P1-13 / D. H. Kim
IMID 2009 DIGEST •
Abstract
We introduce the new modification approaches of photocrosslinkable poly (4-vinylphenol) (PVP) for low hysteresis organic thin film transistors (OTFTs). The dielectric layers were composed of different PVP resin, low molecular melamine, and halogen free photo-initiator. The low hysteresis OTFT from one of the organic gate dielectrics has been realized. The electrical performance of low hysteresis OTFT with photocrosslinkable PVP exhibited a fieldeffect mobility of 0.2 cm2/Vs, a threshold voltage of -0.04V, hysteresis of 0.4V.
1. Introduction
Organic thin film transistors (OTFTs) are in interest for flexible displays and low cost radio frequency identification tags (RFIDs) [1,2]. However, because of high curing temperature, poor chemical resistance and instability of the organic materials against atmospheric moisture (H2O) and oxygen (O2), the issues of stability of OTFTs are still key hurdles for real product applications. [3,4]
To lower a curing temperature and to suppress the hysteresis in organic thin film transistors with bottom contact structure (BC-OTFTs), we have developed new photocrosslinkable OGI which are the photo-curable poly (4-vinyl phenol) (PVP) with less-hydroxyl group (-OH), and investigated the performance of BC-OTFTs using the modified photocrosslinkable PVPs as a gate dielectric materials.
2. Experimental
As shown in Figure. 1, we fabricated bottom contact
Figure.1 A Schematic cross-sectional view of bottom
contact pentacene based TFTs. (channel length L=5 - 40μm and width W=200μm).
pentacene OTFT devices on 3 different photocrosslinkable PVPs. For comparison with modified PVPs, PVP-A was synthesized with conventional PVP resin and photo-initiator. PVP-B and PVP-C were synthesized with poly vinylphenol-styrene based resin and modified resin, respectively.
Prior to the deposition of chromium (Cr) gate electrode, the glass substrates were successively cleaned up with acetone, methanol, isopropanol and distilled water cleaning for each 10 min in ultra sonic bath. The Cr gate electrodes were prepared by dc magnetron sputter and then were patterned by photo lithography. Various photocrosslinkable PVPs were coated by two-step spin coating and cured in vacuum oven at 120℃ - 200 ℃ for 1hr. After curing process, samples were exposed by UV lamp, which of intensity was 50mJ/cm2. Thickness of PVPs was about 400nm, which was measured by surface profiler (Alpha-step). Gold (Au) source and drain electrodes were patterned by conventional photolithography method. Gold films were deposited by thermal evaporation in a vacuum chamber and final thickness was 60nm. Channel width (W) and length (L) of OTFTs were 200 and 40μm,
Low Hysteresis Organic Thin Film Transistors with
Modified Photocrosslinkable Poly (4-vinylphenol)
Doohyun Kim
1, Hyoungjin Kim
1, Byunguk Kim
2, Weyong Kim
2, Hojin Kim
2,
and Munpyo Hong
1*
1
Department of Display Semiconductor Physics, Korea University, Chungnam 339-700, Korea
2
Materials Business dept. 1, Dongjin Semichem, co. ltd., Yodang-ri 625-3, Hwasung-si, Gyeong-do, 445-931, Korea
TEL: 82-41-860-1321, e-mail: [email protected]
P1-13 / D. H. Kim
• IMID 2009 DIGEST
Figure.2 FTIR absorption spectra of PVPs on
glass substrate.
respectively. Pentacene was deposited by vacuum thermal evaporation at 1.0 * 10-7 torr and deposition rate was fixed 0.2 Å/s and final thickness was 40nm.
We have investigated the dielectric property of polymers by means of C-V measurements and the formation of hydrogen bonds by the Fourier transform infrared spectroscopy (FTIR). The electrical performances of OTFTs were measured by semiconductor parameter analyzer (HP4156C) in dark spaced probe station.
3. Results and discussion
Figure.2 shows the FTIR absorption spectra of PVPs. PVPs shows O-H stretching bands at 3540 and 3340 cm-1, which correspond to free (non hydrogen-bonded) and hydrogen-bonded O-H groups, respectively [5,6]. Therefore, non hydrogen bonded group at 3540 may easily respond to the applied electric field and attract charge carriers. From these phenomena, hysteresis has been observed by slow polarization. As shown in figure.2, O-H group of PVP-B and PVP-C is less than that of PVP-A. Therefore, low hysteresis OTFTs with PVP-B and PVP-C would be expected. Electrical performances of OTFTs with 3 different PVPs are in good agreement with FTIR spectra. Figure.3 shows the transfer curve of OTFTs with 3 different PVP. Hysteresis of OTFT with PVP-A is 12.78V. On the other hand, PVP-B and PVP-C are 3.08 and 0.41 V, respectively. Performance parameter was summarized in Table.1.
-40 -20 0 20 40 0.0 500.0 1.0m 1.5m 2.0m 2.5m 3.0m (-Drain cu rren t) 1/ 2 ( A 1/ 2 ) (a) PVP-A Gate Voltage (V) 10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 - Dr ain Cur re nt (A) -40 -20 0 20 40 0.0 500.0 1.0m 1.5m 2.0m 2.5m 3.0m (-Drain curre n t) 1/ 2 ( A 1/ 2 ) (b) PVP-B Gate Voltage (V) 10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 - Dr ain Cu rr en t (A) -40 -20 0 20 40 0.0 500.0 1.0m 1.5m 2.0m 2.5m 3.0m (-Drain c u rren t) 1/ 2 ( A 1/ 2 ) (c) PVP-C Gate Voltage (V) 10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 - Drain Cu rren t (A)
Figure. 3 The electrical performance of OTFT
with 120℃ cured (a) PVP-A, (b) PVP-B, (c) PVP–C.
P1-13 / D. H. Kim
IMID 2009 DIGEST •
4. Summary
In this paper, bottom contact OTFTs with 3 different PVPs have been fabricated. To lower the curing temperature and to suppress the hysteresis, PVP-B and PVP-C were synthesized with poly vinylphenol-styrene based resin and modified resin, respectively. And also, these PVPs have a low curing temperature.
Acknowledgement
This research was supported by a grant (F0004041-2008-31) from information display R&D Center, one of the 21st Century Frontier R&D Program funded by the Ministry of Knowledge Economy of Korean Government and Program for the Training of Graduate Students in Regional Innovation which was conducted by the Ministry of Commerce Industry
5. References
[1] C. Drury, C. Mutsaers, C. Hart, M. Matters, D. de Leuw, Appl. Phys. Lett.73 (1998) 108.
[2] R. Rotzoll, S. Mohapatra, V. Olariu, R. Wenz, M. Grigas, K. Dimmer, O. Shchekin, and A.
Dodabalapur, Appl. Phys. Lett. 88, (2006) 123502. [3] Po-Yuan LO, Zing-way PEI, Feng-Yu YANG,
Yu-Rung PENG, Yu-Chung LIN, and Yi-Jen CHAN JJAP. Vol.46 2714 - 2716 (2007).
[4] Sangyun Lee, Bonwon Koo, Joonghan Shin, Eunkyoung Lee, and Hyunjeong Park, Appl. Phys. Lett. 88, 162109 (2006).
[5] L. F. Scantena, M. G. Brown, and G. L. Richmond, Science 292, 908 (2001).
[6] S. H. Kim, J.Y. Jang, H.Y. Jeon, W.M. Yun, S.J. Nam, and C.E. Park, Appl. Phys. Lett. 92 (2008).
Table.1 Electrical characteristics of BC-OTFTs with 120℃cured PVPs
Gate insulator Performance parameter PVP_MC PVP_MC73 PVP_NMC Vth (hysteresis) [V] -0.55 (12.78) 2.98 (3.08) 0.04 (0.41) Mobility [cm2/V•s] 0.147 0.126 0.143 S-Slope [V/Dec] 2.56 3.0 1.69 Ion/Ioff ∼ 7 ∼ 7 ∼ 7
