 2019; Received in revised form 19 July 2019; Accepted 24 August 2019 Available online 25 August 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
Organic Electronics 75 (2019) 105431
O.O. Ogunleye, et al.
output from the pressure load. Interestingly, conventional dual-gate OFETs demonstrate that a top-gate voltage could deplete the charge carrier concentration accumulated by a bottom-gate OFET [17,18]. The organic semiconducting layers are sandwiched between the top and bottom gate electrodes in addition to corresponding organic insulating layers. When one of the gates (i.e., bottom gate) is biased at a specific voltage, a conducting channel is formed with accumulated charges at the dielectric/semiconductor interface, while the other gate (i.e., top gate) modulates the charge carriers accumulated in the channel leading to a change in threshold voltage (Vth). Piezoelectric materials generate charges or voltages in response to pressure load; one example of such material is P(VDF-TrFE) [19,20]. When polarized, its dipoles are aligned parallel in a way that surface charges are generated when force is exerted on it. When used for sensing applications, these surface charges modulate charges in the induced channel of the transistor similar to the functionality of a top-gate voltage in a dual-gate OFET [21]. Based on the piezoelectric property exhibited by P(VDF-TrFE), we recently developed a dual-gate OFET based pressure sensor, with a low operation voltage of −5 V. This novel device configuration consists of a piezoelectric P(VDF-TrFE) film laminated on the channel of the OFET [22]. The device achieved a significant ΔI/I0 of 155 when under a pressure load of 300 kPa. In contrast to a conventional dual-gate OFET, our dual-gate OFET based pressure sensor employs the piezoelectric dielectric layer as the top gate insulator, which would generate this topgate bias voltage when pressure is applied. A qualitative description for the operation mechanism of the dual-gate OFET based pressure sensor suggests that the pressure-induced voltage of the piezoelectric layer might cause shifts in Vth of the dual-gate OFET. In addition, our previous research focused on the performance of the organic pressure sensor based on its sensing capability and operation conditions. However, a quantitative clarification of the device's operation mechanism has not been proved yet. In this study, we investigate the operation mechanism of the dualgate organic pressure sensor. We demonstrate that the operation of the dual-gate OFET based pressure sensor was due to the piezoelectric behavior of the polarized P(VDF-TrFE) sensing layer. Using a conventional dual-gate, we found out that the charges in the channel of the OFET could be controlled. This is consistent with the performance of the OFET used for pressure sensing. In addition, the shift in transfer characteristics, as well as the shift in Vth, is consistent with that of the results from the dual gate pressure sensor. We could then estimate the amount of charge carriers depleted in the channel of the OFET by the piezoelectric layer when force is applied on it. Furthermore, results obtained from both devices were used to deduce the piezoelectric constant (d33) of the sensing layer to be 72 pC/N, which was compared with the value obtained when the piezoelectric constant of the sensing layer was measured directly.
Fig. 1. Device structure of (a) Dual-gate pressure sensor (b) Conventional dualgate OFET.
ethanol. Ethanol was used to rinse the electrodes thoroughly followed by drying on a hot plate. To complete the fabrication of the bottom-gate OFET, a blend of TIPS-pentacene (Ossila) and Polystyrene (Mw = 600,000, Sigma Aldrich) solution in chlorobenzene at a volume ratio of 3:1 and concentration 10 mg/ml was spin coated at 1000 rpm to form a semiconducting layer, followed with annealing for 30 min at 100 C. ͦ For the fabrication of the dual-gate OFET, a 950 nm-thick top gate dielectric layer was formed by spin coating CYTOP (CTL-809 M) on the semiconducting layer at 2000 rpm, followed by annealing on a hot plate for 20 min at 100 ͦC. To fabricate the sensing layer of the dual-gate OFET based pressure sensor, P(VDF-TrFE) solution (0.15 g/ml) in N-methyl-2pyrrolidinone solvent was blade coated on a Si wafer, followed by annealing at 140 ͦC for 1 h. The molar ratio for VDF to TrFE is 75:25 (Kureha Corp.). The crystallized 12 μm-thick polymer film was polarized at −1000 V to improve its piezoelectric properties. This was done by sandwiching the bladed coated Si/P(VDF-TrFE) film with another Si substrate. The area of P(VDF-TrFE) film was accurately measured to be 0.945 cm by 0.920 cm using a Vernier calliper. The poling voltage was applied to the bottom Si electrode while the top Si electrode was grounded. The top Si electrode was removed after poling. The p-type Siwafer used has resistivity range from 0.1 to 100 Ωcm. To complete the dual-gate organic pressure sensor, the P(VDF-TrFE) layer was superimposed on the active layer of the low-voltage OFET with the piezoelectric layer facing the active layer of the OFET. An image of the dualgate low-voltage organic pressure sensor can be found in Fig. S1 (a) of the supporting information. This device configuration is same as previously reported by our group [22]. Independently fabricating the OFET and P(VDF-TrFE) sensing layer prevents orthogonality problem arising from the N-methyl-2-pyrrolidinone solvent and the TIPS-pentacene/ polystyrene semiconducting layer of the OFET. The piezoelectric constant of the polarized P(VDF-TrFE) films were measured with a quasistatic (belincourt) piezoelectric measurement system [23]. Briefly, an 8.0 mm diameter disk-shaped metallic probe, which also acted as a top electrode, loaded periodically on the films with an interval of approximately 2.9 s, where the electric charge flowed between the probe and the Si substrate was measured through a charge amplifier. The preload and periodically applied load was 4.0 N and 6.0 N, respectively. Dividing the value of the generated charge by the difference of the loads (2.0 N) provided d33. The electrical properties of the devices were characterized using a semiconductor characterization system (Keithley 4200) in a glove box filled with dry nitrogen at room temperature. For the dual-gate OFET, the top gate bias voltage was applied with a DC power supply (Matsusada P4K40–0.6) connected to a probe.
2. Experimental methods Fig. 1(a) and (b) shows the schematic structures of the dual-gate organic pressure sensor and the dual-gate OFET, respectively. CYTOP (CTL-809 M) is used for the top-gate dielectric for the dual-gate OFET, while polarized P(VDF-TrFE) is used as the top-gate dielectric for the organic pressure sensor. We employed precisely the same structure for the bottom-gate OFET. For the bottom-gate OFET fabrication, a 30 nm Aluminium gate electrode layer was evaporated in a vacuum on a clean glass substrate. Poly (vinyl cinnamate) (PVCN) dissolved in chlorobenzene solvent at concentration 40 mg/ml was spin-coated at 3000 rpm to form the dielectric layer with thickness of 300 nm. The dielectric layer was cross-linked by UV treatment for 50 min followed by heating on a hot plate for 1 h. Thermally evaporated Ag source/drain electrodes (50 nm) with a shadow mask defined a channel of 50 μm in length and 2 mm in width. The source and drain Ag electrodes were modified with pentafluorobenzenethiol solution (0.005 mol L−1) in
3. Results and discussion The bottom-gate OFET was first electrically characterized. Fig. 2(a) 2
Organic Electronics 75 (2019) 105431
O.O. Ogunleye, et al.
immediately measured. Then the magnitude of pressure load is rapidly increased and the transfer characteristics of the OFET simultaneously measured. No significant discharge occurred because of the corresponding shift in transfer curve which indicates that charges were generated by the piezoelectric layer at the increasing magnitude of pressure. These measurements were carried out at room temperature condition to avoid pyroelectric effect on the sensor. The top-gate electrode was grounded during measurements. An increase in the pressure load ranging from 113 kPa to 451 kPa leads to a corresponding shift in the transfer curve and it returns to the initial position when pressure is released, indicating that this device operates as the pressure sensor. Fig. 3(c) shows a linear relationship between the pressure load and Vth. The shift in Vth to the negative VG direction is typical of a dual-gate OFET in which the top-gate positive voltage is applied [26,27]. These results suggest that the top-gate voltage generated in the top-gate P (VDF-TrFE) layer resulted in depletion of the accumulated charge carriers in the channel of the bottom-gate OFET which gives rise to a decrease in ID as well as the shift in Vth according to the pressure load increment. Furthermore, the pressure sensors’ operation behaviour involving change in the amount of charges in the channel is similar to the conventional dual-gate OFET. The pressure sensor device seems to have a sensing voltage induced by the deformed polarized P(VDF-TrFE) layer as the top-gate voltage [28]. These voltages would have a similar function to deplete the amount of charges in the channel of OFETs resulting in the shift of the transfer curve. In order to unveil the operation mechanism in the dual-gate OFET based pressure sensor, d33 of P(VDF-TrFE) layer in the dual-gate OFET based pressure sensor was calculated from the correlation between the applied pressure and Vth shift. As discussed above, the Vth shift would be due to the change in magnitude of the generated sensing voltage as a function of the applied pressure, suggesting that the Vth shift would be due to the piezoelectricity of P(VDF-TrFE). If this is the case, the calculated d33 value of P(VDF-TrFE) based on pressure response of the dual-gate OFET based pressure sensor would be consistent with the measured d33 value of P(VDF-TrFE) film. In order to calculate d33 of P (VDF-TrFE) layer in the dual-gate OFET based pressure sensor, we used equation (1) which states that the quantity of charges (Q) generated by the piezoelectric sensing layer is proportional to force applied (F), and d33 is the piezoelectric constant [19,29,30].
Fig. 2. (a) Transfer characteristics of the OFET (b) Output characteristics of the OFET.
shows the typical transfer characteristics of the bottom-gate OFET. The gate voltage (VG) was swept from 1 to −5 V in steps of 0.1 V at a constant drain voltage (VD) of −5 V. Fig. 2(b) shows the output characteristics of the OFET. The gate voltage was changed from 1 to −5 V in steps of −1 V while the VD was swept from 1 to −5 V in steps of 0.1 V. The on-off ratio, field-effect mobility, Vth and subthreshold swing (SS) value were extracted to be 1.1 × 106, 0.8 cm2/Vs, 0.18 V, and 115 mV/ dec, respectively. The device clearly operated in a low-voltage range based on these results. This result is consistent with low-voltage OFET devices using PVCN as dielectric and a blend of TIPS-pentacene/polystyrene as the semiconducting layer [24]. Stacking a polarized P(VDFTrFE) layer on top of the bottom-gate OFET leads to a shift in the Vth compared with that of the OFET with a unpolarized piezoelectric P (VDF-TrFE) layer (Fig. 3(a)). In addition, with a polarization bias of 1000 V, the shift in transfer curve was to the right; thus, indicating a switch in the polarization direction in the P(VDF-TrFE) film [25]. In other words, the surface potential on the polarized P(VDF-TrFE) layer induces the initial shift in Vth. An investigation of the correlation between the shift and the magnitude of the surface potential of the P (VDF-TrFE) layer will be reported elsewhere. Fig. 3(b) shows the transfer characteristics at different pressure loads on the device. ID-VG sweeps were carried out at each pressure value. When certain pressure load is exerted on the piezoelectric film, the transfer characteristics is
Q = d33 F … … …
(1)
Considering the relation between Q and F per unit area A, equation (1) is rearranged to give equation (2) below: Q/A = d33 F/A … … …
(2)
Where F/A is pressure on the material. In the device, F/A is the pressure load that induced the Vth shift. The value of pressure to obtain per unit amount of change in the Vth shift can be described as F/A × Vth−1, which is the slope of Fig. 3(c) and calculated as c.a. 11.2 × 105 Pa/V.
Fig. 3. (a) Threshold shift when polarized P(VDF-TrFE) was placed on the active layer (b) Transfer curve shifts corresponding to pressure load (c) graph of pressure load against threshold voltage. 3
Organic Electronics 75 (2019) 105431
O.O. Ogunleye, et al.
Fig. 4. (a) Threshold shift of the OFET corresponding to top-gate voltage (b) graph of Top gate voltage against threshold voltage (c) graph of charge per unit against threshold voltage.
similar fashion like a conventional dual-gate OFET when the top-gate is positively biased. Hence, we concluded that the pressure sensor is a dual-gate OFET based pressure sensor with the low-voltage bottom-gate OFET and a polarized P(VDF-TrFE) generating a top-gate voltage or sensing voltage when pressure is exerted on its Si top-gate electrode. These findings will open up the possibility for pressure sensors using dual-gate configuration, with one gate controlling the FET operation in response to the sensing voltage from another gate dielectric that corresponds to pressure exerted on it.
While Q/A describes the quantity of charges per unit area generated by the piezoelectric sensing layer under pressure application, which is accumulated at the interface between P(VDF-TrFE) layer and the TIPSPentacene active layer. As shown in Fig. 3(b), the charges seemed to cause the Vth shift, which is the same function as the top-gate voltage (VTop) application to the conventional dual-gate OFET. In order to obtain the quantity of Q/A, the conventional dual-gate OFET were fabricated and characterized. Although the top-gate dielectric layer of the conventional dual-gate OFET is different from that of the dual-gate organic pressure sensor, the bottom-gate OFET used for the two devices are similar as well as the operating condition of the bottom-gate OFET. Thus, the quantity of Q/A that causes the Vth shift of the OFET is considered to be consistent. Fig. 4(a) shows the transfer curves of the conventional dual-gate OFET. When positive VTop is applied to the dualgate OFET, the transfer curve shifts according to the magnitude of VTop. When a positive bias is applied to this dual-gate OFET, the electrostatic potential changes without shielding by the semiconductor [17]. This leads to the depletion of an amount of charge carriers in the bottom channel of the OFET. Fig. 4(b) shows a linear relationship between the Vth and VTop. The Vth shift caused by the positive VTop is typical of a dual-gate OFET. We estimated (Q/A) accumulated at the interface between the top-gate dielectric (Cytop) and TIPS-Pentacene active layer by the VTop from equation (3) [18]. Q/A = CT VTop … … …
Acknowledgment This work is partially supported by JSPS KAKENHI Grant Number JP16K21061 and JP18K05256 (to H.S.) Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2019.105431. References [1] S. Lai, P. Cosseddu, A. Bonfiglio, M. Barbaro, IEEE Electron. Device Lett. 34 (2013) 801–803. [2] G. Schwartz, B.C.-K. Tee, J. Mei, A.L. Appleton, D.H. Kim, H. Wang, Z. Bao, Nat. Commun. 4 (2013) 1–8. [3] Y. Zang, F. Zhang, D. Huang, X. Gao, C. Di, D. Zhu, Nat. Commun. 6 (2015) 6269. [4] D.-I. Kim, T.Q. Trung, B.-U. Hwang, J.-S. Kim, S. Jeon, J. Bae, J.-J. Park, N.-E. Lee, Sci. Rep. 5 (2015) 12705. [5] S.C.B. Mannsfeld, B.C.K. Tee, R.M. Stoltenberg, C.V.H.H. Chen, S. Barman, B.V.O. Muir, A.N. Sokolov, C. Reese, Z. Bao, Nat. Mater. 9 (2010) 859–864. [6] K. Takei, T. Takahashi, J.C. Ho, H. Ko, A.G. Gillies, P.W. Leu, R.S. Fearing, A. Javey, Nat. Mater. 9 (2010) 821–826. [7] M. Ramuz, B.C.-K. Tee, J.B.-H. Tok, Z. Bao, Adv. Mater. 24 (2012) 3223–3227. [8] V. Maheshwari, R.F. Saraf, Science 312 (2006) 1501–1504. [9] A.N. Sokolov, B.C.K. Tee, C.J. Bettinger, J.B.H. Tok, Z. Bao, Acc. Chem. Res. 45 (2012) 361–371. [10] D.J. Lipomi, M. Vosgueritchian, B.C.K. Tee, S.L. Hellstrom, J.A. Lee, C.H. Fox, Z. Bao, Nat. Nanotechnol. 6 (2011) 788–792. [11] B.C.K. Tee, C. Wang, R. Allen, Z. Bao, Nat. Nanotechnol. 7 (2012) 825–832. [12] A. Chortos, J. Liu, Z. Bao, Nat. Mater. 15 (2016) 937–950. [13] L. Li, L. Pan, Z. Ma, K. Yan, W. Cheng, Y. Shi, G. Yu, Nano Lett. 18 (2018) 3322–3327. [14] S. Yao, Y. Zhu, Nanoscale 6 (2014) 2345–2352. [15] S. Harada, W. Honda, T. Arie, S. Akita, K. Takei, ACS Nano 8 (2014) 3921–3927. [16] Y.H. Lee, M. Jang, M.Y. Lee, O.Y. Kweon, J.H. Oh, Chem 3 (2017) 724–763. [17] S. Iba, T. Sekitani, Y. Kato, T. Someya, H. Kawaguchi, M. Takamiya, T. Sakurai, S. Takagi, Appl. Phys. Lett. 87 (2005) 023509. [18] M.-J. Spijkman, K. Myny, E.C.P. Smits, P. Heremans, P.W.M. Blom, D.M. de Leeuw, Adv. Mater. 23 (2011) 3231–3242. [19] F. Maita, L. Maiolo, A. Minotti, A. Pecora, D. Ricci, G. Metta, G. Scandurra, G. Giusi, C. Ciofi, G. Fortunato, IEEE Sens. J. 15 (2015) 3819–3826. [20] S. Hannah, A. Davidsona, I. Glesk, D. Uttamchandani, R. Dahiya, H. Gleskova, Org. Electron. 56 (2018) 170–177. [21] R.S. Dahiya, G. Metta, M. Valle, A. Adami, L. Lorenzelli, Appl. Phys. Lett. 95 (2009) 034105. [22] Y. Tsuji, H. Sakai, L. Feng, X. Guo, H. Murata, APEX 10 (2017) 021601. [23] Y. Ishii, S. Kurihara, R. Kitayama, H. Sakai, Y. Nakabayashi, T. Nobeshima, S. Uemura, Smart Mater. Struct. 28 (2019) 08LT02. [24] L. Feng, W. Tang, J. Zhao, R. Yang, W. Hu, Q. Li, R. Wang, X. Guo, Sci. Rep. 6
(3)
2
Where CT (1.9 nF/cm ) is the capacitance of the Cytop layer and A is the unit area of the CYTOP layer. From this calculation, the relation between Vth and VTop can be replotted as the relation between Q/A and VTop (Fig. 4(c)). The value of Q/A to obtain per unit amount of change in the Vth shift can be described as Q/A×Vth−1, which is the slope of Fig. 4(c) and calculated as 8.1 nC/cm2V. Now, the quantitative correlation Q/A and F/A in equation (2) with respect to the per unit amount of change in the Vth shift is obtained. By substituting the values into equation (2), the value of d33 was estimated to be 72 pC/N. The estimated value is higher than the measured d33 of 53 pC/N. We speculate that the reason for the high value of d33 estimated from the device analysis could be due to overestimation of Q/A. For instance, the capacitive coupling of CYTOP dielectric to the bottom gate dielectric (PVCN) layer of the dual-gate OFET led to a reduced Vth shift, hence leading to a high Q/A estimated as charges generated by the P(VDFTrFE) sensing layer. Further detail studies using P(VDF-TrFE) as the top-gate dielectric for the dual-gate OFET may give accurate d33 values estimated. 4. Conclusion We described the operation mechanism a dual-gate pressure sensor by estimating the piezoelectric constant, d33 of the P(VDF-TrFE) film used as a sensor, to be 72 pC/N. Measuring d33 of the film directly gave an average value of 53 pC/N. The piezoelectric behavior of the film when compressed gave a sensing voltage that caused a shift in Vth in 4
Organic Electronics 75 (2019) 105431
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D.M. de Leeuw, P.W.M. Blom, B. de Boer, Org. Electron. 9 (2008) 839–846. [28] R. Pfattner, A.M. Foudeh, S. Chen, W. Niu, J.R. Mathews, M. He, Z. Bao, Adv. Electron. Mater. 5 (2018) 1800381 19. [29] R.S. Dahiya, G. Metta, M. Valle, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 56 (2009) 2. [30] ANSI/IEEE, IEEE standard on piezoelectricity, IEEE Stand. 176 (1987) 1987.
(2016) 20671. [25] T. Ishikawa, H. Sakai, H. Murata, IEICE Trans. Electron E102 (2019) 188–191. [26] M.-J. Spijkman, J.J. Brondijk, T.C.T. Geuns, E.C.P. Smits, T. Cramer, F. Zerbetto, P. Stoliar, F. Biscarini, P.W.M. Blom, D.M. de Leeuw, Adv. Funct. Mater. 20 (2010) 898–905. [27] F. Maddalena, M. Spijkman, J.J. Brondijk, P. Fonteijn, F. Brouwer, J.C. Hummelen,
5
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Investigation of the sensing mechanism of dual-gate low-voltage organic transistor based pressure sensor
T
Olamikunle Osinimu Ogunleyea,b, Heisuke Sakaia,1, Yuya Ishiic, Hideyuki Murataa,∗ a
School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa, 923-1211, Japan Department of Physics, Federal University Lokoja, Lokoja, P.M.B 1154, Nigeria c Faculty of Fiber Science and Engineering, Kyoto Institute of Technology, Kyoto, Kyoto, 606-8585, Japan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Dual-gate pressure sensor Dual-gate organic field-effect transistor Threshold voltage P(VDF-TrFE) Piezoelectric constant
Dual-gate pressure sensors consist of a piezoelectric polyvinylidene fluoride and trifluoroethylene P(VDF-TrFE) sensor and a low-voltage OFET read-out element. The pressure-induced voltage from the piezoelectric layer depletes charge carriers accumulated in the channel of the OFET, causing a shift in threshold voltage. By a quantitative analysis of results obtained from both the dual-gate organic pressure sensor and a conventional dual-gate OFET, we show that the piezoelectric constant of the sensing layer is estimated to be 72 pC/N. By comparing this value with that measured directly with piezoelectric measurement system, we concluded that the operation mechanism of the dual-gate pressure sensor was due to the piezoelectric behavior of the P(VDF-TrFE) layer.
1. Introduction The demand for wearable healthcare devices, electronic skin and flexible touch displays has given rise to an unprecedented interest in research and development of low-cost, large area, low-power electronic sensors [1–12]. Development of electronic sensors using organic materials has been explored over the years. These materials are compatible with high-throughput solution-processing methods; thus, could be used for low-cost production of sensors in large areas [13–15]. In the development of these organic electronic sensors, organic field effect transistors (OFETs) have been used for transduction and amplification of physical or chemical signals from sensing elements such as relative humidity, gas, and pressure [16]. The sensing mechanism could be resistive, capacitive or piezoelectric depending on the material properties, which would change in response to pressure applied. In connection with the active layer of an OFET, the change in such physical property induces the modulation of charge carriers in the semiconducting channel of the OFET leading to a change in electrical output of the OFET at different pressure magnitudes. In recent years, various architectures of pressure sensors using OFETs as the read-out element have been demonstrated; for example, a pressure sensor with PDMS film attached to a floating gate and a control gate to switch on a low-voltage OFET [1]. Under pressure, the capacitance of the film changes, leading to charge distribution in the floating
gate. This, in turn, modulates the field effect of the OFET's semiconducting channel. However, the change in output current (ΔI/I0) was approximately 0.04 at pressure magnitude of 78 kPa, where ΔI is the change in drain current (ID) when pressure is applied and I0 is the ID with minimum pressure. This was due to the small change in capacitance. To enhance the capacitance change, Schwartz et al. [2] used a PDMS microstructure in a top-gate, bottom-source and drain configuration. Although, this led to a significant on-current increase of factor 2.5 at pressure magnitude of 46 kPa from 6.8 kPa, the operation voltage of the OFET was quite high (−200 V). In addition, the elastic limit of PDMS hampers the magnitude of the capacitance change, therefore, limiting the sensitivity of the pressure-sensing device. Devices utilizing a suspended gate electrode configuration with an air gap to the dielectric has been reported to show an enhancement of on-current of more than 3 orders of magnitude at a low-pressure magnitude of 2 kPa exerted on the gate [3]. However, the OFET's high operation voltage (ranging from −60 V to −80 V), makes these pressure sensors unsuitable for practical application such as for low-power wearable devices. Despite progress in increasing capacitance changes by optimizing pressure-sensing device configurations, the OFET required high operation voltages to transduce these capacitance changes. To utilize OFETs with low operation voltages for pressure sensors, the sensor unit would be required to directly control charge carriers accumulated in the semiconducting channel in response to signal
∗
Corresponding author. E-mail address: [email protected] (H. Murata). 1 School of Science and Engineering, Kokushikan University, 4-28-1, Setagaya-ku, Tokyo 154-8515, Japan. https://doi.org/10.1016/j.orgel.2019.105431 Received 21 April 2019; Received in revised form 19 July 2019; Accepted 24 August 2019 Available online 25 August 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
Organic Electronics 75 (2019) 105431
O.O. Ogunleye, et al.
output from the pressure load. Interestingly, conventional dual-gate OFETs demonstrate that a top-gate voltage could deplete the charge carrier concentration accumulated by a bottom-gate OFET [17,18]. The organic semiconducting layers are sandwiched between the top and bottom gate electrodes in addition to corresponding organic insulating layers. When one of the gates (i.e., bottom gate) is biased at a specific voltage, a conducting channel is formed with accumulated charges at the dielectric/semiconductor interface, while the other gate (i.e., top gate) modulates the charge carriers accumulated in the channel leading to a change in threshold voltage (Vth). Piezoelectric materials generate charges or voltages in response to pressure load; one example of such material is P(VDF-TrFE) [19,20]. When polarized, its dipoles are aligned parallel in a way that surface charges are generated when force is exerted on it. When used for sensing applications, these surface charges modulate charges in the induced channel of the transistor similar to the functionality of a top-gate voltage in a dual-gate OFET [21]. Based on the piezoelectric property exhibited by P(VDF-TrFE), we recently developed a dual-gate OFET based pressure sensor, with a low operation voltage of −5 V. This novel device configuration consists of a piezoelectric P(VDF-TrFE) film laminated on the channel of the OFET [22]. The device achieved a significant ΔI/I0 of 155 when under a pressure load of 300 kPa. In contrast to a conventional dual-gate OFET, our dual-gate OFET based pressure sensor employs the piezoelectric dielectric layer as the top gate insulator, which would generate this topgate bias voltage when pressure is applied. A qualitative description for the operation mechanism of the dual-gate OFET based pressure sensor suggests that the pressure-induced voltage of the piezoelectric layer might cause shifts in Vth of the dual-gate OFET. In addition, our previous research focused on the performance of the organic pressure sensor based on its sensing capability and operation conditions. However, a quantitative clarification of the device's operation mechanism has not been proved yet. In this study, we investigate the operation mechanism of the dualgate organic pressure sensor. We demonstrate that the operation of the dual-gate OFET based pressure sensor was due to the piezoelectric behavior of the polarized P(VDF-TrFE) sensing layer. Using a conventional dual-gate, we found out that the charges in the channel of the OFET could be controlled. This is consistent with the performance of the OFET used for pressure sensing. In addition, the shift in transfer characteristics, as well as the shift in Vth, is consistent with that of the results from the dual gate pressure sensor. We could then estimate the amount of charge carriers depleted in the channel of the OFET by the piezoelectric layer when force is applied on it. Furthermore, results obtained from both devices were used to deduce the piezoelectric constant (d33) of the sensing layer to be 72 pC/N, which was compared with the value obtained when the piezoelectric constant of the sensing layer was measured directly.
Fig. 1. Device structure of (a) Dual-gate pressure sensor (b) Conventional dualgate OFET.
ethanol. Ethanol was used to rinse the electrodes thoroughly followed by drying on a hot plate. To complete the fabrication of the bottom-gate OFET, a blend of TIPS-pentacene (Ossila) and Polystyrene (Mw = 600,000, Sigma Aldrich) solution in chlorobenzene at a volume ratio of 3:1 and concentration 10 mg/ml was spin coated at 1000 rpm to form a semiconducting layer, followed with annealing for 30 min at 100 C. ͦ For the fabrication of the dual-gate OFET, a 950 nm-thick top gate dielectric layer was formed by spin coating CYTOP (CTL-809 M) on the semiconducting layer at 2000 rpm, followed by annealing on a hot plate for 20 min at 100 ͦC. To fabricate the sensing layer of the dual-gate OFET based pressure sensor, P(VDF-TrFE) solution (0.15 g/ml) in N-methyl-2pyrrolidinone solvent was blade coated on a Si wafer, followed by annealing at 140 ͦC for 1 h. The molar ratio for VDF to TrFE is 75:25 (Kureha Corp.). The crystallized 12 μm-thick polymer film was polarized at −1000 V to improve its piezoelectric properties. This was done by sandwiching the bladed coated Si/P(VDF-TrFE) film with another Si substrate. The area of P(VDF-TrFE) film was accurately measured to be 0.945 cm by 0.920 cm using a Vernier calliper. The poling voltage was applied to the bottom Si electrode while the top Si electrode was grounded. The top Si electrode was removed after poling. The p-type Siwafer used has resistivity range from 0.1 to 100 Ωcm. To complete the dual-gate organic pressure sensor, the P(VDF-TrFE) layer was superimposed on the active layer of the low-voltage OFET with the piezoelectric layer facing the active layer of the OFET. An image of the dualgate low-voltage organic pressure sensor can be found in Fig. S1 (a) of the supporting information. This device configuration is same as previously reported by our group [22]. Independently fabricating the OFET and P(VDF-TrFE) sensing layer prevents orthogonality problem arising from the N-methyl-2-pyrrolidinone solvent and the TIPS-pentacene/ polystyrene semiconducting layer of the OFET. The piezoelectric constant of the polarized P(VDF-TrFE) films were measured with a quasistatic (belincourt) piezoelectric measurement system [23]. Briefly, an 8.0 mm diameter disk-shaped metallic probe, which also acted as a top electrode, loaded periodically on the films with an interval of approximately 2.9 s, where the electric charge flowed between the probe and the Si substrate was measured through a charge amplifier. The preload and periodically applied load was 4.0 N and 6.0 N, respectively. Dividing the value of the generated charge by the difference of the loads (2.0 N) provided d33. The electrical properties of the devices were characterized using a semiconductor characterization system (Keithley 4200) in a glove box filled with dry nitrogen at room temperature. For the dual-gate OFET, the top gate bias voltage was applied with a DC power supply (Matsusada P4K40–0.6) connected to a probe.
2. Experimental methods Fig. 1(a) and (b) shows the schematic structures of the dual-gate organic pressure sensor and the dual-gate OFET, respectively. CYTOP (CTL-809 M) is used for the top-gate dielectric for the dual-gate OFET, while polarized P(VDF-TrFE) is used as the top-gate dielectric for the organic pressure sensor. We employed precisely the same structure for the bottom-gate OFET. For the bottom-gate OFET fabrication, a 30 nm Aluminium gate electrode layer was evaporated in a vacuum on a clean glass substrate. Poly (vinyl cinnamate) (PVCN) dissolved in chlorobenzene solvent at concentration 40 mg/ml was spin-coated at 3000 rpm to form the dielectric layer with thickness of 300 nm. The dielectric layer was cross-linked by UV treatment for 50 min followed by heating on a hot plate for 1 h. Thermally evaporated Ag source/drain electrodes (50 nm) with a shadow mask defined a channel of 50 μm in length and 2 mm in width. The source and drain Ag electrodes were modified with pentafluorobenzenethiol solution (0.005 mol L−1) in
3. Results and discussion The bottom-gate OFET was first electrically characterized. Fig. 2(a) 2
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immediately measured. Then the magnitude of pressure load is rapidly increased and the transfer characteristics of the OFET simultaneously measured. No significant discharge occurred because of the corresponding shift in transfer curve which indicates that charges were generated by the piezoelectric layer at the increasing magnitude of pressure. These measurements were carried out at room temperature condition to avoid pyroelectric effect on the sensor. The top-gate electrode was grounded during measurements. An increase in the pressure load ranging from 113 kPa to 451 kPa leads to a corresponding shift in the transfer curve and it returns to the initial position when pressure is released, indicating that this device operates as the pressure sensor. Fig. 3(c) shows a linear relationship between the pressure load and Vth. The shift in Vth to the negative VG direction is typical of a dual-gate OFET in which the top-gate positive voltage is applied [26,27]. These results suggest that the top-gate voltage generated in the top-gate P (VDF-TrFE) layer resulted in depletion of the accumulated charge carriers in the channel of the bottom-gate OFET which gives rise to a decrease in ID as well as the shift in Vth according to the pressure load increment. Furthermore, the pressure sensors’ operation behaviour involving change in the amount of charges in the channel is similar to the conventional dual-gate OFET. The pressure sensor device seems to have a sensing voltage induced by the deformed polarized P(VDF-TrFE) layer as the top-gate voltage [28]. These voltages would have a similar function to deplete the amount of charges in the channel of OFETs resulting in the shift of the transfer curve. In order to unveil the operation mechanism in the dual-gate OFET based pressure sensor, d33 of P(VDF-TrFE) layer in the dual-gate OFET based pressure sensor was calculated from the correlation between the applied pressure and Vth shift. As discussed above, the Vth shift would be due to the change in magnitude of the generated sensing voltage as a function of the applied pressure, suggesting that the Vth shift would be due to the piezoelectricity of P(VDF-TrFE). If this is the case, the calculated d33 value of P(VDF-TrFE) based on pressure response of the dual-gate OFET based pressure sensor would be consistent with the measured d33 value of P(VDF-TrFE) film. In order to calculate d33 of P (VDF-TrFE) layer in the dual-gate OFET based pressure sensor, we used equation (1) which states that the quantity of charges (Q) generated by the piezoelectric sensing layer is proportional to force applied (F), and d33 is the piezoelectric constant [19,29,30].
Fig. 2. (a) Transfer characteristics of the OFET (b) Output characteristics of the OFET.
shows the typical transfer characteristics of the bottom-gate OFET. The gate voltage (VG) was swept from 1 to −5 V in steps of 0.1 V at a constant drain voltage (VD) of −5 V. Fig. 2(b) shows the output characteristics of the OFET. The gate voltage was changed from 1 to −5 V in steps of −1 V while the VD was swept from 1 to −5 V in steps of 0.1 V. The on-off ratio, field-effect mobility, Vth and subthreshold swing (SS) value were extracted to be 1.1 × 106, 0.8 cm2/Vs, 0.18 V, and 115 mV/ dec, respectively. The device clearly operated in a low-voltage range based on these results. This result is consistent with low-voltage OFET devices using PVCN as dielectric and a blend of TIPS-pentacene/polystyrene as the semiconducting layer [24]. Stacking a polarized P(VDFTrFE) layer on top of the bottom-gate OFET leads to a shift in the Vth compared with that of the OFET with a unpolarized piezoelectric P (VDF-TrFE) layer (Fig. 3(a)). In addition, with a polarization bias of 1000 V, the shift in transfer curve was to the right; thus, indicating a switch in the polarization direction in the P(VDF-TrFE) film [25]. In other words, the surface potential on the polarized P(VDF-TrFE) layer induces the initial shift in Vth. An investigation of the correlation between the shift and the magnitude of the surface potential of the P (VDF-TrFE) layer will be reported elsewhere. Fig. 3(b) shows the transfer characteristics at different pressure loads on the device. ID-VG sweeps were carried out at each pressure value. When certain pressure load is exerted on the piezoelectric film, the transfer characteristics is
Q = d33 F … … …
(1)
Considering the relation between Q and F per unit area A, equation (1) is rearranged to give equation (2) below: Q/A = d33 F/A … … …
(2)
Where F/A is pressure on the material. In the device, F/A is the pressure load that induced the Vth shift. The value of pressure to obtain per unit amount of change in the Vth shift can be described as F/A × Vth−1, which is the slope of Fig. 3(c) and calculated as c.a. 11.2 × 105 Pa/V.
Fig. 3. (a) Threshold shift when polarized P(VDF-TrFE) was placed on the active layer (b) Transfer curve shifts corresponding to pressure load (c) graph of pressure load against threshold voltage. 3
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Fig. 4. (a) Threshold shift of the OFET corresponding to top-gate voltage (b) graph of Top gate voltage against threshold voltage (c) graph of charge per unit against threshold voltage.
similar fashion like a conventional dual-gate OFET when the top-gate is positively biased. Hence, we concluded that the pressure sensor is a dual-gate OFET based pressure sensor with the low-voltage bottom-gate OFET and a polarized P(VDF-TrFE) generating a top-gate voltage or sensing voltage when pressure is exerted on its Si top-gate electrode. These findings will open up the possibility for pressure sensors using dual-gate configuration, with one gate controlling the FET operation in response to the sensing voltage from another gate dielectric that corresponds to pressure exerted on it.
While Q/A describes the quantity of charges per unit area generated by the piezoelectric sensing layer under pressure application, which is accumulated at the interface between P(VDF-TrFE) layer and the TIPSPentacene active layer. As shown in Fig. 3(b), the charges seemed to cause the Vth shift, which is the same function as the top-gate voltage (VTop) application to the conventional dual-gate OFET. In order to obtain the quantity of Q/A, the conventional dual-gate OFET were fabricated and characterized. Although the top-gate dielectric layer of the conventional dual-gate OFET is different from that of the dual-gate organic pressure sensor, the bottom-gate OFET used for the two devices are similar as well as the operating condition of the bottom-gate OFET. Thus, the quantity of Q/A that causes the Vth shift of the OFET is considered to be consistent. Fig. 4(a) shows the transfer curves of the conventional dual-gate OFET. When positive VTop is applied to the dualgate OFET, the transfer curve shifts according to the magnitude of VTop. When a positive bias is applied to this dual-gate OFET, the electrostatic potential changes without shielding by the semiconductor [17]. This leads to the depletion of an amount of charge carriers in the bottom channel of the OFET. Fig. 4(b) shows a linear relationship between the Vth and VTop. The Vth shift caused by the positive VTop is typical of a dual-gate OFET. We estimated (Q/A) accumulated at the interface between the top-gate dielectric (Cytop) and TIPS-Pentacene active layer by the VTop from equation (3) [18]. Q/A = CT VTop … … …
Acknowledgment This work is partially supported by JSPS KAKENHI Grant Number JP16K21061 and JP18K05256 (to H.S.) Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2019.105431. References [1] S. Lai, P. Cosseddu, A. Bonfiglio, M. Barbaro, IEEE Electron. Device Lett. 34 (2013) 801–803. [2] G. Schwartz, B.C.-K. Tee, J. Mei, A.L. Appleton, D.H. Kim, H. Wang, Z. Bao, Nat. Commun. 4 (2013) 1–8. [3] Y. Zang, F. Zhang, D. Huang, X. Gao, C. Di, D. Zhu, Nat. Commun. 6 (2015) 6269. [4] D.-I. Kim, T.Q. Trung, B.-U. Hwang, J.-S. Kim, S. Jeon, J. Bae, J.-J. Park, N.-E. Lee, Sci. Rep. 5 (2015) 12705. [5] S.C.B. Mannsfeld, B.C.K. Tee, R.M. Stoltenberg, C.V.H.H. Chen, S. Barman, B.V.O. Muir, A.N. Sokolov, C. Reese, Z. Bao, Nat. Mater. 9 (2010) 859–864. [6] K. Takei, T. Takahashi, J.C. Ho, H. Ko, A.G. Gillies, P.W. Leu, R.S. Fearing, A. Javey, Nat. Mater. 9 (2010) 821–826. [7] M. Ramuz, B.C.-K. Tee, J.B.-H. Tok, Z. Bao, Adv. Mater. 24 (2012) 3223–3227. [8] V. Maheshwari, R.F. Saraf, Science 312 (2006) 1501–1504. [9] A.N. Sokolov, B.C.K. Tee, C.J. Bettinger, J.B.H. Tok, Z. Bao, Acc. Chem. Res. 45 (2012) 361–371. [10] D.J. Lipomi, M. Vosgueritchian, B.C.K. Tee, S.L. Hellstrom, J.A. Lee, C.H. Fox, Z. Bao, Nat. Nanotechnol. 6 (2011) 788–792. [11] B.C.K. Tee, C. Wang, R. Allen, Z. Bao, Nat. Nanotechnol. 7 (2012) 825–832. [12] A. Chortos, J. Liu, Z. Bao, Nat. Mater. 15 (2016) 937–950. [13] L. Li, L. Pan, Z. Ma, K. Yan, W. Cheng, Y. Shi, G. Yu, Nano Lett. 18 (2018) 3322–3327. [14] S. Yao, Y. Zhu, Nanoscale 6 (2014) 2345–2352. [15] S. Harada, W. Honda, T. Arie, S. Akita, K. Takei, ACS Nano 8 (2014) 3921–3927. [16] Y.H. Lee, M. Jang, M.Y. Lee, O.Y. Kweon, J.H. Oh, Chem 3 (2017) 724–763. [17] S. Iba, T. Sekitani, Y. Kato, T. Someya, H. Kawaguchi, M. Takamiya, T. Sakurai, S. Takagi, Appl. Phys. Lett. 87 (2005) 023509. [18] M.-J. Spijkman, K. Myny, E.C.P. Smits, P. Heremans, P.W.M. Blom, D.M. de Leeuw, Adv. Mater. 23 (2011) 3231–3242. [19] F. Maita, L. Maiolo, A. Minotti, A. Pecora, D. Ricci, G. Metta, G. Scandurra, G. Giusi, C. Ciofi, G. Fortunato, IEEE Sens. J. 15 (2015) 3819–3826. [20] S. Hannah, A. Davidsona, I. Glesk, D. Uttamchandani, R. Dahiya, H. Gleskova, Org. Electron. 56 (2018) 170–177. [21] R.S. Dahiya, G. Metta, M. Valle, A. Adami, L. Lorenzelli, Appl. Phys. Lett. 95 (2009) 034105. [22] Y. Tsuji, H. Sakai, L. Feng, X. Guo, H. Murata, APEX 10 (2017) 021601. [23] Y. Ishii, S. Kurihara, R. Kitayama, H. Sakai, Y. Nakabayashi, T. Nobeshima, S. Uemura, Smart Mater. Struct. 28 (2019) 08LT02. [24] L. Feng, W. Tang, J. Zhao, R. Yang, W. Hu, Q. Li, R. Wang, X. Guo, Sci. Rep. 6
(3)
2
Where CT (1.9 nF/cm ) is the capacitance of the Cytop layer and A is the unit area of the CYTOP layer. From this calculation, the relation between Vth and VTop can be replotted as the relation between Q/A and VTop (Fig. 4(c)). The value of Q/A to obtain per unit amount of change in the Vth shift can be described as Q/A×Vth−1, which is the slope of Fig. 4(c) and calculated as 8.1 nC/cm2V. Now, the quantitative correlation Q/A and F/A in equation (2) with respect to the per unit amount of change in the Vth shift is obtained. By substituting the values into equation (2), the value of d33 was estimated to be 72 pC/N. The estimated value is higher than the measured d33 of 53 pC/N. We speculate that the reason for the high value of d33 estimated from the device analysis could be due to overestimation of Q/A. For instance, the capacitive coupling of CYTOP dielectric to the bottom gate dielectric (PVCN) layer of the dual-gate OFET led to a reduced Vth shift, hence leading to a high Q/A estimated as charges generated by the P(VDFTrFE) sensing layer. Further detail studies using P(VDF-TrFE) as the top-gate dielectric for the dual-gate OFET may give accurate d33 values estimated. 4. Conclusion We described the operation mechanism a dual-gate pressure sensor by estimating the piezoelectric constant, d33 of the P(VDF-TrFE) film used as a sensor, to be 72 pC/N. Measuring d33 of the film directly gave an average value of 53 pC/N. The piezoelectric behavior of the film when compressed gave a sensing voltage that caused a shift in Vth in 4
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