P1-100 / S. H. Lee
• IMID 2009 DIGEST
Abstract
We have applied self-assembled monolayer to make high performance and stable OTFT on the organic gate dielectric. The β-phenethyltrichlorosilane (SAM) was coated on the organic gate dielectric and then active layer was printed. Significant improvements in on-currents and threshold voltage shift were achieved for the SAM treated devices compared to device without SAM.
1. Introduction
Organic thin film transistors (OTFTs) are of increasing attention for the low cost electronic devices such as RFID tags, flexible displays, smart cards. [1-5] Recently, OTFTs have focused on the development of practical OTFT materials which have not only high performance but also stability in the operational environment. [6] Although OTFT with pentacene or soluble pentacene derivatives showed high electrical performance such polyacenes have the serious disadvantage of readily undergoing air and photo-oxidation, leading to degraded OTFT performance. [7]
To overcome oxidation stability of p-type organic semiconductors, many groups developed organic semiconductors with deep HOMO level and wide band gap. However the OS with deep HOMO level has a large injection barrier to Au electrode. Because of large injection barrier to contact electrode, the OTFT has the disadvantages of large threshold voltage and high large contact resistance. Kano’s group improved the injection barrier by using MoOx at the
contact between electrode and organic semiconductor. [8]
In this work, we have synthesized a new active material (4-pentylphenylethynyl)-di-thienyl-anthr acene (PPEAnt) having a wide band gap and deep HOMO level and made a printed OTFT with the organic semiconductor.
We compared the OTFT properties with that with pentacene derivatives. It is found that the PTS treatment on the gate insulator decreases the threshold voltage and increases the field effect mobility significantly.
2. Experimental
We have synthesized wide band gap materials, PPEAnts with starting material 2, 6-dibromo anthraxquinone that is purified by re-crystallization in methylene chloride. Figure 1 shows synthesis scheme of PPEAnts which is pentacene like structure and have same pentylphenylethynyl side groups to study electrical properties of OTFT and expected PPEAnt have deeper HOMO level and wider band gap than that of (4-pentylphenylethynyl)pentacene (PPE PEN). [9]
We have fabricated bottom contact transistor with PPEAnt. Polyethylenenaphthalate(PEN, Dupont Teijin) was used as a substrate. AlNd was deposited on the substrate as a gate electrode by sputtering and patterned by photolithography process.
Threshold Voltage Shift in
(4-pentylphenylethynyl)-di-thienyl-anthracene Organic Thin-film Transistor with
Self-assembled Monolayer.
Sun Hee Lee
1,2, Sung Hoon Kim
1,2, Seung Hoon Han
1,2, Min Hee Choi
1,2,
Yong Bin Jeong
1,2, Dong Joon Choo
1,3and Jin Jang
1,1Advanced Display Research Center, Kyung Hee University, 1 Hoegi dong,
Dongdaemoon-gu, Seoul 130-701, Korea
2Dept. of Information Display, Kyung Hee University, 1 Hoegi dong, Dongdaemoon-gu,
Seoul 130-701, Korea
2Dept. of Chemistry, Kyung Hee University, 1 Hoegi dong, Dongdaemoon-gu, Seoul
130-701, Korea
Tel.:82-2-961-0270, E-mail: [email protected]
P1-100 / S. H. Lee
IMID 2009 DIGEST •
(a) 2-(Tributylstannyl)-thiophene, Pd(PPh3)4, Toluene, 100 ℃, 21h
(b) 1-Ethynyl-4-pentylbenzene, n-BuLi, THF, -78 ℃ → rt, 3h ; SnCl2, 10 % HCl, 60℃, 30min
Fig. 1. Synthesis scheme of PPEAnt by 2-step reaction
Gate dielectric of poly(4-vinylphenol)(PVP) (550 nm) with cross-linking agent was spin-coated on the substrate and cured in a vacuum oven at 180 oC for 4
hrs. Then, PVP was etched to make contact holes. As source/drain electrodes Cr/Au (5 nm/50 nm) were deposited by sputtering and patterned by photolithography. After source/drain patterning process, poly(vinylalcohol) (450 nm) solution was spun on the substrate and patterned by UV exposure to make bank pattern.
To reduce injection barrier we treated Au electrode surface with pentafluorobenzenethiol (PFBT) which has ~ 5.3 eV work function value however injection barrier still remain. [10] And we treated gate dielectric surface with PTS by immersion process. After PTS treatment we deposited PPEAnt solution by spin-coating and ink-jet printing, respectively. And printed TFTs were annealed on the hotplate at 80 oC for 30 min to eliminate residual solvent in the active layer.
3. Results and discussion
Figure 2(a) shows UV-spectrum of PPEAnt solution in 1, 2-dichlorobenzene. The HOMO level was measured by cyclic voltammetry (C-V) and the energy of PPEAnt was shown in Figure 2(b). PPEAnt has wider band gap and deeper HOMO level compared to PPE PEN so that we expect that PPE Ant has large injection barrier to Au contact electrode.
In order to reduce injection barrier between Au and PPEAnt, we treated Au surface with PFBT but injection barrier still remain.
Figure 3 shows the transfer characteristics of printed PPEAnt OTFT with and without PTS treatment. Fig. 3(a) shows spin-coated PPEAnt TFT characteristics rely on SAM treatments.
We made PPEAnt TFT by spin-coating and ink-jet printing, respectively. Regardless of active layer deposition method, both of devices show similar
(a) (b)
Fig. 2 UV absorption spectrum of PPEAnt in 1, 2-dichlorobenzene (a) and its HOMO and LUMO levels (b)
Fig. 3 Transfer characteristics of PPEAnt TFT with and without PTS treatment spin-coated PPEAnt TFT
behavior with SAM treatment. (W=500 µm, L=8 µm) Both of PTS treated devices were improved than that of without ones. Significant improvements in on-currents and threshold voltage shift were achieved for the SAM treated devices compared to device without SAM treatment. Devices without PTS layer have large threshold voltage of -37.9 V (spin-coated device) or 44.8 V (ink-jet printed device) which value caused an energy barrier to form at the metal-semiconductor interface disturbing carrier injection. In contrast devices with PTS layer show improved threshold voltage of -1.2 V (spin-coated device) or -3.6 V (ink-jet printed device)
In OTFT, SAMs at the organic semiconductor/ gate dielectric interface influenced electrical properties, significantly. The presence of PTS layer improved threshold voltage and on-current characteristics in
P1-100 / S. H. Lee
• IMID 2009 DIGEST
PPEAnt device, that is similar behavior of doping effect. [11]
The threshold voltage of the PPEAnt device which has deep HOMO level semiconductor can be control by insertion of PTS layer at the organic semiconductor/gate dielectric interface. It seems that doping effect of active layer. We can observe this effect larger with deeper HOMO level active layer than that of smaller one in OTFT.
4. Summary
We have synthesized wide band gap materials, PPEAnts and have fabricated OTFT with SAM layer to make high performance and stable OTFT.
Regardless of active deposition methods, devices with SAM show improved on current and threshold voltage compared to without ones.
Acknowledgement
This research was supported by the Ministry of Knowledge Economy of Korean government.
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