Solution-Processable Components for Organic Electronic Devices E-Book

• Herausgeber: Wiley-VCH Verlag GmbH & Co. KGaA
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 1291

Veröffentlichungsjahr: 2019

Beschreibung

Provides first-hand insights into advanced fabrication techniques for solution processable organic electronics materials and devices

The field of printable organic electronics has emerged as a technology which plays a major role in materials science research and development. Printable organic electronics soon compete with, and for specific applications can even outpace, conventional semiconductor devices in terms of performance, cost, and versatility. Printing techniques allow for large-scale fabrication of organic electronic components and functional devices for use as wearable electronics, health-care sensors, Internet of Things, monitoring of environment pollution and many others, yet-to-be-conceived applications. The first part of Solution-Processable Components for Organic Electronic Devices covers the synthesis of: soluble conjugated polymers; solution-processable nanoparticles of inorganic semiconductors; high-k nanoparticles by means of controlled radical polymerization; advanced blending techniques yielding novel materials with extraordinary properties. The book also discusses photogeneration of charge carriers in nanostructured bulk heterojunctions and charge carrier transport in multicomponent materials such as composites and nanocomposites as well as photovoltaic devices modelling. The second part of the book is devoted to organic electronic devices, such as field effect transistors, light emitting diodes, photovoltaics, photodiodes and electronic memory devices which can be produced by solution-based methods, including printing and roll-to-roll manufacturing.
The book provides in-depth knowledge for experienced researchers and for those entering the field. It comprises 12 chapters focused on:

? novel organic electronics components synthesis and solution-based processing techniques
? advanced analysis of mechanisms governing charge carrier generation and transport in organic semiconductors and devices
? fabrication techniques and characterization methods of organic electronic devices

Providing coverage of the state of the art of organic electronics, Solution-Processable
Components for Organic Electronic Devices is an excellent book for materials scientists, applied physicists, engineering scientists, and those working in the electronics industry.

Leseprobe

Table of Contents

Cover

Preface

1 Precision and Purity of Conjugated Polymers – To be Ensured Before Processing

1.1 Polymer Design

1.2 Polymer Synthesis

1.3 Molecular Structure, Supramolecular Structure, and Interfaces

1.4 Beyond Solution Synthesis

1.5 Conclusions

Acknowledgments

References

2 Synthesis of Solution‐Processable Nanoparticles of Inorganic Semiconductors and Their Application to the Fabrication of Hybrid Materials for Organic Electronics and Photonics

2.1 Synthesis and Characterization of Colloidal Semiconductor Nanocrystals

2.2 Primary Ligand Identifications in Colloidal Nanocrystals of Inorganic Semiconductor – Methodology and Investigation Techniques

2.3 Exchange of Primary Ligands for Functional Ones

2.4 Preparation and Applications of Photopatternable Nanocrystals in the Microfabrication of 2D/3D Functional Structures

2.5 Energy and Charge Transfer in Semiconducting Nanocrystal‐Based Hybrid Materials

2.6 Synthesis, Electrical/Optical Properties, and Applications of Perovskite Nanomaterials

Acknowledgments

References

3 Synthesis of High

k

Nanoparticles by Controlled Radical Polymerization

3.1 Introduction to Controlled Radical Polymerization

3.2 Surface‐Initiated Controlled Radical Polymerization

3.3 SI‐CRP from Nanoparticles

3.4 Materials Prepared via SI‐CRP

3.5 High

k

Nanoparticles

3.6 High

k

Hybrid Materials Prepared via SI‐CRP

3.7 Summary

Acknowledgments

References

4 Polymer Blending and Phase Behavior in Organic Electronics: Two Case Studies

4.1 Introduction

4.2 Calculation of the Ternary Phase Diagram of an Amorphous Mixture

4.3 Elimination of Charge Trapping in OLEDs by Polymer Blending

4.4 Vapor‐Induced Demixing in Polymer Films for Organic Memory Devices

4.5 General Outlook

Acknowledgments

Appendix Experimental Procedures Relevant to Section 4.3

Appendix Experimental Procedures Corresponding to Section 4.4

References

5 Photogeneration of Charge Carriers in Solution‐Processable Organic Semiconductors

5.1 Introduction

5.2 Photogeneration in Single‐Component Systems

5.3 Photogeneration in Donor–Acceptors Systems

5.4 Discussion of Contemporary Results

5.5 Conclusions

Acknowledgments

References

6 Charge Carrier Transport in Organic Semiconductor Composites – Models and Experimental Techniques

6.1 Introduction

6.2 Basic Concepts

6.3 Exciton States

6.4 Charge Carriers in Semiconductors

6.5 Density of States in Amorphous Organic Semiconductors

6.6 Models of Charge Carrier Transport in Organic Semiconductors

6.7 Steady‐State Currents

6.8 Influence of Semiconductor Morphology on Field‐Effect Mobility

6.9 Drift–diffusion Current in Organic Heterostructure

6.10 Selected Experimental Techniques for Investigation of Charge Carriers Transport

6.11 Concluding Remarks

References

7 Organic Field‐Effect Transistors Based on Nanostructured Blends

7.1 Introduction

7.2 Binary Semiconducting Blends

7.3 Semiconducting Blends of Conjugated Semiconductors and Polymer Insulators in Organic Field‐Effect Transistors

7.4 Summary

Acknowledgments

References

8 Organic Light‐emitting Diodes Based on Solution‐Processable Organic Materials

8.1 Introduction

8.2 Basic Characteristics and Recent Trends in Organic Light‐emitting Diodes

8.3 Photoactive Organic Materials

8.4 Fabrication of Solution‐Processable OLEDs

8.5 Conclusions

References

9 Organic Photovoltaics Based on Solution‐Processable Nanostructured Materials: Device Physics and Modeling

9.1 Introduction to Modeling of Electronic Devices

9.2 Simulation Examples

9.3 Conclusions

Acknowledgments

References

10 Solution‐Processed Organic Photodiodes

10.1 Introduction

10.2 Basic Principles of Organic Photodiodes

10.3 State of the Art of Solution‐Processed Organic Photodiodes

10.4 Device Modeling of Organic Photodiodes

10.5 Conclusions

Acknowledgments

References

11 Electronic Memory Devices Based on Solution‐Processable Nanostructured Materials

11.1 Introduction

11.2 Various Principles of Data Storage

11.3 Resistive Memory Operation Mechanisms

11.4 Materials for Printable Memory Devices

11.5 Final Remarks

Acknowledgments

References

12 Intelligent Roll‐to‐Roll Manufacturing of Organic Electronic Devices

12.1 Introduction

12.2 Polymer‐Based Organic Photovoltaics

12.3 Manufacturing of Organic Electronic Devices by Roll‐to‐Roll Pilot‐to‐Production Lines

12.4 In‐line Optical Metrology for Quality Control

12.5 Conclusions

Acknowledgments

References

Index

End User License Agreement

List of Tables

Chapter 2

Table 2.1 Band gaps, exciton Bohr radii, crystal structures, and lattice paramet...

Table 2.2 Summarized reaction conditions (composition of the reaction mixture an...

Table 2.3 Summarized experimental conditions of core/shell nanocrystal preparati...

Chapter 3

Table 3.1 Summary of polymer‐grafted metal oxide nanoparticles prepared by SI‐AT...

Table 3.2 Static dielectric permittivity (

k

), experimental band gap (Gap), and c...

Chapter 4

Table 4.1 Relative degree of polymerization, monomeric molar volume, and solubil...

Table 4.2 Flory–Huggins interaction parameters for the MEH‐PPV:insulator:chlorob...

Chapter 8

Table 8.1 The thickness, surface roughness (RMS), and the electrical characteris...

Chapter 9

Table 9.1 Examples of open‐source software and resources for simulation of photo...

Table 9.2 Extracted parameters: values of parameters that were optimized in the ...

Table 9.3 Optimized parameters: values of parameters that were optimized in the ...

Chapter 10

Table 10.1 Performances of solution‐processed organic photodiodes operating in t...

Table 10.2 Near‐infrared organic photodiode performances.

Table 10.3 Summary of simulation parameters used in this section.

Table 10.4 Correspondence between Figure 10.17 and work function modification.

Table 10.5 Pool Frankel mobility parameters used in simulation illustrated in Fi...

List of Illustrations

Chapter 1

Scheme 1.1 Structures of classical conjugated polymers.

Scheme 1.2 Representative metathesis polymerization of cycloolefins. ...

Scheme 1.3 Examples of milestones in organic electronics.

Scheme 1.4 Dimer and tetramer of thiophene and pentacene (

5

).

Scheme 1.5 Resonance structures of a thiophene radical cation [108].

Scheme 1.6 Formation of a radical cation salt illustrated by the intera...

Scheme 1.7 Radical cation stabilization, dimer formation, and oxidative...

Scheme 1.8 Commonly applied monomers for oxidative polymerization.

Scheme 1.9 Oxidative coupling of benzene and possible side reaction. ...

Scheme 1.10 The quinoid structure of PPP enforced by doping or by termi...

Scheme 1.11 Compromised extended conjugation due to increased nonplanar...

Scheme 1.12 Biphenyl synthesis by Ullmann coupling.

Scheme 1.13 4,4′‐Dimethyl‐1,1′‐biphenyl synthesis using a Grignard reag...

Scheme 1.14 Yamamoto coupling of 1,4‐dibromobenzene with chelated zero‐...

Scheme 1.15 Polymerization of 1,4‐dihalobenzene with end‐capping by mon...

Figure 1.1 Plot of the degree of polymerization

as a function of th...

Scheme 1.16 Bifunctional tin and boronate reagents and a corresponding ...

Scheme 1.17 Selected donor and acceptor building blocks for donor–accep...

Scheme 1.18 Synthesis of AB‐thiophene monomer

28

to yield regioregular ...

Scheme 1.19 Mechanism of chain growth polymerization proposed by Yokoza...

Scheme 1.20 Different poly‐

para

‐phenylene structures.

Scheme 1.21 Synthetic route toward LPPP.

Scheme 1.22 Fluorene (

34

) and indenofluorene (

36

) polycondensation afte...

Scheme 1.23 Regioselective monomer synthesis as demonstrated for 3,6‐di...

Scheme 1.24 Pyrene (

43

) in different reaction protocols.

Scheme 1.25 Use of bifunctionalized cyclopentadithiophene (

47

) and the ...

Scheme 1.26 Alkylation of fluorene (

50

) and poly‐2,7‐fluorene containin...

Scheme 1.27 1,4‐Bis(phenyldichloromethyl)benzene (

54

) yielding poly‐1,4...

Scheme 1.28 Concepts of PPV synthesis through σ‐bond formation (i), con...

Scheme 1.29 Wessling–Zimmermann route toward PPV.

Scheme 1.30 Side reactions within the Wessling–Zimmermann route [221]....

Scheme 1.31 Side products in the Gilch polymerization depending on the ...

Scheme 1.32

syn

‐1,4‐Bis(4‐halophenyl)‐1,4‐dimethoxycyclohexadienes (

65

)...

Scheme 1.33

syn

‐1,4‐Bis(4‐halophenyl)‐1,4‐dimethoxycyclohexadienes lead...

Scheme 1.34 Ring‐closing metathesis reaction to achieve a ladder‐type p...

Scheme 1.35 PA through the Durham route [232].

Scheme 1.36 Ring‐opening metathesis polymerization of benzvalene and re...

Scheme 1.37 Ring‐opening metathesis polymerization of ladderene

77

and ...

Scheme 1.38 Random hydrogen and bromo end groups and defined amino term...

Scheme 1.39 End‐capping protocol used to terminate a Suzuki polymerizat...

Scheme 1.40 D–A‐polymer with and without precise tolyl end‐capping.

Figure 1.2 Schematic description of solvent‐assisted film formation of ...

Figure 1.3 Schematic illustration of lamellar packing of chains and col...

Scheme 1.41 10,10′‐Dibromo‐9,9′‐bianthryl precursor monomer to achieve ...

Scheme 1.42 Fullerene, armchair carbon nanotube (

85

), and zigzag carbon...

Scheme 1.43 Fundamentally different ways to obtain graphene.

Chapter 2

Figure 2.1 Molecular structure of InP nanocluster (1.3 nm) (In

37

P

20

(O

2

C...

Figure 2.2 Two binding modes of thiol on copper sulfide nanocrystals. ...

Figure 2.3 Effect of quantum confinement on the band structure of semic...

Figure 2.4 Size‐dependent absorption and emission spectra of CdSe nanoc...

Figure 2.5 The effect of CdSe nanocrystal shape on their band gap (a) a...

Figure 2.6 Comparison of the electrochemically determined HOMO and LUMO...

Figure 2.7 Computer simulation of

hot‐injection

and

heating‐up

...

Figure 2.8 Schematic illustration of the optimization of InP nanocrysta...

Figure 2.9 Reactions of sulfur with oleylamine (OLA) (a) and 1‐octadece...

Figure 2.10 Proposed reaction pathway for precursor conversion to

monom

...

Figure 2.11 TEM images of 6 nm (a), 8 nm (b), and 9 nm (c) sized PbS na...

Figure 2.12 Variation of the shapes of CdSe nanocrystals by changing of...

Figure 2.13 Shape control of colloidal nanocrystals, (a) the high‐energ...

Figure 2.14 TEM images of CdSe nanorods synthesized using mixtures of T...

Figure 2.15 Illustrating the reactivity of dichalcogenide precursors.

Figure 2.16 Schematic of localized surface plasmon resonance polarizati...

Figure 2.17 TEM images of initial CdSe (a), Ag

2

Se transformed from the ...

Figure 2.18 Structure of

chalcopyrite

,

zinc blende

, and

wurtzite

CuInS

2

Figure 2.19 Cyclic voltammograms of Cu–In–S nanocrystals with average d...

Figure 2.20 Schematic illustration of the synthesis of

chalcopyrite

and...

Figure 2.21 Schematic of the growth mechanism with corresponding TEM im...

Figure 2.22 Schematic illustration of the Kirkendall effect on Cu

2−x

...

Figure 2.23 Cyclic voltammograms of In

2

S

3

and CuInS

2

nanocrystals (29 ±...

Figure 2.24 TEM images of chalcopyrite CuInSe

2

nanocrystals prepared us...

Figure 2.25 TEM images of CuInSe

2

nanocrystals (a, b), HRTEM image of a...

Figure 2.26 TEM and HRTEM images of

wurtzite

‐Cu

2

ZnSnS

4

(a) and

kesterit

...

Figure 2.27 Schematic illustration of the typical alloyed and core/shel...

Figure 2.28 Schematic representation of the energy‐level alignment in d...

Figure 2.29 Evaluation of the absorption (a) and photoluminescence (b) ...

Figure 2.30 TEM images of CdSe plain core nanocrystals and the correspo...

Figure 2.31 Self‐assembly of CdSe/CdS nanorods prepared by the seeded g...

Figure 2.32 Photoluminescence spectra of InP(Zn) nanocrystals prepared ...

Figure 2.33 TEM images of InP/CdS core/shell nanocrystals after 1 (a), ...

Figure 2.34 Schematic diagrams of the electronic energy levels of donor...

Figure 2.35 X‐ray diffraction patterns of the

zinc blend

(a) and

wurtzi

...

Figure 2.36

1

H NMR spectra of 1‐dodecanethiol (DDT), oleylamine (OLA), ...

Figure 2.37 Types of nanocrystal ligation according to Green's covalent...

Figure 2.38 Typical methods used to efficiently remove long‐chain organ...

Figure 2.39 Schematic illustration of nanocrystal initial ligand remova...

Figure 2.40 The valence and conduction band edge energies of PbS nanocr...

Figure 2.41 Structures of the ligands used for transfer of nanocrystals...

Figure 2.42 Schematic pathways for the preparation of the bifunctional ...

Figure 2.43 Schematic representation of the different strategies for th...

Figure 2.44 Oligothiophenes with different anchoring groups used for th...

Figure 2.45 Chemical structure of electroactive ligands introduced to C...

Figure 2.46 Cyclic voltammograms of free ligands

L4

–

L7

and Cu–In–Zn–S n...

Figure 2.47 Chemical structure of conjugated polymers with different an...

Figure 2.48 Molecular recognition between 1‐(6‐mercaptohexyl)thymine ca...

Figure 2.49 (a) Chemical amplification reaction of

t

‐BOC‐capped QD nano...

Figure 2.50 Optical microscopy images of negative 2D patterns fabricate...

Figure 2.51 (a) Current–voltage curves of photovoltaic devices fabricat...

Figure 2.52 (a) Synthetic route of photopolymerizable green CdSe/ZnS QD...

Figure 2.53 Photopatternable QD and its applications: (A) Scheme of pho...

Figure 2.54 Synthetic route of Cd‐free photopolymerizable QD In(Zn)P/Zn...

Figure 2.55 (a) Top view and side view of designed 3D structures. (b, c...

Figure 2.56 Photocurrent density as a function of applied voltage in de...

Figure 2.57 Schematic energy band diagram of QD‐hybridized rubrene NSs ...

Figure 2.58 (a) Schematic illustration of optical waveguiding experimen...

Figure 2.59 (a) LCM PC spectra of the P3HT/PCBM without QDs and with va...

Figure 2.60 (a) LCM PC spectra of pristine and O‐QD‐infiltrated and M‐Q...

Figure 2.61 (a) Schematic illustration of SPR‐assisted FRET effect in a...

Figure 2.62 (a) Synthetic routes for the preparation of SWNTs coupled w...

Figure 2.63 Schematic synthetic procedures and chemical structures of Q...

Figure 2.64 (a) LCM PL spectra of green QDs and QD‐P3000. The dotted cu...

Figure 2.65 (a) LCM PL spectra of green QD, QD‐P3HT‐3000, QD‐P3HT‐6000,...

Figure 2.66 (a) Schematic illustration of the reaction system and proce...

Figure 2.67 Schematic representation of the derivation of lower dimensi...

Figure 2.68 (a) Schematic representation of the water‐induced direct tr...

Figure 2.69 (a) Schematic of CsPbBr

3

perovskite lattice, (b) transmissi...

Figure 2.70 (a) Schematic representation of interparticle anion exchang...

Figure 2.71 Steady‐state PL spectra from aligned OPC film photoexcited ...

Figure 2.72 (a) Perovskite nanocrystal LED. (b) Energy‐level diagram. (...

Chapter 3

Figure 3.1 Overview of new ATRP techniques with ppm amounts of Cu catal...

Scheme 3.1 Illustration of “ligand exchange” of octylamine (OA)‐capped ...

Scheme 3.2 Typical SI‐CRP reactions. (a) SI‐ATRP. X: Br or Cl; L: ligan...

Scheme 3.3 General mechanism of SI‐ATRP with low‐ppm Cu catalyst and/or...

Scheme 3.4 Three main approaches of SI‐RAFT polymerization.

Scheme 3.5 Examples of anchoring groups for surface modification (a) an...

Scheme 3.6 Illustration of the population of dormant and radical specie...

Figure 3.2 Illustration of the transition from CPB to SDPB. (a) A hybri...

Figure 3.3 Illustration of hydrogen bonding interactions between the am...

Figure 3.4 Correlation of dielectric permittivity with a band gap of ca...

Figure 3.5 Temperature dependence of dielectric parameters of the BaTiO

Figure 3.6 2‐D simulated electric field distributions for 200 particles...

Figure 3.7 Schematic illustration for the fluoropolymer@BaTiO

3

nanopart...

Figure 3.8 Frequency‐dependent dielectric properties of BaTiO

3

‐PS/PMMA ...

Figure 3.9 Variation of the permittivity of the TiO

2

‐PS composite versu...

Chapter 4

Figure 4.1 Schematic overview of the phase separation processes that ma...

Figure 4.2 Exemplary ternary phase diagram for a solvent‐borne binary p...

Figure 4.3 Ternary phase diagrams of the blends MEH‐PPV/PS35/chlorobenz...

Figure 4.4 AFM surface topography (a, b, e) and CLSM (c, d, f) images o...

Figure 4.5 Current density (

J

) of positive (lines) and negative (dots) ...

Figure 4.6 Luminous efficiency plotted as a function of voltage (

V

) for...

Figure 4.7 Calculated ternary phase diagrams for polymer (P)/water (W)/...

Figure 4.8 Numerical simulation of water vapor‐induced spinodal decompo...

Figure 4.9 Tapping mode AFM height images of P(VDF‐TrFE) thin films coa...

Figure 4.10 AFM topography images of the polymer/substrate interface in...

Figure 4.11 AFM height images of P(VDF‐TrFE) thin films coated from cyc...

Figure 4.12 (a) Layer thickness and (b) rms roughness plotted as a func...

Figure 4.13 (a) Device yield as a function of water miscibility of solv...

Figure 4.A.1 Schematic cross‐sectional images displaying the architectu...

Chapter 5

Figure 5.1 (a) External quantum efficiency (EQE) measured under short‐c...

Figure 5.2 (a) Internal quantum efficiency (IQE) for differently prepar...

Figure 5.3 Coulomb binding energy (

E

coul

), as a function of the excit...

Figure 5.4 Schematic illustrating the process of eh‐separation in a bul...

Figure 5.5 (a) Short‐circuit photocurrent spectrum (solid line) and abs...

Figure 5.6 IQE of a single‐layer solar cell made in the structure ITO/M...

Figure 5.7 Sketch of the energy diagram of the

INDO

‐calculated (

interme

...

Figure 5.8 (a) Sketch of the energy levels in a solar cell under short‐...

Figure 5.9 Schematic illustrating the different states involved in the ...

Figure 5.10 Experimental transient absorption signals (open symbols) as...

Figure 5.11 The internal quantum efficiency (IQE) of photogeneration in...

Figure 5.12 (a) The chemical structure of the poly(

p

‐phenlyene)‐based p...

Figure 5.13 The dissociation probability as a function of the electric ...

Figure 5.14 The current–voltage characteristics obtained for a bilayer ...

Figure 5.15 (a) The calculated dissociation probability

ϕ

(

F

)

as a...

Figure 5.16 (a) The probability density

|

ψ

n

(

y

)|

2

of measuring ...

Figure 5.17 (a) Schematics of the 1‐D (top panel) and 2‐D (bottom panel...

Figure 5.18 (a) 1‐D and 2‐D Monte Carlo simulations for

m

eff

= 0.1 an...

Figure 5.19 (a) Fill factor for a polymer thickness of 14, 36, and 66 n...

Figure 5.20 Schematic illustrating the competition between recombinatio...

Figure 5.21 Experimental TDCF photocurrent transients (open squares) me...

Figure 5.22

j

(

V

)

curves for (a) as‐cast, (b) annealed, and (c) DIO‐tre...

Figure 5.23 (a) Probability that in a 100‐nm‐thick BHJ‐OSC with asymmet...

Figure 5.24 The dependence of the fill factor on the ratio between the ...

Chapter 6

Figure 6.1 Illustration of density of electrons per energy unit in Ferm...

Figure 6.2 Schematic illustration of exchange energy as a function of d...

Figure 6.3 Visualization of formation of σ and π orbitals in ethylene. ...

Figure 6.4 Schematic presentation of energy levels of bonding and antib...

Figure 6.5 The band model for the intrinsic semiconductor illustrating ...

Figure 6.6 (a) Energy scheme for

N

noncoupled molecules

i

that are in t...

Figure 6.7 Schematic illustrating the shift and broadening of spectral ...

Figure 6.8 Schematic representation of the proposed three‐phase morphol...

Figure 6.9 Energy levels in n‐ and p‐type doped semiconductors.

Figure 6.10 Splitting of the Fermi level in a p‐type semiconductor in t...

Figure 6.11 The thermal dependence of the mobility for a polaron; solid...

Figure 6.12 Temperature dependence of the electrical conductivity (norm...

Figure 6.13 Sheet conductivities (

σ

) of (a) poly(3‐hexylthiophene)...

Figure 6.14 Molecular structures of (a) pentacene and (b) F4TCNQ; (c) i...

Figure 6.15 Energy levels in molecular semiconductors comprising positi...

Figure 6.16 Evolution of electronic structure with increasing doping le...

Figure 6.17 Schematic view of the effect of state filling (gray area) i...

Figure 6.18 Normalized Gaussian DOS [

g

(

E

) in units

N

t

/

σ

DOS

, ...

Figure 6.19 Schematic representation of transport mechanisms for (a) ba...

Figure 6.20 Schematic illustration of organic field‐effect transistor (...

Figure 6.21 An energy band diagram for a compositionally graded semicon...

Figure 6.22 Microstructure of conjugated polymer films: (a) semicrystal...

Figure 6.23 Chemical structures of (a) (poly(3,4‐ethylenedioxythiophene...

Figure 6.24 PEDOT:PSS composite submersed in electrolyte; (a) the polar...

Chapter 7

Figure 7.1 Chemical structures of compounds exhibiting hole‐dominating ...

Figure 7.2 Chemical structures of compounds exhibiting electron‐dominat...

Figure 7.3 (a) Scheme of an OFET working in the ambipolar regime in whi...

Figure 7.4 (a) Optical microscopy images of ambipolar blends of P3HT:PC

Figure 7.5 AFM images of MDMO‐PPV:PF:PC

61

BM (1 : 1 : 2) blend deposited...

Figure 7.6 Scheme of (a) bilayer and (b) bulk heterojunction film struc...

Figure 7.7 Schemes of (a) OFET with TiO

x

separation layer between p‐ ...

Figure 7.8 (a) Transfer characteristics of a BHJ OFET based on BBL:PTHQ...

Figure 7.9 (a) Height and (b) phase AFM images of P(NDI2OD‐T2):P3HT ble...

Figure 7.10 (a) Optical absorption spectra of P3HT, PDPPTTT, and BHJ P3...

Figure 7.11 AFM height images of P3HT:P(NDI2OD‐T2) bends cast from (a) ...

Figure 7.12 (a) XPM images and (b) schematic cross sections showing the...

Figure 7.13 (a) X‐ray diffractograms of zone‐cast TIPS‐pentacene, NBI‐4...

Figure 7.14 (a) Schematic representation of a capillary tube‐driven for...

Figure 7.15 (a) Scheme of a TIPS‐TAP:TIPS‐pentacene 1 : 1 cocrystal. At...

Figure 7.16 Chemical structures of polyethylene (PE), poly(dimethylsilo...

Figure 7.17 Scheme of phase separation and output characteristics of OF...

Figure 7.18 (a) Schematic representation of phase separation induced by...

Figure 7.19 (a) Scheme of a zone‐casting apparatus and vertical phase s...

Figure 7.20 (a) Schematic representation of the Marangoni effect disrup...

Figure 7.21 (a) Micrograph of PC doped with 1 wt% of TTT‐TCNQ

2

, (b) cro...

Figure 7.22 SEM cross sections of zone‐cast films consisting of (a) 1 :...

Figure 7.23 Morphology of P3HT film on an elastic substrate stretched t...

Figure 7.24 (a) Morphology of amorphous P3HT film exposed to external s...

Figure 7.25 (a) Schematic representation of DPPT‐TT embedded in an elas...

Chapter 8

Figure 8.1 Schematic structure of solution‐processable OLED devices. ...

Figure 8.2 Polymer with conjugated backbone, which by itself fulfills t...

Figure 8.3 Selected examples of conjugated rod and coil structures of s...

Figure 8.4 Block copolymer OLEDs are spun‐coated on glass/ITO/PEDOT:PSS...

Figure 8.5 Absorption (a) and PL (b) spectra of (36–41) in acetonitrile...

Figure 8.6 Phosphorescent emission principles in host/dopant systems [1...

Figure 8.7 Schematic drawing of RGB‐based (a) or two complementary (cya...

Figure 8.8 Bisterpyridines and their corresponding Zn

2+

coordinatio...

Figure 8.9 (a) The preparation of GdeL1eL2 multicomponent nanoparticles...

Figure 8.10 Chemical structures of the di‐UPy functionalized chromophor...

Figure 8.11 (a) Photoluminescence spectra of titration experiments in C...

Figure 8.12 A schematic diagram of a: (a) simple form and (b, c) multil...

Figure 8.13 Scheme of the electroluminescence mechanism in an OLED. Ele...

Figure 8.14 Spin‐coating process. (a) Schematic description. (b) Repres...

Figure 8.15 Gravure printing process. (a) Schematic description. (b) Re...

Figure 8.16 Inkjet printing process. Schematic description of the (a) c...

Figure 8.17 Chemical structure of PEDOT:PSS [206].

Figure 8.18 Commonly used components in the electron transport layer. P...

Figure 8.19 CIE (

x

,

y

) chromaticity diagram.

Figure 8.20 (a) Schematic architecture of the OLED devices. (b) AFM ima...

Figure 8.21 (a) The experimental 〈

ε

(

ω

)〉 (symbols) measured by...

Figure 8.22 (a) The PL spectra of the studied emissive films. (b) The E...

Chapter 9

Figure 9.1 Hierarchy of standard simulation methods applicable to model...

Figure 9.2 Electrostatics of a p–n junction in thermal equilibrium: (a)...

Figure 9.3 p–n junction in thermal equilibrium: (a) Net concentrations ...

Figure 9.4 p–n junction in thermal equilibrium, calculated assuming Bol...

Figure 9.5 Band energies and quasi‐Fermi levels in p–n junction: (a) re...

Figure 9.6 p–i–n junction in thermal equilibrium: (a) doping profile; (...

Figure 9.7 p–i–n junction under illumination: (a) short‐circuit conditi...

Figure 9.8 Gauss–Fermi integral ( 9.34) plotted for several values of n...

Figure 9.9 p–n junction in thermal equilibrium, calculated assuming Gau...

Figure 9.10 Comparison between

–

characteristics of otherwise ide...

Figure 9.11 Band bending at metal semiconductor junction at

: (a) th...

Figure 9.12 Transfer matrix simulation of optical multilayer stack cons...

Figure 9.13 Steps in a computer simulation using the drift–diffusion mo...

Figure 9.14 Comparison between transient photocurrent measurement on or...

Figure 9.15 Analysis of parameter estimation. (a) Correlation matrix of...

Chapter 10

Figure 10.1 Schematic representation of the processes of exciton creati...

Figure 10.2 Typical schematic representation of a bulk heterojunction s...

Figure 10.3 Simplified representation of the different steps from light...

Figure 10.4 Simplified band diagram of metal/active layer/metal photodi...

Figure 10.5 Direct (or normal) architecture versus inverted architectur...

Figure 10.6 Typical current–voltage curves in dark condition (black) an...

Figure 10.7 Example of measured EQE spectra on a P3HT:PC

60

BM blend syst...

Figure 10.8 External quantum efficiency (EQE) versus dark leakage curre...

Figure 10.9 Chemical structure of PC

60

BM acceptor molecule and regioreg...

Figure 10.10 Simulations illustrating the impact of active layer thickn...

Figure 10.11 Band diagram illustration of the concepts of “injection” b...

Figure 10.12 Band diagram illustrating common blocking layers in organi...

Figure 10.13 Principle of an X‐ray imager based on organic photodiodes,...

Figure 10.14 Cartoon of silicon (A and B) and silicon–organic hybrid (C...

Figure 10.15 External quantum efficiency (EQE) versus wavelength (a) an...

Figure 10.16 Direct architecture used in simulation.

Figure 10.17 Simulation of

I–V

characteristics under dark and ill...

Figure 10.18 Example of the field‐dependent mobility model on

I–V

Figure 10.19 Example of deep trap influence on

I–V

curve characte...

Figure 10.20 Impact of parallel resistance (4 MΩ m) simulation of

I–V

...

Figure 10.21 Impact of series resistance (20 Ω m). Simulation of

I–V

...

Chapter 11

Figure 11.1 Flexible organic floating gate OFET memory array (16 × 16 O...

Figure 11.2 Main classification of the electronic memory devices accord...

Figure 11.3 (a) Simplified structure of the ReRAM, (b) unipolar ReRAM, ...

Figure 11.4 Four fundamental two‐terminal passive circuit elements: res...

Figure 11.5 Possible outline of the read/write circuit designed for non...

Figure 11.6 Schematic circuitry of capacitor DRAM or ferroelectric FeRA...

Figure 11.7 Various principles of field‐effect transistor‐based memory ...

Figure 11.8 Typical current–voltage characteristics of the n‐type OFET ...

Figure 11.9 The cross point of perpendicular electrodes separated by th...

Figure 11.10 Rewritable memory based on the reversible conducting filam...

Figure 11.11 (a) Typical

I–V

characteristics and (b) the ON/OFF c...

Figure 11.12 Chemical structure of (a) PDI derivative with carbazoyl do...

Figure 11.13 Electrical characteristics of the sandwich device with PDI...

Figure 11.14 Polyimide with two side chain dipoles yielding OFET memory...

Figure 11.15 (a) Structure of the memory device based on the ambipolar ...

Figure 11.16 Schematic cross‐sectional diagram of the vertical transist...

Chapter 12

Figure 12.1 An OPV module consisting of several nanolayers printed on a...

Figure 12.2 Principle of operation of a bulk heterojunction OPV.

Figure 12.3 Structure of a representative inverted architecture of full...

Figure 12.4 Calculated extinction coefficient (

k

) of pristine P3HT as a...

Figure 12.5 Calculated extinction coefficient (

k

) of the pristine PC

60

B...

Figure 12.6 Unique r2r pilot line of LTFN that includes in‐line laser p...

Figure 12.7 P3HT:PC

60

BM stripes printed by slot‐die on PET/ITO/ZnO web ...

Figure 12.8 Schematic representation of the laser patterning processes ...

Figure 12.9 SEM images of the (a) P1 patterning of ITO and ZnO, (b) P2 ...

Figure 12.10 Photograph of the r2r‐printed OPV module with the P1‐, P2‐...

Figure 12.11 Three‐dimensional plot of the calculated thickness of the ...

Figure 12.12 Three‐dimensional plot of the calculated electronic transi...

Figure 12.13 Imaginary part of the in‐line measured

spectra from th...

Figure 12.14 Evolution of thickness of printed P3HT:PC

60

BM nanolayer on...

Figure 12.15 Evolution of characteristic electronic transitions of repr...

Figure 12.16 Schematic representation of the theoretical model used for...

Figure 12.17 Schematic representation of the theoretical model used for...

Guide

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Solution-Processable Components for Organic Electronic Devices

Edited by

Beata ŁuszczyńskaKrzysztof MatyjaszewskiJacek Ulański

Copyright

Editors

Dr. Beata Łuszczyńska

Lodz University of Technology

Department of Molecular Physics

Zeromskiego Street 116

90‐924 Lodz

Poland

Prof. Krzysztof Matyjaszewski

Carnegie Mellon University

Department of Chemistry

4400 5th Avenue

Pittsburgh, PA 15217

United States

Prof. Jacek Ulański

Lodz University of Technology

Department of Molecular Physics

Zeromskiego Street 116

90‐924 Lodz

Poland

Cover

Dr. Łukasz Janasz

Lodz University of Technology

Department of Molecular Physics

Zeromskiego Street 116

90‐924 Lodz

Poland

Mr. Zbigniew Sieradzki

QWERTY Sp. z o.o.

Siewna 21

94‐250 Lodz

Poland

All books published by Wiley‐VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

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© 2019 Wiley‐VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany

All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.

Print ISBN: 978‐3‐527‐34442‐0

ePDF ISBN: 978‐3‐527‐81494‐7

ePub ISBN: 978‐3‐527‐81495‐4

oBook ISBN: 978‐3‐527‐81387‐2

Preface

This book has been inspired by a growing interest in an emerging technology – printed organic electronics. We have realized that there is a considerable demand for a book that would present achievements and – perhaps even more importantly – challenges in the synthesis and processing of solution‐processable materials required for organic electronics as well as in assembling them into functional devices. Such a demand results from the fact that the industrial‐scale production of printed (and preferably also flexible) large‐area organic electronics still remains an unfulfilled promise. Several prominent scientists have kindly accepted our invitation to contribute with insights into the synthesis and processing of different materials used in organic and hybrid electronics (with particular emphasis on nanostructured materials) and also with the discussion of basic physical mechanisms governing optoelectronic properties of organic semiconductors (OSCs) and solution‐based methods of fabrication of organic electronic devices.

In the introductory chapter entitled “Precision and Purity of Conjugated Polymers – to be Ensured before Processing,” Klaus Müllen and his coworker, Thorsten Prechtl, discuss the basic design principles for conjugated polymers as well as the potential pitfalls of the synthetic methods used. The central message of this chapter is: “do not commence processing and device fabrication before having optimized and ensured the integrity of the macromolecular structure as well as its reliable and reproducible synthesis.”

The chapter on the synthesis of solution‐processable nanoparticles of inorganic semiconductors applicable in organic electronics and photonics written by two groups of authors led by Adam Pron and by Kwang‐Sup Lee presents a broad range of semiconducting nanocrystals with a detailed analysis of the role of ligands. The authors describe recently developed advanced materials such as photopatternable nanocrystals or perovskite nanomaterials, analyze energy and charge transfer phenomena in semiconducting nanocrystal‐based hybrid materials, and present possible applications.

Krzysztof Matyjaszewski with coworkers reviews the most advanced method of synthesis of inorganic high k nanoparticles by means of controlled radical polymerization, which allows precise control over material composition, dispersibility, and interfacial interactions. Incorporation of high k nanoparticles into polymeric matrices with high breakdown strength yields novel solution‐processable materials with excellent dielectric performance. Control and tuning of interfacial interactions allows improvement of each parameter of the dielectric material in order to satisfy the requirements of specific applications.

In the chapter on polymer blending and phase behavior in organic electronics, Paul Blom with coworkers presents two excellent examples of beneficial use of blending materials to develop novel materials with extraordinary properties. The applied innovative blending procedures employ liquid–liquid demixing during solution casting of the active layer. The authors represent an approach enabling a construction of a phase diagram based on Flory–Huggins theory. The first of the described cases illustrates how trapping phenomena in semiconducting and electroluminescent polymers can be reduced by blending them with an insulating polymer, which results in doubling of the organic light‐emitting diode (OLEDs) efficiency. In the second case, it is shown how the nonsolvent‐induced phase separation of solutions of the ferroelectric polymer influences the quality of materials used for the production of organic capacitive memory devices. On the basis of these examples, the authors present general observations on the role of liquid–liquid demixing in thin film device processing.

Anna Köhler and Heinz Bässler, in their chapter on photogeneration of charge carriers in solution‐processable OSCs, explain how photoexcitation can result free electrons and holes. After a brief introduction into established concepts to account for photocurrent generation in a single OSC phase, the authors discuss the photogeneration of charges when a more electronegative acceptor material with a less electronegative electron donor material is combined to form a bulk heterojunction or bilayer, as is common in organic photovoltaics (OPVs). There, photogenerated electron–hole pairs should still be strongly electrostatically bound because of low dielectric permittivity of OSCs. This is in apparent contradiction with the fact that the power efficiency of modern OPVs has reached values above 10%. The authors discuss this currently most intriguing issue emphasizing the role of the initial interfacial charge transfer, subsequent dissociation of the geminately bound electron–hole pair into free charge carriers, and their collection at the electrodes or geminate or nongeminate recombination.

Another issue that is critical for the performance of organic electronic devices – charge carrier transport in OSCs – is discussed by Jaroslaw Jung and Jacek Ulanski in the subsequent chapter, with a special emphasis placed on multicomponent materials such as heavily doped semiconductors, composites, and nanocomposites. The obstacles hindering progress and widespread application of organic electronics are connected with insufficient knowledge of physical processes responsible for charge carrier transport. The basic concepts, such as density of states, Fermi energy, Fermi level or charge carrier mobility, drift currents, drift–diffusion currents, or space–charge‐limited currents, must be better recognized and understood. Some models of charge carrier transport in OSCs are presented together with selected experimental techniques allowing verification of these models.

The second part of the book, its remaining six chapters, is devoted to organic electronic devices that can be produced by solution‐based methods, including printing and roll‐to‐roll manufacturing.

In the seventh chapter, Lukasz Janasz, Wojciech Pisula, and Jacek Ulanski indicate that the overall development of organic electronics relies on progress in the research on organic field‐effect transistors (OFETs), as they (particularly ambipolar transistors) constitute basic building blocks of almost any electronic device. The authors describe methods based on mixing of n‐type and p‐type OSCs and the relationships between fabrication procedures, film morphology, and performance of ambipolar OFETs. Another approach aims to improve the properties of the active layers in OFETs, such as stretchability/bendability, air stability, and thin film processability that can be achieved by mixing a conjugated semiconductor and insulator. It is shown that fabrication of efficient and multifunctional active layers requires a controlled phase separation between insulating and semiconducting fractions. In order for this development to happen, it is crucial that comprehensive understanding of the role of the blend morphology in the overall behavior of the transistors be gained.

Fabrication of OLEDs based on solution‐processable organic materials is described by Joannis K. Kallitsis and coworkers. They demonstrate the importance of selection of optimal device components, not only OSCs but also electrodes and carrier injection layers. Proper choice of the materials with appropriate properties results in custom‐made OLEDs exhibiting desired performance levels. As a consequence of the tuning and tailoring of the OSC properties and of the device preparation process, it is possible to produce both rigid and flexible OLED devices. The authors argue that the fabrication method also plays an essential role in the commercialization of OLEDs as it affects the total manufacturing cost. From this point of view, wet fabrication processes such as spin‐casting or inkjet printing are preferable especially when addressing large‐area display devices.

Marek Zdzislaw Szymanski and Beata Luszczynska, in their chapter on OPVs, focus on the device physics and modeling. After brief introduction to device modeling, it has been shown how the simulations can be run using a free open‐source software such as oedes (www.oedes.org). Such simulations can provide critical insights into the performance of emerging OPV devices, enabling the estimation of parameters that cannot be directly measured. The authors focus on the drift–diffusion modeling as it is the most fundamental approach to modeling of photovoltaic devices and is applicable to inorganic, organic, dye‐sensitized, and perovskite solar cells. The chapter also presents several examples of simulations. The authors underline the overall importance of concepts of electrical and optical modeling in the development of all electronic devices.

In the chapter on solution‐processed organic photodiodes, Raphaël Clerc et al. present an introduction to this class of devices from chemistry and processing to device physics and applications. The main materials, processes, device architectures, operation principles, and figures of merits are described, followed by an analysis of state of the art of solution‐processed organic photodiodes. This comprises not only photodiodes working in the visible spectral domain but also application of organic photodiodes to X‐ray imaging, photodiodes working in the NIR spectral domain, and organic photodiodes integrated into complementary metal oxide semiconductor (CMOS) imagers. Additionally, the chapter presents the main current approaches to modeling of performance of the organic photodetectors, including the drift–diffusion models.

Jiri Pfleger has contributed with a chapter on organic electronic memory devices based on solution‐processable materials. He argues that in spite of the importance of data storage in electronic applications, the main effort in the development of printed organic electronics has so far been on devices such as OFETs, OLEDs, OPVs, and sensors. For this reason, the progress in printable organic memory devices is less spectacular. The chapter presents basic information on classification of electronic memory devices and principles of data storage with the main focus on the resistive memory operation mechanisms. What follows is the characterization of the materials for printable memory devices, including resistive RAM device, polymer ferroelectrics for OFET memory devices, OFET memory devices with charge‐trapping mechanism, OFET memory with floating electrode, and vertical organic transistors memory devices. It has been predicted that although currently the organic electronic memories can predominantly introduce an added value in rather simplified structures in low‐cost, low‐end consumer electronics, there are promising emerging applications for organic materials in multilevel logic in neuromorphic systems.

The last chapter, written by Stergios Logothetidis and Argiris Laskarakis, presents the latest state of the art in intelligent manufacturing of organic electronic devices by roll‐to‐roll (r2r) manufacturing processes. The described unique r2r pilot line was designed for the fabrication of the large‐area and flexible OPVs and equipped with robust in‐line optical metrology and ultrafast pulsed laser patterning processes. The authors demonstrate that one of the most promising methods to be used as a quality control tool for the r2r manufacturing processes is spectroscopic ellipsometry. This approach has opened a way for introducing intelligence into production lines in nanomanufacturing of a vast variety of organic electronic devices as it afforded the capability of real‐time monitoring and control of the end properties of the printed nanomaterials.

We do hope that this book will be helpful for scientists and students involved in the field of organic electronics and that it will inspire and guide engineers in their efforts for industrial implementation of printed organic electronics technology.

Beata Łuszczyńska

Krzysztof Matyjaszewski

Jacek Ulański

1Precision and Purity of Conjugated Polymers – To be Ensured Before Processing

Thorsten Prechtl and Klaus Müllen

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128, Mainz, Germany

1.1 Polymer Design

The performance of electronic or optoelectronic devices fabricated from conjugated polymers depends on three key factors: the structure of the macromolecule, its packing and morphology in a solid thin film, and the interfacing of the film with the outside world. The present book is mainly concerned with devices made by polymer deposition from solution. This formation of thin films is a complex process comprising a subtle interplay of scientific and engineering issues and thus asks for quite different competences. Complications arise when additional criteria come into play such as costs or the speed of the processing upon printing. There is, however, a serious caveat, which must be obeyed right at the beginning of such research and development: do not commence processing and device fabrication before having optimized and ensured the integrity of the macromolecular structure as well as its reliable and reproducible synthesis. Understandably, not every reader might be interested in the fine details of synthesis, but every reader, even if leaning toward the physics side of the field, would greatly benefit from knowing

(i) the basic design principles for conjugated polymers as well as

(ii) the concepts behind and potential pitfalls of the synthetic methods used.

Comprehensive reviews describing conjugated polymer synthesis in all its variations can be found elsewhere [1–9]. Such collections are beyond the scope of this introductory chapter, which is, instead, restricted to preparing the reader's mind before proceeding to polymer processing.

In a conjugated polymer, unsaturated aromatic, olefinic, or acetylenic building blocks are covalently connected by single bonds to create a “box” of delocalized π‐electrons. Although some torsion about single bonds is tolerated, too large deviation from coplanarity would hamper π‐conjugation. Clearly, π‐conjugation would also be interrupted by sp3‐hybridized carbon centers [10]. It is this large domain of mobile, polarizable electrons that qualifies an organic material to interact with light or to undergo electron transfer [11]. The energy levels of the electronic bands (corresponding to HOMO [highest occupied molecular orbital] and LUMO [lowest unoccupied molecular orbital] energies of small molecules) predict whether a polymer can readily be reduced or oxidized, and thus if it is a p‐type or n‐type material [ 10,12,13]. The band gap determines whether a material is an insulator, a semiconductor, or a conductor. The extended π‐conjugation has originally been considered as being responsible for electrical conductivity, but the pioneering work of Heeger, MacDiarmid, Shirakawa, and coworkers has shown that conductivity requires the formation of charge carriers by doping [14–16], which is nothing other than partial oxidation or reduction [17]. The operation of devices implies an additional process, that is, electron transfer between external electrodes and the active organic component [18], and the efficiency of this interfacial process will again depend on the energy levels of the materials. Finally, the band gap is decisive not only for electron transfer but also for the optical properties in determining the wavelength of light absorption and emission [11].

The classical examples of conjugated polymers are polyacetylene (PA) with an alternating array of single and double bonds, poly‐1,4‐phenylene (PPP), and its electron‐rich, though less stable congener polythiophene (PT). There are also “hybrid” structures such as polyphenylenevinylene (PPV) and polyphenylenethinylene (PPE) comprising both aromatic and olefinic or acetylenic moieties, respectively (see Scheme Scheme 1.1). The key difference between PA and PPP is that the former is obtained as an insoluble film upon catalyzed polymerization of acetylene whereby the catalyst can also act as a dopant to yield electrical conductivity, whereas PPP, in an alkyl substituted form, is more commonly synthesized in solution. Solubility in organic solvents is thus a central issue for the success of both synthesis and processing. What should not be ignored is the issue of stability. Thus, PA, for example, is known to readily interact with air with the formation of oxygen‐containing functional groups, which hamper the flexibility and conductivity of the material [ 18,20].

Scheme 1.1 Structures of classical conjugated polymers.

Source: Müllen et al. 2013 [19]. Reproduced with permission of Royal Society of Chemistry.

It is impossible not to be impressed by the huge available “toolbox” for conjugated polymer synthesis including

(i) the broad choice of aromatic building blocks,

(ii) the doping of such hydrocarbons with heteroatoms,

(iii) the (regular or irregular) incorporation of different repeat units into one polymer,

(iv) the modes of their connection (e.g. 1,4‐, 1,2‐, or 1,3‐phenylene), and

(v) the attachment of substituents.

All this creates an enormous structural and thus functional manifold. Even if one ignores here their structurally related oligomers and other small conjugated molecules, which would certainly widen the wealth of organic electronic materials, it is this versatility of polymer structures that constitutes one strength of organic electronics and that opens the possibility of tailoring optical and electronic properties. This variety can be further extended into new directions by, for example, co‐incorporation of metal centers or by synthesis in confining geometries to furnish discrete nanoparticles [21–27]. The latter are of tremendous use in photonics [28–31].

Materials chemistry has developed powerful rules, either empirical or guided by theory, to predict and explain the chromophoric and electrophoric properties of macromolecules, and it can clearly be understood that these are also determining electrical or optical device function. The art of thus “synthesizing” desired functions and performances is based on reliable structure–property relationships. Clearly, this fundamental concept becomes obsolete without reliable structures. Deviations from an idealized structure of the polymer, whether detectable or not, and impurities will thus not only compromise structure–property relations but also diminish device performance by trapping charges or excited states [32].

Understandably, the early science and technology of conjugated polymers was more concerned with exploring unknown territory by making new materials, rather than with ensuring whether they possessed sufficiently high structural precision and purity. This was accentuated by the slow rate at which the competences of cutting‐edge synthetic organic chemistry were applied to this field. Improvements of synthetic protocols toward conjugated polymers and better analytical methods have, however, tremendously strengthened the validity of the materials science of conjugated polymers. More powerful synthetic protocols included metal‐catalyzed bond formation [33–41] and metathesis polymerization [42–46] (see Scheme Scheme 1.2), whereas on the analytical side, established methods became more sensitive and new ones were additionally employed. Thus, the power of nuclear magnetic spectroscopy has been significantly enhanced by the availability of higher magnetic fields and multidimensional techniques [47–49]. More importantly, solid‐state nuclear magnetic resonance (NMR) measurements [50,51] have provided insights into the packing of polymers, which could not be ascertained by X‐ray scattering [52]. Classical elemental analysis [53] remains of undiminished value but has become complemented by X‐ray fluorescence microscopy [54–56].

Scheme 1.2 Representative metathesis polymerization of cycloolefins.

Source: Albertsson and Varma 2003 [44]. Reproduced with permission of American Chemical Society.

Consequently, optimizing precise synthesis, scrupulous purification, and sensitive detection of defects must be carefully approached in order to make solution processing of polymers meaningful. Failure to meet these requirements would put the validity of the science and technology of (opto)electronics at risk. Accordingly, no reader should lightly excuse himself by setting aside the chemical fundamentals of the field and by relying only on expertise in physics.

There is, indeed, a tight connection between molecular structure and key physical processes in devices. Historically, conjugated polymers were considered to be candidates for developing electrically conducting “plastics,” which was best illustrated by the notions of obtaining “conducting polymers” or even “synthetic metals” [17]. As has been mentioned, conductivity is bound to the formation of charge carriers and this, in turn, leads to chemical instability. This obstacle together with unintentional doping of low band gap, and thus chemically reactive, polymers has caused loss of interest in this field, and other optical and electronic functions have moved into the limelight. This has then redefined the needs of synthesis, so that, for example, the first use of polymers as emitters in LEDs (light emitting diodes) by the group of Friend required a suitable synthesis of PPV by the Holmes group [57,58]. In an LED, opposite charges injected from different electrodes must recombine to create excitons, so good device efficiency requires high and balanced concentrations of holes and electrons [59]. High charge carrier mobility is less important, whereas that is the decisive parameter for a polymer‐based field effect transistor (FET) [60]. Thus, the availability of a regioregular poly‐3‐alkylthiophene, first synthesized by McCullough and Lowe [61], was instrumental in enabling Sirringhaus' work [62] on polymer FETs. A polymer solar cell, in turn, is different from LEDs and FETs because a second component will be required. In order for a solar cell to become efficient, an electron‐donating polymer must be combined with an electron acceptor to facilitate charge separation. Here, a breakthrough came when Heeger, Sariciftci, and coworkers [63] added C[60] fullerene (1) to a substituted PPV (2) to enhance the desired light‐induced charge separation (see Scheme Scheme 1.3). All these cases nicely document the crucial role in obtaining good device performance of the nature of the material, although, as cannot be stressed enough, supramolecular order in the solid state and morphology of films come into play as well.

Scheme 1.3 Examples of milestones in organic electronics.

Source: Müllen et al. 2013 [19]. Reproduced with permission of Royal Society of Chemistry.

Although we will be looking in this chapter at polymer chemistry as the basis of device research, one must admit that efficient organic LEDs and solar cells have also been fabricated from small molecules [64]. Films of small molecules, but not of polymers, can be fabricated by vacuum deposition procedures. Although deposition from solution is cheaper, it requires, of course, sufficiently soluble materials [65]. Additional problems will arise if multilayers are needed. This asks for orthogonal solubility of different materials in different solvents, or loss of solubility by cross‐linking before deposition of the subsequent layer [66,67]. The advantages and disadvantages of employing polymers or small molecules and oligomers as active components of devices must be considered separately for each case. The latter can be purified by vacuum sublimation, and the electronic properties of oligomers, when plotted against the number of repeat units, converge toward those of the corresponding polymers [68–70]. On the other hand, solution‐processed polymers give more homogeneous films over large areas, whereas small molecules tend to form crystalline domains, which may not only cause undesired light scattering but also obstruct charge transport because of the presence of grain boundaries [71]