Dispositivi e sensori a semiconduttore organico - Dieeannalisa/didattica/biomedica/Organici.pdf ·...

Dispositivi e sensori a semiconduttore organico

Annalisa Bonfiglio University of Cagliari, Italy

[email protected]

1: Semiconduttori organici

2: Organic Thin Film Transistors (OTFT)

3: Sensori basati sugli OTFT

Outline prima parte

Proprieta� degli orbitali del Carbonio

Molecole coniugate

Semiconduttori organici: orbitali molecolari e bandgap

Portatori di carica e trasporto nei semiconduttori organici

Influenza della morfologia sul comportamento elettrico dei film sottili organici

E

2s

2p

Fundamental state

hybrid sp3

sp3

hybrid sp

2p

sp

hybrid sp2

2p

sp2

c c c c c

1σ" 1π "

134 pm 1σ "

154 pm

1σ "2π "

120 pm

Hybridization of orbitals in Carbon

sp3

C = [He]2s22p2

H = 1s1

2s

Ibrido sp3 stato fondamentale

E

2s

2p

2px 2py

AB4: tetraedrica

2pz

For instance: CH4

109°28’


fig.1.1: un atomo di carbonio ibridizzato sp2 [3].

Hybrid orbital sp2 Top view Side view


fig.1.2: la sovrapposizione orbitalica nel doppio legame carbonio-carbonio [3].

Molecular Orbital= linear combination of atomic orbitals (LCAO)

Carbon-Carbon bonding

C C

H

H

H

H

Π bonding"

σ bonding" σ bonding"

C C H

H

H

H H H C C H

H

Carbon-Carbon bonding

C C

H

C

C C

H

C H

H H

H

C C

H

C

C C

H

C H

H H

H

C C

H

C

C C

H

C H

H H

H C6H6

C C

H

C

C C

H

C H

H H

H

Real molecule

Limit formula

Coniugated molecules

Limit formula

MEH-PPV LPPPT PTV

A B C

D E F fig.1.6: struttura molecolare di: A) trans-poliacetilene (PA); B) poli-para-fenilene (PPP); C) poli-fenile-vinilene (PPV); D) poli-pirrolo (PPy); E) poli-tiofene (PT); F) poli-furano (PF) [4].

Coniugated molecules

fig.1.7: orbitali molecolari π leganti e antileganti [3].

Band Gap

Legame Energia di legame (kJ • mol-1)

C—C C—H C—O C—S C—F C—Cl C—Br C—I

350 410 350 260 440 330 280 240

Legame Energia di legame

(kJ • mol-1) Si—Si Si—H Si—O Si—S Si—F Si—Cl Si—Br Si—I

180 300 370 230 540 360 290 210

fig.1.10: funzioni d’onda dell’elettrone π nella buca di potenziale profonda infinito [6].

L = coniugation length = length of the shortest molecular segment with a perfect alternation of single and double bonds; N= number of atoms (each with 2 electrons); d = atomic distance

2

22

8mLhnEn =

( )2

22

82)(Ndm

hN

HOMOE!"

#$%

&

=

( )2

22

8

12)(Ndm

hN

LUMOE!"

#$%

& +=

( )( ) Nmd

hNdmhNHOMOELUMOEEG 2

2

2

22

881)()( ≈

+=−=

The highest N, the lowest the gap

Band Gap

Charge transport in organic semiconductors

Å

nm

µm

Intrachain Interchain Intergrain

+

3 coesistent mechanisms+ charge injection from contacts

Measurements difficult to explain

Charge transport in organic semiconductors

Doping and charge transport

In inorganic semiconductor, doping is substitutional and causes an increase in free charge carrier concentration. These carriers move in a periodic potential with no interaction with the crystal (band model). In organic, doping is not substitutional: supplementary charge physically interact with molecules causing a perturbation in the coniugated chain. This perturbation (charge + deformation induced by the charge) is called polaron

In analogy with inorganic semiconductors, the polaron can be roughly described as a charge free of moving along the chain strongly interacting with the semiconductor lattice

m eff (polaron) >> m eff (free charge carrier)

! mobility (polaron) << mobility (free charge carrier)

Doping and charge transport

Intermolecular transport: hopping

hopping = tunneling between crossing chains

To move, the charge carrier needs energy that can be provided by phonons (increasing with temperature)

TkdeBJ

D ⋅⋅

⋅=

τµ

2

2σ( T )= σo ( T )⋅exp[-( To /T )1/(1+d)]

But, in organic materials there is a great variety of behaviour from material to material. A unifying theory has not yet been developed.

Intermolecular transport: hopping

Materials for “Plastic Electronics”

Intergrain transport: Pentacene (C22 H14)

•  5 benzene rings •  Non soluble. Deposited by evaporation

•  Molecules stand perpendicular to the substrate. • Molecules form separated domains (grains)

15Å

15Å

Conduction: - hopping -  thermically activated -  limited by traps at grain boundaries

Correlation btw grain dimensions and mobility

trapsbulk µµµ

111+=

Intergrain transport: Pentacene (C22 H14)

8 10 12 14 16 18 20

0.000

0.005

0.010

0.015

0.020

0.025

0.030

Freq

uenc

y

height (nm)

500 nm

Influence of surface morphology

1 µm 1 µm

Pentacene on Mica Pentacene on SiO2


1 µm

Pentacene on SiO2 Pentacene on Mylar

1 µm


Outline seconda parte

Organic Thin Film Transistors (OTFTs)

Contatti metallo-semiconduttore organico

Comportamento elettrico degli OTFT

Modello elettrico del dispositivo

Tecnologia

++++++++++++++++++++++++++

Source Drain semiconductor

Insulating layer

OTFT = ORGANIC THIN FILM TRANSISTOR

Field Effect Transistors

Interface metal - semiconductor

Gate

Charge injection from metal contacts

TFT model was first developed for poorly conductive semiconductors like amorphous Si

Tipically p-type!

Thin Film Transistor

++++++++++++++++++++++++++

Source Drain semiconductor

Insulating layer

0 -20 -40 -60 -80 -1000

-10µ

-20µ

-30µ

-40µ

-50µ a) Vg=100V Vg=80V Vg=60V Vg=40V Vg=20V Vg=0V Vg=-20V Vg=-40V Vg=-60V Vg=-80V Vg=-100V

I D(A

)

VD(V)

How an OTFT works

Typical values: µ ~ 10-2: 10-1

Ion/Ioff ~104 :105

VT ~ -10:+10 V

Linear region:

Saturation region:

( )

( )221

TGoxD

DTGoxD

VVLZ

CI

VVVLZ

CI

−=

−=

µ

µ

Toff

on VII ,,µ

Important parameters:

( )TGoxsatG

D VVLZ

CVI

−=∂

∂µ

100,0 50,0 0,0 -50,0 -100,00,0

-500,0n

-1,0µ

-1,5µ

-2,0µ

-2,5µ Bottom Contact Au electrodesVt=-7.5Vµ=5.2x10-3cm2/VsZ/L=53

I D(A

)

VG(V)

Ioff

Ion

Equations and extraction of parameters

Structures and fabrication techniques

Bottom gate

Top gate

Si/SiO2 substrate Substrate-free

Free-standing insulating

layer

OSC S D

G

I Contact scheme

channel

Top contact Bottom contact Bottom contact Bottom contact

OSC S D

G

I Contact scheme

channel

OSC

Materials and fabrication techniques OSC

S D

G

I Contact scheme

channel

Materials: Electrodes: metals, conductive polymers Insulating layers: polyimide, PVA, PVP,…

Organic semiconductors

Solution processable Polymers

Small molecules

(deposited by evaporation)

Low cost technique for liquid phase materials

Deposition

Spin up

Spin off

Spin down

Process parameters: 1.  Spin up time 2.  Spin speed 3.  Spin off time 4.  Spin down time 5.  Dewetting time 6.  Dewetting T

Thermal annealing (dewetting) Adv: simple and low cost

Fabrication techniques - Spin Coating

1) 2)

3) 4)

5) 6)

Fabrication technique – Soft Lithography

Inks instead of resist for patterning metals


MicroContact Printing


Printing & Roll to Roll Printing

Possible applications for OTFTs

Outline terza parte

Sensori per variabili meccaniche

Sensori per variabili chimiche: ISOFET

Prospettive applicative

OFET based mechanical sensors

0 -20 -40 -60 -80 -100

0

-10µ

-20µ

-30µ

-40µ

-50µ

-60µID[VG=40V]ID[VG=20V]ID[VG=0V]ID[VG=-20V]ID[VG=-40V]ID[VG=-60V]ID[VG=-80V]ID[VG=-100V]Id

(A)

Vd(V)0 -20 -40 -60 -80 -100

0

-10µ

-20µ

-30µ

-40µ

-50µ

-60µ P=0 KPa P= 5.5 KPa

Id(A

)Vd(V)

Vg=-100V

Vg=40V

Vg=-20 V

Nice molecular ordering, even when deposited on “non ideal” substrate as Mylar®

I. Manunza et al., Appl. Phys. Lett. 89, 143502 (2006) I. Manunza et al., Bios. Bioelec. 22, 2775, (2007)

ID always decreases when mechanical stimulus is applied

Different OS can be employed

Tri-isopropylsilylethynyl (TIPS) Pentacene Soluble

Pentacene Not soluble

poly(3-hexylthiophene) (P3HT) Soluble

Pressure sensors: set up

Pressure Sensors: pentacene

100 50 0 -50 -100

0.0

0.2

0.4

0.6

0.8

1.0 P=0KPa P=3.7KPa P=-4.2KPa

I D n

orm

aliz

ed fo

r P=0

kP

a

VG(V)

0 100 200 300 400 500 600

-450n

-375n

-300n

-225n

-150n

0.0

4.0

ID

I D[A

]

Time[s]

VG=-40VVD=-10V

Pressure

Pre

ssur

e[K

Pa]

Current decreases when pressure is applied both for positive and negative P High sensitivity around 45 to 60% Response, rise and fall time ≈ 100ms Short TS > around 5-10s

> around 5-10s Hysteresis increases for P>2.5kPa

-5 -4 -3 -2 -1 0 1 2 3 4 5

0

10

20

30

40

50

60

70

Sens

itivi

ty %

Pressure [KPa]

Pressure Sensors: P3HT

100 50 0 -50 -1000.0

0.2

0.4

0.6

0.8

1.0 P=0KPa P=3.65 KPa P=-4KPa

I D n

orm

aliz

ed fo

r P=0

kP

a

Vg(V)0 100 200 300 400 500

-2.3µ

-2.2µ

-2.1µ

-2.0µ

-1.9µ

-1.8µ

0.0

1.4

2.8

4.1

5.5

6.9

8.3

9.7 Id

Id[A

]

Time[s]

Vg=-0VVd=-10V

P

Pre

ssio

ne[K

Pa]

Current decreases when pressure is applied both for positive and negative P Low sensitivity, around 15% Response, rise and fall time ≈ 100ms Short TS > around 5-10s

> around 5-10s Hysteresis increases for P>2.5kPa

-5 -4 -3 -2 -1 0 1 2 3 4 5

0

2

4

6

8

10

12

Sen

sitiv

ity %

Pressure [KPa]

Pressure Sensors: dynamic characterization

40 41 42 43 44 45

-1300

-1250

-1200

-1150

-1100

F = 2Hz

mV

Seconds60 61 62 63 64 65

-1200

-1175

-1150

-1125F = 4 Hz

mV

seconds

100 101 102 103 104 105-1140

-1150

-1160

-1170

-1180

-1190

-1200

mV

F = 6 Hz

Seconds62,0 62,5 63,0

'1720

'1710

'1700

'1690

'1680

'1670

F "= "10"Hz

mV

S econds

Some Examples: Matrixes of sensors

Up to 16 elements in 4cm2 Area

For pressure distribution analyses

Common Ground (Source) Rows " Common Gate Columns " Common Drain Every single element can be indipendently investigated


4 5 6


1 4 7

Some Examples: Breathing monitoring

Sensor applied to a chest b a n d a g e f o r b r e a t h i n g monitoring

0 10 20 30 40 50 -8.8n -8.6n -8.4n -8.2n -8.0n -7.8n -7.6n -7.4n

I D (A

)

Time (s)

V D = -5V

V G = -40V

Regular breathing

Some Examples: Posture detection

Sensor applied to a shoe sole for posture detection and monitoring

As can be noticed, output current varies as soon as pressure changes across the active surfaces


Prototype of the main board and measurement set up

Chemical sensors: Device structure

#  The sensor structure is based on a fully flexible organic field-effect transistor (OFET)

#  The device is assembled on a flexible film (Mylar®), which acts as gate insulator

source drain

organic semiconductor

gate

insulating flexible film

control gate

functionalized sensing area

Working principle

1. Electric charge immobilized on the active area 2. Positive and negative charge

separate inside the floating gate

3. The accumulation of charge affects the channel formation

floating gates

Gold

Device processing

Pentacene

drain electrode

control-gate electrode

source electrode

drain electrode

Fabricated device

Flow cell

floating gates

#  A custom flow cell hosting two chambers with inlet and outlet channels, was developed to handle solutions involved in the assay

Flow cell

#  A custom flow cell hosting two chambers with inlet and outlet channels, was developed to handle solutions involved in the assay

#  pH sensitivity is achieved functionalizing the floating gate by anchoring NH2 groups on its surface

#  The amino groups immobilized on the sensor protonize proportionally to the concentration of H3O+ ions in solution

Ion-sensitive device: functionalization

#  The variation of the ionization state results in the formation of a positive charge sheet on the actual gate

#  The positive charge obstructs the channel formation in the semiconductor higher threshold voltage



Amino protonation curve - Graphical representation

3 4 5 6 7 8

1000

1200

1400

1600

1800

2000Rct-[Ω

]

pH

Amino protonation curve – Electrochemical Impedence Spectroscopy (EIS) measurements


•  The device has the typical behavior of an organic p-type field effect transistor, working in accumulation mode

•  The ion-sensitivity was tested measuring the variation in the drain-source current induced by the change in the pH value of the solution

•  The testing was carried out by replacing the solution under investigation by means of the custom-developed fluidic system

Ion-sensitive device: experimental results

0 "20 "40 "60 "80 "1000,0

"2,0µ

"4,0µ

"6,0µ

"8,0µ

"10,0µ s ens or&& & & & &re ference &&

I D&[A

]

VD&[V ]

pH &7pH &7

0 "20 "40 "60 "80 "1000,0

"2,0µ

"4,0µ

"6,0µ

"8,0µ

"10,0µ !s ens or!! ! ! ! !!re ference !!

I D![A

]

VD![V ]

pH !2 .5pH !7

•  The testing was performed by placing solutions at different pH values on the sensor, while keeping a solution at pH 7 on the reference


0 "20 "40 "60 "80 "1000,0

"800,0n

"1,6µ

"2,4µ

"3,2µ

"4,0µ

"4,8µs ens or&& &pH &2.5

I D&[A

]

VD&[V ]

0 "20 "40 "60 "80 "1000,0

"800,0n

"1,6µ

"2,4µ

"3,2µ

"4,0µ

"4,8µ

I D#[A

]V

D#[V ]

s ens or## #pH #3.5

0 "20 "40 "60 "80 "1000,0

"800,0n

"1,6µ

"2,4µ

"3,2µ

"4,0µ

"4,8µ

I D#[A

]

VD#[V ]

#s ens or## #pH #4

0 "20 "40 "60 "80 "1000,0

"800,0n

"1,6µ

"2,4µ

"3,2µ

"4,0µ

"4,8µ

I D#[A

]

VD#[V ]

#s ens or## #pH #5

0 "20 "40 "60 "80 "1000,0

"800,0n

"1,6µ

"2,4µ

"3,2µ

"4,0µ

"4,8µ

I D#[A

]

VD#[V ]

s ens or## #pH #6


•  Maximum drain current measured on both the sensor and the reference with varying pH (currents values normalized with reference to the current at pH 6)

2 3 4 5 60

20

40

60

80

100

))s ens or))re ferenceI D

)/ID@

)pH)6)[%

]

pH


•  Variation of the effective threshold voltage of the sensor with varying pH of the solution

2 3 4 5 6

&52

&48

&44

&40

&36

&32

VTH,[V

]

pH

amino groups protonation

3 4 5 6 7 8

1,0k

1,2k

1,4k

1,6k

1,8k

2,0k

Rct/[Ω

]

pH


2 3 4 5 6 720

40

60

80

100

120

I D,/,I D,@

,pH7

pH

#  Trend of the maximum sensor drain current with varying the pH towards more acid solutions


– Electronic skin – Biomedicine

Physiological parameter monitoring

–  Smart textiles – Wearable electronics

Conclusions: OFETs based sensors perspectives

Dispositivi e sensori a semiconduttore organico - Dieeannalisa/didattica/biomedica/Organici.pdf ·...

Documents

Transcript of Dispositivi e sensori a semiconduttore organico - Dieeannalisa/didattica/biomedica/Organici.pdf ·...