polymer heterointerface organic field effect transistor

Thin Solid Films 556 (2014) 495–498

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Hybrid carbon nanotube/polymer heterointerface organic field effect transistor C.L. Chua, K.H. Yeoh, K.L. Woon ⁎ Low Dimensional Materials Research Center, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e

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Article history: Received 9 July 2013 Received in revised form 18 December 2013 Accepted 19 December 2013 Available online 28 December 2013 Keywords: Organic field effect transistor Poly(3-hexylthiophene) Carbon nanotube

a b s t r a c t Hybrid carbon nanotube/polymer heterointerface field effect transistor is developed by drop-casting carbon nanotubes on top of the semiconducting polymer. This method results in enhancement of mobility by a factor of 10 compared with the control device. In contrast with most devices that integrating carbon nanotubes into the polymer, the on/off ratio of our device actually improves. The enhancement of on/off ratio is interpreted as a result of density of carbon nanotube approaching the percolation threshold. We show that such improvement can be possible only when the charge carriers travel throughout the bulk of the active layer. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Organic field effect transistors (OFETs) using polymeric semiconductors offer potential to be used as radio frequency identification tags [1], chemical sensors [2], display drivers [3] and photosensors [4]. The solution processability of such materials enabled low cost high volume manufacturing. In general, OFETs offer high on/off ratio and sufficient reliability to be adopted in flexible electronics [5]. However, polymeric semiconductors suffer from relatively poor mobility that limits the range of possible applications. In contrast with carbon nanotubes (CNTs) which exhibit very high mobility, the on/off ratio of field effect transistors fabricated using as received CNTs is poor [6]. Although highly purified network semiconducting CNT transistors can give very high on/off ratio, the purification process is very time consuming, complicated and expensive [7]. Pristine CNTs can be incorporated into organic electronics to enhance the electrical properties. This can be done by mixing CNTs network inside the organic semiconducting materials. However, it is a challenge to mix CNTs with polymeric semiconductors homogeneously as a result of strong Van der Waals interaction between them [8]. The dispersion of CNTs often greatly depends on the dispersing agent [9]. Introduction of dispersing agent is usually undesirable as it could result in charge traps. Furthermore, the use of long sonication time degrades the polymers. The loading of pristine CNTs into organic semiconductors can be low and enhancement of mobility remains poor. To circumvent the problem, more CNT loading is required. In order to increase the loading of CNTs into the polymeric semiconducting materials, Park et al. chemically functionalized the CNTs [10]. Although the mobility is increased by an order of magnitude, ⁎ Corresponding author. E-mail address: [email protected] (K.L. Woon). 0040-6090/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.12.053

the on/off ratio decreases drastically as a result of the presence of parasitic metallic CNTs. Other approaches involve electrochemical deposition of conductive polymer on to the CNTs or by coating vertically aligned CNTs with polymer without the need of forming homogeneous dispersion in CNTs and polymer solution [11,12]. In this report, we present another approach that uses a simple method to overcome the above-mentioned problems. By using hybrid CNT/polymer heterointerface FETs, it is possible to enhance both the charge mobility and the on/off ratio. 2. Experiment A 300 nm thermally grown oxide layer of Si/SiO2 substrate was used to fabricate a bottom gated OFET. The heavily doped Si acts as a back gate. The substrate was cleaned with acetone, ethanol and de-ionized water with the aid of ultrasonic agitation followed by nitrogen purge to get rid of dust particles prior to spin coating. It is then treated with phenyltrichlorosilane (PTCS). The detailed method of PTCS treatment is reported elsewhere [13]. The active materials, RR P3HT and CNTs (SWeNT SG65), are purchased from American Dye Source Inc. and SouthWest Nanotechnologies Inc. respectively. The regioregular poly(3-alkylthiophene) (RR P3HT) is dissolved in chloroform (CF) in a range of concentration from 0.1% to 1% by weight to produce films of thickness from 10 nm to 80 nm. The organic solution prepared from CF was filtered through a 0.45 μm pore size nylon filter and deposited on the substrate via spin coating with speed of 1800 rpm for 1 min to form a thin film. It is then heated at 120 °C for 10 min to eliminate residue solvent and to induce better film crystallinity. The heterointerface in the OFET is formed by drop-casting the solution of 1 mg CNTs in 15 ml N-methyl-2-pyrrolidone (NMP) on top of the RR P3HT film. The NMP, being an orthogonal solvent, doesn't dissolve the RR P3HT thin

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Fig. 1. Schematic illustration of (a) drop-casted CNTs in NMP on a RR P3HT film and (b) CNTs network formed within the OFET channel.

film. The droplet is allowed to rest on the surface for a designated period as shown in Fig. 1. Since NMP has very long evaporation time, after a designated period, the film was spun at 8000 rpm for 30 s to remove excess NMP and CNTs. The thin film is then heated at 120 °C for 10 min to remove residue solvent. The experiment was then followed by patterning 50 nm thick gold source–drain electrodes on the thin film via thermal vacuum evaporation employing a shadow mask. The channel width and length of the device are 1 mm and 30 μm respectively. To compare the performance of the device fabricated using this approach, a control device consisting of only pristine P3HT is also fabricated. The whole process was performed at room temperature under N2 environment. The semiconducting channel is then encapsulated with a layer of poly(methyl methacrylate) prior to electrical characterization. Two Keithley 236 source-measurement units (SMUs) are used to bias the device and measure the current resulted from the device, where one unit of the SMUs is used to establish the source–drain voltage Vds and measure the drain current Ids while another unit of SMU is used to supply the gate voltage Vg and measure the gate current Ig.

3. Results and discussion We fabricated devices with different polymer film thickness and found that a 22 nm thick gives the best performance. The effect of semiconducting polymer thickness on the device performance had been studied elsewhere [14]. Therefore, we fix the thickness of the RR P3HT and focus on the effect of treatment period towards the device performance. Immersing the RR P3HT OFET in NMP for several hours doesn't change the device performance. This indicates that NMP does not damage the organic semiconducting layer. NMP being an excellent dispersing solvent can be used to disperse the CNTs without the use of surfactant. Fig. 2 shows the Atomic Force Microscopy images of CNT-RR P3HT thin films for 2-hour incubation and 4-hour incubation. It is clearly seen that CNTs are deposited on to the RR P3HT thin film probably due to the weak interaction of CNTs and RR P3HT. The density of CNTs is higher for a longer treatment time.

Fig. 3 shows the transfer characteristic of control device (device 1), hybrid carbon nanotube/polymer FET soaked with CNTs-NMP solution for 2 h (device 2) and 4 h (device 3). The charge carrier mobility, μ, can be extracted from transfer characteristic curve from Fig. 3(a). All the devices operate at enhancement mode with hole as major charge carrier which is typical for P3HT based OFET. High conductivity is observed in device 3. Field effect mobility, μ [15], is extracted from the saturation regime at Vg = −60 V from Fig. 2(a) using the equation below. Ids ¼

2 WCi  μ V g −V t 2L

ð1Þ

Ids represents the drain current at saturation regime, W and L are the channel width and length respectively, Ci is the capacitance of the insulator, Vg is the applied gate voltage and Vt is the threshold voltage. From Eq. (1), we take the average value of, μ, between Vg = + 40 V and Vg = + 60 V, the highest value of μ comes from device 3 (1.13 × 10−3 cm2/Vs) which is a pronounce improvement compared to device 2 (1.77 × 10−4 cm2/Vs) and device 1 (1.07 × 10−4 cm2/Vs). For devices 1 and 2, the on/off ratio is 103 and device 3 is 104 for Vds = − 60 V. Incubation period of NMP-CNTs solution longer than 4 h resulted in drastic reduction of on/off ratio. When there are enough drop-casted CNTs on top of the organic semiconductor film, the mobility is drastically enhanced just before the density of the CNTs appears to approach the percolation threshold. Beyond the percolation of CNTs, the on/off ratio is drastically decreased for such a hybrid/ CNTs/polymer transistor. Similar effect is observed in polymeric semiconductor/graphene hybrid field-effect transistors by Jia Huang et al. whereby they mixed the organic solvent soluble graphene flakes inside the organic semiconducting materials [15]. We also repeat the same experiment without the use of P3HT by immersing Si/SiO2 substrate for 4 h. We found that there is little current flow (~ 0.1 nA) through the device indicating that the density of CNTs is less than percolation threshold. Fig. 3(b) shows typical source–drain current vs. source–drain voltage plots at various gate voltages for the control device and device 3

Fig. 2. Atomic Force Microscopy image of (a) device 2 and (b) device 3.

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Fig. 3. (a) Transfer curve of device 1 (triangle), device 2 (circle) and device 3 (square). Filled symbol representing semilogarithm curve of Ids while the open symbol representing square root of Ids at Vds = 60 V. (b) Output characteristic of device 3 (dashed line) and device 1 (continuous line).

operating in enhancement mode. The 4-hour treatment increases the drain current of the OFET by a factor of 5 at Vds = 60 V, from 30 to 170 nA. The addition of CNTs on top of the RR P3HT enhances the electrical properties of OFET. SS is the subthreshold swing and it is the gate voltage swing required to change the drain current by one decade. It is a measure of how easily a transistor can be switched from the off-state to the on-state and hence can be related to density of trap states, Nt [16]. It can be extracted from the logarithmic curve from Fig. 2(a) based on Eq. (2) [17]. SS ¼

δV g ∝Nt δ logI d

ð2Þ

SS values are 6.13, 7.09 and 5.03 V/decade for devices 1, 2 and 3 respectively which are proportional to the density of trap state, Nt. The mobility in device 3 with smaller Nt is found to be less varied with respect to changing gate-voltage than that in the device with larger Nt. Device 2 has the highest trap density and the mobility is almost unchanged compared with device 1 even though it is covered with a layer of CNTs. This can be explained by the fact that for device 2, there aren't sufficient CNTs to connect the multicrystalline domains resulting CNTs to act as traps. Furthermore, the highest occupied molecular orbital (HOMO) of RR P3HT is quite similar with average work function of CNTs, but since the pristine CNTs contain a wide range of chirality it is likely that valence band of some of the species would be located above the HOMO of RR P3HT [18–20]. This can result in higher density of shallow traps whereby dominant charge carriers are in the bulk of RR P3HT. Hence the mobility of device 2 shows little improvement. The on-set of “saturated” mobility for device 2 occurred at a higher gate voltage as shown in Fig. 4 as more carriers are needed to fill the traps. RR P3HT consists of multicrystalline domains. The grain boundaries can act as a charge trap [21]. When there is sufficient network of CNTs, the charge carriers can travel across the grain

Fig. 4. The mobility as a function of gate voltage for pristine, device with 2-hour treatment and 4-hour treatment.

boundaries via the CNTs to the source–drain electrodes. This results in lower SS and reduced trap density. One would argue that there is preferential interaction of semiconducting CNTs with RR P3HT which could explain the enhancement of on/off ratio. This preferential interaction has been observed with certain semiconducting polymer such as poly(9,9-dioctylfluorenyl-2,7-diyl)-co-(bithiophene) [22]. MicroRaman spectroscopy as shown in Fig. 5 confirmed that the distribution metallic and semiconducting CNTs are relatively unchanged between the as received CNTs and CNTs on the 4-hour treated thin film. It is well known that in OFET, charge carriers generally travel along the interface of semiconductor/dielectric [17]. However, it was found that charge carriers could travel at some distance from the interface or at the hetero-junction [22–25]. Here we show that the charge can travel in both CNTs and P3HT even though the CNTs are located at the surface of semiconducting polymer. Such phenomenon occurs when there is sufficient density of CNTs but below the percolation threshold with CNTs connecting the multicrystalline domains of P3HT. These results also imply that the presence of conductive channel throughout the bulk rather than confining at the interface between the polymer and dielectric layer as usually thought.

4. Conclusions In conclusion, a method to enhance the mobility of a semiconducting polymer with pristine CNTs by 10 without degrading the on/off ratio is developed. This is a result of CNTs forming fast connecting bridges at sufficient density just before the percolation threshold. In addition, the enhancement is possible only if the charge carriers can actually travel throughout the bulk rather than the polymer/dielectric interface. This simple approach should prove useful to the development of high on/off ratio and high mobility of OFETs by integrating CNTs network.

Fig. 5. The distribution of metallic and semiconducting CNTs obtained by MicroRaman spectroscopy.

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Acknowledgments This work was financially supported by the IPPP Grant (PV120/ 2012A), HIR chancellery (J-00000-73581), MOHE-HIR (HIR F0006) and UMRG Grant (RG06/AFR) from the University of Malaya. References [1] P.F. Baude, D.A. Ender, M.A. Haase, T.W. Kelley, D.V. Muyres, S.D. Theiss, Appl. Phys. Lett. 82 (2003) 3964. [2] J.T. Mabeck, G.G. Malliaras, Anal. Bioanal. Chem. 384 (2006) 343. [3] Y. Kato, T. Sekitani, M. Takamiya, M. Doi, K. Asaka, T. Sakurai, T. Someya, IEEE Trans. Electron Devices 54 (2007) 202. [4] R. Zakaria, K.L. Woon, C.L. Chua, Appl. Phys. Express 5 (2012) 082002. [5] C.T. Lin, C.H. Hsu, I.R. Chen, C.H. Lee, W.J. Wu, Thin Solid Films 519 (2011) 8008. [6] R.G.S. Goh, J.M. Bell, N. Motta, P.K.H. Ho, E.R. Waclawik, Superlattice. Microst. 46 (2009) 347. [7] H.J. Lee, S.J. Oh, J.Y. Choi, J.W. Kim, J. Han, L.S. Tan, J.B. Baek, Chem. Mater. 17 (2005) 5057. [8] B.F. Jogi, M. Sawant, M. Kulkarni, P.K. Brahmankar, J. Encapsulation Adsorpt. Sci. 2 (2012) 69.

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