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SAÜ Fen Edebiyat Dergisi (2011-II) O. ÖRNEK
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KAPI DİELEKTRİĞİ SiO2 OLAN
POLY(3HEXYLTHIOPHENE) ORGANİK ALAN ETKİLİ
TRANSİSTÖR
Osman ÖRNEK
Sakarya University, Faculty of Education, Department of Science Education, Sakarya / Turkey
osmano@sakarya.edu.tr
ÖZET
Bu çalışmada Organik İnce Film Transistör (OTFT) üretiminde kullanımı
elverişli olan inorganik dielektrik katman olarak SiO2 kullandık ve bu dielektrik
katman üzerine döndürerek kaplama yöntemi ile Poly(3-Hexilthiophene) P3HT
organik yarı iletken (OSC) molekülünü kapladık. Kapı, kaynak ve oluk
kontaklar için yine termal buharlaştırma yöntemi ile Ag kapladık. P3HT OSC
için µFET=0.103 cm2/Vs alan etkili mobilite değerini, Ion/Ioff=101 açma/kapama
(Ion/Ioff) akım oranını VT=8V eşik voltajı değerini bulduk
Anahtar Kelimeler: OTFT, P3HT, SiO2, mobility
POLY(3-HEXYLTHIOPHENE) BASED FIELD-EFFECT
TRANSISTORS WITH GATE SiO2 DIELECTRIC
ABSTRACT
In this study we have used SiO2, which is actually a useful material as a
inorganic dielectric layer for Organic Thin Film Transistor (OTFT) production,
and then we have coated the Poly(3-Hexilthiophene) P3HT organic
semiconductor (OSC). We coated the gate, source and drain contacts with Ag
using thermal evaporation method. Our study has shown that on mobility
value (µFET), treshold voltage (VTh) and on/off current ratio (Ion/Ioff). For P3HT
OSC, we have reached the value as following; µFET=0.103 cm2/Vs field effect
mobility value, Ion/Ioff=101 current ratio, VT=8V threshold voltage.
Key Words: OTFT, P3HT, SiO2, mobility
O. ÖRNEK SAÜ Fen Edebiyat Dergisi (2012-II)
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1. INTRODUCTION
Organic field-effect transistors (OFET) are receiving significant attention
because of their potential use in low-cost flexible electronic applications
such as: smart pixels, radio frequency identification tags, drivers for
electronic papers, and driving circuits for flat-panel displays [1].
Conjugated polymers such as poly(3-hexylthiophene) (P3HT) are
becoming a valid alternative to amorphous silicon for the active layer in
OFETs for large area and low cost applications [2]. Regi-regular P3HT (rr-
P3HT) shows high field-effect mobility [3]. The most commonly used
device geometry is bottom gate with top contact partly because of
borrowing the concept of thin-film silicon transistor (TFT) using
thermally grown SiO2 oxide as gate dielectric. Due to advantage of being
commercially available high quality Si/SiO2 substrate, it has dominated
the whole community. Transistor parameters such as mobility and
threshold voltage are all deeply related to the chemical structure and the
dielectric properties of the insulating materials [4].
2. EXPERİMENTAL SETUP
In this study, P3HT (Fig. 1.) based OFET was produced in which SiO2
was used as a dielectric layer.
SAÜ Fen Edebiyat Dergisi (2011-II) O. ÖRNEK
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Figure. 1. Structure of P3HT
A bottom-gate with top contact device structure was shown in the Fig.2.
At the first stage of the production, some SiO2 was removed by
hydrofluoric acid from one side of the silicon surface. Then, Si wafers
were cleaned by acetone, methanol, ethanol and deionized water
respectively. After cleaning processes, Ag was deposited by thermal
evaporating under 5x10-6 mbar pressure on the silicon layer as gate
contact. At the later stage, P3HT which was dissolved in chloroform
solvent (10 mg/ml) was coated on SiO2 layer by spin-coating (1800 rpm
for 30s) and annealed 150 oC for 10 minutes. Finally, Ag source and drain
contacts were deposited via thermal evaporating method. The current-
voltage characteristics of our device were investigated by using a
Keithley 4200 semiconductor characterization system (SCS).
O. ÖRNEK SAÜ Fen Edebiyat Dergisi (2012-II)
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Figure. 2. Scheme of common top contact geometry for P3HT-OFET
3. DISCUSSION
The thickness of the oxygen layer of SiO2 is 100 nm and this layer’s
capacitance value is 96.8 nF/cm2. W/L ratio is 1000µm/17µm. The device
exhibits saturation characteristics at -40 V operation voltage. Field-effect
mobility (µFET=0.103 cm2/Vs), thershold voltage (VT=8V) and current
on/off ratio (Ion/Ioff=101) were obtained from output (Fig. 3.) and transfer
characteristics (Fig. 4 (a).).
Finally, as shown in Fig. 4(b), the field effect mobility μFET can be
determined from the slope of a plot between |IDS|0.5 versus VGS output
curves derived Eg.1. The extrapolated x-intercept of this plot yields the
value of the threshold voltage.
2
2i
DS FET GS T
WC
I V V
L
(1)
SAÜ Fen Edebiyat Dergisi (2011-II) O. ÖRNEK
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We have calculated the mobility value of the SiO2/Pentacene based OTFT
in its saturation zone from the slope () of the best fitted curve applying
the formula shown in Eq. 2.
12
2i
WC
L
(2)
Figure. 3. IDS-VDS output characteristic of P3HT-OFET with SiO2 as gate
dielectric layer.
O. ÖRNEK SAÜ Fen Edebiyat Dergisi (2012-II)
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Figure. 4. (a) IDS-VGS transfer characteristic and (b) (IDS)0.5-VGS characteristic
of P3HT-OFET with SiO2 as gate dielectric layer
(a)
(b)
SAÜ Fen Edebiyat Dergisi (2011-II) O. ÖRNEK
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4. RESULT
We calculated the values of field effect mobility and on/off current ratio
of fabricated OFET in the experimental section. The values we obtained
are smaller compared to those found in the literature. In literature, the
reason that rr-P3HT is found higher by using same materials is due to
some interface improvements [5]. The reason why the SiO2 dielectric
layer did not show good interface characteristics with rr-P3HT thin layer
in our OTFT devices is related to the organic/inorganic incompatibility of
the interface. On the other hand, the mobility of our devices and their
performances is improved and it is considerably high compared to the
polymeric based transistors since dielectric layer was inorganic.
5. REFERENCES
[1] S.P. Tiwari, W. J. Potscavege, T. Sajoto, S. Barlow, S. R. Marder, B.
Kippelan, Organic Electronics 11 (2010) 860-863
[2] G. Scavia, W. Porzio, S. Destri, L.Barba, G. Arrighetti, S. Milita, L.
Fumagalli, D. Natali, M. Sampietro, Surface Science 602 (2008) 3106-3115
[3] H. Yang, T.J. Shin, L. Yang, C.Y. Ryu, Z. Bao, Adv. Funct. Mater. 2005,
15, No.4, April
[4] A.Wang, I. Kymissis, V. Bulović and A.I. Akinwande, Appl. Phys.
Lett. 89 (2006) 112109-112109.
[5] Vibha Saxena, A.K. Chauhan, N. Padma, D.K. Aswal, S.P. Koiry,
Shashwati Sen, R.B. Tokas, S.K. Gupta, C. Sürgers, J.V. Yakhmi, Thin
Solid Films, 517, 6124, 2009