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• Introduction: Organic electronics

• Fabrication and characterization of organic thin films

• Devices: solar cells, OLEDs, OFETs

blackboard part: OFET

Organic Electronics

MatWi II summer term 2015

Priv. Doz. Bert Nickel (nickel@lmu.de)

Organic Electronics

AMOLED displays

OLEDs for lightening

AP photo

Anil Duggal, who heads up GE Global

Research's Organic Electronics Project, says

sheets of organic light-emitting diodes, such as

the one above, might be the future of lighting.

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OLED TV … after 15 years R&D …

… ca 2000 EUR (LG & Samsung)

55 Zoll (140 cm)

O-Solar cells

Alan Heeger / University of California - Santa

Barbara

Science 2007

"The result is six and a half

percent efficiency," said

Heeger. "This is the highest

level achieved for solar cells

made from organic materials. I

am confident that we can make

additional improvements that

will yield efficiencies sufficiently

high for commercial products."

He expects this technology to

be on the market in about three

years.

Note: Today we are at 10%.

After using more than 150 Mil

USD investment capital,

Konarka filed bankruptcy.

Wikipedia

Copper-Indium-Gallium-Diselenid (CIGS)

How is this possible,

or what do we need for organic

electronics ?

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Silicon: microstructering and doping

Intel 80486DX2

Strukturierter Si-wafer

Organic materials (hydrocarbons) in

everydays life …

… are good insulators

all electrons covalently bound

Polyethylen (PE)

electron density

MatWi I

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electron mobility

(Drude Model)

sp2 hybridisation (C2H4):

p-orbital

s

s

s

s

s

conjugated and aromatic molecules:p-electrons

molecular orbitals (MO) of aromatic

molecules: Benzol

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Naming orbitals: HOMO and LUMO

• Nobel Price Chemistry 2000: Alan J. Heeger

poly(styrenesulfonate),

Doped polymers are conducting: PDOT : PSS

Nobel prices for aromatic and conjugated materials

Graphene 2010 physics

Polyacetylene 2000 chemistry

Fullerenes 1996 chemistry

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Linear acenes:

HOMO – LUMO band gap

Bsp. Pentacene: Absorption

Translation

valence band, conduction band, electron-hole pair, doping, traps for charge carriers, phonons, energy bands, Drude model, electroluminescence, surface states

LUMO, HOMO, excitonic states, chemical impurities, vibrations, hopping, fluorescence and phosphorescence, singulets, triplets, anhilation, oxidation, reduction

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The vision of organic electronics: mass printing

Status Quo

• ... 90% of OLEDs, by far the most

important application of organic electronics

at the moment, are produced by vapor

deposition of small molecules

[Source: ICB 10 July 2008 ]

Organic materials deposition and

growth

1422HC

Properties:

•hole mobility larger than electron

mobility

•forms well-ordered layers when

evaporated in vacuum at RT

•gold contact are reasonably well

matched

Record material: Pentacene

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Pentacene-deposition by vacuum

sublimation, accuracy ca. 0.1 nm (QMC)

0

10 mm

0

10 mm

H HH H H H

Si

OH OHOH OH OH OH

Si

SiO2

Pentacene film growth

Phys. Rev. B 67 125406 (2003)

diffusion nucleation

island formation

= Diffusionsenergie

= Diffusionskonstante

• temperature

• surface modification

• deposition rate

Diffusion limited Aggregation (DLA)

R = D / F

Molecular structure: Bragg-scattering

APL (2004)

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Pentacene growth (thick films)

Comparison: Coronene films

M. Huth (LMU), diploma thesis (2006)

1.4µm

1.4

µm

3µm

1.5

µm

surface energy determines growth mode (Wulf Konstruktion)

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Summary: Deposition and structure

• Growth mode of small molecules largely determined by molecular shape and surface energy

• molecular arrangement can be determined by x-ray experiments

Carrier mobility

Güte des Kristalls

little overlap - bands are flat – mass of the carriers is high – mobility is low

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Measurement of mobility: Time-Of-Flight:

1. Generation of Elektron-Hole pairs by

hard light pulse (pulse 0.76ns)

Charge carriers are generated at

the surface due to adsorption

2. measurement of displacement

current

injection free method

electron and hole current

separately

Experiment

TOF-GeometryKepler & LeBlanc 1960 for

Anthracene crystals

typical

mobility

for organics

m = 1 cm2/Vs

same as

amorphous

Si

traps reduce mobility (Shockley-Read-Hall)

mobility, in the presence of

shallow traps:

where m0(T) ~ T-n

n 1.5 – 3 depending on material,

scattering mechanisms, etc.

m(T) =

m0(T)

1 + [exp( ) -1]NT

N0

ET

kBT

µ0(T): intrinsic mobility

ET : trap energy

NT/N0: trapping vs. conduction states

grain boundaries reduce mobility

Horowitz Adv. Mat. (2000)

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Comparison of organic and inorganic Semiconductors

Evac

Ecb

Evb

Egap

0

E [eV]

2.2

5.8

EF

x

narrow bands

( ca. 100meV at 300K )

high mass

bandgap Egap 3.6eV

(diamond Egap = 5.5eV)

kBT = 26meV at 300K

no free carriers in thermal equilibrium

Compared to tetracene:

strong influence of traps

Summary – electronic properties• conjugated and aromatic molecules have delocalized

electrons (p electrons)

• small organic molecules form highly ordered crystals, while polymer films are only partially ordered

• Details of the arrangement of the molecular orbitals and symmetry determine the electronic properties of organic crystals

• organic molecules have large band gaps (typically 2 eV), few or no intrinsic carriers at RT

• bands are flat, dispersion typically 100 meV

• crystal quality matters for mobility (grain boundaries, traps)

Thin-film devices

• transistors

• p-n contacts (diodes, solar cells, oLEDs)

Thin film transistors

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Thin film transistors - design

Contacts

(60 nm Au + 3nm Ti)

Thermal SiO2 (200nm) )/108.1( 28 cmFCox

-

~ 48 nm Pentacene

Gold-Structure(bottom-contact)

250 µm

200 µm

750 µm250 µm

m

750 µm250 µm

m

750 µm

Channel

w = 10 µm

l = 20 µm

2.5 µm

5 µm25 µm

12.5 µm

Transistor channel Pentacene on SiO2

Fieldeffect-Transistors from pentacene molecules

0 -5 -10 -15 -20 -25 -300.0

-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7

VG = 0 V

VG = -10 V

VG = -20 V

VG = -30 V

I SD *

10

-5 [

A]

VSD

[V]

Precise saturation behaviour

Ohmic contacts

Linear regime

p-type pentacene bottom-contact OTFTs

Organic field effect transistors

(see blackboard for characteristic curve)

Pentacene TFTs : Trap density

Hysteresis V shallow traps

• Energy level near HOMO level

• Can be released thermally

ID 0099:

Threshold shift VT deep traps

• Energy level far from HOMO level

• Fixed interface charges

ID 0099:

1st measurement

last measurement

M. Fiebig (LMU, LS Kotthaus), Diploma thesis 2005

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Diodes, solar cells

Si – the p/n- junction energy band deformation by doping

Singlet excitons diffuse within the organic crystal until they decay fluorescent or

radiationless . Lifetime is very short (fs-ps) , therefore diffusion length is only 10-20 nm.

Optical excitations in organic crystals

Exciton splitting at hetero-junctions

Appl. Phys. Lett. 48, 183 (1986)

Two‐layer organic photovoltaic cell

C. W. Tang

Modellsystem: Pentacene/C60

(C60 erst ab 1985 bekannt)

p n

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Organic solar cells

http://www.adsdyes.com/solarcells.html

e

h

Summary

• all important devices can be made by organic electronics:

oLEDsfield-effect-transistors (FETs)solar cells

• Materials science aspects have huge influence on performance:structural defects, chemical impurities, lifetime and stability

Organic Electronics summer term 15

Pentacene Linear Acenes:

Comparison Pentacene-Gold

MOSFETMetal-Oxide-Semiconductorfield-effect-transistorhere: p-type, normally off

similar: MISFET (metal insulator...) IGFET (insulator-gate...)

working principle of an OFET

plate capacitorgate electrodecontact + semicond. film

I-V curve of a MOSFET

conceptionally, L>>d (channel length >> oxide thickness)

Moors law requires thin oxides as main technologicalchallenge for small MosFETS with Si

with organic FETs, parasitic problems dominate so far:contact barriers, traps, contaminations, ...

linear range: amplifier saturation regime: logic