Organic Optoelectronics

Nir Tessler

EE Dept. Microelectronic Center & Nanoelectronic Center

Technion

The material:

Organic semiconductors = molecular materials

small molecules to long polymer chains.

Length scale: Typical layer thickness – 50-100nm Inter chain distance ~ 0.5nm

Semiconducting Organic Polymers

Soluble Optoelectronic materials

New Functional Materials New/Improved Devices

Chemically madeQuantum Dots

Semiconducting-polymer devices

Chemical SynthesisOf semiconductingMolecules/polymers

BiologyGenetic engineering

Uri Banin Nir Tessler Yoav Eichen Gad Schuster

New Methods for

Material Assembly

Molecular SizeDevices

Libraries & precisionIn Polymersynthesis

Macro- Devices

New Functionalities“plastic” emitting at

1.3-1.5m

Nano Science and Technology Interdisciplinary Collaboration

Chem. Dept Chem. Dept Biology DeptEE Dept

New Functionalities

Polymers emit visible light only! But optical communication elements operate with near infrared light can’t use polymers?

OK – Lets mix

ZnSe

0.99

1.26

CdSe

InAs

Shell

0.46

V

Eg

0.92

5 nm

U. Banin, Hebrew University,Jerusalem

Functional

Glass

Ca\Al (cathode)

PEDOT/ITO (Anode)

nMeO

O

nMeO

O

Polymer

nanocrystal

V-

+

Current/Energy is first injected into the polymer

Energy/ChargeTransfer to the nanocrystal

Light Emission

What do we hope to achieve by mixing

Composite Emission in the NIR(>20V%NC)

1000 1200 1400 1600 1800

Lum

ines

cenc

e (

a.u

.)

Wavelength (nm)

Size A Size B Size C~1% electroluminescence external efficiency.

N. Tessler et. al., Science, 2002

Tessler et. al.,Science, 2002

Plastics for telecomm

The potential Impact

We are not yet capable to accurately predict the optoelectronic properties associated with a given chemical structure

A Known Solution:Make many structures (libraries)Screen and cross correlate properties structure

A TIP:In biology, Peptide synthesis, is a fully developed automatic method.

Libraries

Libraries of -Conjugated Polymersfrom Fiction to Science?

S

O

O

S

SO2NNH

O

O

NH

S

O

O

NH

SO2N

O

Trimer

Dimer

SNO

O H

H

fmoc

(4)

Solid State Peptide SynthesisFirst Attempt

S. Tal, Y. Eichen

will the material remain a semiconductor when such unit is used as a linker?

Peptide synthesis valid for conjugated molecules/polymers

Gate voltage dependence

5 10-10

1 10-9

1.5 10-9

2 10-9

2.5 10-9

3 10-9

3.5 10-9

-30 -20 -10 0 10 20 30

I D (

A)

VGS

(V)

VDS

=10

ID IS

VG

Peptide

SiO2

Au Au

Au

Field Effect Transistor

=0VG > 0

What Do we (in EE) contribute to this

material/chemistry dominated field?

• Investigate the internal processes.

• Learn how the material property affect the device performance.

• Know how to extract the material properties as manifested in a working device.

• Provide feedback to materials/chemistry

Level 1

• The material is made of discrete molecules.• Charge is hopping between sites that are

distributed in space and energy.

Charge Transport

Monte-Carlo Simulation

Gaussian Density of States(width )

E

Y. Bar

What do we extract from it?

• Can we apply statistics to describe the motion.

• Can we use Fermi-Dirac Statistics?

• Can we define a quasi-equilibrium?

Can we use the notion of: mobility () diffusivity (D)

If yes we can use/create device models

Assume:1. Yes, we can use D and 2. Energy distribution of sites- Gaussian

What value/function (E,n) should we use in a device model?

Ratio Between D and (Einstein relation)

The semiconductor is degenerate for (almost) every practical density

Built-in charge density dependence of the transport phenomena

LED device

FET device

0

2

4

6

8

10

12

14

10 10 10 10 10 1015 16 17 18 19 20

Charge Density (cm-3)

Ein

stei

n R

elat

ion

(q

/kT

)

=10 kT

=5 kT=3 kT

Y. Roichman

Use the Physical picture to study A very practical problem

HOMO Level(Valence “band”)

LUMO Level(Conduction)

Metal

Energy

E

Standard text book Model :

kT

E

e

Contact region Bulk region

x

Hot carriers thermalization

Cold carriers thermalization

Carriers diffusion at potential

Organic LED transport in the presence of energy disorder

B

Device Model of a contact region

3-10nm

Y. Preezant

The contact as simulated within a DEVICE model

Introducing the effect of the Einstein Relation Only

=4kT

=2kT

<<kT

Using text book analysis:

1. E is temperature dependent

2. Contact phenomena is unpredictable

1012

1013

1014

1015

1016

1017

1018

1019

1020

0 20 40 60 80 100

Ca

rrie

rs c

on

cen

tra

tio

n [

cm

-3]

Distance From Metalic Contact [nm]

Using the NEW model:

1. Good Agreement between Theory and Experiment

2. Contact phenomena is predictable

(As was shown on the poster)

The contact phenomena(Through 2D modeling of Field Effect Transistor)

polymer

Cha

rge

Den

sity

(10

18cm

-3)

Insulator

Length (m)

Depth (m)

Drain Source

S’D’

t=t

D’ S’

Cha

rge

Den

sity

(10

18cm

-3)

Length (m)

Depth (m)

polymer Insulator

Drain

Source

Channel Charge at steady state VS = 0VVD = -3VVG = -5V

D’S’

Track the charges, Potential drops (associated with the structure), ….

Know how to translate Device performance to Material Performance

Close the loop with the Material/Chemistry part

Chemistry/Materials

Device Modeling

Device Design & measure

Analysis and extraction ofproperties

New Functionalities Novel Materials

Thanks To

Yoav EichenShay Tal

Uri BaninMiri KazesShi-hi Kan

Vlad MedvedevYevgeni PreezantYhoram BarYohai RoichmanNoam RapaportOlga SolomeshchAlexey RazinYair GanotSagi Shaked

Avecia Chemicals

Israel Science Foundation

European Union FW-6$