University of Groningen Photophysics of nanomaterials for opto-electronic applications Kahmann, Simon

(1)

Photophysics of nanomaterials for opto-electronic applications

Kahmann, Simon

IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from

it. Please check the document version below.

Document Version

Publisher's PDF, also known as Version of record

Publication date:

2018

Link to publication in University of Groningen/UMCG research database

Citation for published version (APA):

Kahmann, S. (2018). Photophysics of nanomaterials for opto-electronic applications. Rijksuniversiteit

Groningen.

Copyright

Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).

Take-down policy

If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum.

(2)

Photophysics of nanomaterials for

opto-electronic applications

Simon Kahmann

November 8, 2017

University of Groningen

(3)

(4)

O-List of Figures

2.1 Semiconductor density of states with respect to dimensionality . . . 5

2.2 Hybridisation of atomic orbitals in carbon . . . 6

2.3 Molecular orbitals and their energy levels . . . 7

2.4 Overview of different semiconducting polymers . . . 8

2.5 Energy levels of donor-acceptor polymers . . . 9

2.6 Jablsonski scheme . . . 10

2.7 Frank-Condon principle and vibronic side peaks . . . 11

2.8 Wannier-Mott and Frenkel excitons in semiconductors . . . 12

2.9 Polaron stabilistion mechanisms . . . 13

2.10 Polaron energy levels and optically allowed transitions . . . 15

2.11 Energy levels in different states of condensed matter . . . 16

2.12 Mechanisms for exciton migration . . . 18

2.13 Exciton motion through a Gaussian distribution of states . . . 18

2.14 Charge transfer state formation and dissociation . . . 21

2.15 CNT chirality indices and lattice vectors from graphene . . . 23

2.16 Construction of the CNT band dispersion . . . 24

2.17 DOS and band gap of metallic and semiconducting CNTs . . . 25

2.18 Optically allowed and forbidden transitions in CNTs . . . 25

2.19 Excited many body states reported for CNTs . . . 26

2.20 Absorption- and 2D-PL spectrum of CNTs . . . 28

2.21 Energy gap for PbS CQDs with respect to their size . . . 32

2.22 Impact of disorder and coupling on the transport in CQD solids . . . 35

2.23 Surface chemistry of PbS CQDs . . . 38

3.1 Main components of a Michelson interferometer . . . 54

3.2 Explanation of the Fourier transformation . . . 56

3.3 Illustration of the transient PL set-up . . . 58

3.4 Measurement principle and signals in PIA spectroscopy . . . 59

3.5 Steady state set-ups for PIA spectroscopy . . . 61

3.6 Transient absorption spectroscopy set-up . . . 62

(7)

4.3 Calculated absorption spectra for C-/Si-PCPDTBT oligomers . . . 67

4.4 Electron-hole density of a PCPDTBT oligomer and aggregate . . . 69

4.5 Explanation of polymer IRAVs in the MIR spectral region . . . 70

4.6 Different exciation PIA spectra of neat polymers . . . 71

5.1 Materials and solar cell characterisation for the ternary device . . . 80

5.2 PL spectra of neat and blended organic films . . . 81

5.3 PL spectra for polymer blends of different concentration . . . 83

5.4 Steady state PIA spectra of the organic blends . . . 84

6.1 Energy levels and absorption spectra of polymer wrapped CNTs . . . 90

6.2 Photoluminescence spectra of polymer wrapped CNTs . . . 91

6.3 NIR PIA and TA of polymer wrapped CNTs . . . 92

6.4 MIR PIA of polymer wrapped CNTs . . . 94

6.5 Calculated first excitation for a polymer wrapped CNT . . . 96

6.6 Representative molecular orbitals for P3DDT wrapped CNTs . . . 97

7.1 Energy levels and absorption spectra of PbS CQDs and PCPDTBT . . . 105

7.2 PL spectra of neat and blended films of PCPDTBT and PbS CQDs . . . 107

7.3 TA and EQE of neat and blended films of PCPDTBT and PbS . . . 109

7.4 J-V curves of fabricated solar cells and AFM images of their ALs . . . 110

8.1 Sketch and TEM images of employed ligands and CQDs . . . 116

8.2 Absorbance spectra of CQDs in solution and as a film . . . 117

8.3 PIA spectra of PbS CQDs with different size and ligands . . . 118

8.4 PL upon ligand variation and TEM image of degraded CQDs . . . 119

8.5 Data ffor degradaed or differently washed PbS_OA . . . 120

8.6 Vibrational spectra and Fano calculations for PbS CQDs . . . 122

A1 Additonal electrical charactersation of organic solar cells . . . 133

A2 PL spectra of PDCBT:PC70BM . . . 133

A3 Absorption spectra of PF12-related samples . . . 134

A4 MIR PIA spectra of neat P3DDT . . . 134

A5 PIA spectra of a P3DDT:PCBM blend with different energy . . . 135

A6 PIA spectra of P3DDT wrapped CNTs . . . 135

A7 PIA spectra of P3DDT wrapped CNTs with excess polymer . . . 135

A8 PIA spectra of neat PF12 upon above and below gap excitation . . . 136

A9 PIA spectra of a PF12:PCBM blend . . . 136

A10 PIA spectra of PF12 wrapped CNTs . . . 136

P HO T OP H YS IC S OF NAN OM A TE R IALS F OR OPT

(8)

O-List of Tables

2.1 Exciton Bohr radii of relevant semiconductors. . . 33

5.1 Electrical parameters of organic solar cells . . . 79

5.2 Extracted PL lifetimes for organic films . . . 81

5.3 Extracted PL lifetimes for polymer blends . . . 84

7.1 PL lifetimes of hybrid blends . . . 108

A1 Calculated transition energies for PCPDTBT aggregates . . . 132

A2 Calculated transition energies for trions in CNTs . . . 137

A3 Composition of excited states for P3DDT wrapped CNTs . . . 137

A4 Peak positions and widts of PIA bands of Pbs CQDs . . . 138

A5 Molecular vibrations associated with oleic acid . . . 138

A6 Molecular vibrations associated with 1,4-benzenedithiol . . . 139

(9)

(10)

1 Introduction

Nanotechnology is the exciting field of research dealing with objects smaller than approximately 100 nm. On this scale, matter can display phenomena not encountered on a macroscopic scale. Many such aspects involve the electronic properties of materials and their interaction with light. Photophysics describes the investigation of these interactions.

This thesis is separated into five chapters of experimental work. To allow for properly under-standing them, the relevant material classes carbon nanotubes (CNTs), colloidal quantum dots (CQDs) and conjugated polymers will be introduced in Chapter 2. The two former are prime examples for nanoscale objects, in which the limited size confines the free motion of charge car-riers and thereby leads to the emergence of surprising features. In colloidal quantum dots, for example, carriers are confined in all three spatial dimensions, which is why these objects are of-ten referred to as zero-dimensional. Assembling such CQDs in films can give rise to matter with remarkable properties: optically behaving like isolated CQDs and electrically almost like a bulk crystal.

In CNTs, charge carriers move freely in one direction, but are confined in the remaining two. These objects are consequently referred to as being one-dimensional. This, again, has a pro-found effect on the properties of the material. Carriers encounter hardly any resistance when moving longitudinally to the tube axis, but are confined within the narrow diameter of these tu-bes. One-dimensional semiconductors also give rise to narrow optical transitions, found in the near infrared spectral region, in case of CNTs.

Conjugated polymers and organic molecules attract considerable attention in the research com-munity for applications in flexible electronics.

Fabrication and eventual commercialisation of opto-electronic products, such as solar cells, photodetectors or light emitting devices, is one major aspect driving research on these three ma-terial classes. In order to successfully apply them, though, the photophysical processes occur-ring in single materials or in blends and at their interfaces need to be properly understood. Their interaction is here investigated with several optical techniques – mostly in the infrared region of the electromagnetic spectrum. Chapter 3 is thus devoted to explaining basic concepts of these techniques, as well as the specific set-ups used in this thesis. Most prominently, photo-luminescence- (PL) and photoinduced absorption (PIA) spectroscopy will be discussed, as these are the main tools of investigation. A special focus is devoted to the introduction of the Fourier

(11)

this field of research. Often, spectroscopic studies are limited to the near infrared (NIR) spectral region, down to 1.1 or 0.55 eV, depending on the detector material. Using FTIR spectroscopy, processes at low energy (few tens of meV), in the mid infrared (MIR) spectral region, can easily be monitored and considerably improve the understanding of light-matter interaction in the systems discussed herein.

This MIR PIA technique is then used to more closely examine the spectral signatures of charge carriers in donor-acceptor polymers in Chapter 4. By comparing the similar polymers C-PCPDTBT and Si-PCPDTBT and their blends with PCBM, differences and similarities are investigated. Using density functional theory (DFT) simulations, it is shown that the PIA signatures in the MIR are due to positive polarons on the polymer. This examination adds to the previously existing lite-rature on similar experiments for homopolymers. The concept of intra- and interchain deloca-lisation of polarons is finally invoked to explain the PIA at lowest energy, as previously observed for P3HT. It is shown, though, that instead of an extended crystallinity, the partial alignment of neighbouring chains already gives rise to these transitions.

In Chapter 5, the working mechanism of a ternary blend comprising two conjugated polymers with PC70BM leading to a high power conversion efficiency (PCE) in solar cells is investigated.

Contrary to the expectations from the bulk energy levels of the components, this system does not work as an energy cascade, but is governed by an efficient energy transfer from the crystal-line wider band gap polythiophene to the narrow band gap donor-acceptor polymer. The energy transfer is observed via a delayed maximum in PL intensity of the latter. It is furthermore obser-ved that the former, despite its supposedly lower lying HOMO level, acts as the hole transporting polymer in the ternary blend.

Based on the discoveries reported in Chapters 4 and 5, the photophysical behaviour of polymer wrapped semiconducting single walled carbon nanotubes is investigated in Chapter 6. Even when exciting this composite material at an energy below the polymer band gap, polymer po-larons can be monitored in the MIR spectral region. This surprising observation is a clear indi-cation that the two materials act as a single hybrid. Additional quantum chemical calculations indeed reveal states to be partially shared between the polymers and the CNTs.

The excited states interaction of a narrow band gap polymer with lead sulphide colloidal quan-tum dots (CQDs) is investigated in Chapter 7. These materials exhibit a complementary absorp-tion, thus seem to be prime candidates for hybrid photovoltaics. A favourable charge transfer interaction is observed if the long insulating ligands capping the CQDs after synthesis are ex-changed for shorter entities. Despite this promising observation, fabricated solar cells, suffer from hampered charge carrier transport through an unfavourable microstructure and conse-quently exhibit a poor performance.

P HO T OP H YS IC S OF NAN OM A TE R IALS F OR OPT O-EL

(12)

quantum dots. These nanocrystals exhibit two distinct trap distributions below 0.5 eV. Their energy increases for smaller CQDs but is shown to be independent of the surface ligands. The ligand exchange from native oleic acid to shorter entities, however, leads to a significant incre-ase in PIA signal. Fano resonances are furthermore observed between the broad trap-related absorption bands and the vibrational modes associated with the surface ligands.

These investigations help to develop a deeper understanding of excited states and their inte-raction in different materials relevant for opto-electronic applications. The MIR PIA techni-que, in particular, allows for accessing information beyond reach for more commonly employed techniques. Its power is shown here by investigating vastly different aspects ranging from the identification of polymers through their vibrational fingerprints, deduction of information about the environment of charge carriers as well as polymer chain alignment or the monitoring of sur-face trap states in colloidal quantum dots. With its versatility proven, this thesis also aims at a popularisation of this technique.

(13)

2 Materials

This chapter introduces basic properties of the different material classes employed in this thesis. After a brief introduction into nano-sized semiconductors, conjugated polymers, carbon nanotu-bes and colloidal quantum dots will be discussed in more detail. This includes their basic proper-ties with a focus on photophysical behaviour, as well as aspects of property modification. Impor-tant concepts necessary to understand these materials, such as excitons, triplets or charge carrier transport, will be introduced in the first section on organic solids and later on discussed specifi-cally for CNTs and CQDs.

2.1 Nanoscale Semiconductors

The focus of this thesis lies on the study of nano-sized semiconducting materials. Unlike metals, for which the Fermi energy (EF) lies within a band of allowed states, semiconductors exhibit EF in a gap of their energy bands. Similar to insulators, this leads to a comparably small density of free charge carriers under equilibrium conditions (at finite temperature) – states below EF are occupied and empty above. The distinction between insulators and semiconductors is arbitra-rily chosen with respect to the width of the gap between the highest fully occupied band (valence band, VB) and the lowest fully unoccupied band (conduction band, CB). Typically, semiconduc-tors are said to have a band gap smaller than 3 eV.

The density of states of an arbitrary semiconductor is shown in Figure 2.1 with respect to its di-mensionality. A macroscopic or bulk semiconductor allows the charge carriers to move freely in all three spatial directions and one finds a square root shaped density of states (DOS) for the conduction- and the valence band respectively. If one dimension is drastically decreased, the carriers can only move freely in the two unchanged dimensions and are confined in the third. This geometry is said to be a quantum well (QW), which exhibits a stepwise constant DOS. Furt-her confinement in a second dimension leads to the formation of quantum wires (QWR) and finally, if the confinement acts in every dimension, a zero-dimensional quantum dot (QD) is for-med.[1]Ideal QDs exhibit a discrete DOS spectrum and are therefore sometimes referred to as artificial atoms.[2]Their discrete energy states are hence commonly denoted using the same for-malism as for the quantum numbers of atoms. Typically, the confinement manifests strongest in

(14)

Ener Density of states HOMO HOMO-1 HOMO-2 LUMO LUMO+1 LUMO+2 Conduction band Valence band 1se 1sh 1ph 1pe CB VB C1 C2 V1 V2 E_F

Figure 2.1: Density of states for an arbitrary semiconductor with respect to its dimensionality. From left

to right, the confinement increases leading to a DOS that is proportional topE for the bulk material, a

step-function for a quantum well and marked by van Hove singularities with an exponential decay for a quantum wire. Quantum dots and molecules on the right ideally exhibit discrete energy levels.

the separation of states close to the original band gap. Higher lying states lie increasingly close together and might be quasi-continuous.

Of these classes, quantum wires and quantum dots will be investigated in the form of carbon nanotubes and lead sulphide colloidal quantum dots. Additionally, the large class of organic se-miconductors will be addressed, which exhibit discrete energy states as well, when isolated, as depicted on the right in Figure 2.1.

The critical length from which on confinement sets in is linked to the Bohr radius rBof the exci-ton describing the size of the formed electron hole pair.

rB=²0²r

h2

πm∗_e2, (2.1)

where_²0and²r are the permittivity of vacuum and the material and m∗is the exciton effective mass. If the size of the nanocrystal is smaller than the Bohr radius in one direction (the charac-teristic length), confinement is present.

2.2 Organic Semiconductors

2.2.1 Conjugated Polymers

Organic materials are predominantly based on hyrdogen and carbon atoms. Carbon, in its ground state, exhibits the electron configuration 1s22s22p2, i.e. two filled s-orbitals and two half occupied and degenerate 2p-orbitals (Hund’s rule). In order to bind to the commonly found four partners, instead of just two as suggested from this configuration, one of the 2s-electrons is

(15)

2s2 1s2 _1s2 2px 2py 2pz sp3-hybrid orbitals (a) (b) Hybridisation x x p_z s z y x 109.5° sp3

Figure 2.2: Scheme of energy levels (a) and shape (b) of s-, p- and sp3-orbitals in carbon. Due to the Coulomb repulsion, sp3-orbitals form a tetrahedron with an angle of 109.5◦between them.

promoted into the unoccupied p-orbital and the atomic orbitals (AO) of carbon hybridise (mix). This process requires energy (Figure 2.2 (a)) that is significantly lower than what is freed when forming molecular bonds. In the simplest case of methane (CH4), for example, all outer

elec-trons bind to one hydrogen atom, each then located in a molecular orbital (MO) shared between the atoms. These so-calledσ-bonds exhibit their highest electron density between the C and H atom and the MOs arrange as a tetrahedron with an angle of 109.5◦between them due to their mutual Coulomb repulsion (Figure 2.2 (b)). All four AO from C participating in these bonds are equal sp3-orbitals, each exhibiting 1/4 s-character and 3/4 of p-character.

Similarly, carbon can also form sp2- or sp-orbitals, the former of which represent the back-bone of conjugated materials. In sp2-carbon, the three hybrid orbitals (with 1/3 s- and 2/3 p-character) lie in a plane (e.g. x-y), forming an equilateral triangle with an angle of 120◦between them. The semi-occupied pz-orbital aligns perpendicularly to this plane. In the simple case of ethene (C2H4), for example, each carbon atom forms twoσ-bonds via the sp2-electrons to

hyd-rogen atoms and an additionalσ-bond between the two carbons (Figure 2.3 (a)). The double bond between the carbon atoms stems from the two pz-orbitals, which form overlapping mo-lecular orbitals above and below the sp2-plane – so-calledπ-bonds. In contrast to electrons in theσ-bond, which are fixed between the two participating atoms, electrons in these π-orbitals are delocalised and can move freely in theπ-system. These electrons are responsible for charge carrier conduction and related phenomena discussed throughout this thesis.[3]

In a quantum mechanical picture, molecular orbitalsΨMO are formed approximately by the linear combination of all involved atomic orbitalsφi (LCAO)[3]

ΨMO=X

i

ciφi, (2.2)

where c is a weighting factor. Depending on the sign (or phase) of the involved AO, binding or

(16)

C

σ-bond (a) (b) (c) E C₁ C=C C₂ sp2 pz π* σ π σ* C1 C2 C₁ C2 Bonding π Anti-bonding π Ψ |Ψ Ψ*_|

Figure 2.3: Alignment of orbitals in ethene (a) leading toσ- and π-bonds between the two carbon atoms

(H-atoms are omitted for clarity). Corresponding energy levels (b) of involved bonding and anti-bonding molecular orbitals and wavefunctionsΨ of the atomic and molecular orbitals (c). The probability to find an electron in the centre between the carbon atoms vanishes for antibonding orbitals.

anti-binding MO form. The latter are characterised by a lower energy and high probability of finding an electron between the atoms (consider Figure 2.3 (b) and (c), anti-binding orbitals are denoted with a star). Since anti-bonding orbitals exhibit zero probability of carriers in the centre between the atoms, these are typically more unstable than binding orbitals and their formation can give rise to bond dissociation. The highest molecular orbital occupied with electrons is cal-led HOMO and the lowest unoccupied accordingly LUMO – together, they form the so-calcal-led frontier orbitals. Their difference in energy is typically associated with the band gap energy Eg (although, strictly speaking, there are no bands present here).

In most cases,π-bonds are responsible for the interaction of organic materials with light. The absorption of a photon promotes an electron from theπ- to the π∗-orbital. For ethene, the diffe-rence in energy amounts to 6.7 eV. Increasing the number of carbon atoms involved, i.e. forming a chain with alternating single and double bonds, leads to an extension of the so-called conjuga-tedπ-electron system along the carbon backbone. Analogously to the particle in a box problem known from quantum mechanics, a longer chain (greater conjugation length) leads to states with a smaller energy separation.

For an infinite conjugation, this would lead to a HOMO and LUMO at the same energy, thus the formation of a semi-occupied band and the material, e.g. a polymer chain, would behave like a metal. Due to a redistribution of the bond lengths in such a system, however, this does not occur and the band gap opens to give the chain semiconducting properties (Peierls distortion).[4] In real polymers, the actual chain is much longer than the conjugation length. This is due to de-fects and structural deformations. Torsion, for example, limits the overlap of the pz-orbitals, for which only the parallel components overlap with each other in theπ-bonds. A single polymer chain can thus include several distinct chromophores (conjugated segments).[5,6]

(17)

O O N S S O N O n C10H21 C 10H21 C8H17 C8H17 N N S n n n S _n N H O N H O C8H17 C8H17 n O O n S n O O n S S N N S n N H S N O N _O S S S S C8H17 C8H17 C10H21 C10H21 n S N S n Heteroatoms Sidechains O _n P3HT MDMO-PPV MEH-PPV PFO PA PP PPV PT PF PC BTD DPP PCPDTBT PCDTBT DPP-TT-T _N2200 A D A

Figure 2.4: Conceptual progress from purely CH-based semiconducting polymers as used in the 1970’s and

80’s (left top), first included the addition of sidechains to increase the polymer solubility and the insertion of heteroatoms in both the backbone and sidechain (left). Nowadays, electron rich and -poor groups (centre) are commonly used as building blocks in donor-acceptor polymers. Abbreviations explained in the glossary or main text.

only. The prime examples are poly-acetylene (PA) and poly-phenylene vinylene (PPV),[7–9]as depicted in Figure 2.4. In 2000, the Nobelprize in chemistry was awarded to Shirakawa, McDi-armid and Heeger for the observation that oxidation (i.e. p-doping) is possible for polyacetylene, which is considered a key moment of the research field into semiconducting polymers. The ob-servation of charge transport through conjugated polymers was followed by growing research efforts into material synthesis as well as into their application. Throughout the 1990’s and early 2000’s, significant improvements in device performance, e.g. in organic solar cells, was achie-ved by using monomers involving heteroatoms. Oxygen, nitrogen or sulphur were for example used in materials such as MEH-PPV, MDMO-PPV, P3HT (Figure 2.4, left) More intricate hydro-carbon chains, such as in polyfluorene, also improved the device characteristics. Crucially, the side chains of these materials significantly increase their solubility in common solvents, facilita-ting their handling. Moreover, as will be discussed further below, the side chains strongly affect the morphology of formed films. In blends with the electron acceptor PCBM, the power conver-sion efficiency (PCE) of PPV-based materials was limited to about 3% – mostly due to its wide band gap (thus poor coverage of the solar spectrum) and a relatively low charge carrier mobility. Thiophene-based materials, especially poly(3-hexylthiophene) (P3HT), have been the working horses for organic photovoltaics (OPV) for a long time. Their typical PCE amounts to 3.5-4% and the record was set at 5.2%.[10,11]The band gap of P3HT, at about 1.9 eV, is still not optimal, but the polymer exhibits good transport properties and is easy to process. Aforementioned materials (sometimes referred to as homopolymers) generally exhibit a band gap that is not smaller than approximately 2 eV. For photovoltaic purposes, however, a smaller gap would be beneficial. De-velopment of new conjugated polymers has therefore often been driven by the desire to reduce the band gap in order to cover a broader range of the sun spectrum for applications in OPV.

(18)

HOMO LUMO

Donor-unit DA-polymer Acceptor-unit

Egap

Figure 2.5: Depiction of the formation of the narrower band gap in donor-acceptor polymers. The orbitals

of the sub-units overlap, leading to a higher lying HOMO and a lower lying LUMO of the D-A-polymer with respect to either sub-unit.

As a remedy, heteropolymers or donor-acceptor (D-A) polymers have been synthesised, in which electron rich (donor) and -poor (acceptor) sub-units are coupled together (consider Figure 2.4, centre). As depicted in Figure 2.5, the orbitals of the two sub-units mix and form a gap that is smaller than the one of either sub-unit.[12]In this way, the absorption onset of polymer films could be shifted from approximately 560 nm for MEH-PPV, 630 nm for P3HT to 830 nm in case of PCPDTBT (consider Figure 2.4, right). The incorporation of the D-A groups furthermore leads the HOMO to be predominantly located on the donor group and the LUMO on the acceptor group, which is assumed to be beneficial for exciton dissociation (vide infra).[13,14]

Some important building blocks for D-A polymers are depicted in Figure 2.4. Common electron donor groups include polycarbozole (PC), which, together with the acceptor group benzothiadi-azole (BTD) and a bridging thiophene, forms the material PCDTBT, currently often used to study basic properties in (opto)-electrical devices.[15]In solar cells, it typically offers a PCE of 6-7%.[16] A similar polymer, in which the BDT unit is connected with a bridged dithiophene instead of the carbozole, is PCPDTBT.[17,18]This prototypical D-A polymer led to efficiencies of up to 5.5% and was studied extensively to understand excited state phenomena and the impact of spacer groups between the D and A group.[13]Furthermore, as will be discussed in more detail in Chapter 4, it is possible to significantly improve the ordering of this material by replacing the bridging atom of the dithiophenes by silicon, to form Si-PCPDTBT (also known as PSBTBT).[19]

Another acceptor group leading to promising performances of up to 8% is the diketopyrrolopyr-role (DPP) class of materials, often linked to thiophene rings.[20]Recently, some of the new wor-king horses have been D-A polymers based on the benzodithiophene coupled to thienothiop-hene units, such as in PTB7 or its variant PTB7-th, which will be employed in Chapter 5.[21] Alt-hough these D-A materials achieved impressively high power conversion efficiencies in devices, they are often also prone to severe degradation upon illumination. This signifies that despite significantly improving key properties through chemical tailoring, the challenges involved for

(19)

A b sorp tio n Intersystem Crossing Fluor esc ence In tern al Con ver sio n

S

0

S

1

S

2

T

3

T

1

T

2 Phosph o resc ence

Figure 2.6: Jablonski scheme depicting the electronic singlet and triplet states with their vibrational levels.

Solid lines denote optical transitions. (Figure modified from Ref.[23].)

commercialisation of organic photovoltaics remain demanding.

Materials introduced so far share that they are all of p-type, i.e. they generally exhibit signifi-cantly higher hole than electron conductivity. Synthesising good n-type materials has for a long time been very challenging, but one of the few examples of a well-performing n-type material is N2200, based on a naphthalene diimide (A) linked to thiophene sub-units (D) as shown in Figure 2.4 right.[22]

2.2.2 Interaction with Light

The Jablonski diagram, as shown in Figure 2.6, allows to depict possible photophysical transi-tions in a simple way. If a system (e.g. a molecule or polymer segment) is in its ground state (S0), the electrons fill up all states beginning from the deepest. Each state will be occupied by

two electrons of opposite spin (up and down) giving rise to a multiplicity of one – therefore cal-led singlet state. The absorption of a photon occurs quasi-instantaneously and can promote the electron to a higher lying singlet state Sn. Each of these singlet states includes vibrational and possibly rotational sublevels. The latter of which are omitted in Figure 2.6 for clarity. If a transition ends in a vibrationally excited state, the electron quickly (few ps) undergoes internal conversion, commonly due to non-radiative relaxation, and moves to the ground state of that

(20)

0 1 2 3 4 0 1 2 3 4

S

₀

S

₁ Nuclear Coordinate Pot en tial Ener gy R0 R’ Absorption Fluorescence Energy In tensity 2-0 3-0 4-0 5-0 6-0 0-3 0-4 0-5 0-6 0-2 characteristic of S1 characteristic of S0 (a) (b) 0-1 1-0 Stokes shift

Figure 2.7: The Frank-Condon principle describes that vibronic transitions occur vertical with respect to

the nuclear coordinate R (a). Vibrationally excited states relax quickly into the ground state of the electro-nic level. Transitions thus predominantly start from this level and end in the vibrational manifold of the electronic final state and the respective spectra can be mapped in optical spectroscopy (b). Adapted from Refs.[3,24]

electronic level. In thermal equilibrium, transitions thus predominantly start in an electronic ground state (Kasha’s rule). From these excited singlet states, also the reverse process of pho-ton emission can occur. This is termed fluorescence. The emission is hence generally lower in energy than the absorption, which is manifested in the so-called Stokes shift. The lifetime of the excited singlet states typically lies in the region of nanoseconds. Similar to the relaxation of vibrational levels, electronic states can also be deactivated non-radiatively –the yield of which strongly depends on the respective material and environment.

For two electrons with parallel spin, there is a multiplicity of three. Such states are thus called triplet states. Transitions from singlet to triplet states, i.e. inter system crossing (ISC), are spin-forbidden, but can occur due to spin-orbit coupling (especially in presence of heavy atoms). Triplet states also include vibrational sublevels as discussed for singlets. Photon emission from triplet states due to a transition back to the S0(phosphorescence) is a forbidden process as well,

leading to a significantly longer lifetime of triplets (up to milliseconds). Triplet states generally lie lower in energy than singlets, which is due to a lower Coulomb repulsion between two electrons of the same spin: as stated by the Pauli principle, they cannot occupy the same state and are thus on average farther apart from each other than electrons in singlet configuration (exchange interaction).

The probability for vibronic (vibrational and electronic) transitions can be determined by the Frank-Condon principle as explained in Figure 2.7 (a). Since the excitation into the S1state is

accompanied by a change in arrangement of the electrons, this also leads to a change in nuclei conformation (here described as a changed nuclear coordinate R). The non-harmonic oscilla-tors of the excited singlet states do thus not exhibit the same nuclear coordinate as their ground state (R06= R0). The curve corresponding to S1is usually located at a larger nuclear coordinate,

(21)

Wannier-Mott Frenkel (a) (b) + +

-

- kBT Inorganic εr=15 Organic ε_r=4 r_C,O r_B,O r_C,I r_B,I

Figure 2.8: Depiction of a Wannier-Mott and a Frenkel exciton (a), showing that the latter are strongly

lo-calised due to the Coulomb attraction. Dependence of Coulomb and Bohr-radius on the permittivity for a typical organic and inorganic value (b). Adapted from Ref[25]. The Bohr radius in inorganic semiconduc-tors significantly exceeds the Coulomb radius, leading to the formation of free charge carriers.

since electronically excited states tend to have more anti-bonding character than ground sta-tes (consider section 2.2.1). The transitions are much faster than the nuclear motion and thus run vertically with respect to R (Born-Oppenheimer approximation). While doing so, they cut through several vibrational levels, to which a transition is possible. The highest probability is found for the greatest overlap of the initial and final vibrational wavefunction |〈νf|νi〉 |2, which leads to an intensity profile for absorption and emission as illustrated in Figure 2.7 (b). The sub-peaks superimposed on the broad bands in absorption and emission hence map the vibrational structure of the S1and S0state. Often, the 0-0 transition is not observable and the separation

between the first maximum of the two spectra is termed Stokes-shift.

2.2.3 Excited States in Organic Solids

Inorganic semiconductors, such as silicon or germanium, are also based on atoms with hybri-dised orbitals. In marked contrast, however, the interacting species forming organic solids are the polymers or molecules, bound together by van der Waals forces, instead of atomic bonds. Covalent bonds between silicon atoms, for example, are much stronger than these temporary dipole-interactions, which is why these types of solids exhibit marked differences, e.g. in their toughness, melting temperature or degree of order. Moreover, the permittivity²r of organic films lies between two and four, whereas for inorganic semiconductors it is significantly hig-her (²r(Si ) ≈11.7, ²r(P bS) ≈18). Due to this low permittivity, phenomena become dominant in organic materials, which have a negligible impact in classical inorganic semiconductors. One major characteristic of organic solids is the formation of strongly bound, so-called Fren-kel excitons. Electronic excitation leads to the promotion of an electron from the HOMO to the

(22)

+

+ + + + +

-

+

Electronic polarisation Deformation

Figure 2.9: Schematics of the effects leading to polaron formation in organic solids. A charge depicted

on the central molecule creates dipoles in its environment, thus inducing an electronic polarisation. This successively also leads to a deformation of the charge carrier’s environment.

LUMO, leaving a vacancy or hole in the former and forming an electron-hole pair. Excitons are quasi-particles that describe electron-hole pairs which are not yet dissociated into free charge carriers, but still interact with each other through mutual Coulomb attraction. Their attraction leads to an exciton radius of about 1 nm and a binding energy E_{bi n}X of around 0.4 eV in orga-nic solids, which needs to be overcome in order to form free charge carriers.[23]Excitons are also formed in inorganic solids, but the larger permittivity in these materials leads to a stronger shielding of the Coulomb attraction, thus to a significantly lower binding energy and larger size (Figure 2.8 (a)). Such weakly bound excitons are described as Wannier-Mott excitons and the typical distance between the electron and hole is an order of magnitude larger than the lattice constant.[26]

This difference may be better understood with the example given in Figure 2.8 (b). A covalent so-lid with a high permittivity (²r= 15) and a typical value for an organic solid (²r= 3) are assumed, and the exciton wavefunction is approximated with the Bohr radius rBaccording to equation 2.1. The Coulomb radius rCis found at the distance, for which the Coulomb potential is equal to the thermal energy kBT , thereby illustrating that in the latter case thermal energy is sufficient to split excitons. Both, exciton dissociation into free carriers and exciton motion, will be treated further below. First, however, the nature of charge carriers in organic solids will be discussed more closely.

Similar to excitons, also the properties of electrons and holes are affected by the low permittivity and softness of organic materials. The dominant difference, as opposed to classical inorganic se-miconductors, besides the commonly encountered absence of band transport, is the formation of polarons, i.e. charge carriers surrounded by a cloud of polarised environment.[27]Adding or removing a charge, for example to/from a polymer segment, leads to three distinct mechanisms of stabilisation. Firstly, the change in the intra-chain (or intra-molecular) electron configuration in theπ-system also affects the electron distribution in the σ-orbitals, thus leading to a bond

(23)

relaxation energy, sometimes denoted asλ. Secondly, and more important, the electric charge leads to an electronic polarisation of its environment (Figure 2.9, top), which in return also de-forms the material in the vicinity of the charge (bottom).

Although polarons have been studied intensively in organic semiconductors, uncertainties re-main in fundamental aspects of their nature. The energetic position of negative and positive polarons is often discussed as being shifted into the fundamental band gap of the polymer. This shift is assumed to be due to the aforementioned relaxation of the environment that stabilises the charge carrier.[28,29]As depicted in Figure 2.10 (a), the removal of an electron from an un-charged polymer to form a positive polaron was for a long time assumed to lead to the formation of two new levels in the band gap. Of these the lower one is semi-occupied due to the remo-val of the electron and the upper completely empty and due to a down-shift of the previously unperturbed LUMO. The analogous process for an electron addition and a negative polaron is depicted alongside the former and also exhibits two levels within the fundamental band gap – the lower fully occupied and the upper semi-occupied. This discussion is originally based on the Su-Schrieffer-Heeger (SSH) model,[30]and has some descriptive power, but is significantly limited by the neglection of many body effects.

Despite its crude simplifications and objections from experiments,[31] this model is the most commonly employed explanation for the optical transitions of polarons in conjugated polymers. For partially crystalline polymers with high order, it was also extended to allow for the descrip-tion of polarons delocalised over several adjoining chains (inter-chain polarons), giving rise to transitions as depicted on the right hand side of Figure 2.10 (a).

Nevertheless, the model fails to account for a key observation, which is that it falsely predicts the energy needed for the addition (removal) of a second charge carrier, i.e. the second electron affinity (EA) (ionisation potential, IP) of the neutral polymer, to be larger (smaller) than the first one. Intuitively, this should not be the case, for if an additional electron really were to be locali-sed on a polymer chain, the addition of yet another electron to this chain segment would have to overcome the Coulomb repulsion of these two carriers, which reduces the EA. Experimentally, this was observed for organic materials via XPS spectroscopy[32]_{and led to the formulation of an}

alternative scheme of energy positions as depicted in Figure 2.10 (b).[33]It is important to note that the relevant transport levels of electrons and holes remain shifted into the fundamental gap of the unperturbed material, but the actual occupation changes – also the position depending on the charge carrier spin.

In both pictures, the polaron formation leads to two optically active transitions below the band gap energy. The low energy transition, often termed P1, is commonly found in the mid infrared

spectral region between 0.3 and 0.5 eV and the higher energy transition (P2) is located slightly

below the band gap energy of the unperturbed polymer. These features will be studied experi-mentally in Chapters 4, 5 and 6.

The order of polymer chains in films can vary significantly. Amorphous materials are fully

dis-P HO T OP H YS IC S OF NAN OM A TE R IALS F OR OPT O-EL

(24)

S₁ P₂ P₁ + polaron Neutral LUMO HOMO Delocalised + polaron - polaron + polaron P2 P₁ _DP 1 DP2 Ener (a) (b) Neutral S₁ P2 P₁

Figure 2.10: Polaron energy levels and occupation with respect to the fundamental band gap according

to the classical representation based on the SSH model (a) and as recently proposed by Heimel (b).[33] Blue arrows indicate the optically allowed transitions accessible in doping- or photoinduced absorption spectroscopy.

ordered with neither an extended conjugation of theπ-system in chain direction (intra-chain) nor overlappingπ-systems in inter-chain direction. Highly ordered phases, however, are gene-rally considered to be beneficial for the transport of charges and excitons (vide infra), since the strongπ-π-interaction facilitates the transition between adjacent chains. An increased degree of crystallinity is thus often desired for manufactured devices and was one of the key aspects for an improving power conversion efficiency in OPV. Specifically, it was observed that in P3HT an increased head-to-tail ratio of the hexyl-chain leads to a significant improvement of the cry-stallinity from a completely amorphous film in case of regio-random P3HT to a high degree of crystalline domains for the regio-regular variant.[34]Besides a better planarisation of the chain,

i.e. more extended_{π-system in the chain direction, the regular alignment of the tail also leads}

to an improved overlap of theπ-systems, i.e. delocalisation over adjacent chains.[35]A strong interaction of adjoiningπ-systems commonly leads to a higher carrier mobility in inter-chain direction than along the polymer backbone, where kinks and torsion tend to hamper the free movement of charge carriers between conjugated segments.

The strong inter-chain interaction can often be observed by a red-shift in the absorption onset for a higher regio-regularity. The intermolecular coupling furthermore leads to the formation of a pronounced vibrational structure in the absorption spectrum.[36,37]Agglomeration of po-lymer chains can additionally generate so-called J- or H-aggregates. The optical properties in these confirmations can significantly vary with respect to those of isolated chains, e.g. in solu-tion.[38,39]

Further strategies to increase the film crystallinity of polymer films include an increased mo-lecular weight (longer chains are able to fold back into themselves and thereby increase the order[40]), slow film drying by using slowly evaporating solvents or saturated atmospheres,[41] thermal annealing above the polymer glass temperature[42,43]or the mixing with high-boiling

(25)

Molecule Single crystal Disordered solid HOMO LUMO IP CB VB 2σ Ph P_e S1 T₁ S0 E_opt E_trans E_gas Ebin EX Jablonski Scheme

Figure 2.11: Energy levels for isolated molecules, single crystals and disordered organic solids with respect

to vacuum (from left to right). The polarisation energies Pe/Ph shift the levels for organic solids closer

together than for isolated molecules. Transitions in the added (reduced) Jablonski scheme occur relative to the ground state S0. The energy difference between the optical and transport gap amounts to the exciton

binding energy.

additives in order to induce a de-mixing in blends.[44,45]

Figure 2.11 gives an overview of effects in organic solids as discussed so far. The energies are defined relative to vacuum, which is set to zero. The left hand side displays the energy levels for an isolated chromophore.[23]The states are occupied up to the HOMO, which lies at a posi-tion corresponding to the ionisaposi-tion potential. This ionisaposi-tion potential is the energy required to remove one electron from the material. Similarly, the energy gained by adding an extra elec-tron to the molecule is termed the elecelec-tron affinity and describes the difference between the vacuum energy and the LUMO. The difference between IP and EA is the band gap in gas phase

Eg as. For an ordered or crystalline organic solid, as depicted in the centre, the valence and con-duction level shift closer together, giving rise to the transport gap Et r ans. In other words, the EA is increased and IP is reduced by the polarisation energies P−_/P+_{respectively. This is the}

stabilising energy leading to the formation of polarons instead of free charge carriers discussed above. Additionally, in contrast to the discrete states of single molecules in gas phase, crystalline solids exhibit a broadening of the valence and conduction state due to the formation of electro-nic bands. The right hand side, finally, depicts the case for a material in presence of disorder. The states are commonly assumed to exhibit a Gaussian distribution leading to a broadening of electronic states that is not due to their superposition. In the two solid cases, the energy diffe-rence between the valence and conduction level is termed the transport gap, because this is the energy separation of the transport levels of electrons and holes (or the polarons, to be precise). In order to understand the optical processes, the states introduced in the Jablonski diagram (Fi-gure 2.6) have been included on the right hand side in Fi(Fi-gure 2.11. Note that these two concepts are not compatible. The molecular energy levels are defined with respect to vacuum and in the Jablonski scheme everything is scaled relative to the ground state S0. S0is here aligned with the

(26)

the energy difference between the S0→ S1transition (optical band gap Eopt), and the transport gap Et r ans.

2.2.4 Carrier Transport in Disordered Systems

Once excitons or polarons are generated in organic solids, these particles can move through the film and recombine via different mechanisms or can be extracted at external contacts. The following section is devoted to discussing the concepts of transport, which, in large parts, are also applicable for the material classes discussed farther below.

Exciton migration can occur trivially due to the emission and re-absorption of photons, but more typical for solids is non-radiative diffusion. In disordered materials, this process occurs as in-coherent hopping from one conjugatedπ-segment to the next. The classical descriptions are through a Förster-like process or through Dexter transfer.[46,47]The former, also known as fluo-rescence resonance energy transfer (FRET), is a resonant process between an emitter (the inital segment) and a receiver and can occur if the emission spectrum of the former overlaps with the absorption spectrum of the latter and if their dipole moments are non-orthogonal (Figure 2.12 (a) top). This resonant process involves the exchange of a virtual photon and can occur over a certain distance; its efficiency typically decreases with r−6. An efficient interaction is commonly observed for distances between 2 and 5 nm.[48,49]Since the FRET conserves the spin of both the emitter and receiver, only singlet excitons can be transferred with this mechanism. Notably, the FRET was initially formulated for diluted solutions of point-like chromophores – an approx-imation, which might not hold in solid films of chromophores with extended conjugation. Ne-vertheless, FRET offers a good qualitative understanding of transport through receiver-emitter systems.[48]

Dexter transfer describes the double exchange of electrons due to the wavefunction overlap of adjacent sites. Given the necessary overlap, it is limited to short distances (approx. 1 nm) and its efficiency decreases exponentially with site separation. It can be thought of as a simultaneous exchange of one electron from the LUMO of the emitter to the LUMO of the receiver and a back transfer of an electron from the HOMO of the receiver (Figure 2.12 (a) middle and bottom). This process also allows for the migration of triplets as long as the overall spin is conserved. Similar to the Förster process, though, it is favoured when the exciton energy of the emitter is larger than that of the receiver.

Figure 2.12 (b) summarises the processes an exciton may undergo in its lifetime. During the random migration due to hopping, it possesses a finite probability to eventually recombine (ge-minately) either non-radiatively through thermalisation or radiatively through the emission of a photon. Furthermore, excitons might dissociate into free charge carriers, for example in the

(27)

(a) (b) D* _A _D _A* D* _A _D _A* D* _A _D _A* Singlet Dexter-transfer Triplet Dexter-transfer HOMO LUMO Hopping Hopping Dissociation R ec ombina tion Diffusion length Exciton

Figure 2.12: Transfer of excitons, including the Förster and Dexter mechanism (a) and processes excitons

can undergo in solid films (b). The average distance travelled before recombination is described as the exciton diffusion length LD. Adapted from Ref.[50].

E # 0 σ2_/k BT Density of states Carrier density Transport energy Space, energy T=0K T>0K Relaxation x Rapid down-hill migration Iso-energetic transport

Figure 2.13: Gaussian distribution of exciton hopping sites and transport regimes. A rapid down-hill

mi-gration is followed by an iso-energetic hoppping via around the transport energy. At low temperatures, the transport may freeze and excitons only migrate down-hill towards the lowest lying states. Adapted from Refs.[50,55].

presence of an energy offset for the HOMO/LUMO levels of adjacent sites. The average distance travelled by an exciton is the diffusion length LD, which is an especially critical parameter in so-lar cells, and commonly lies below 20 nm.

As illustrated in Figure 2.13, all excitations would undergo a rapid downhill migration to sites with the lowest energy, when at zero Kelvin (observable as red-shifts in PL).[51]In presence of finite temperature, however, excitons are also able to carry out small up-hill jumps in energy and therefore move iso-energetically on a dynamic equilibrium transport level after the initial fast down-hill migration.[52,53]The density of exciton states is typically assumed as a Gaussian distribution with a width of 50-110 meV.[54]

Once free charge carriers are formed, they can also move through random diffusion analogously

(28)

be delocalised over theπ-systems of polymer segments, their transport does not occur through Bloch oscillations. In most cases, a phonon mediated hopping between polymer sites is ob-served, a concept initially introduced for impurity mediated conduction in inorganic materials (thus neglecting polaronic effects).[56,57]For the hopping rateν from a site i to a site j, the Miller-Abrahams formalism is often employed, which distinguishes between hopping events with a gain or a loss in energy:[58]

νMi l l er i j (∆Ei j, ri j) = ν0exp · −2γrri j− ∆Ei j+ |∆Ei j| kBT ¸ . (2.3)

Here,ν0is the maximum hopping rate and r is the spatial- and∆E is the energetic separation

respectively. The first exponential term describes the wavefunction overlap between adjoining sites via the inter-site distance and the inverse localisation radiusγr. The second term determi-nes whether the process is thermally assisted in case of upwards jumps.

This Miller-Abrahams formalism is a simplified version of the Marcus rate. The latter is com-monly used to describe electron transfer reactions in chemistry and – crucial for organic solids – also accounts for polaronic effects.[59]For the hopping rate one finds

νM ar cus i j = I2 i j ħ r π λkBT exp · −(∆G + λ) 2 4λkBT ¸ , (2.4)

with the reorganisation energyλ, Gibbs free energy ∆G (which includes the energy difference ∆Ei j from equation 2.3)) and the overlap integral Ii jbetween the sites. The Marcus expression assumes the initial and final site to be located in a potential well separated by an energy bar-rier corresponding to the reorganisation energyλ. This mimics the polaronic nature of charge carriers in organic solids. Both approaches mentioned are semi-classical and break down for

T → 0.[60] Alternatively, quantum mechanical descriptions such as nuclear tunnelling[61]are able to describe transport at that limit, but reduce to the semi-classical Marcus expression for the experimentally relevant temperatures.

Analogous to the discussion for exciton migration above, the site-disorder is commonly model-led as a Gaussian distribution:[62]

g (E ) =pNt 2πσ2exp · −(E − E0) 2 2σ2 ¸ , (2.5)

where E0is the centre of the DOS, Ntis the density of hopping sites andσ is the variance in energy (diagonal disorder). Typically, the transport through polymers is assessed via the charge carrier mobilityµ, which characterises the drift velocity of the carrier when applying an electric field. In the so-called Bässler or Gaussian disorder model (GDM) for high electric fields, the

(29)

µGD M= µ∞exp · − µ ₂σ 3kBT ¶2¸ ×    exp¡C [(σ/2kBT )2− Σ2 p F¢ Σ ≥ 1.5 exp¡C [(σ/2kBT )2− 2.25] p F¢ Σ < 1.5, (2.6) whereµ_∞is the mobility in the limit of infinite temperature, C is a constant depending on the inter-site distance (typically 1-2 nm),Σ is the disorder in site-separation (off-diagonal disorder) and F is the magnitude of the electric field. Since the hopping is phonon assisted, the mobility increases with temperature (contrary to band transport). Moreover, also an increased electric field assists the transport.

An additional aspect, which is neglected in the GDM, is the dependence on the charge carrier density. As evident from the sketch in Figure 2.13, an increased carrier density leads to the filling of the DOS from the bottom, for which hopping events are less likely than in the middle of the DOS. The mobility thus increases with the carrier density.[63–65]Since the carrier density is also affected by the temperature, the observed trend of the mobility typically follows an Arrhenius-like behaviour instead of l n(µ) ∝ T−2, as predicted by Bässler. The GDM furthermore assumes the energy of adjacent sites to be uncorrelated, which is not necessarily true.[66]

A result of this model is that, similar to excitons, carriers brought into the DOS rapidly thermalise (as shown in Figure 2.13) and reach an equilibrium transport energy, which lies_kσ2

BT below the

DOS maximum.[62]Charge carriers might also (non-geminately) recombine when encountering free carriers of opposite sign (bimolecular recombination) or mediated by trap states (Shockley-Read-Hall recombination).

2.2.5 Charge Generation

Charge photogeneration in organic solids is most commonly discussed in the context of organic photovoltaics. Although this thesis does not focus on OPV itself, it is helpful to briefly discuss the underlying ideas of the community. As detailed above, the small permittivity of organic so-lids is commonly expected to not allow for prompt charge formation upon photon absorption, but expected to create strongly bound Frenkel excitons. In order to effectively dissociate exci-tons, materials are blended together to create what is termed a bulk heterojunction.[67] Histo-rically, this was most commonly carried out by using a strongly absorbing polymer as electron donor and a fullerene derivative (PCBM) as acceptor material. Quite recently, also non-fullerene acceptors attracted much attention with well superior performance than in classical systems.[68] In the simplest picture, as depicted in Figure 2.12 (b) on the right, the energy offset between the respective HOMO and LUMO level of acceptor and donor are assumed to promote the exciton dissociation.

In more refined discussions, a charge transfer state (CTS) is often invoked to explain the disso-ciation in more detail. As depicted in Figure 2.14, this intermediate state forms at the interface

(30)

+

-

Charge transfer state

-

+ + E_CT LUMO_A HOMO_A LUMOD HOMOD CTS formation Diffusion to interface CTS dissociation

(i) (ii) (iii)

Figure 2.14: Exciton diffusion towards the donor-acceptor interface (i) can lead to the formation of a charge

transfer state (ii). The CTS dissociation promotes a free electron into the acceptor LUMO and a hole into the donor HOMO (iii).

between two phases and was observed experimentally in many different blends for example via high sensitivity absorption, photo- and electroluminescence, or photocurrent spectroscopy.[69–72] Nonetheless, there is still an intense debate about the precise mechanism and different impacts on the exciton dissociation. For example, it was observed that charge formation may also occur quasi-instantaneously during several tens of fs,[73]which is in contrast to exciton diffusion to-wards an interface, as this process would require several ps.[74]It was thus proposed that addi-tionally to the neat donor and acceptor phases, blended materials also form finely intermixed regions, wherein a significant exciton population is generated directly at an interface.[75]A de-tailed discussion of charge generation at donor-acceptor interfaces and the role of the CTS may be found in the reviews included in the bibliography.[76–78]

Nevertheless, photogeneration of charges is generally also observed in neat polymers, albeit with a significantly lower yield. Sources of charge generation are chain defects, such as kinks, which lead to an energetic driving force for exciton dissociation.[79]Similarly, the mixture of crystalline and amorphous regions within a polymer film can create internal homojunctions, at which dri-ving forces can be present as well.[80,81]

Charge formation is again also observed at ultrafast timescales.[82,83]While there is still room for debate on the precise mechanism, several reports indicate that excitons, when formed, are initially delocalised over a large part of the polymer backbone (up to 9 nm after 100 fs were re-ported[84]) and then quickly localise or self-trap.[85,86]It was argued that this process occurred as coherent energy transfer through a series of delocalised states[5,74,87]and that dissociation was the result of incomplete tunnelling.

For donor-acceptor polymers, the initially formed excitons were furthermore reported to alre-ady exhibit a partial charge transfer character, which might be a precursor for exciton dissoci-ation.[13,14,88]Fittingly, the polaron yield in these materials is often observed to be significantly larger than for homopolymers.

(31)

2.3.1 Opto-electronic properties

Carbon nanotubes (CNTs) can conceptually be considered as a sheet of graphene, i.e. a honey-comb layer of sp2-hybridised carbon, rolled-up into a hollow cylinder. The hexagonal lattice of graphene with its unit vectors~a1,~a2and the translational indices (n, m) is then used to describe

the so-called chirality of the nanotube (Figure 2.15 (a)). A carbon atom in the virtual rolling-up process will be projected onto another atom on the circumference.[89]This projection can be described using a vector

~

C = n~a1+ m~a2; n ≥ m, (2.7)

which points perpendicularly to the tube axis. This translation forms an angleθ between ~a1and

~

C , which may lie between 0 and 30◦_{and comprises all possible chiralities due to the six-fold}

rotational symmetry of the graphene lattice. For the two extreme cases, the tubes are strictly speaking not chiral and forθ = 0◦_{one finds n = 0 and for θ = 30}◦_{n = m. The former are referred}

to as zigzag- and the latter as armchair tubes due to the shape of the CC-bonds limiting these tubes (examples are given in Figure 2.15 (b)). The chiral angle can be determined using

cosθ = sin−1 Ã p 3m 2pn2_{+ nm + m}2 ! . (2.8)

It is furthermore possible to directly determine the circumference |~C | and diameter dtof a CNT from the chiral indices (n, m) using

|~C | = a0· p n2_{+ nm + m}2 _(2.9) and dt=|~C | π = a0 π p n2_{+ nm + m}2_, _(2.10)

where a0is the graphene lattice constant, connected to the distance between two carbon atoms

aCCas a0=

p

3aCC= 0.246 nm.

Commonly, CNTs of different chirality and with a number of walls are formed during synthesis. While multi walled CNTs (MWCNTs) can form , they are generally not employed in electronic applications. The following discussion thus solely focuses on single walled CNTs (SWCNTs).1 CNTs are marked by their large aspect ratio, leading them to be virtually one-dimensional ma-terials. In their axis direction, CNTs can be as long as several centimetres and with a diameter range between 0.5 and 3 nm, their circumference gives rise to a strong confinement in off-axis direction.

1_{Unless stated otherwise, the term CNT will be used in this thesis to refer to SWCNTs to maximise the readability.}

(32)

(0,0) Zigzag (8,3) a₁ a₂ θ (1,0) (2,0) (3,0) (2,2) (1,1) (3,3) Chiral (n-m)mod3 = 1 or 2 (n-m)mod3 = 0 Semiconductor Metal _(7,3) Chiral (20,0) Zigzag (10,10) Armchair (a) (b)

Figure 2.15: Graphene lattice with the construction of CNTs and the assignment of the chirality indices.

emiconducting tubes are indicated by orange dots and blue dots denote metallic tubes (a). Adapted from Ref.[90]Top and side view of exemplary CNTs with different chirality (b).

When describing the electronic properties of SWCNTs, it is again useful to consider the graphene lattice (zone folding approach) as depicted in Figure 2.16 (a).[91]The corresponding reciprocal lattice, shown in (b), is defined by the vectors b1and b2and the first Brillouin zone (blue shaded

region) includes the characteristic pointsΓ, K and M. The relevant point for optical transitions is the K -point, at which the valence and conduction band meet and exhibit a linear dispersion – so-called Dirac cones – as displayed in (c).

Given the extended nature of CNTs in axis direction, the wave vector component parallel to the tube ~k_∥is continuous and follows the dispersion of graphene. In circumferential direction, ho-wever, the wave vector ~k_⊥obeys periodic boundary conditions, giving rise to quantised values ~

C ·~k⊥= 2π j , where j is a non-zero integer. This set of vectors is indicated as parallel lines in

Figure 2.16 (c) and their projection onto the graphene band dispersion determines the allowed states in CNTs. If the parallel lines cut through the Fermi point K or K0_{, the resulting tubes}

exhi-bit a finite DOS at the Fermi level, thus behave like a metal. In all other cases, a band gap forms and the tubes behave like semiconductors. Since this dependence is solely governed by the tube chirality, the indices (n, m) can be used to determine the CNT electronic character as follows:

(n − m)mod3 =    0 : met al l i c el se : semi cond uc t i ng (2.11)

Statistically, a third of CNTs is hence metallic and two thirds are semiconducting (indicated by coloured circles in Figure 2.15 (a)). Consequently, all armchair tubes are metallic. To be precise, if curvature effects are taken into account (which are neglected in the zone folding approxima-tion), a small secondary band gap opens for non-armchair tubes and scales inversely with the tube diameter.[92,93]