Donor-acceptor block copolymers in organic electronics [Elektronische Ressource] : spectroscopy, charge transport, morphology and device application / vorgelegt von Sven Hüttner

Sven Huettner

Sujets

• Physik

Informations

• Publié par: universitat_bayreuth
• Publié le: 01 janvier 2010
• Nombre de lectures: 25
• Langue: Deutsch
• Poids de l'ouvrage: 34 Mo

Extrait

Donor-Acceptor Block Copolymers
in Organic Electronics
Spectroscopy, Charge Transport, Morphology and
Device Application
Dissertation
zur Erlangung des akademischen Grades
eines Doktors der Naturwissenschaften (Dr. rer. nat.)
im Fach Chemie der Fakultät für
Biologie, Chemie und Geowissenschaften der Universität Bayreuth
vorgelegt von
Sven Hüttner
geboren in Hof
Bayreuth 2010iiDie vorliegende Arbeit wurde in der Zeit von Januar 2006 bis Januar 2010 am Lehrstuhl
Angewandte Funktionspolymere / Makromolekulare Chemie I unter der Betreuung von
Prof. Dr. Mukundan Thelakkat angefertigt.
Vollständiger Abdruck der von der Fakultät für Biologie, Chemie und Geowissenschaf-
ten der Universität Bayreuth genehmigten Dissertation zur Erlangung des akademi-
schen Grades Doktor der Naturwissenschaften (Dr. rer. nat.).
Amtierender Dekan: Prof. Dr. Stephan Clemens
Dissertation eingereicht am: 19. Januar 2010
Wissenschaftliches Kolloquium am: 5. Mai 2010
Prüfungsausschuss:
Prof. Dr. Mukundan Thelakkat (Erstgutachter)
Prof. Dr. Peter Strohriegl (Zweitgutachter)
Prof. Dr. Anna Köhler
Prof. Dr. Jürgen Senker (Vorsitzender)
iiiivContents
1 Summary 1
2 Zusammenfassung 3
3 Introduction 5
3.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2 Organic Photovoltaics . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3 Polymer Blends and Block Copolymers . . . . . . . . . . . . . . . . . . 11
3.4 FullyFunctionalisedBlockCopforOrganicElectronicApplications 14
3.5 Organic Field Effect Transistors . . . . . . . . . . . . . . . . . . . . . . 16
4 Overview 27
5 N-type Organic Field Effect Transistors from Perylene Bisimide
Block Copolymers and Homopolymers 43
6 Controlled Solvent Vapour Annealing for Polymer Electronics 51
7 Intermolecular Interactions in Perylene Bisimide Polymer Architec-
tures 65
8 Organic Field Effect Transistors from Triarylamine Side-Chain Poly-
mers 91
9 Tunable Charge Transport using Supramolecular Self-assembly of
Nanostructured Crystalline Block Copolymers 101
10 InfluenceofMolecularWeightontheSolarCellPerformanceofDouble-
Crystalline Donor-Acceptor Block Copolymers 123
11 Photophysics of Double-Crystalline Donor-Acceptor Block Copoly-
mers containing P3HT and Perylene Bisimide based Polymers 133
12 Annex: Block Copolymers as Compatibilisers for Binary Blends 157
List of Publications 165
Acknowledgements 167
Erklärung 169
vviChapter 1
Summary
Organicelectronic deviceshaveattracted increasingattentionoverthe lastdecade. The
use of organic materials allows the creation of large area, flexible and low-cost light-
emitting devices, transistors and photovoltaics. The development of new organic ma-
terials contributes to a successful commercialisation. The present work deals with the
characterisation of novel donor-acceptor block copolymers and their constituent poly-
mer blocks that are well-suited for organic photovoltaics. In conventional approaches,
blends of polymers are used with limited morphological control. Block copolymers,
however, phase-separate and self-assemble into nanostructured morphologies due to
the covalent linkage of the two blocks. The interplay between intermolecular interac-
tions, mesoscopic crystalline structures and the block copolymer microphase separation
determine the material properties and therefore the device characteristics. Thus, these
block copolymers offer a unique platform to study the electronic and photophysical
properties of confined donor-acceptor systems. This work is concerned with the fun-
damental characterisation of these properties as well as the application in organic field
effect transistors and organic solar cells.
The acceptor polymer block poly(perylene bisimide acrylate) (PPerAcr) consists
of perylene bisimide (PBI) units that are linked to a polyacrylate backbone. We have
investigated the homopolymer PPerAcr, a model block copolymer in conjunction with
polystyrene (PS), as well as fully functionalised block copolymers with a donor block
eithermadeofpoly(vinyltriphenylamine)(PvTPA)orpoly(3-hexylthiophene)(P3HT).
These polymers offer a set of electronically active materials with several hierarchical
structures: The PBI moieties feature intermolecular 𝜋−𝜋 interactions that lead to
crystalline side chains of PPerAcr that form a monoclinic lattice of one-dimensional
stacksofPBI.FurthernanoscopicstructuresareinducedbythecombinationofPPerAcr
with another amorphous block or another semi-crystalline block such as P3HT due to
phase separation.
Since PPerAcr is used as an electron transporting material in all subsequent
block copolymers, its structural, optical and electronic properties are investigated in
detail. The planar PBI moieties feature strong 𝜋−𝜋 interactions, rendering PPerAcr
crystalline, which is important for charge transport. As tested in organic field effect
transistors, PPerAcr exhibits excellent electron transport properties for an n-type poly-
2−3 cm
mer with mobilities around 10 . Furthermore, a cylindrical block copolymer of
Vs
PS-b-PPerAcr shows the same charge transport properties as the homopolymer PPer-
Acr, indicating that the incorporation of PPerAcr into a block copolymer does not
necessarily reduce its charge transport performance.
1Summary
The intermolecular interactions of the PBI moieties favor not only charge trans-
port, but also affect the optical properties, due to the electronic coupling of the tran-
sition dipole moments. Thus, optical spectroscopy such as absorption and fluorescence
spectroscopy give access to information about the intermolecular packing, which is
correlated with temperature dependent X-ray diffraction studies. The strong inter-
molecular packing of the PBI units can be overcome by solvent-vapour exposure. This
is specially helpful to induce polymer chain mobility, enabling the completion of block
copolymerphaseseparationforexample. Thismethodwasstudiedindetailbymeansof
in-situ spectroscopy and ellipsometry during controlled solvent-vapour exposure. Spin-
coated films of PvTPA-b-PPerAcr exhibit an incomplete phase separation and can be
transformed into an ordered lamellar morphology by solvent-vapour annealing.
In addition to PvTPA, we have characterised further poly(triarylamines) with
different electron-rich substituents at the TPA units in OFETs. All these polymers
are amorphous side-chain polymers. We found the charge carrier mobility to be in-
dependent of the molecular weight, though allowing an adjustment of their thermal
properties for device fabrication.
This is in contrast to P3HT, which is a semi-crystalline, conjugated main chain
polymer. X-ray diffraction, steady state and time-resolved spectroscopy, as well as the
transistor device characterisation were employed to establish a charge transport - mor-
phology relation for the donor-acceptor block copolymers P3HT-b-PPerAcr containing
two crystalline blocks. Controlling the crystallisation preferences of the two blocks
leads to a new processing route for OFETs with tunable p-type, ambipolar, or n-type
transport through a one-time thermal annealing step.
The application of P3HT-b-PPerAcr in organic photovoltaic devices showed also
very promising results. Maximum external quantum efficiencies of up to 31% have
been measured. The performance was strongly dependent on the molecular weight. In
block copolymers with the same block ratio but with only half the molecular weight,
the performance decreased by almost a factor of 10. The reduced domain size caused
by the lower segment length and the reduced P3HT crystallinity led to a decreased hole
carrier mobility which was responsible for the large difference in performance between
the two block copolymers.
Subsequently, the photophysics of P3HT-b-PPerAcr by means of absorption and
fluorescence spectroscopy as well as time-resolved transient absorption spectroscopy
were investigated. All block copolymers exhibited an ultra-fast charge-pair forma-
tion and a strongly reduced photoluminescence, suggesting domain sizes of only some
nanometres. Although efficient charge separation could be accomplished, a good charge
percolation was lacking due to small domain sizes. Furthermore the herein presented
results emphasis the fundamental importance of morphology and interfacial properties
such as crystallinity. These findings motivate the further use of block copolymers as
compatibilising agents for polymer blends to improve their interface and morphology.
2Chapter 2
Zusammenfassung
Organische Elektronik hat sich über das letzte Jahrzehnt zu einem viel beachteten
Gebiet entwickelt. Organische Materialen ermöglichen die Entwicklung großflächiger,
flexibler und preisgünstiger Anwendungen von lichtemittierenden Bauelementen, Tran-
sistoren und Solarzellen. Die stetige Entwicklung neuer organischer Materialien trägt
maßgeblich zu diesem Erfolg bei. Die vorliegen Arbeit befasst sich deshalb mit der Cha-
rakterisierung von neuen Donor-Akzeptor Blockcopolymeren, die sich zur Anwendung
inorganischenSolarzelleneignen.HerkömmlichePolymersolarzellenbestehenauseinem
BlenddesAkzeptor-undDonormaterials,welchesnureinesehreingeschränkteKontrol-
le über deren Phasenseparation erlaubt. Im Gegen