P2-14 / H. J. Kim
IMID 2009 DIGEST •
Novel Approaches of Modified Poly (4-vinylphenol) for Low Hysteresis
Organic Thin Film Transistors
Hyoungjin Kim
1, Doohyun 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]
Abstract
We have investigated the new modification of poly (vinyl phenol) (PVP) for low hysteresis organic thin film transistors (OTFTs). In order to suppression of hysteresis phenomenon, synthesized various backbone structure polymeric gate dielectric. The modified polymeric dielectric was synthesized by inducing ring-shape phenol backbone structure instead of conventional chain. We could be observed that relieved hysteresis and excellent air stability from ring-shape phenol backbone structure.
1. Introduction
Organic thin-film transistors (OTFTs) have attracted considerable attention because of their high potential such as flexible display backplane, radio frequency identification tags, and smart cards (1; 2). Over the last decade, there has been noticeable progress in study of the OTFTs with organic gate insulators (OGIs) as poly (4-vinyl phenol)(PVP)(3), polyimide (PI)(4; 5), poly (vinyl alcohol)(PVA)(6; 7), and polyacrylates(8). Among these OGIs, PVP has been the most frequently chosen for high performance OTFTs (3; 9). However, the large hysteresis has observed in OTFTs with PVP because of the slow polarization (10). In general, when organic gate dielectric is used, slow polarization in OGI is due to residual polar solvent in OGI or water absorption from air. Particularly, hydroxyl groups in OGIs may affect to the slow polarization. Hence, there have been lots of reports to understand phenomenon and to reduce the hydroxyl groups of PVP for low hysteresis OTFTs. (11-14) However, there has not been studied enough to understand the effect of backbone structure of OGI on the hysteresis. In this paper, we fabricated the OTFTs, using conventional PVP and modified PVP to
understand how the backbone structure of PVP influence the hysteresis
2. Experimental
We fabricated the bottom-contact organic thin film transistor (OTFTs) on a glass substrate, and used various poly (vinyl phenol) (PVP) for gate dielectric and gold for the source and drain electrode and the pentacene for the semiconductor. Prior to spin coating of PVP solution, synthesizing the organic gate insulator mixed solution by two species of PVP, conventional poly (vinyl phenol) (PVP) (molecule weight 20,000) and modified PVP, and low molecule (melamine formaldehyde)
Figure.1 A schematic cross-sectional view of bottom
contact pentacene based TFTs. (Channel length L=20 µm and width W=200µm)
with the solvent propylene glycol mono-methyl ethere acetate (PGMEA). In order to investigate the effect of backbone structure in gate dielectrics, two different species of phenol resin that has different backbone chain. After prepared the PVP solution, patterning of chrome gate electrode on glass substrate by conventional photo-lithography method. PVP solution was coated on the patterned gate electrode and two-step heat treatment at 95℃ for 2min in hot plate and
P2-14 / H. J. Kim
• IMID 2009 DIGEST
subsequent cross-linking at 200℃ for 1hr in vacuum oven. After curing thickness of insulate layer was approximately 400nm as measured by surface profiler (Alpha-step). Gold source and drain electrode on cross-linked PVP was patterned by conventional photolithography method. Gold films were deposited by thermal evaporation in a vacuum chamber and final thickness is 60nm. Nominal channel width (W) and length (L) of OTFTs were 200 and 40µm. Pentacene were deposited by thermal evaporation in a vacuum chamber that had been in base pressure of 1.0 * 10-7 torr and deposition rate was fixed 0.1Å/s and final thickness is 40nm. Current-voltage (I-V) measurements for OTFTs were performed with a Semiconductor parameter analyzer (HP 4156C, Agilent Technologies), and capacitance versus frequency (C-f) measurements were made with an AC impedance analyzer (HP 4284A, Agilent Technologies). All electrical parameter measurement was measured at room temperature air condition in dark space.
3. Result and discussion
In order to investigated the effect of backbone structure in polymeric dielectrics. Measures the electrical properties of the conventional PVP film and modified PVP film were shown in Fig.2 and Fig.3. Fig.2 show comparative current-voltage (I-V) transfer curves obtained from our pentacene OTFTs with different dielectrics, modified PVP and conventional PVP, and Table Ι summarizes the transistor parameters extracted from the measurement. Fig.2 (a) and (b) shows interesting result that both PVP film exhibit different hysteresis. Conventional PVP TFTs shows exhibit larger hysteresis than modified PVP TFTs.
In order to observe the absorption of polar molecules, such as water or mobile ion, one more measure the same device which keep in air condition for 60 hours. Fig.2 (b) shows transfer characteristics measurement plot after air exposure. Conventional PVP TFT shows large hysteresis (0.82V to 9.0V) but modified PVP TFT was not much changed (-1.7V to -1.67V).
-40 -20 0 20 40 0.0 500.0 1.0m 1.5m 2.0m 2.5m 10-14 10-12 10-10 10-8 10-6 10-4 (a) (VDS=-10V) Gate Voltage (V) - D ra in C u rren t ( A ) (-D ra in cu rren t) 1/2 ( A 1/ 2) ConventionalSQRTID Conventional ModifiedSQRTID Modified -40 -20 0 20 40 0.0 500.0 1.0m 1.5m 2.0m 2.5m 10-14 10-12 10-10 10-8 10-6 10-4 (b) (V DS=-10V) - Dra in Cu rre n t (A ) (-D ra in cu rre n t) 1/ 2( A 1/ 2) ConventionalSQRTID Conventional ModifiedSQRTID Modified Gate Voltage (V)
Figure.2 Transfer (ID-VG) characteristics pentacene based
OTFTs with VDS = -10V, swept in both directions, of device with Length (L=40µm) and Width (W=200µm). (a) Without air exposure, (b) Air exposure (60hrs)
Table.1 Electrical characteristics of BC-OTFTs Without air exporsure Air exposure (60hrs) Electrical
Parameter Modified Conventional Modified Conventional Hysteresis [V] - 1.7 + 0.82 -1.67 + 9.0 Vth [V] -2.9 4.2 -1.7 3.05 Mobility [cm2/V·s] 0.222 0.215 0.202 0.203 S-Slope [V/Dec] 1.88 1.93 2.15 2.3 Ion / Ioff 6 > 6 > 6 > 6 >
The permittivity was modified PVP film (kmod= 4.16) and conventional PVP film (kcon= 3.89) However, two species of PVP films OH absorbance peak different each other as shown in Fig.3
P2-14 / H. J. Kim IMID 2009 DIGEST • 2000 2500 3000 3500 4000 0 5 10 15 20 25 30 A b so rb a n ce (a rb. uni t) 3340 3540 Wave number (cm-1) Conventional Modified Fig ure.3 Fourier transform infrared spectroscopy (FTIR)
absorption spectra of conventional PVP and novolak PVP film to examine the existence of hydroxyl (O-H) bond.
Fig.3 shows FTIR measurement plot for conventional and modified PVP film. Conventional and modified PVP film has O-H stretching band at 3340cm-1 and 3540cm-1, which correspond to non-hydrogen bonded O-H and hydrogen bonded O-H group, respectively. [15, 16] Both PVP resin exhibit non-hydrogen bond O-H and non-hydrogen bonded O-H group easily respond to polar molecule and attract charge carriers which cause slow polarization and large hysteresis. [17-27] Modified PVP film has non bonded OH group and lager permittivity, not exhibit particular slow polarization effect. According to the experiment result, modified PVP resin could be suppresses hysteresis phenomenon.
4. Summery
In this paper, bottom contact OTFTs were fabricated with modified PVP and conventional PVP. In other to suppress the hysteresis, modified PVPs were synthesized by different backbone that has phenol backbone chain. Modified backbone structures could be one of method to suppression of the hysteresis phenomenon.
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