Enhancing the Performance of Organic Thin-Film Transistor using a Buffer Layer

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3 Proceedings of the 9th International Conference on Properties and Applications of Dielectric Materials July 19-23, 29, Harbin, China L-7 Enhancing the Performance of Organic Thin-Film Transistor using a Buffer Layer Chung-Ming Wu 1, Shui-Hsiang Su 1 *, Shu-Yi Kung 1, Meiso Yokoyama 1, Tsung-Syun Huang 2, Yan-Kuin Su 2 1 Department of Electronic Engineering, I-Shou University Kaohsiung, 84, Taiwan, ROC 2 Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan, 71, Taiwan * shsu@isu.edu.tw Abstract: Vanadium oxide ( ) and 4,4',4''-tris(N-3-methylphenyl-N-phenyl-amino)-triphen ylamine (m-mtdata) doped film, respectively, is used as a buffer layer between source/drain electrodes and organic active layer in organic thin film transistors (OTFTs) to improve the electrical characteristics. Results show that the saturation current, mobility, threshold voltage, and on/off ratio are improved of the proposed OTFT with a buffer layer, especially on/off ratio is about five times than that of the device without a buffer layer. Keywords: buffer layer; OTFTs; on/off ratio Introduction Organic thin film transistors (OTFTs) have attracted much attention in developing the commercial electronics including smart cards, sensor, radio frequency identification tags (RFID), e-paper, and flat panel displays.[1-4] Due to the potential of low manufacturing cost, low process temperature, and serving as the driving circuits of a display pixel so as to facilitate the reduction of optical energy loss.[5,6] However, the crucial parameters, on/off ratio and field-effect mobility, of OTFT are lower than the amorphous silicon thin-film transistors (α-si TFTs) and low temperature poly silicon (LTPS). Improving the performance of OTFT using a buffer layer is extensively used. Hu et al. applied 4,4',4''-tris{N,(3-methylpheny)-Nphenylamino}- triphenylamine) (m-mtdata) to enhance charge-injection similar to the hole injection layer in organic light-emitting diodes (OLEDs).[7] Chu et al. inserted a transition metal oxide, MoO 3, as a hole injection layer between the source-drain contact and organic active layer and the current were significantly improved.[8] Moreover, vanadium oxide (V2O5) was recently demonstrated that it can be used as the anodic buffer layer and an interfacial layer in OTFTs.[9] In this study, the influences of various materials, m-mtdata, and :m-mtdata (9:1), were used as a buffer layer between source/drain electrodes and organic active layer in OTFTs to improve the electrical characteristics. Experiment Figure 1 shows the OTFT configuration examined in this work. The device s structure is glass/indium-tin oxide (ITO)/poly(methyl methacrylate) (PMMA)/pentacene/buffer layer/au (source/drain)., m-mtdata and m-mtdata doped films, respectively, are used as the buffer layer. Prior to dielectric layer coating, ITO coated glass substrates with a sheet resistance of 2 Ω/square were solvent cleaned by ultrasonic baths in acetone and isopropyl alcohol then in deionized water, followed by UV ozone treatment for ten minutes. The gate dielectric layer, PMMA, was dissolved in chlorobenzene with a concentration of 6 wt %, and then was coated on the ITO glass substrates by spin-coating, followed with an annealing treatment at 1 C for one hour. Organic active layer, buffer layer and cathode material were sequentially deposited through a shadow mask by thermal evaporation. Deposition began with 6 Å-thick pentacene as an organic active layer. Finally, 12 Å-thick Au was deposited as source and drain electrodes. The channel length and width of the device was 5 and 5 μm, respectively. The organic layers and source/drain electrodes were evaporated at a pressure below torr, and the evaporation rate was controlled at 1-2 Å/s. The thickness of the evaporation layers was controlled by oscillating quartz monitors and further calibrated by ellipsometric measurements. The Electrical characteristics of OTFTs were determined by a Keithley 24 programmable voltage-current source system. All measurements were at a glove box. Fig. 1 Schematic architectures of the OTFT with a buffer layer between pentacene and source/drain electrodes. Results and discussion Figures 2 (a) ~ (d) show the source-drain current ( ) versus source-drain voltage ( ) curves of OTFT without a buffer layer and with a buffer layer of, /9/$ IEEE 1241

4 m-mtdata, and :m-mtdata (9:1 in wt. ratio), respectively. The OTFTs using pentacene as an active layer work in a accumulation mode. From these data, the highest of -4 μa was obtained from the OTFTs using :m-mtdata (9:1) at a of -6 V. At the same, the OTFTs without a buffer layer and with a buffer layer of and m-mtdata have the of -15.9, -4 and μa, respectively. It was found that the of pentacene-based OTFTs with as a buffer layer is larger than that of the OTFT without a buffer layer. It suggests that created a more homogeneous adhesion between the active layer and S/D electrodes.[9] (ua ) -25 = V =-1 V (a) -2 =-2 V =-4 V =-3 V =-5 V V -15 GS =-6 V = V = -2 V = -4 V = -6 V (V ) (b) = V = -2 V = -4 V = -6 V = -1 V = -3 V = -5 V = -1 V = -3 V = -5 V (c) = V = -2 V = -4 V = -6 V = -1 V = -3 V = -5 V (d) Fig. 2. The source-drain current ( ) v.s. source-drain voltage ( ) curves of OTFTs (a) without a buffer layer and with a buffer layer of (b), (c) m-mtdata and (d) :m-mtdata (9:1), respectively. Figures 3 (a) and (b) show the transfer characteristics of versus and the square root of the versus curve at a fixed of 5 V. I on /I off is the ratio of the highest current and the lowest current. Threshold voltage and filed-effect mobility were extracted from Fig. 3(b) with Eq. 1 in the saturation regime. 2 ID = W μc i (VG - VT ) 2L Eq. (1) where μ is the field-effect mobility, L and W are channel length and width, respectively, Ci is the insulator capacitance per unit area, and V T is the threshold voltage. The characteristics of in the saturation regime, I on/off, the V T and the field effect mobility for various buffer layer materials for all OTFTs are summarized in Table I. It was found that inserting a buffer layer of, m-mtdata, and :m-mtdata (9:1) can significantly improve the electrical characteristics of the pentacene-based OTFTs, especially in mobility and on-current. It might be attributed to the slight mismatch between the work function of Au and the highest occupied molecular orbital (HOMO) level of pentacene. Besides, it might be caused by metals deposited onto the pentacene surface either doping the upper layer of pentacene, or diffusing into pentacene to form a metallic. The modified buffer layer interface provides protection against metal diffusion into the organic layer and a chemical reaction between organic and metal electrodes. The improvements in I on/off and V T of OTFTs by inserting m-mtdata suggest that m-mtdata plays an important role to reduce the contact resistance and enhance the hole-injection. The :m-mtdata (9:1) buffer layer provides not only a more homogeneous adhesion but the lower contact resistance between the active layer and S/D electrodes. 1242

5 - (A) 1E-4 1E-5 1E-6 1E-7 1E-8 1E-9 without a buffer layer (a) m-mtdata :m-mtdata (9:1) TABLE I. Comparative electrical characteristics of OTFTs without and with a buffer layer of, m-mtdata, and :m-mtdata (9:1), respectively. Buffer layer (μa) I On/Off ( 1 4 ) V T μ (cm 2 /Vs) Without a buffer layer m-mtdata :m-mtdata (9:1 ) /2 (A 1/2 ) 1E V without a buffer layer (b) m-mtdata :m-mtdata (9:1) -13 V -11 V Fig. 3 (a) v.s. and (b) of -5 V for OTFTs. I DS v.s. at a fixed Conclusion We have investigated the electrical characteristics of OTFTs with a, m-mtdata and m-mtdata doped film, respectively, as the buffer layer between source/drain electrodes and organic active layer. Comparing with the OTFT without a buffer layer, a buffer layer formed by : m-mtdata (9:1 in wt. ratio) in an OTFT leads to I on/off increases from to , field-effect mobility increases from.168 to.45 cm 2 /V-s, and increases from 15.9 to 4 μa, respectively. The results shown in this work demonstrate that : m-mtdata (9:1) film can be one of the potential candidates as a buffer layer for fabricating high performance OTFTs. Acknowledgement The authors would like to thank the National Science Council of the Republic of China, for financially supporting this research under Contract No. NSC E and NSC E The authors would also like to thank the MANALAB at ISU, Taiwan. Reference [1] G. Horowitz et al, Adv. Mater. (Weinheim, Ger.) 1 (1998) 365. [2] C.J. Dury et al, Appl. Phys. Lett. 73 (1998) 18. [3] G. Horowitz et al, Synth. Met. 54 (1993) 435. [4] C. Chih-Wei et al, Appl. Phys. Lett. 87 (25) [5] T. Takenobu et al, Appl. Phys. Lett. 88 (26) [6] W. Jun et al, JOURNAL Appl. Phys. Lett. 97, (25) 2616 [7] H. Wei, Microelectronics Journal 38 (27) 59 [8] C. Chih-Wei, Appl. Phys. Lett. 87, (25) [9] C. Chih-Wei et al, Appl. Phys. Lett. 86, (25)