• Introduction: Organic electronics

• Fabrication and characterization of organic thinfilms

• Devices: solar cells, OLEDs, OFETs

blackboard part: OFET

Organic Electronics

MatWi II summer term 2016

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

Organic Electronics

AMOLED displays OLED TVs

Sony

10 Mio. :1 Kontrastverhältnis0,1ms Reaktionszeit

Freier Betrachtungswinkel15 Zoll / 38 cm Diagonale40 Watt1.6 kg1400 EUR (14.10.2010)

OLED TV … 15 years R&D55 Zoll (140 cm)

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.

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 12%. After

using more than 150 Mil USD

investment capital, Konarka filed

bankruptcy.

Wikipedia: Solar cell

Grätzel

CIGS: Kupfer-Indium-(Gallium)-Schwefel-Selen

Ultrathin and lightweight organic solar cells with high flexibilityMartin Kaltenbrunner, Matthew S. White, Eric D. Głowacki, Tsuyoshi Sekitani, Takao Someya, Niyazi Serdar Sariciftci, Siegfried BauerNature Communications 3 (2012)

How is this possible,or what do we need for organic

electronics ?

Silicon: microstructering and doping

Intel 80486DX2

Strukturierter Si-wafer

Organic materials (hydrocarbons) in everydays life …

… are good insulators

all electrons covalently bound in sigma bonds

Polyethylen (PE)

electron density

MatWi I

electron mobility(Drude Model)

sp2 hybridisation (C2H4):

π−orbital

σ

σ

σ

σ

σ

conjugated and aromatic molecules:ππππ-electrons

molecular orbitals (MO) of aromaticmolecules: Benzol

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 materialsGraphene 2010 physicsPolyacetylene 2000 chemistryFullerenes 1996 chemistry

Nobel prices for aromatic and conjugated materialsGraphene 2010 physicsPolyacetylene 2000 chemistryFullerenes 1996 chemistry

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

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 andgrowth

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: PentacenePentacene-deposition by vacuum sublimation, accuracy ca. 0.1 nm (QMC)

0

10 µµµµm

0

10 µµµµm

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)

Pentacene growth (thick films)

Comparison: Coronene films

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

3µm

1.5

µm

surface energy determines growth mode (Wulf construction)

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

scattering time depends on sample quality

little overlap - bands are flat – mass of the carriers is high – mobility is low Measurement of mobility: Time-Of-Flight:

1. Generation of elektron-eole pairs by hard light pulse (τpulse ≈ 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

typicalmobilityfor organicsµ = 1 cm2/Vs

same as amorphousSi

traps reduce mobility (Shockley-Read-Hall)

mobility, in the presence of

shallow traps:

where µ0(T) ~ T-n

n ≈ 1.5 – 3 depending on material,

scattering mechanisms, etc.

µµµµ(T) =

µµµµ0(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)

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

equilibriumCompared to tetracene:

strong influence of traps

Summary – electronic properties• conjugated and aromatic molecules have delocalized

electrons (π 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

Thin film transistors - design

Contacts

(60 nm Au + 3nm Ti)

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

ox

−⋅=

~ 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

Field effect-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

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

Organic solar cells

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

e

h

Summary

• active elements (diode, transistor) made from organic electronics materials:

OLEDs, solar cells, field-effect-transistors (FETs)

• materials science aspects have huge influence on performance: structural defects, chemical impurities, lifetime and stability need to be optimized

• the molecular character is usually not completely lost in crystals: strongly bound excitons, low epsilon

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