OFET (Organic Field Effect Transistor)
Elektronika Organik maulana.lecture.ub.ac.id maulana.lecture.ub.ac.id
Transistor Transistor Organik Organik – – OFET OFET (Organic Field Effect Transistor) (Organic Field Effect Transistor) Basic Structure of OFET
Skematik OFET (a) metal-insulator-semiconductor FET (MISFET); (b) metal-semiconductor FET (MESFET); (c) thin-film transistor (TFT). Struktur dan Mekanisme OFET
Adachi lab.-kyushu University
Organic Transistors
OFET
1. Organic electrochemical transistors (OECTs)
2. Organic field effect transistors (OFETs)
3. Electrolyte-gated OFETs
4. Sensors Application based on OFET
1. Organic Electrochemical
Transistors (OECTs)• Reversible oxidation and reduction switching
• Electrochemical devices uses both electrons
and ions as charge carriers
reduction
PEDOT PSS + M + e PEDOT + M PSS oxidation
Conducting Semi-conducting Transparent
Deep blue colored
1.1.The dynamic configuration
(one area of conducting polymer) Structure 1
- Reduction at the negatively biased side of electrode
• oxidation at the positively biased side of electrode
- Dynamic behavior
- + M + M + M -
-
-
e
red ox
e e - +
V
1.2 The bi-stable configuration
(two areas of conducting polymer) Structure 2
• Reduction at the negatively biased electrode
- Oxidation at the positively biased electrode
- Bi-stable behavior
red
PEDOT PSS + M + e PEDOT + M PSS ox
- +
M
+ M + M - e - - e
red ox
- - +
V
Flexible substrates
A flexible organic electrochemical transistorThe first transistor (1947) Size: 2.5cm
1.3 The three-terminal transistor
Common ground
Structure 2 G
V G Structure 1
V D S D
PEDOT:PSS
Electrolyte
Pinch-off
- Pinch-off due to the decrease of charge
- - +
carriers at the drain side S
D of the channel
- Almost all resistance is located within 100µm of the channel edge
- Effect of structure 1
Potential Absorption
Svensson et al. (2003) POLYTRONIC
Chronoamperometric response
• Comparison between lateral and vertical
design
Nafion
- Cation conductor, mainly protons.
- Forms inverted micelle clusters with sulphonic acid groups on the inner surface
- The micelles are joined through canals.
- Charge transport by cations wandering between –SO
- groups.
3
- Ion conduction increases with water content due to swelling and dissociation of ions.
Increase of water content Humidity sensor
- + V
G
- Transducer part: EC-transistor
G
Nafion
- Sensitive part: Nafion
D S
V D
- - G
2. Organic Field-Effect Transistors (OFETs)
semiconduct or gat e
- – – – – – – – – – – – – – – – – – –
V G sour ce dr ain insulat or
V D conduct ing channel
I D
Structure of an Organic Thin Film Transistor
2.1. Current-voltage characteristics
2.1.1. Transfer characteristic
- 5
10
- 6
ON = conduction channel open
10
)
- 7
10
(A
The charge in the channel is
t n
- 8
modulated by adjusting Vg, so
e
10 Vd=-25 V
rr
that the device behaves as a
- 9
cu
10
in variable resistance. ra -10
10 D
- 11
10 OFF = No conduction channel
- 12
10
- 20
20
40
60 80 100 Gate voltage (V)
A FET is basically a capacitor, where one plate is constituted by the gate
electrode, and the other one by the semiconductor film. When a voltage
V is applied between source and gate, majority carriers accumulate at
gthe insulator-semiconductor interface, leading to the formation of a
conduction channel between source and drain.
A potential signal V is transformed in a current signal I
g d2.1.2. Current-voltage
V W D
I C V dV V
D i G T
L No analytical solution, unless the mobility is assumed to be constant.
If V small, the charge is nearly constant over the channel and the drain current is : d
W
I C
V V
V
D i G T D
Linear L
If Vd > Vg, the channel is pinched-off:
W
2 I C
V V Sat urat ion
D sat i G T
,
L
2
2.1.3. Output characteristic
Linear regime: For a given V >0, the current provided by the conduction channel increases with V .
g d
The drain electrode inject the charge carriers passing through the channel, the channel let pass as many charges the drain electrode injects.
V controls the doping level N in the conduction channel: large V large current I g g d
- <
- 5 10
0 V
- 6
W and L= channel width and length
- 20 V
V g
)
C = capacitance of the insulator layer
i
- 40 V
- 6
(A
- 60 V
μ = field-effect mobility
t -3 10 n
- 80 V
V = threshold voltage (accounts for voltage
e T
- 6
rr
- 2 10
drops of various origin across the insulator-
u c
semiconductor interface)
in
- <
- 1 10
ra D No conduction channel
- 6
1 10
W
20 -20 -40 -60 -80 -100 -120
I C
V V
V
D i G T D
Drain voltage (V)
L
Saturation regime: For a given V , when V =V , the electrical potential between drain and gate is zero.
g d g This destroys the capacitor created between the doped channel and the gate : pinch off. The channel is then interrupted close to the drain.
- <
- 5 10
0 V
- 6
Saturation
- 20 V
V g
)
- 40 V
- 6
(A
- 3 10
t W n
2
- 80 V
e
I C
V V
- 6
rr
D sat i G T
,
- 2 10
u c L
2 in
- <
- 1 10
ra D
- 6
1 10 20 -20 -40 -60 -80 -100 -120 Drain voltage (V)
Out put charact erist ic
2.1.4. How to get the field effect mobility?
1) If V small, the charge is nearly constant over the channel and the drain current is :
d Z=channel width
Z
I C
V V
V D i G T D Linear
L
- The channel conductance g can be expanded to first order:
d 2) A further step of the method consists of introducing a contact series resistance R , which leads to s
2.2. Film morphology versus field-effect mobility
The mobility measured with a FET is characteristic for the whole film. It is thus
expected to depend on the quality of the organic film; especially the quality of
the first mono-layers deposited on the insulator n o ti u b ri ist d l a ti n te o P2.2.1. The distribution of charge in the channel
(from Poisson’s equation):
= permittivity of the organic ε
s
semiconductor q = electron charge C = capacitance (per unit area) of the
i
insulator
The first molecular layer is important!
Dimitrakopoulos, Adv. Mater. 2002, 14, 99
The channel reduces to the first monolayer
The organic TFT is a 2D device
Structural order in the first monolayer is
crucial
Monolayer thickness= 1.25 nm for tetracene
High mobility along the layers
2.2.2. Grain size dependence mobility
Polycrystalline film Grain Grain boundary (GB) (G)
1 0,8
) s
2
/V
2 1cm
/Vs G
0,6
m c ( ty
2 0,4
ili 0.01cm
/Vs b
GB o M
0,2 Length of the GB
10nm L GB
2000 4000 6000 8000 10000 Charge transport in polycrystalline media
divide the material into high (crystal grains) and low (grain boundaries) conductivity region.
As grains and grain boundaries are connected in series: R =R +R tot G GB
R= ρL/S (ρ=resistivity)
for the same surface S =S (active thickness in the FET), we can write G GB L L
- G G GB GB
ρL= ρ ρ
Conductivity σ= 1/ρ÷ pμe
if the concentration in charge carrier ”p” is similar in both regions, the effective mobility of the medium is given by
L L L L G GB G GB
G GB
2.3. Mobility and architecture evolution
Organic material can have a mobility larger than amorphous silicon Saturation with oligoacene
maybe with another molecule, mobility will go higher… Mobility for OTFT (at RT)
Discotic liquid crystals
A. M. van de Craats et al, Adv. Mater., 2003, 15, 495
Ink-jet Printed OFET’s
H. Sirringhaus, Science,290 (2000)
Fig. 1. (A) Schematic diagram of high-resolution IJP onto a prepatterned substrate. (B) AFM showing accurate alignment of
inkjet-printed PEDOT/PSS source and drain electrodes separated by a repelling polyimide (PI) line with L = 5 µm. (C) Schematic diagram of the top-gate IJP TFT configuration with an F8T2 semiconducting layer (S, source; D, drain; and G, gate). (D) Optical micrograph of an IJP TFT (L = 5 µm). The image was taken under crossed polarizers so that the TFT channel appears bright blue because of the uniaxial monodomain alignment of the F8T2 polymer on top of rubbed polyimide. Unpolarized background illumination is used to make the contrast in the remaining areas visible, where the F8T2 film is in an
H. Sirringhaus, Science,290 (2000)
A) Transfer characteristics of an IJP TFT with F8T2 aligned uniaxially parallel to the current flow (L = 5 µm, W = 3000 µm)
measured under an N2 atmosphere. Subsequent measurements with increasing (solid symbols) and decreasing (open symbols) gate voltage are shown. (B) Scaling of the output characteristics of IJP F8T2 TFTs normalized by multiplying the drain current by the channel length (dashed lines with open symbols, L = 20 µm; solid lines with solid symbols, L = 5 µm). Subsequent measurements with increasing (upward triangles) and decreasing (downward triangles) gate voltage are shown.
3. Electrolyte-gated OFETs
The use of a polyelectrolyte allows combining the advantages of the electrochemical transistors ( low- voltage <1V, robustness, less sensitive to thickness ) and the advantage of the OFETs ( fast response < 0.3ms ).
Low-cost plastic transistors
- 1) For portable applications: compatible with printable batteries (~1.5V)
Low voltage
• 2) For “one-use” applications: compatible with roll-to-roll
printing techniques
Robustness, printable electrodes, thicker layers
- 3) For logic applications:
Fast response, low capacitive currents The Challenge: To combine those properties
P(VPA-AA)
Proton migration
- 5
- 45°
- P h a s e a n g le ( d e g re e )
~0.1 V polyelectrolyte
R
170 kHz (6 μs)
Capacitive behavior EDLC builds up at the electrodes Resistive behavior Protons migrate away from the polymer chains
C
Frequency (Hz)
2 )
ff e c ti v e c a p a c ita n c e ( F /c m
90 E
60
30
10
10 Electric double layer capacitor gated OFETs
6
10
5
10
4
10
3
10
2
10
Electric double layer capacitors (EDLCs)
- 6
1. Protons migrate to the gate and form an EDLC
2. Simultaneously, holes injection at Au/P3HT contact and formation of an EDLC at the P3HT-polyanion interface 3. The channel is open
V G =
Transistor characteristics
- 1.0 V
RR-P3HT
SiO
I D ( µ A )
Au Au Si wafer
54 nm P(VPA-AA) Au
2 Ti
- 0.5
- 1.0
- 1.5
- 0.6 V -0.8 V
- 0.2 V -0.4 V
0.0 V
- 1
V G
s
2 V
Extracted mobility: ~0.012 cm
I
V V C L W
2 T G i sat D
2
)
1 -3 A ½
½ (
D )
(
(V)
D ( A )
V D (V)
1.5 0.0 -0.2 -0.4 -0.6 -0.8 -1.0
0.0
L = 9 µm, W = 200 µm
- I
- I
- 1
- 1
0.0
0.5
1.0
10 -8
= -1 V
10 -7
10 -6
V D sweep rate: 0.03 V s
0.0 -0.2 -0.4 -0.6 -0.8 -1.0
I on
/I off = ~140
V D
V T
Response Time V = -1 V; V = -
1 V when 0 s ≤ t ≤ 0.5 s
D G
- 0 V otherwise <>11.5
Rise: 60% in ~0.1 ms ) A µ
- 1.0
(
1
2 D
I
- 2
Fall: switch-off
- 0.5
90% in <0.3 ms
- 1
0.0
0.0
0.2
0.4
0.6
0.8
1.0 Time (s)
500 501 502
Time (ms) Towards the mechanism OFET is ON
A. Field-effect vs. Electrochemistry
S
1
)
V
- + H
( G
- 1
D
V P(VPA-AA)
Immobile anions
OFET is OFF S
- 1.0 -0.05
) A µ
0.00 (
D
5
10
15
20 I -0.5
0.0 D
5
10
15
20 Immobile anions
Time (s) Towards the mechanism Electrochemical transistor is ON
A. Field-effect vs. Electrochemistry
S
- 4
ClO
1
1
) )
V V ( ( G
Li G
+
- 1 -1
V V
P(VPA-AA) P(VPA-AA)
D
P(VPA-AA) + 1wt% LiClO
4
- 1.5 -1.5
Penetration of anions
- 0.10 -0.10
V =0, but not G
- 1.0 -0.05
- 1.0 -0.05
) ) A A completely OFF
µ µ (
0.00
0.00 (
D D
5
5
10
10
15
15
20
I I -0.5 -0.5
20 S
- 4
ClO
0.0
0.0
5
10
15
20
5
10
15
20 Time (s)
Time (s) D
B. EDLC builds independently of the channel-gate distance
1.1 mm
Laterally gated OFET Hemispherical PE D S G
~0.2 mm
D S G
=
V G
- 1.0 V
- 1.0 V
- 0.6 V -0.8 V
- 0.2 V -0.4 V
- 0.6 V -0.8 V >0.1
- 0.2
0.0 V
(V)
V D
I D ( n A )
0.0 -0.2 -0.4 -0.6 -0.8 -1.0
=
V G
0.0 V
(V)
V D
I D ( µ A )
- 10
- 20
- 30
0.0
0.0 -0.2 -0.4 -0.6 -0.8 -1.0
Planar water gated organic field effect transistor Organic field-effect transistor with extended indium tin oxide gate structure for selective pH sensing Organic Electronics, Volume 15, Issue 3, March 2014, Pages 646-653
Organic Electronics, Volume 12, Issue 11, November 2011, Pages 1815- 1821 Fabrication Method Organic Electronics, Volume 15, Issue 3, March 2014, Pages 646-653
Planar water gated Organic Field Effect Transistor
Design and Fabrication Organic Electronics, Volume 15, Issue 3, March 2014, Pages 646-653
Planar Water Gated Organic Field Effect Transistor
Organic Electronics, Volume 12, Issue 11, November 2011, Pages 1815-1821 Organic Electronics, Volume 13, Issue 4, April 2012, Pages 533-540 Transparent and flexible organic field-effect transistor for multi-modal sensing Organic field-effect transistor with extended indium tin oxide gate structure for selective pH sensing
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Issue 5, May 2014, Pages 1050-1055 Organic Electronics, Volume 14, Issue 12, December 2013, Pages 3453-3459