biblía - polímeros semicondutores

8/8/2019 Bibla - Polmeros Semicondutores

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Contents:

0 Introduction

I Concepts and Materials

I.1 Chemistry of Carbon

I.1.a Hybrid orbitals

I.1.b The benzene ring

I.1.c Conjugated molecules

I.2 Examples of organic semiconductors

I.3 Excitations in organic semiconductors

I.3.a Polarons and excitons

I.3.b Light emission from organic molecules

I.3.c Controlling the bandgap

I.4 Charge carrier injection

II Organic semiconductor applications and devices

II.1 Synthetic Metals

III.1.a Water- based synthetic metals

III.1.b Applications of synthetic metals

II.2 Organic field effect transistors

III.2.a Description of organic FET operation

III.2.b Requirements on OFET materials

II.3 Organic light emitting devices


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II.3.a Overview over basic phenomena

II.3.b Bipolar carrier injection

III.4.c Exciton formation


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0 Introduction

Knowledge of the following concepts and terms

from solid state physics is essential and will bepresumed:

Metal, semiconductor, Fermi level, work function,

conduction band, valence band, band gap, doping,

hole, chemical / electrochemical potential, wave

vector, quasiparticle, polaron, (Wannier)

exciton, conductivity, (positive / negative)temperature coefficient (PTC / NTC).

Please read up in a solid state physics textbook

about any of these concepts you feel you are not

familiar with.


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I.1 Chemistry of Carbon (C)

For our context, the key difference between

organic and inorganic solid matter is that

excitations in inorganic matter are delocalised

and best described by a wave vector k, while in

organic matter, excitations usually are localised

and k is not a good quantum number. To understand

organic semiconductors (and, maybe synthetic

metals?), we have to understand how something

like a bandgap can arise within a single

molecule. The key to this understanding lies in

the chemistry of carbon.

The most common carbon isotope is 12C (nucleus has

6 neutrons, 6 protons), but there is a natural

abundance of 1.2% the 13C isotope with 7 neutrons,

6 protons. This has a nuclear magnetic momentum,

which is used in NMR. In atomic carbon, the 6

electrons occupy the following orbitals, table

I.1:

Table I.1

Orbital: 1s 2s 2px 2py 2pz

No. of electrons: 2 2 1 1 0

Table I.1: Electronic configuration of carbon. 1smeans principle quantum number (QN) n=1, Orbital

QN l = 0, consequently magnetic QN 0; 2 electrons

go into 1s due to 2 spins. Briefly, this is

written as 1s22s

22p

2.


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I.1.a Hybrid orbitals

Carbon, like most chemical elements, forms

covalent bonds. The driving force for chemical

reaction is the desire to share electrons between

different atoms to complete electronic shells.Thus, usually,

Atomic Orbitals Molecular Orbitals

Carbon should form 2 bonds to add 2 electrons to

complete the vacancies in the two incomplete p

orbitals (px and py): Carbon should be divalent(form 2 single bonds).

In reality, carbon forms 4 bonds. In C (and some

other atoms), chemical bonding proceeds via

intermediate steps: Promotion and

Hybridisation:

Atomic orbitals Hybrid Orbitals MolecularOrbitals

For hybridisation, carbon promotes one 2s

electron into the empty pz orbital, we arrive at

1s22s12p3. Then, C combines (hybridizes) the

remaining 2s electron and either:

three 2p orbitals sp3 hybrids

or two 2p orbitals sp2 hybrids

or one 2p orbital sp hybrid


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sp3

hybrid orbitals have 4 fingers pointing

into space symmetrically, i.e. into the corners

of a tetraeder. The angle between the fingers

is 109.5o. In this form, C can form 4 bonds, e.g.

by sharing electrons with Hydrogens 1s shell:CH

4(methane), or with other sp

3carbon (e.g. H

3C-

CH3, ethane). The C-C bond in ethane is called a bond. bonds are very strong: Diamond consistsof carbon held together by bonds entirely.

sp2hybrid orbitals have 3 fingers in a plane,

with 120

o

to each other, plus one remaining porbital perpendicular to the plane. In this form,

C needs another sp2 hybrid C to form a molecule,

e.g. H2C=CH2: 2 of the 3 fingers of each C bond

to H, as before, the third overlaps with another

C sp2 orbital to form a bond ( bond). The

remaining p orbitals of either C overlap, as

well, to form another carbon/carbon bond, the so-

called bond. (1+1 bond: Carbon double bond)This is a weaker bond, and the respective orbital

is more delocalized, i.e. occupies relatively

large space rather far away from its original

carbon.

sp hybrid orbitals have 2 fingers along one

axis (say, x) 180

o

to each other, plus 2remaining p orbitals (along y and z axis). In

this form, C can bond e.g. with 2 H and another

sp hybrid. It forms one bond between the sp

orbitals, plus the remaining 2 p orbitals of each


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molecule overlap to form two bonds (carbon

triple bond). This is Ethene (acetylene), HCCH.

The bond lengths are: C-C 1.45 , C=C 1.33 ,

CC z

(1 = 10-10 m)

I.1.b The Benzene ring

sp2 hybrid orbitals have an angle of 120o

with

respect to each other. Hence, by -bonding 6 sp2

carbons we can form a regular hexagon. Each Cwill form 2 bonds, one with each of its

neighbours. There remains one sp2 orbital per C

to be capped, e.g. by a H. The remaining p

orbitals will again overlap to form bonds. The

resultingbenzene molecule may look like shown in

Fig. I.1:

Fig. I.1: The two possible borderline

structures of benzene.

It is not quite clear where the bonds should

be. In reality, a quantum mechanical

superposition of the two borderline states is

adopted, wherein it is impossible to assign

double bonds: The electrons are completely

Or


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delocalized to form a cloud that spans the

entire molecule.

Fig. I.2: The structure of benzene. The side of a

ring is 1.39 in length, intermediate between C-

C and C=C bond lengths.

The benzene ring is one of the most important and

versatile building blocks of organic chemistry.

Its delocalized electrons have remarkable

properties with respect to their interaction with

light, and many molecules containing benzene

rings can donate or accept electrical charges

with relative ease. Much of molecular physics,

including organic semiconductor physics, is

concerned with molecules containing benzene

rings.

I.1.c Conjugated Molecules

The benzene ring is the prototype of conjugated

molecules, that is molecules with alternating

single/double or single/triple carbon bonds. In

conjugated molecules, electrons delocalize

= 1 /2


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throughout the entire molecule and are relatively

loosely bound.

We can think of conjugated molecules being built

step-by-step by binding hybridised carbons

together (linear combination of atomic orbitals,

LCAO). In the molecular orbitals (MO)

description, we instead imagine a given, rigid

set of points at which atomic nuclei are fixed,

and fill that skeleton with electrons to arrive

at the molecule. The LCAO and MO approach

correspond to two schools of quantumchemistry/computer simulation. Both approaches,

of course, should lead to the same molecules, but

in the MO picture, the correspondence to

semiconductors is easier to see. We have to put N

electrons into the molecule to balance N positive

charges. The first electrons will cluster closely

to the atomic nuclei, resulting in almost

undisturbed atomic orbitals that is equivalentto saying that e.g. the carbon 1s electrons do

not participate in chemical bonds. But the last

few electrons will go into what we have called

delocalized orbitals. Although we can trace

orbitals to the hybridised atomic orbitals of

carbon, we have seen that in a conjugated

molecule, they may delocalize far from their

original carbon hence, for the cloud, the

MO picture is more appealing.


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Here follows a reference list of conjugated

molecules and polymers, which are of importance

in the field of organic semiconductors, together

with acronyms by which they will be referred to

throughout this text. The sheer length of this(hopelessly incomplete!) list underscores one of

the key assets of organic semiconductors: The

practically unlimited diversity of synthetic

organic chemistry allows tailoring materials with

a large property portfolio.

Historically, the organic semiconductordiscipline distinguishes between polymeric and

low molecular weight organic semiconductors.

This distinction is nowadays blurred due to the

advent of a number of hybrid materials, that

combine properties and attributes of low-

molecular weight and polymeric materials. A few

examples of these are included in the following

list, table I.2. We will refer to these materialsthroughout the text, often with their acronyms

only as given in table I.2.

Table I.2

a.) Low molecular weight organicsemiconductors

S

S

S

S

S

S6T


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Pentacene

Perylene

N

CH3

N

CH3

TPD

O

NN

PBD

C60


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N

O

Al

N

O

N O

Alq3

N

PtN N

N

PtOEP

N

S

2

IrO

O

btpacac


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N

N

Eu

O

O 3

ADS053RE

N

S

C7H15O

SC12H25 7O-PBT-S12

S

S

S

S

S S

C6H13

C6H13

C6H13

C6H13

C6H13

C6H13 HHTT

N

N

O

OH

OABTo

Ru

N

N

O

OH

OABTo

N

N

S

S

N

Ru

N

N

O

OH

O

ABTo

N

O

ABTo

N

N S

S

S

N3 Black dye


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O

NO2

O2N

O2N

TNF

b.) polymeric organic semiconductors

*

*n

PPV

MEH-PPV

C

N

n CN-PPV

* *n

PPE

O

O


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** n

PPP

MeLPPP

S

R

* *n

PAT

SS *n

PTV

PTAA

N

n

H3C

H3C

C10H21

C10H21

* *


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R R

** n

PF

C8H17 C8H17

*

N N

S

*n

F8BT

C8H17 C8H17

*S

S

*n

F8T2

c.) hybrid materials

N

**n

PVK


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N

N

N N

ST 638

sQP

N N

O(CH2)n

O

O

(H2C)nO

O

O

oxTPD


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N

NDSB Dendron

(G2)

d.) synthetic metals

*

*n

PA

**n

PDA


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NH

N

N

NH

PAni

S*

*

OO

n

PEDOT

Table I.2: A selection of organic semiconductors

and synthetic metals.

Let us briefly discuss the attributes of the

listed materials immediately. This discussion

will preview many of the topics that are

introduced more systematically later in thedescription of organic semiconductor devices;

nevertheless the reader is invited to immerse

her/himself into the fascinating world of

molecular diversity at this stage.

a.) low molecular weight materials:

Hexithiophene (6T), Pentacene, Perylene, and TPD

(N,N'-bis-(m-tolyl)-N,N'diphenyl-1,1-biphenyl-

4,4'-diamine) are hole transporting and more or

less strongly fluorescent organic semiconductors

(Perylene more, 6T and TPD less). 6T is one

representative of the thiophene family of organic


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semiconductors, which are known for their fast

hole mobilities and are often used in organic

transistors. PBD (2-(biphenyl-4-yl)-5-(4-tert-

butylphenyl)-1,3,4-oxadiazole) is an electron

conductor. Both TPD and PBD have been used ascarrier injection layers in multilayer device

architectures. C60 is a material with very high

electron affinity, and C60

derivatives have been

used as electron acceptors in organic

photovoltaic devices. However, electron transport

in C60 is very sensitive to even traces of oxygen,

which limits its practical potential. Alq3

(tris(8-quinolinolato)aluminum(III)) is an

organometallic Al chelate complex with efficient

green electroluminescence and remarkable

stability. Alq3 was used as the emissive material

in the first double layer organic light- emitting

device. PtOEP is a red phosphorescent porphyrine

derivative. The central Pt atom facilitates spin-

orbit coupling that allows light emission from

triplet excitons. btp2Ir(acac) (bis(2-(2'-

benzothienyl)-pyridinato-N,C-3')

iridiumacetylacetonate) is a representative of a

family of highly efficient phosphors that have

been used successfully as triplet- harvesting

emitters in efficient electrophosphorescent

devices. ADS053 RE is a trade name for the red-

emitting organolanthanideTris(dinapthoylmethane)mono (phenanthroline)-

europium(III). Organolanthanides transfer both

singlet- and triplet excitons to an excited

atomic state of the central lanthanide, resulting

in very narrow emission lines, i.e. spectrally


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pure colours (here, the 612 nm red line of

Europium). 7O-PBT-S12 and HHTT are hole-

transporting calamitic and discotic liquid

crystals, respectively. Due to the stacking of

conjugated cores in smectic (7O-PBT-S12) and somediscotic (HHTT) liquid crystalline phases, both

can display rather high charge carrier

mobilities. N3 and black dye are

organometallic dyes with broad absorption spectra

spanning the red and near infrared, which are

often used for the harvesting of solar photons in

dye- sensitized photovoltaic cells (Grtzel

cells). Trinitrofluorenone (TNF) is often used

as electron acceptor for the formation of charge

transfer complexes with conjugated molecules.

b.) polymeric organic semiconductors

Poly(para-phenylene vinylene) PPV played an

outstanding role in the development of organic

electroluminescence. MEH- PPV and Cyano- PPV (CN-

PPV) are sidechain- substituted PPVs. Sidechains

promote solubility and also can change the

bandgap, and the type of transported charge

carrier. Poly(phenylene ethynylene) PPE and

poly(para phenylene) PPP are variations on a

similar theme. Methylated ladder- type PPP

(MeLPPP) is similar to PPP, but with all backbonerings forced to be coplanar. PPVs, PPP, PPE, and

MeLPPP have been explored extensively in organic

light emitting devics. Poly(alkylated thiophene)

(PAT) and poly(thienylene vinylene) (PTV) are

less emissive, but have higher hole mobilities


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and low ionisation potentials, which qualifies

them for use in organic field effect transistors.

However, thiophene- containing materials are

generally sensitive to oxygen, which for

practical organic transistor applications can notbe excluded. Instead, variations of

poly(triarylamine) (PTAA) are being developed for

hole- transporting transistors. Polyfluorene (PF)

is a blue emitter that has recently competed

successfully with PPV as organic light emitting

material. Typically, PF is copolymerized rather

than sidechain- substituted to modify its

properties. F8BT is an electron transporter and

efficient green emitter. F8T2 is a hole

transporter that works well in transistors. PF,

F8BT, and F8T2 also display interesting liquid

crystalline phases.

c.) hybrid materials:

Poly(vinyl carbazole) PVK is historically one of

the first (the first?) polymeric organic

semiconductors. PVK clearly is a polymeric

material, with the film forming and morphological

properties typical of polymers. However, the

semiconducting carbazole units dangle laterally

from a non- conjugated backbone, and are isolated

from each other. The electronic properties of PVKare therefore very similar to those of low

molecular weight carbazole. PVK is used in

photocopiers, and was the first polymer for which

electroluminescence (EL) was reported. ST 638 is

the tradename for 4,4,4-Tris(N-(1-naphthyl)-N-


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phenyl-amino)-triphenylamine. This is a low

molecular weight material, but due to its

sterically hindred starburst architecture it

has a very high glass transition temperature and

does typically not crystallise when spincast fromsolution, but forms a glassy film, like many

polymers. The glassy morphology has considerable

advantages for device applications; a tendency to

crystallise is a major problem with hole

transporting small molecules such as TPD. The

same structural theme was employed for the design

of electron transporting starburst- type

phenylquinoxalines (not shown here). Another

structural theme that can be used to suppress

crystallisation in non- polymeric materials is

the use of spiro- links between two (or more)

para-phenylene units, here exemplified by a

spiro- linked pair of quaterphenyls (sQP). Note

the cross- shaped 3- dimensional architecture of

spiro compounds that is difficult to sketch on

paper. oxTPD is clearly a low- molecular weight

compound, but via the oxetane functions that are

attached with flexible spacers it can be

crosslinked in- situ with the help of a suitable

(photo)initiator. The result is a highly

crosslinked, inert hole transporting film with no

crystallisation tendency that has been used

successfully in multilayer devices. NDSB Dendron(G2) is a second generation, nitrogen- cored

distyryl benzene dendrimer. The core displays

visible absorption and emission, the meta- linked

dendronic sidegroups have a bandgap in the UV and


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for the purposes of charge injection, transport,

and light emission can be considered as inert.

d.) synthetic metals:

The distinction between organic semiconductors

and synthetic metals is somewhat arbitrary, as

the synthetic metals shown here are in the

undoped state, when they display semiconducting

rather than metallic or quasimetallic properties.

Metallic properties are observed only after

chemical doping. PA (poly(acetylene)) is the

classic example, the (chance) discovery thatiodine- doped PA displays metallic conductivity

was a milestone discovery that earned the 2000

chemistry Nobel prize. Poly(diacetylene) (PDA)

has a widely tunable bandgap if substituted with

suitable sidegroups. Both PA and PDA are of

historic, but no longer of practical interest.

Poly(aniline) (PAni, here shown as emeraldine

base) and poly(3,4-ethylenedioxythiophene) PEDOT

are more modern developments. These are made

metallic by acid- rather than redox doping.

Water- based PAni and PEDOT preparations are now

commercially available. PEDOT that is acid- doped

with poly(styrene sulfonic acid) (PSS)

(PEDOT/PSS) is now very popular in the OLED

community to modify (or replace) the commonlyused transparent ITO anodes.

As a general remark on the above list of

materials, it is very important to keep in mind

that materials that nominally are the same often


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show very different performance in devices.

Device performance can be very sensitive to low

levels of impurities and/or chemical defects, and

often, different chemical routes that lead to the

same material introduce different levels andtypes of defects and/or impurities. Experience

shows that the Gilch route is superior to Wittig,

Horner, and Stille coupling for high quality

poly(arylenevinylene)s, while Suzuki coupling is

to be preferred over Yamamoto or Kovacic coupling

for poly(arylene)s. The highest regioregularity

in poly(alkylthiophene)s are achieved with the

help of Rieke Zinc. Companies or research groups

that are able to provide conjugated materials in

the quality required for practical devices are

few and far between. Even if chemistry is

ideal, the same material can still display very

different properties when prepared in different

ways. The solvent and casting method used for

solution processing, or deposition rate, type and

temperature of substrate for evaporated films,

thermal treatment cycles, and the presence or

absence of even trace amounts of oxygen and/or

water can have a decisive impact on the resulting

device.

I.3 Excitations in organic semiconductors

In any semiconductor application, the material

will not be in its ground state. To transport

charge, and/or emit light, the semiconductor

needs to sustain excitations, and in the case of

charge transport, these excitations also need to


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illustrates that in the format of a chemical

reaction:

Eq. I.1: DIp

D+

+ e

A + e Ea A

Ip

and Ea

are conceptually closely related to

electrochemical Redox potentials. The main

difference is that Ip and Ea are defined with

respect to electrons in vacuum, while Redox

potentials are normalised with respect toelectrons in a reference electrode.

Experimentally it is much more common to measure

a molecules Redox potentials with a technique

called cyclic voltammetry (CV).

Note that in a metal, there is only one frontier

orbital, namely the Fermi level EF

. The energy

required to remove an electron from a metal is

called work function . As the frontier orbitalin a metal is the Fermi level, the nave

expectation is = - EF. This expectation,

however, is not met. The work function is a

surface property, while the Fermi level is a bulk

property. For a more detailed discussion, see

[T4, Ch. 18]. As a practical consequence, when

two metals are in contact, their Fermi levels

will equilibrate, but the work functions will

not. Instead, the work function will be that of

the metal that constitutes the particular surface

through which the electron leaves the metal.


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As a practically important example for an organic

radical ion, a neutral and a positively charged

polythiophene segment are sketched in Fig. I.3.

Note how the missing electron (hole) leads to a

redistribution of the -bonds and hence, to

different bond lengths, bond angles, and nuclear

positions.

Fig. I.3: A polythiophene segment and the derived

radical cation. See [T1, Ch. 14] for moreinformation.

Apart from polarons, the most important

excitation in an organic semiconductor is known

as exciton. This can be visualized as an electron

that is removed from the HOMO, but is placed into

the LUMO instead of being removed entirely. Atypical way of lifting an electron from the

HOMO into the LUMO is via the absorption of a

photon. Note that the exciton is electrically

neutral. Alternatively, an exciton can result

S S

S

S

S

SS

SS

S

S

S S

S

S

S S

S

Holeinjection

(oxidation)


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from the combination of a hole- and an electron

polaron.

After exciton formation, the electronsredistribute into the excited * orbitals, which

are also known as antibonding orbitals, as they

destabilise the molecule. The strong bonds are

crucial in keeping the molecule intact

nevertheless: The presence of a - bonded

backbone makes the difference between

photophysics and photochemistry. Again, theexcitation leads to a related structural

relaxation of the surrounding molecular geometry.

Fig. I.4 shows the geometric relaxation and

redistribution of electron density in an excited

phenylene- vinylene segment:

Excitation

Fig.I.4: The transition from an aromatic,

bonding phenylene- vinylene system to a

quinoidal, antibonding * system on optical or

electrical excitation.


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The size of the exciton is about 3 repeat

units, or 10 nm, and the exciton has clearly

intramolecular, one- dimensional character. This

makes organic excitons Frenkel excitons, while in

inorganic semiconductors, excitons typically are

more delocalised Wannier excitons. Due to the

mutual attraction of electron and hole in the

exciton, and structural relaxation of the

molecule, the energy difference between the

excitonic state and the ground state is lower

than the difference between Ip

and Ea

suggests

(which in turn is lower than the difference

between HOMO and LUMO). This energy difference is

known as exciton binding energy Eb. E

bis larger

in Frenkel than in Wannier excitons; typical

organic (Frenkel) exciton binding energies are in

the range 0.2 to 0.5 eV. Note how considerable

ambiguity arises in the term bandgap when

applied to organic semiconductors, which can meaneither the energy difference between LUMO and

HOMO, or Ip- E

a, or (I

p E

a) E

b.

Fig. I.5 gives an alternative, more schematic

representation of the electronic ground state,

radical ions (here called polarons), and excitons

in organic semiconductors.


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Fig. I.5: Level diagrams for excitations in

organic semiconductors

Fig. I.5 shows two different types of exciton,

singlet and triplet excitons. The different types

of excitons result from the fact that electrons

as well as holes posses a spin. The quasiparticle

exciton has an overall wavefunction that

contains a spatial and a spin part, and the spin

part of the wavefunction results from a

combination of the respective electron- and hole

spins in a way that is consistent with the basic

LUMO

HOMOGround state

Holepolaron

Electronpolaron

Tripletexciton

Singletexciton


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rules of quantum mechanics, in particular the

Pauli principle. There is three ways in which

hole and electron spin can combine so that the

resulting overall spin part of the wavefunction

is symmetric under particle exchange, and hastotal spin S = 1 - namely >,>, and 1/2(>

+ >). Excitons with that property are called

triplet excitons. One combination of spins,

namely 1/2(> - >), results in a spin part

of the wavefunction that is antisymmetric under

particle exchange, and total spin S = 0. This

combination is called a singlet exciton. Theproperties of excitons are summarised in table

I.3:

Table I.3

Spin state ket S Sz

symmetric(+)/

antisymmetric(

-)

> 1 1 +> 1 -1 +

1/2(> +>) 1 0 +1/2(> ->) 0 0 -Table I.3: The possible spin combinations (spin

state kets) of electron / hole pairs (excitons),

the resulting total spin, spin z- component, and


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symmetry property under particle exchange.

Excitons with a spin ket symmetric under particle

exchange are triplet excitons, excitons with spin

ket antisymmetric under particle exchange are

singlet excitons.

I.3.b Light emission from organic molecules

In most simplistic terms, the absorption of a

photon that has generated an exciton on an

organic molecule can in some cases be reversed

(ignoring some intermediate steps to be discussed

below): The excited electron drops back from theLUMO into the HOMO, emitting a photon in the

process. This phenomenon is known as

fluorescence. Since excitons can also be

generated electrically by the combination of

polarons; this paves the way to organic

electroluminescence (EL). First, we will discuss

fluorescence in some detail, using a framework

based on groundstate and excited state

molecular orbitals. Then, we will return to

discuss a striking difference between

fluorescence and EL, which is best understood in

the more schematic picture used in Fig. I.5.

Fluorescence in the molecular orbitals picture

The following diagram Fig. I.6 is used to discuss

fluorescence in the MO picture. The two curves in

the potential energy / distance diagram describe

the ground state and first excited state of a

chemical bond, with the equilibrium distance


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being the bond length. Bond length is longer for

the excited state due to the presence of

antibonding * orbitals. The horizontal lines

represent the vibrational states of the bonds.

Nuclear distances oscillate around the

equilibrium bond length within the limits of the

intersection of the horizontal line with the

potential curve. Vibrational levels are quantised

with a vibronic spacing in the order 0.1 eV

1100 K; consequently, at room temperature, almost

all bonds are in the lowest vibronic level of the

ground state. From there, they may be lifted intothe first excited state by absorption of a

photon.

Fig. I.6: Energy diagram for molecules in ground-

and excited states, showing potential energy E

vs. nuclear distance. See e.g. [T6, Ch. 17]

E

r

S0

S1

R0 R1


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The absorption process is governed by the Franck-

Condon principle: Electronic transitions are much

faster than nuclear rearrangement. Hence,

transitions always occur vertically in the

diagram, from ground state equilibrium position

to the turning point of a vibrational mode of the

excited state that is close to the ground state

equilibrium distance. Transitions from the 0

vibronic level of the ground state into the 0 / 1

/ 2 / vibronic level of the first excited state

are known as 0-0 / 0-1 / 0-2 / transitions. The

relative intensity of these is controlled by the

overlap integral between the electronic ground

state / vibronic ground state (0 vibronic state)

wavefunction and excited state / vibronic state

0, 1, 2, wavefunctions. The overlap integral can

be separated into an electronic and a vibronic

integral; the square of the vibronic integral is

known as the Franck-Condon factor of thetransition. A detailed discussion is in [T6, Ch.

17]. If the equilibrium bond lengths in the

ground- and first excited state were equal, the

0-0 transition would have unit Franck-Condon

factor, and all higher vibronic transitions would

have Franck-Condon factor zero, i.e. they would

be forbidden. In reality, bond lengths for

excited states are longer than for the groundstate, and higher vibronic transitions become

allowed, the so- called vibronic satellites.

Usually, however, the 0-0 transition will be the

most intense.


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In the excited state, there will be a rapid

(order 10-12

s), radiationless relaxation of the

excited state into its lowest vibronic level,

this is known as internal conversion. From there,

the photon may be re- emitted after typically 1

to 10 ns with a transition to the lowest vibronic

level of the ground state (0-0 transition), the

first vibrational state (0-1 transition), second

vibrational state (0-2 transition), etc. Vibronicspacing is similar in ground- and excited state,

therefore these often appear like mirror images.

Fig. I.7 shows absorption- and fluorescence

spectra of perylene as an example. Perylene is a

molecule with pronounced vibronic satellites.

Less rigid molecules, e.g. para- phenylenes,

often do not show resolved vibronic sidebands,

but a single, broadened peak.

Fig. I.7: Absorption and emission spectra of

perylene dissolved in cyclohexane. Both spectra

are normalized to unit maximum peak. Note that

when spectra are represented against a wavelength

scale, absorption spectra appear on the left at

0

0.2

0.4

0.6

0.8

1

0

0.2

0.4

0.6

0.8

1

300 350 400 450 500 550 600norma

lise

d

absor

bance

(o

)

norm

ali

sedfl

uor

es

cen

ce

(x

)

Wavelength [nm]


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38

shorter wavelengths (higher energies), emission

spectra on the right (lower energies). When

represented against an energy or wavenumber

scale, the situation is reversed.

In Fig I.7, the 0-0 absorption and 0-0

fluorescence peak at (virtually) the same

wavelength, as it is expected from the previous

discussion. In many cases, however, 0-0 emission

is shifted by a few nm (order 5 to 10) to longer

wavelength (lower energy). This phenomenon is

known as Stokes shift. Stokes shift results fromthe interaction of molecules with their

environment; we had so far considered isolated

molecules. No details here.

Either the onset of absorption, or the

intersection of absorption and emission spectra

in a normalised plot like Fig. I.7, is often

called the optical bandgap. In the absence of

Stokes shift, of course the intersection of

absorption- and emission spectra occurs at the

absorption / emission maximum.

Perylene has a fluorescence quantum yield FL

=

0.94 in cyclohexane solution, and many modern

conjugated polymers exceed 50 % quantum yield

even in the solid state. However, some conjugated

polymers display rather low fluorescence quantum

yields.


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39

In organic light emitting devices, light is

generated not by the absorption of a photon, but

by the combination of an electron- and a hole

polaron: An electron in the LUMO and an electron

vacancy (hole) in the HOMO can combine to form anexciton. The fluorescence emitted from an

electrically generated exciton is called

electroluminescence (EL). However, there is a

fundamental difference between the formation of

excitons by absorption of light, and by

combination of polarons.

When light is absorbed, only singlet excitons are

formed, and all of them are in principle capable

of fluorescence. The respective transition is

denoted as S0S

1transition (singlet ground state

to singlet 1st

excited state). Since the photon

carries a unit h of orbital angular momentum L,

S0 and S1 have orbital angular momentum quantum

numbers l differing by 1 (one). Inversely, the

S0S1 fluorescence transition does fulfil the

selection rule l = 1. The photons orbital

angular momentum provides the required angular

momentum for the S0S1 transition; conversely,

the S0S

1transition provides the orbital angular

momentum needed for photon emission. Such a

transition is called dipole allowed.

When excitons are generated electrically, naively

only 1 in 4 excitons will be a singlet exciton

(S1state). 3 out of 4 will be triplet excitons,


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corresponding to the T0

state. This is simply

because there is 3 ways to combine 2 spins into a

triplet state and only one way to combine them as

a singlet. Both S0and T

0have the same angular

momentum quantum number l, hence l = 0 for theS

0T

0transition: The triplet- to- singlet

transition is dipole forbidden. Triplets can not

emit fluorescent light, because the necessary

angular momentum h can not be supplied from the

orbital angular momentum difference between

excited and ground state. Obviously, it is bad

news for organic electroluminescence that

apparently only 1 in 4 electrically generated

excitons can emit light.

However, T0

does carry angular momentum in its

spin: Remember triplet excitons have total spin S

= 1, singlets 0. In some molecules, triplet spin

angular momentum can be transferred to a photons

orbital angular momentum. The conversion of spin

into orbital angular momentum is facilitated by

the interaction of the magnetic fields that both

electron spin and orbital angular momentum

generate. These fields interact with each other,

and interaction is proportional to the product of

orbital and spin angular momentum, L.S. This

interaction is therefore known as spin- orbit orLS coupling. The emission of light from T

0 via LS

coupling is known as phosphorescence. To have

strong LS coupling, we need to incorporate atoms

into our molecules that have filled atomic shells

with high orbital quantum number l. This will


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41

generally be heavy atoms (i.e., heavier than

carbon). Phosphorus may work the clue is in the

name but modern phosphors used in organic EL

are heavy metal organometallic complexes using

e.g. Iridium.

I.3.c Controlling the bandgap

In the previous chapter, we have seen how the

characteristic vibronic structure of organic

spectra arises. However, the overall location of

absorption- and emission of a semiconductor are

controlled by the size of its bandgap. Theperceived colour of light, in turn, is controlled

by the location of the emission band within the

visible spectrum. In particular for full- colour

displays, as well as for photovoltaics, the

control of the bandgap is a key requirement.

Synthetic chemistry is extremely versatile in

manipulating molecular architecture in a way that

allows bandgap tuning throughout the visible

spectrum. Bandgaps reduced or increased by

molecular engineering are often referred to as

redshifted andblueshifted, respectively. We will

discuss the two key approaches to bandgap

control: The steric approach and the

electronic approach. In the latter approach, it

is in fact possible to manipulate Ip or Ea only,leaving the respective other unchanged. It thus

becomes possible to design hole- or electron

transporting materials.

The steric control of bandgap


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42

When orbital overlap is improved by forcing

rings into a coplanar arrangement by chemical

bonds, bandgap will be reduced. Example:

Oligomers of para-phenylene (PPP), fluorene

(PFO), and methylated ladder-type poly(para-

phenylene) (mLPPP):

Table I.4

Number of

Benzene

Rings

Oligo-PP

[eV]

Oligofluore

ne

[eV]

Oligo-mLPP

[eV]

3 4.44 3.704 4.25

5 4.15 3.18

6 4.03 3.56

7 3.00

8 3.43

910 3.35

Table I.4: Absorption maxima for oligo(para-

phenylene)s, Oligofluorenes, and oligo(ladder-

para- phenylene)s. Data compiled from [J Grimme,

M Kreyenschmidt, F Uckert, K Mllen, U Scherf,

Adv. Mater.7, 292 (1995)] and [D Klaerner, R DMiller, Macromolecules31, 2007 (1998)].


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43

It is evident that at a given number of rings,

the more planarized the backbone, the lower the

bandgap.

The electronic control of the bandgap.

By introducing either electron- withdrawing or

electron-donating chemical groups into a

conjugated molecule, electron affinity and

ionization potential will be affected, hence, the

bandgap will change. Such groups can be

introduced in two ways, namely as sidechains or

in the mainchain. We will discuss the followingexamples: Alkoxy- sidechains attached to PPV

rings (MEH- PPV), CN- groups attached to alkoxy-

PPV vinylene bonds (CN- PPV, a case somewhat

intermediate between sidechain- and mainchain

modification) and fluorene copolymers (mainchain

modification).

MEH- PPV is an example for alkoxy- substituted

PPVs. Sidechains make it soluble in organic

solvents such as THF or chloroform. Sidechains

also somewhat isolate chain backbones from each

other in the solid film, which increases quantum

yield over unsubstituted PPV. An interesting

molecular engineering motif that was introduced

in the design of MEH- PPV is the use of a

branched sidechain. The branch constitutes a

chiral centre, MEH- PPV chains thus contain a

racemic mixture of right- and left handed

monomers. However, backbone conjugation is not

affected by the branch in the sidechain. This


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44

gives MEH- PPV the photophysical properties of a

homopolymer, but crystallisation is suppressed as

in a random copolymer. As crystallisation often

is detrimental to fluorescence efficiency, this

was a welcome step forward in the development ofconjugated polymers. The alkoxy- linkage of MEH-

PPVs sidechains to the phenyl backbone ring also

changes the electronic structure of the backbone.

Alkoxy links have a tendency to donate electrons

to the backbone, which changes the shape and

location of the HOMO. As a result, emission is

redshifted compared to PPV, from green to orange.

The case of CN- PPV is somewhat intermediate

between sidechain- and mainchain modification. In

addition to the alkoxy- sidechains as in MEH-

PPV, highly electron- withdrawing cyano groups

are attached to the vinylene bonds. This leaves

the conjugated backbone highly electron

deficient, thus considerably increasing theelectron affinity (by about 0.6 eV [R1]). Due to

their high electron affinity, CN- PPVs are n-

type semiconductors. CN- PPVs typically are red

emitters.

Another approach to bandgap control is

copolymerization of different conjugated unitsinto the polymer backbone. Copolymers between

alkane- and alkoxy substituted PPV- type polymers

are discussed in [R1]. Here, we will focus on

copolymers of fluorene. Polyfluorenes display a

blue bandgap that is almost indifferent to the


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45

type of sidechain attached. Note that sidechains

are attached pairwise at a position which itself

is not part of the conjugated backbone, and these

pairs are widely spaced, so that there is neither

steric nor electronic impact of the sidechains onthe backbone properties. However, strictly

alternating copolymers of fluorenes with

comonomers with different electronic properties

have been prepared, such as F8BT (alternating

copolymer between dioctyl fluorene with

benzothiadiazole) and F8T2 (alternating copolymer

of dioctyl fluorene with two thiophenes). For

both F8BT and F8T2, the resulting bandgap is

reduced, and they both emit in the green- yellow

region. The reduction of the bandgap has

different reasons: In the case of F8BT, the

benzothiadiazole comonomer has a higher electron

affinity Ea than fluorene, thus leading to a

polymer with higher Ea. In the case of F8T2, the

two thiophene groups have a lower ionization

potential Ip than fluorene, thus leading to a

polymer with lower Ip. Copolymerization thus does

not only allow control of the bandgap, but of

both Ip and Ea in a predictable manner, and a

large number of fluorene copolymers have been

synthesized and studied. For a review, see [R13].

Both polymers have found interesting

applications: Some of the most efficient organicEL devices have been built from blends of a

minority amount of F8BT as electron injecting /

transporting material in hole injecting /

transporting polyfluorene host material. F8T2, on


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46

the other hand, is an excellent material for p-

type OFETs.

I.4 Charge carrier injection

The injection of charge carriers is an issue of

immense practical importance for semiconductor

devices: Transistors require the injection of one

type of carrier from an electrode, and rather

fast transport of that carrier. Light emitting

devices require the injection of carriers of both

types from different electrodes. Photovoltaic

devices need to separate excitons and transportthe resulting carriers to opposite electrodes. It

is thus paramount that we discuss the factors

controlling carrier injection and transport.

Carrier injection from a metal electrode into a

semiconductor is controlled by the work function

of the metal relative to the electron affinityEa of the semiconductor for electron injection,

and relative to the ionisation potential Ip of

the semiconductor for hole injection. A level

diagram as in Fig. I.13 is often used to

illustrate carrier injection e.g. into an organic

light emitting device(OLED). In a level diagram,

electrons minimise their energy by moving

downwards to lower energy; holes minimise their

energy by moving uphill because that implies an

electron going downwards.


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47

Energy [eV]

ITOPPV

Al

Ca

Ip4.7

5.2

2.52.8

4.2

Ea

Vacuumlevel

0V a c u u mlevel

Cathode

th

Location

te

Location

ITO

Fig. I.13: Energy levels for a PPV layer

sandwiched between unlike electrodes. Indium tin

oxide (ITO) is a transparent metallic material

that is commonly used as anode for OLEDs. Left:

No bias voltage applied. Right: A voltage is

applied in forward bias.

Let us clearly state the assumptions that have to

be made when Fig. I.13 is used to discuss carrier

injection. Firstly, we assume the near complete

absence of dopant- induced charge carriers in the

semiconductor. In electrical engineering, undoped

materials are often called insulators rather

than semiconductors even when they have a small

bandgap. The absence of dopant- induced charge

carriers implies that bands remain straight at

all times - bands may tilt, but they will not

bend (show curvature). Thus, Fig. I.13 describes

ametal- insulator junction. Secondly, we assume

that the levels drawn in Fig. I.13 metal work


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functions, ionisation potential, and electron

affinity are the same at the metal- organic

semiconductor interface as they are in the

isolated metal, or bulk semiconductor. Note that

due to the molecular nature of organicsemiconductors, there are no surface dangling

bonds which do usually distort bulk energy

levels in inorganic semiconductors. However, in

metal- organic semiconductor contacts, interface

dipole moments often are present (sometimes,

deliberately engineered) which will modify

barriers. Also, the work function of a metal

coated with an organic semiconductor can be

different from that of the same metal with

respect to vacuum. An example is the gold-

pentacene interface, which displays a

surprisingly large energy barrier. These issues

are discussed e.g. in [N Koch, A Elschner, J

Schwartz, A Kahn, Appl. Phys. Lett. 82, 2281

(2003)]. In the presence of such effects, levels

in Fig. I.13 have to be re- drawn at a different

energy.

After these cautional remarks, let us inspect the

salient features of Fig. I.13, that illustrates

the level scheme at the example of PPV sandwiched

between ITO- and Al (or Ca) electrodes. On the

left, we see that a hole would have to move

downwards by 0.5 eV on injection from ITO to PPV.

However, holes will voluntarily move uphill

rather than downwards. The necessary step into

the wrong (energetic) direction is called an


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49

injection barrier, here of 0.5 eV for holes from

ITO into PPV. Electron injection from Ca or Al

into PPV needs to overcome a barrier of 0.3 or

1.7 eV for electron, respectively. On the other

hand, for electron injection from ITO, therewould be a large barrier of 2.2 eV. Thus, the use

of electrodes made from unlike metals defines a

forward and reverse bias for the OLED. To

minimise barriers, a high workfunction anode and

a low workfunction cathode are required. Table

I.7 lists the work functions of some metals, as

well as the highly doped semiconductor ITO, and

the synthetic metal PEDOT/PSS.

Table I.7

Metal Work function [eV]Cs 1.81

Ca 2.8

Mg 3.64

Al 4.25

Ag 4.3

Au 4.7

Cu 4.4

ITO 4.7

PEDOT/PSS 5.05.2


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Table I.7: The work function of a number of

metals, most data from [T4, Table 18.1, Ch. 18].

High work function metals are known as noble

metals. Low work function metals are highly

reactive and usually need to be protected fromambient atmosphere to avoid oxidation.

The right- hand part of Fig. I.13 shows the same

device (assuming a Ca cathode) under a forward

bias. Carriers can now overcome injection

barriers by tunnelling from the electrodes so far

into the device that energy at that location ofthe tilted band is as low or lower than in the

electrode. Also, carriers can be injected

thermally. For barriers of 0.5 eV as shown here,

the current density in a device in forward bias

will usually be controlled by the injection of

carriers across the barrier (injection limited

current), rather than by the transport of

carriers across a device. The tunnelling currentdensity j(Vbias) is described by the equation of

Fowler and Nordheim (Fowler- Nordheim (F-N)

tunnelling):

(eq.I.4.1) jFN =C

V(Vbias

d)2exp[B

dV3/2

Vbias]

with B = 8(2m*)/(2.96eh) with an effective

electron mass m*. For a detailed discussion, see

e.g. [T1, Ch. 12]. Note that jFN

depends on

applied voltage Vbias and film thickness d only in


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the combination Vbias/d, i.e. jFN scales with the

applied field E. Thermally activated or

thermionic injection is described by the

Richardson- Schottky (R-S) equation:

(eq. I.4.2) jRS = AT2exp[(V Vm (E)) /kT]

with A = 4emkb2/h

3, and Vm(E) describing the

field- dependend lowering of the injection

barrier by the attraction of the injected carrier

to its mirror charge; Vm(E) = (eE/40).

Generally, R-S injection will dominate at low

fields, and F-N tunnelling at large fields that

are practically relevant for organic devices.

Experimental j(V) results on MEH- PPV diodes can

qualitatively be described by the F-N eq.n [I D

Parker, J. Appl. Phys.75, 1656 (1993)], however,the absolute current density is several orders of

magnitude lower than described by the F-N eq.n.

This is due to a large backflow of carriers from

the semiconductor back into the metal.

In the case of V 0, i.e. in the case of a

work function that is matched to the respectivesemiconductor level (5.2 eV for hole injection

into PPV), the metal- semiconductor contact is

termed ohmic. For ohmic contacts the F-N and R-S

equations do no longer apply. Instead, current

density will be controlled by the transport of


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charge carriers across the film. Carrier

transport is characterised by charge carrier

mobility , which plays a crucial role fororganic transistors. Practically, a contact can

be considered ohmic whenever carrier transport is

a more restrictive limit on current density than

carrier injection.

Having an ohmic contact means the problem of

injection is solved ohmic contacts are

desirable. Considerable effort has therefore been

devoted to increase the workfunction of thetransparent ITO anode by a variety of

physicochemical treatment cycles [J S Kim, M

Granstrom, R H Friend, N Johansson, W R Salaneck,

R Daik, W J Feast, F Cacialli, J. Appl. Phys. 84,

6859 (1998)]. Recently, it has become common to

coat ITO with a thin film of the high work

function synthetic metal PEDOT/PSS [L

Groenendaal, F Jonas, D Freitag, H Pielartzik, J

R Reynolds, Adv. Mater.12, 481 (2000)] ( = 5.2

eV). As cathodes, low workfunction materials such

as Ca are commonly used. These require protection

from ambient atmosphere, otherwise they would

rapidly degrade. This can be provided by

encapsulation, or by capping with a more stable

metal such as Al. Note that in such an electrode,Fermi levels of Al and Ca will equilibrate, but

the work function will remain that of the metal

through which the electron attempts to leave,

which is Ca when we consider electron injection

into the organic semiconductor.


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II.1 Synthetic Metals

According to a general theorem proven by Peierls,

there cannot be a one- dimensional metal (Peierls

transition). This seems to exclude the

possibility of metallic polymers.

However, inorganic semiconductors can be made

quasimetallic if a high level of doping is

introduced. In 1977, Heeger, MacDiarmid,

Shirakawa, and coworkers discovered a very

similar semiconductor (quasi)metal transition

in the organic semiconductor trans- polyacetylene

(PA) [H Shirakawa, E J Louis, A G MacDiarmid, C K

Chiang, A J Heeger, Chem. Comm. No. 13, 578

(1977)]. Pure polyacetylene is a semiconductor

with a bandgap in the visible. However, when PA

was exposed to halogen vapours, these doped the

semiconductor and conductivity increased

significantly. Iodine vapours were mosteffective, increasing conductivity by several

orders of magnitude up to 38 S/cm. Heeger,

MacDiarmid, and Shirakawa were awarded the 2000

Chemistry Nobel Prize for their discovery.

Besides PA, examples for synthetic metals are

polyaniline (PANi), polypyrrole (PPy), and

poly(3,4-ethylene dioxythiophene) (PEDOT or

PDOT).

Doping in synthetic metals is somewhat different

from doping in inorganic semiconductors, were

heteroatoms of different chemical valencies are


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54

introduced into a host crystal lattice. In

synthetic metals, doping is by a chemical

reaction between semiconducting polymer and

dopant. Both redox- and acid/base reactions

between semiconductor and dopant have been usedsuccessfully; typical examples are PA oxidation

with chloride or iodine, and acid doping of PAni

with camphorsulfonic acid, PPy with phosphoric

acid, and PEDOT with poly(styrene sulfonic acid)

(PSS). Quite high dopant concentrations of (1 to

50)% are typically used. Iodine oxidises

polyacetylene, thus removing electrons from the

HOMO. This opens up mobile vacancies (holes) in

the previously completely full HOMO (valence

band). Conductivity will depend on doping level;

for the highest doping levels PA can be as

conductive as Platinum (105 S/cm at room

temperature). Generally, synthetic metals become

more metal- like in their behaviour as dopant

concentration is increased. Above a criticaldopant level, most synthetic metals remain

conductive in the limit T 0, and display NTC

behaviour at room temperature. Fig. II.1

summarises the behaviour of a number of synthetic

metals. Multiple entries for the same material

correspond to samples with different dopant

levels.

(Fig. 1 from A B Kaiser, Adv. Mater. 13, 927

(2001))


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Fig. II.1: Room- temperature conductivity and

temperature behaviour of conductivity for several

synthetic metals at different temperatures. SWCN

= single- wall carbon nanotubes. Full squares:

NTC at RT and > 0 at T 0 (proper metal);

hollow squares: NTC at RT, but 0 at T 0;

full circles: PTC at RT but > 0 at T 0;

hollow circles: PTC at RT and 0 at T 0

(proper semiconductor). From A B Kaiser, Adv.

Mater. 13, 927 (2001).

To add to the ambiguity in classifying synthetic

metals as proper metals or not, highly doped PA

and PAni show non- zero conductivity in the limit

of zero temperature and NTC behaviour around room

temperature, but PTC behaviour at low

temperatures, with a conductivity maximum in the

region (100250) K. For a detailed discussion,

see [R18].

However, the focus of synthetic metals research

never was to resolve ambiguities in

classification, but the desire to arrive at

practical materials that can be used e.g. as

antistatic coatings, or electrodes. Like many

other mainchain- conjugated polymers, PA isintractable and insoluble. The main obstacle

towards practical applications of synthetic

metals is to prepare highly conductive organic

materials in a versatile formulation.


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56

II.2.a Water- based synthetic metals

Nowadays, mainly PANi and PEDOT are used as

synthetic metals. This is because both of them

are available as aqueous dispersions for

spincoating or ink jet printing. This makes them

easy to process into coatings, or for multilayer

film applications in conjunction with organic

semiconductors: Soluble organic semiconductors

are usually processed from organic solvents, in

which the water- based synthetic metals are not

soluble. The advent of water- based synthetic

metals can be seen as a second breakthrough in

addition to the initial discovery of metallic

polyacetylene. Soluble synthetic metals have

considerably larger potential for applications

than insoluble polyacetylene.

PANi is a very complex material, with several

Redox states. For a discussion, see [T2, Ch.7.9].

Here, we will discuss PEDOT, which typically is

acid- doped with poly(styrene sulphonic acid)

(PSS) to form the highly conductive PEDOT/PSS

complex. PEDOT in that case is synthesized from

EDOT monomer in an aqueous medium that already

contains PSS; in this way PEDOT/PSS become truly

inseparable:


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57

O O

S

Na 2S 2O 8

SO 3H

n

H2O

S

OO

S

OO

S

OO

S

OO

S

OO

SC*

OO

*

n

C

SO3H SO

3H SO

3- SO3HSO3-

**n

SO3H

++

Fig. II.2: The synthesis of PEDOT from EDOT in

aqueous medium in the presence of poly(styrene

sulphonic acid) (PSS) and Na2S2O8. Note how PSS

acts as proton donor (i.e., acid), and PEDOT as

proton acceptor (base).

Na2S2O8 acts as oxidizing agent for the couplingof EDOT monomers. Note how the chemical bonding

pattern of EDOT/PEDOT changes under acid doping:

A C=C bond opens up and the C bonds to an H+

donated by the acid. As a result, there is:


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- A net positive charge on the PEDOT chain that

will strongly attract the negative charge left on

the acid. Since this happens at many points along

the chain, PEDOT and PSS become closely

intertwined and float as a fine dispersion orcolloid in the aqueous solution, rather than

precipitate. They can not separate, and will not

dissolve in organic solvents.

- An unpaired electron remains on the main

chain that is highly mobile along the chain.

PEDOT/PSS aqueous dispersion is commercially

available under the tradename Baytron P. From

Baytron P, thin, highly transparent, conductive

surface coatings can be prepared by spincasting

or dip- coating onto almost any surface. The

resulting conductivity is in the order 1 to 10

S/cm. A typical sheet resistance for spincastPEDOT/PSS coatings is 1 M/square.

II.2.b Applications of synthetic metals

PEDOT/PSS was originally developed as antistatic

coating for photographic films. Large- scale

processing of photographic film (development)

requires the winding of films over reels in the

dark. This leads to static charging, and

discharge sparks may expose the film. A PEDOT/PSS

coating allows charges to disperse and thus

prevents the build- up of high voltages. For this

applications, rather low conductivities are


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59

sufficient. Today, > 108m2of photographic film

is coated with PEDOT/PSS every year.

It should be obvious, however, that an elegant

and versatile off- the- shelve product like

water- based PEDOT/PSS will find a number of

other applications, in particular as it is sold

in high quality at a moderate price. Antistatic

coating of plastic films for packaging

microelectronics components is one of them. Also,

PEDOT/PSS has been used as counter- electrode for

anodised capacitors [F Larmat, J R Reynolds,Synth. Met.79, 229 (1996)].

PEDOT/PSS is now also increasingly used in

organic electronics. PEDOT/PSS displays a high

work function in the order (5 5.2) eV, which

according to I.4 qualifies it as good electrode

for hole injection into a semiconductor. It istherefor used e.g. as contact electrodes and ink-

jet printed wire in organic field effect

transistor circuits, and as anode (or anode

coating) in organic electroluminescent (EL)

devices. Also it is a useful electrode for

organic photovoltaics, were somewhat higher sheet

resistance is not a problem due to the generally

rather low current densities. We will discuss theuse of PEDOT/PSS repeatedly in the following

chapters. The excellent review [R6] is highly

recommended.


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Recently, flexible and transparent PEDOT/PSS-

coated polyester sheets with sheet resistance in

the order 1k/square have become available under

the tradename Agfa OrgaconTM. These sheets can

be patterned by an etching procedure which is

performed in a very similar way to conventional

photoresist patterning, although the chemistry is

rather different. This is the substrate onto

which future organic electronics and displays

will be prepared.


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II.2Organic field effect transistors

The transistor is the mainstay of solid state

electronics, and represents one of the seminal

inventions of the 20th century. All transistors

have three terminals. A current between two of

these three terminals can be controlled by the

input of the third terminal. Transistors can be

categorised into two families according to how

this control is exercised: Those where the

control is through a current at the third

terminal, and those where the control is through

a voltage at the third terminal.

In current- controlled transistors, the three

terminals are calledbase (B), collector (C), and

emitter (E). B controls the current between C and

E. Such transistors are realised by doping the

same semiconductor (typically, silicon) with n-

and p- type dopants in different regions, eitherin npn- or pnp configuration, and are also known

as bipolar transistors. Current- controlled

transistors are typically applied in voltage- or

current amplifiers. However, organic

semiconductors do not lend themselves for the

manufacture of bipolar transistors: Most organic

semiconductors will even without doping transport

only holes, or only electrons. An intrinsicallyp- type material can not be transformed into an

electron transporter by n- doping.


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Voltage- controlled transistors are also known as

field effect transistors (FETs). The three

terminals are called source (S), drain (D), and

gate (G). A voltage at G controls the S- D

current. The basic principle of the field effecttransistor dates back to 1930 [J E Lilienfeldt,

US Patent 1745175, 1930]. FETs are simpler in

design than bipolar transistors, require only one

type of carrier transport material (n or p), and

as we will see, not necessarily need to be doped

at all. Field effect transistors can be

manufactured in high integration densities and

are typically applied for logic gates in digital

electronics.

Due to the possible operation with only one type

of carrier, organic transistors invariably are

field effect transistors (organic FETs or OFETs;

sometimes also known as organic thin film

transistors, OTFTs), and only these shall bediscussed here. One of the major driving forces

behind organic FET (OFET) research is the idea to

make low- performance integrated circuits

completely from plastics on plastic substrates,

that are cheap enough to be discarded after

single use (disposable electronics). A target

application is e.g. an electronic pricetag on a

food wrapper that a supermarket checkout can read

remotely. Such a tag must be cheaper than the

cost of a checkout assistant pulling a barcode

across a scanner. It is now regarded as realistic

that this cost barrier can eventually be


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overcome. Also, higher added- value applications

are envisaged in combination with light emitting

organic semiconductors. Active- matrix addressed

display screens require two FETs and a capacitor

to address each pixel in a screen. Ultimately,organic semiconductor technology aims to make the

transistors as well as the light emitting

material from organics.

II.2.a Description of organic FET operation

Fig. II.3 shows the principle design of an

organic thin film transistor (OTFT) withelectrical connections suitable for basic

measurements. A voltage between source (S) and

drain (D) called drain or source- drain voltage

VDattempts to drive a drain current I

Dthrough

the semiconducting transistor channel. However, a

drain current will only flow if there are mobile

charge carriers in the channel. The channel

semiconductor in OFETs is not chemically doped;

in fact, great care has to be taken to keep

impurity levels in OFET- grade semiconductors as

low as possible to avoid unintentional doping.


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Fig.II.3: A field effect transistor in the

bottom gate, top contact architecture.

Transistors may also be built with bottom

contacts (S, D are placed directly on the

insulator), or completely the other way round

(top gate architecture).

A gate voltageVGwill pull carriers out of the

source into the semiconducting channel. However,

since the gate dielectric is an insulator, these

carriers cannot reach the gate metal. Instead,

they will accumulate in the semiconductor near

the channel / insulator interface where they

represent mobile carriers known as an

accumulation layer. The channel semiconductor

Substrate

Gate (Metal)

Gate Insulator

SemiconductorChannel

Source (Metal) Drain (Metal)

Vd

Vg


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thus gets doped by applying VG. However, other

than in chemical doping, this is quickly

reversible: Switching VG off will switch the

doping off. Hence, VG can switch the channel

conductivity. This behaviour is quantified by theso- called on / off ratio, which in some cases

can be as high as 106. Accordingly, ID at a given

VDwill change with V

G: The FET is an electronic

switch. The accumulation layer in an OFET is

generally very thin, about 5 nm or less [M A

Alam, A Dodabalapur, M R Pinto, IEEE Transactions

on Electron Devices, 44, 1332 (1997)]. This is

much thinner than the typical thickness of the

semiconducting film.

Note that an OFET is switched on by applying VD

and VG of equal polarity opposite to the sign of

the mobile carriers. Thus, OFETs can easily be

used to determine the type of carriers a

particular material sustains: If positive VG, VDswitch the transistor on, carriers are negative

(electrons), if negative VG, V

Dare required,

carriers are holes.

To characterise the gate- and source / drain

voltage dependent drain current in a field effect

transistor beyond the on / off description, twotypes of measurements may be carried out:


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The output characteristic of a FET is the family

of curves ID vs VD with VG fixed during every

single VDscan (drain sweep).

The transfer characteristic is the family of ID

vs VG curves with VD fixed during every single VG

scan (gate sweep).

In principle, both of these contain the same

physical information, as long as a sufficient

number of gate voltages (drain voltages) are

scanned for the output (transfer)

characteristics. Practically, however, output

measurements will usually scan ID vs VD at a

number of gate voltages, while transfer

characteristics often will be taken at only two

fixed VD: One VD will be chosen to be as small as

practically possible; much smaller than the

maximum VG used for the scan; while the second VDwill be chosen to be larger than the maximum VG

used in the VGscan. These two regimes are known

as the linear and saturation regime,

respectively. This terminology will become clear

with the quantitative description of OFET

behaviour.

OFET operation can be described quantitatively if

a number of assumptions are made; leading to an

ideal scenario. These assumptions are: Channel

length shall be considerably longer than device

thickness (long channel limit), source and drain


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make contact to the channel semiconductor without

contact resistance or injection barrier, the

semiconductor shall be trap- free and undoped,

and carrier mobility shall be independent of

gate- and drain voltages. Note that in reality,few if any of these assumptions will be met!

Then, the following equation applies [T1, Ch.

14.2.2.1.2]:

(eq.II.2.1.a) ID =Z

LCi(VG VT)VD for VD


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electric field and DG

the gate electric

displacement. DG has the dimensions of charge /

area, and corresponds to a surface charge density

QS of equal magnitude this is the accumulation

layer. From this consideration, we see it is theelectric displacement DG that switches the

transistor on, not the gate voltage or gate

electric field directly.

While the precise derivation of eq. II.2.1 is

somewhat technical, it can easily be understood

qualitatively if for the moment, we leave thethreshold voltage VT aside. Z/L is an obvious

geometry factor. The conductivity of the channel

will be proportional to charge / area QS= C

iV

G.

Also, for VD


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II.3. At VD = VG, the accumulation layer becomes

triangular and is on average half as strong as

for VD 0; this explains the factor 1/2 in eq.

II.2.1.b. When VD exceeds VG, the pinch- off point

moves away from the drain into the transistorchannel, and the carrier- depleted part of the

channel between pinch- off point and drain will

display very high resistance. Consequently, the

transistor displays drain current saturation for

VD VG, with ID,sat VG2. The only way to increase

ID beyond the saturation is to increase VG.

Fig. II.3: Accumulation layer (black) in a field

effect transistor. Top: VG applied, VD = 0. The

accumulation layer is fully developed. Middle: VG

applied, VD = VG. The accumulation layer is

triangular and pinches off at the drain. Bottom:


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VG

applied, VD 2VG. The accumulation layer

pinches off in the middle of the channel.

This leaves only one important contribution toeq. II.2.1 unexplained, that is the threshold

voltage VT. The theory of VT is intricate; also,

it is often found that VTchanges in transistors

as a result of prolonged operation; this is

termed gate bias stress. Gate bias stress has

been studied e.g. by Katz et al. [H E Katz, X M

Hong, A Dodabalapur, R Sarpeshkar, J. Appl. Phys.

91, 1572 (2002)], who conclude that it may resultfrom both slowly orientating dipoles in the

insulator, or the presence of slowly mobile ions

(electret behaviour). We here will simply take VT

as an empirical constant.

Fig. II.4. shows examples for experimentally

obtained OFET output and transfercharacteristics.

[II.4.a = Fig. 3a of [H Sirringhaus, R J Wilson,

R H Friend, M Inbasekaran, W Wu, E P Woo, M

Grell, D D C Bradley, Appl. Phys. Lett.77, 406

(2000)].


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II.4.b

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60 70

|ID

|1/2

[(microAmp)

1/2]

|V G| [V]

[II.4.c = Fig. 3b of K Fujita, T Yasuda, T

Tsutsui, Appl. Phys. Lett.82, 4373 (2003)]

Fig. II.4: a.) Output characteristic of an OFET

with F8T2 as active material. From [H

Sirringhaus, R J Wilson, R H Friend, MInbasekaran, W Wu, E P Woo, M Grell, D D C

Bradley, Appl. Phys. Lett. 77, 406 (2000)]. b.)

Plot of |ID| vs |VG| with ID taken at VD = VG from

II.6.a. c.) Saturated (VD = -60V) transfer

characteristic of an OFET with pentacene as

active material. The same ID is plotted in two

ways: logI

D(left abscissa, open squares) and

ID (right abscissa, full circles). From [K

Fujita, T Yasuda, T Tsutsui, Appl. Phys. Lett.

82, 4373 (2003)].


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Charge carrier mobility can be determined from

experimental characteristics with the help of

eq.s II.2.1. A very robust method of doing so is

to plot ID vs. VG , with ID in the saturation

regime, VD > V

G. This should result in a

straight line with a slope (Z

2LCi)

1/2and intercept

VT. Such a plot can be constructed in several

ways, both from output- and transfer

characteristics. Saturation current level can be

read for different gate voltages from the

saturated (flat) part of an outputcharacteristic. Alternatively, drain and gate

voltage may be connected to the same source /

measure unit; on ramping up voltage we scan ID(V

= VG = VD), i.e. saturation current at different

gate voltages, but always taken at VG = VD. In

such a measurement, we can directly see how ID

rises parabolically with V (ID VG/D2) [D M

Taylor, H L Gomes, A E Underhill, S Edge, P I

Clemenson, J. Phys. D: Appl. Phys. 24, 2032

(1991)]. The most straightforward method,

however, is the measurement of a saturated

transfer characteristic, i.e. a gate voltage scan

with a fixed drain voltage VD that is larger in

modulus than the gate voltage at all times. In

this way, it is ensured that drain current is

always saturated. Plotting ID vs VG with VD >

VG gives direct access to and VT via slope and

intercept. Another important FET characteristic

is the on / off ratio, defined as I(VG= V, V

D=

V)/I(VG = Voff, VD = V) with an operational voltage


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V that corresponds to the voltage available in

the respective application, and Voff

the gate

voltage that minimises the drain current; usually

Voff 0. On / off ratios typically are large

numbers roughly in the region 103107, and can be

extracted most conveniently from a saturated

transfer characteristic when this is plotted with

the ID axis on a logarithmic scale.

We note that saturated mobility often differs

from mobility at small drain voltage. The

transconductance gm is defined in the linearregime of the output characteristic (VD


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We had seen that short channel length L will

result in higher drain currents, but L is even

more important for switching speed. The maximum

speed at which a transistor circuit can operate

is limited by the time it takes carriers to

cross the FET channel. That is the time the

accumulation layer takes to be emptied of charges

through the drain after VG has been switched off.

At frequencies f > f0= 1/, the output signal

(ID) drifts out of phase with the input signal

(VG), and switching amplitude decreases. isgiven by eq.II.2.4:

(eq.II.2.4) 1

= f0 VD

L2

L enters the equation squared: Obviously the

transit time of carriers through the channel will

be proportional to L. Also, the lateral electric

field that pulls carriers across the channel is

proportional to 1/L. is known as the RC time of

the transistor, and can be calculated

alternatively from the channel resistance and

capacitance, with the same result eq.II.2.4.

Assuming VD = 10 V and = 10-2 cm2/Vs, f0 is 1 kHzfor L = 100m, but 100 kHz for L = 10 m. In

contrast, inorganic computer electronics work

with GHz frequencies. It is obvious that organic


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electronics has to compete on price rather than

performance.

It should be noted with care that eq.II.2.4

represents an upper limit for f0. There may be

other capacitances than the accumulation layer in

the transistor that charge (discharge) when VG is

switched on (off). These parasitic (stray)

capacitances may for example arise from an

overlap of source / drain electrodes with the

gate electrode.

Switching speeds of organic transistor circuits

can be studied with the help of ring oscillators.

A ring oscillator consists of an odd number of

logic NOT gates, which can be made of two FETs.

The NOT operation converts LOW (HIGH) input

voltage into HIGH (LOW) output voltage. The

output of each NOT gate is fed into the input ofthe next, with the output of the last of an odd

number of NOT gates being fed back into the input

of the first NOT gate. Thus, a ring is

completed that has no self- consistent state, see

Fig. II.5. Instead, output will oscillate between

HIGH and LOW with a frequency f that is related

to f0, the switching frequency of an individual

transistor. f will be smaller than f0, as morethan one transistor has to switch, and will

increase with the number of NOT gates or stages

of the oscillator.


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Fig. II.5: A ring oscillator (schematically). If

we assume the output of the bottom NOT gate were

LOW, the output of the top right NOT gate should

be HIGH, the output of the top left NOT gate

should be LOW, leading to HIGH output of the

bottom NOT gate which is inconsistent with the

original assumption. The ring oscillator has noself- consistent state and will oscillate between

HIGH and LOW with frequency f.

When the oscillating output is displayed on an

oscilloscope, f can be extracted. Often, the

inverse of f divided by (2 x number of NOT gates

= total number of transistors) is reported as propagation delay per stage. In the absence of

stray capacitances, this equals = 1/f0. Ring

oscillators allow a dynamic measurement of

carrier mobility. In any case, f is a practically

relevant measure of circuit speed. Fast OFET ring

oscillators with f > 100 kHz have been

demonstrated by the group at Siemens [W Fix, A

Ullmann, J Ficker, W Clemens, Appl. Phys. Lett.

81, 1735 (2002)]. Ring oscillators are not purely

a diagnostic tool, but have important

applications. The group at 3M have demonstrated a

radiofrequency identification tag (rf ID tag)

NOT

NOTNOT


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that uses a pentacene- based organic ring

oscillator with a frequency of about 200 Hz [P F

Baude, D A Ender, M A Haase, T W Kelley, D V

Muyres, S D Theiss, Appl. Phys. Lett. 82, 3964

(2003)]. The rf ID tag has an antenna (LCcircuit) that absorbs an incoming radio signal

(frequency 13.6 MHz), and re- emits a signal of

the same frequency, but with an amplitude

modulation. The amplitude modulation is

facilitated by the ring oscillator periodically

shorting / re- engaging the antenna circuit.

II.2.b Requirements on OFET materials

Good OFETs should display high drain current at

low drain and gate voltages, without reliance in

optimized geometry factor; high on / off ratios,

which means drain current at VG

= 0 should be

extremely low; and fast . This implies strong

requirements on all materials used in OFETs semiconductor, insulator, and metals. In the

field of organic semiconductors, the one property

that has been addressed most is charge carrier

mobility, and that is the one we will discuss in

detail. However, let us just list a few other

material requirements:

- Source metals should make ohmic (barrier- free)

contacts to the semiconductor.

- Gate insulators should be thin without

displaying pinholes (tricky for solution


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processed organics!) or any other current

leakage, and should have a high dielectric

constant.

- organic semiconductors must be free of

unintentional dopants down to very low

concentrations, otherwise the transistor will not

switch off properly.

- organic semiconductors must be free of charge

carrier traps. Trapped carriers are no longer

mobile, which means part of the accumulation

layer DG cannot contribute to the drain current.

- The interface between insulator and

semiconductor is particularly important. Often,

inorganic oxides are used as gate insulators

(SiO2, Al

2O3, Ta

2O

5). These are a found to improve

when they are modified by a self- assembl

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