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Eindhoven University of Technology

MASTER

Photochemical stability of polymers used in LEDs

Romme, S.T.R.

Award date:1995

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Technische Universiteit tû) Eindhoven

University of Technology

Address: Den Dolech 2

P.O.Box 513 5600 MB Eindhoven The Netherlands

Telephone: +40 473110 Telefax: +40 464151

Faculty of Applied Physics

Title: Photochemical Stability of Polymers used in LEDs

Author: S.T.R. Romme

Date: September 28, 1995

Supervisors TUE: Prof. Dr. J.H. Wolter Dr. J.E.M. Haverkort

Supervisor Philips: Dr. A.J.M. Berntsen

This study was carried out at the Philips Research Laboratories. I would like to thank everybody whomade my work experience possible and pleasant. I am grateful for the advice and guidance of Arjan Berntsen and want to thank him for bis time, elfort and helpful comments.

PHILIPS

Abstract

Semiconducting polymers, such as poly(p-phenylene vinylene) or PPV can be used as the active element in a light-emitting diode. The main problem for PPV based EL devices is their short operating lifetimes, due to oxidation of the metal cantacts and polymer. This report is focussed on the photochemical stability of polymers, that can be used as light emitting layer in LEDs.

We present a method to measure the quanturn yield for bleaching,1, which is defined as the number of morromers that are "destructed" by the absorption of a photon. For OC1 C10

PPV in air, 1 = 1.1 · 10-4 .

A fourfold increase in 1 is observed, if the temperature is raised from 20 to 80°C. This shows that it is important to achieve a high EL efficiency in order to reduce heating of the device by dissipating power.

Emission over the whole of the visible spectrum is possible by varying the side chains attached to the PPV. Cyano-PPVs were produced with their luminescence peaks varying from 400 to 750 nm. For a shorter emission wavelength a higher 1 and hence a lower stability is observed.

The quanturn yield for bleaching is highly dependent on the oxygen concentration in the polymer. An encapsulated sample, which is prepared in a nitrogen environment with approximately 5 ppm oxygen, is 1000 times more stabie than a sample measured in air.

The photodegradation mechanism appears to be due to the formation, by an energy transfer reaction from the triplet state of the polymer, of singlet oxygen, which oxidizes the polymer.

We predict the lifetime of the polymer in a 1 cm 2 LED with a brightness comparable to a computer screen (100 cdfm2

) and an emitting area of 1 cm2, using results of the absorption

and photoluminescence measurements. The lifetime for an unencapsulated device (in air) is approximately 6 hours, whereas the lifetime for an encapsulated device (without oxygen) is approximately 5700 hours.

11 Photochemical Stability of Polymers used in LEDs

---- -----------------

Contents

1 Introduetion 2 Theory

2.1 Conducting polymers . . . . . . . . 2.1.1 Band theory . . . . . . . . . 2.1.2 Conductivity models for conjugated polymers 2.1.3 Doping . . . . .

2.2 The poly-LED ..... 2.3 Organic luminescence .

3 Photochemical stability 3.1 Quanturn yield for bleaching 3.2 Experimental setup . . . .

4 Results & discussion 4.1 Determination of 1 . . . . . . . . ..... . 4.2 Parameter dependency of 1 . . . . . . . . . . . .

4.2.1 The bleaching power density dependenee of 1 4.2.2 Wavelength dependency of 1 . . . . . . . . 4.2.3 The influence of bleaching on the photoluminescence 4.2.4 Temperature dependenee of 1

4.3 0 2 concentratien dependency of 1 . . . . . . . . . . 4.4 The photochemical oxidation reaction . . . . . . .

4.4.1 Photochemical degradation reaction mechanism 4.5 Cyano PPVs; the relation between colour and stability 4.6 Polymer degradation in an EL device . . . . . .

5 Conclusions A Derivation of 1

B The transmission vs bleaching time curve

lll

1

3 3 3 4 6 7

10

15 15 17

19 19 22 22 23 25 27 27 32 34 36 38

43

47 51

IV Photochemical Stability of Polymers used in LEDs

-~ --~~-~-------------------,

Chapter 1

Intrad uction

Light-emitting diodes based on conjugated polymers

There has been long-standing interest in the development of solicl-state light-emitting devices. Inorganic semiconductors, such as GaAs1-xP x [1], can be used in light-emitting diodes (LEDs). However they are too expensive for application in large-area displays.

The discovery by the Cambridge group in 1990 that poly(p-phenylene vinylene) or PPV can be used as the active element in a LED [15], led to a considerable amount of research on polymerie light emitters. PPV is a conjugated polymer, which means that its backbone has alternating double bonds. This gives the excited states the ability to move along the polymer chain and allows the conduction of electrical charge. The main advantage of polymerie semiconductors is that they can be deposited inexpensively over a large area.

A simple polymer LED consists of a metal contact deposited on top of a PPV film, which is spincoated on a glass substrate covered with a indium-tin-oxide (ITO) electrode.

The ITO contact is the positive hole-injecting contact and is chosen because of its high work function combined with being transparent for light emission. For the top electrode, which is the negative electron-injecting contact, a metal with a low work function (e.g. calcium) is used. In these devices, the injected positive and negative charges move through the conjugated polymer under the influence of the applied electric field. The opposite charges charge carriers may recombine and produce a photon.

The external quanturn efficiency is measured in photons emitted per injected electron. In 1992 Braun and Heeger [2] reported an external quanturn efficiency of 1.0% for ITO/MEH-PPV /Ca devices. MEH-PPV consistsof a PPV backbone with polar sidechains attached to it, in order to make the polymer soluble in organic solvents. The use of multilayer devices has resulted in even higher external efficiencies. Today, the highest efficiency fora polymer LED is~ 4% [4, 6].

By varying the attached side chains on the PPV backbone, not only the solubility but also the bandgap can be altered. It is possible to produce PPV derivatives, with luminescence peaks varying from 400 nm to 750 nm [5, 7]. This means that the colour of the emitted light can be varied over the hole of the visible spectrum, which can be useful for future applications such as flat, large-area full-colour displays. Computer monitors need a brightness level of 100 cd/m2

• Another range of applications can be found in flexible LEDs, when e.g. a poly( ethylene terephthalate) (PET) substrate is used instead of glass.

The main problem to be solved for the polymer-based electroluminescent (EL) devices is their short operating lifetime. The lifetime is measured at constant driving voltage with decreasing light output and defined by:

L(T) = La/2, (1.1)

1


where L0 is the initiallight output and T the lifetime. A typical stress lifetime for a device operated at a constant driving voltage of about 3 V, with initiallight output of 100 cd/m2 ,

is approximately 100 hours. These lifetimes are achieved in a nitrogen environment, where the metal cantacts and

polymer are protected from oxidation. When air stable metal cantacts are used, the main bottleneck will be the stability of the conjugated polymer.

Photochemical degradation seems to be a property inherent to organic materials. When a polymer absorbs light it has a finite probability to be irreversibly converted into other molecules. In everyday life we witness this effect as the bleaching of dyed fabrics in sunlight. Although the photochemical stability of the polymers is very important for the stability of the LEDs, little is known about their degradation mechanisms and rates. The aim of this study is to develop a method todetermine the photochemical stability in order to compare different polymers, which are potential candidates for application in electroluminescent devices.

This report is therefore focussed on mapping the stability of the different polymers. In addition we will determine the influence of different parameters on the stability , such as temperature, emitted colour and oxygen concentration. The use of calcium does not allow the operation of an electroluminescent device in air. For this reason it is difficult to study polymer degradation during device operation. Photoluminescence and absorption measurements will be used to study the dependency of these parameters. The results of this study will be used to predict the lifetime of an electroluminescent device. Photoluminescence is the excitation of luminescence by ultraviolet or visible radiation and is the most useful process for studying the fundamental behaviour of luminescent materials, because it allows the greatest control over the excitation conditions.

This report is organised as follows. After an introduetion on the conductivity of conjugated polymers, the principlesof electroluminescent devices basedon conjugated polymers are discussed. Chapter 2 is concluded with the basics of organic luminescence. Chapter 3 is concerned with the theoretica! and experimental basis, needed to derive a measure for the photochemical stability of polymers. In Section 3.1, we discuss the model that is used to describe the measured transmission versus bleaching time curves. With this model one can determine the stability of the polymers. The experimental setup is explained inSection 3.2.

In the first section of Chapter 4 we will outline how to determine a quanturn yield for bleaching from the measurements. The quanturn yield for bleaching, 1, is defined as the number of morromers that will be "destructed" by the absorption of one photon. In other words: the inverse of the quanturn yield for bleaching (1/1 ), is the average number of photons, a morromer can absorb before it is destructed or bleached.

The dependency of 1 on the bleaching power density, excitation wavelengthand temperature is discussed in Section 4.2. The influence of both the oxygen concentration and emitted colour on the stability are the topics of Sections 4.3 and 4.5, respectively. In Section 4.6 we calculate the lifetime for a polymer in an electroluminescent device based on the results of absorption and photoluminescence measurements. Conclusions are made in Chapter 5.

Chapter 2

Theory

2.1 Conducting polymers

2 .1.1 Band theory

Consicier a simple diatomic molecule derived from two identical atoms that have only one electron available for bonding. The energy levels of these two electrons before and after the formation of the bond are shown in figure 2.1. Bond formation involves the generation

Figure 2.1: Progression from atomie orbitals in two isolated atoms, through the formation of a bonding orbital in a simple diatomic molecule, and the generation of several bonding orbitals in a medium-sized molecule, to the coalescence of orbital energy levels into bands {8]

of two new energy levels, the bonding level, occupied by two electrons, and an unoccupied antibonding level. The larger the number of atoms that are linked together in a molecule or ultrastructure, the greater the number of bonding and antibonding orbitals will be, until the bonding orbitals become crowded together on the energy scale to form a continuurn of energy levels. This is known as a bonding band, or valenee band. In the same manner band formation occurs with the antibonding orbitals, which are described as the conduction band. The two bands are usually separated by an energy gap, known as the band gap. The magnitude of the band gap and the degree to which the valenee band is filled with electrons determine whether the material is an insulator, a semiconductor or a metal.

Electrical conductivity is attributed to the presence of unpaired electrons. An electron that is unpaired (i.e., is the sole occupant of an energy level) is unconstrained by the presence of a spin-paired partner, and is therefore free to flow in the direction indicated by an applied electrical potential.

Insuiators are materials with a filled valenee band and have band gaps that are so wide that electron jumps of this magnitude are virtually prohibited. Most polymers belong to

3

4 Photochemical Stability of Polymers used in 'LEDs

this category. Materials which have a narrower band gap, so that unpaired electrans can be generated by jumping the band gap from the top of the valenee band to the lower levels of the conduction band, after being activated by thermal energy or light, are called semiconductors. Most polymers that are considered semiconductor have skeletal systems with a highly conjugated backbone ( alternating single and double bonds) see figure 2.2. Metals are materials with an unfilled conduction band and a partly filled valenee band. This situation arises because many metallic atoms contain unpaired electrans that do not associate with their neighbours to form covalent bonds. The electrans occupy the lower levels of the valenee band and have ready access to the unfilled higher levels of that same band. Hence, electrans need not surmount a band gap in order to find an energy level appropriate for migration.

" " " __ffL•._lï'l. -'' ~ lïL ' ~.~~,,~.~\f . " .

Polyacetylene

Polyphenylene

Polypy• rele

Polythioph•ne

Polyoniline

Poly(phenylene

vinyl ene)

Figure 2.2: Chemica} structure of the most important conjugated polymers

2.1.2 Conductivity models for conjugated polymers

In a conjugated system every second bond is a double bond. Single bonds are so-called 0'-bonds, double bonds consist of a 0'-and a 1r-bond. What would happen, if we broke all the 1r-bonds? In the case of trans-polyacetylene the chemica! structure would be an one-dimensional lattice with one unbonded electron per lattice site, see bottorn row of figure 2.3 [9]. So we would have the one dimensional analogue of an alkali metal, a onedimensional metal with a half-filled band, see middle row of figure 2.3. In principle such a structure might be expected to be an excellent metallic conductor, even when it is undoped. In practice, however, the described structure is unstable, because of the Peierls theorem, which states that a manatomie one-dimensional metal with a half-filled band is unstable. The electron-phonon interaction willlead to a lattice distortion, doubling the unit celland opening an energy gap at the fermi level, so that the metal transfarms into an insuiator or semiconductor. In the case of polyacetylene a Peierls distartion is just the formation of the conjugated double bonds, indicated in the top row of figure 2.3. A double bond is shorter than a single bond and consequently the repeat unit of a conjugated system contains two carbon atoms, not just one. The change of the electronic band structure caused by the Peierls transition is demonstrated in figure 2.4 [9], where the density of states function N(E) and the dispersion relation E(k) of the electrans is shown schematically 1 • If the

1 A more complete discussion of the origin of the energy gap in polyacetylene can be found in [10, ll].

Chapter 2. Theory 5

H

H

H H H H H

H H

H H H H H H

H H H H H H H

Figure 2.3: Trans-polyacetylene as a one-dimensional analogue of an alkali metal.

Metallic State

Insuiator State

Figure 2.4: Changes in the electronic density of states and in the electron dispersion relation at the Peierls transition

lattice distartion of the Peierls transition is strictly regular, there is a well defined energy gap. If the distartion occurs in a disordered way, there are localized states in the gap. Special types of such localized states in the gap are solitonic and polaronic states. These are typical for polymers with conjugated double honds. Figure 2.5( a) shows an irregularity in the Peierls distortion. Adjacent to either side of the unpaired electron is a bond of equal length, the Peierls transition is locally suppressed. Consequently there is locally no gap, there is a state in the gap. The conjugational defect of figure 2.5( a) is called a solition. Because of symmetry reasans (electron-hole symmetry) the soliton state is exactly at the midgap. Just like a dislocation in a semiconductor it can piek up electrical charges, and there are positively charged, neutral and negatively charged solitons, depending on the accupation of the midgap state (zero, one, or two electrons, respectively).

A polaron can be envisaged as a 'molecule' of two solitons. Because of the interaction between the parts, in the combined entity the degeneracy of the two midgap states is lifted and one is pushed towards the valenee band, the other towards the conduction band, as shown in figure 2.5(b). Solitons can exist only in trans-polyacetylene, because only there is the lattice energy on both sicles of the soliton the same. In all other polymers solitons would separate domains with different lattice energies and therefore the solitons are unstable. Polarons, however, can exist in all kinds of conjugated polymers. The

6 Photochemical Stability of Polymers used in, LEDs

solitonic and polaronic states play an important role for the conductivity of conjugated polymers sirree they serve as hopping sites for the charge carriers .

• •

l_/i!i!JI!llll_ Conduelion Bond /_////_/ /111/l/_ Conduelion Bond

Soliton State Polaron Stotes -·-\l\l\1\1\l1~ NVV\J~ Valenee Bond YJJX/Xlt!JJfJ( Valenee Bond

Figure 2.5: (a) A soliton and (b) a polaron in trans-polyacetylene

2.1.3 Doping

If a sample with some neutral solitons is treated with an oxidizing agent (acceptor doping) the electrans sitting at the solitonic states are the first ones to be removed. So the first step of doping is just charging the solitons. If all neutral solitons have been used, other electrans can only betaken out of the polymer if honds are braken. This inevitably creates new solitons or polarons, as can been seen from figure 2.6 [9]. Thus the main process of

light doping

heavy doping

Figure 2.6: Light doping charges the already existing neutral solitons, heavy doping creates new solitons by breaking chemica] bands.

doping is the generation of localized states in the gap till finally the band structure of the conjugated system is severely destroyed and the polymer behaves like a disordered metal.

If there were no Peierls transition, polyacetylene would have a metallic band structure and the electrical conductivity should occur via electrans in extended states as in ordinary metals. Solitons and polarons suppress the Peierls transition locally, doping creates solitons or polarons. At high doping concentrations the soliton wavefundions will overlap and the Peierls transition is suppressed globally; the energy gap is closed and the semiconductor has turned into a metal. Because of the high doping level the original band is no longer half-filled and therefore the Peierls theorem does not apply.

However, the temperature dependenee of the conductivity of highly doped polyacetylene is not metal-like: upon cooling the conductivity decreases, thus lattice vibrations are not

Chapter 2. Theory 7

an obstacle for electron motion, they help the conductivity. Apparently the electrans are not in extended states, but in localized ones. This localization is due to disorder, caused by the random distribution of the doping ions and the structural defects of the polymer.

The midgap state of a soliton is a localized electronic state with a finite spatial extension. Since the Fermi level is at midgap the electrans can hop from one solitonic state to another. Su et al. [12] suggested that neutral solitons can move as a whole and calculated an effective mass of only 6 me for this motion. Kivelson [13] assumes that the conductivity of lightly doped polyacetylene samples is due to intersoliton hopping: charged solitons are electrostatically trapped by counter-ions, but neutral solitons are mobile. The neutral solitons move along the chains until they come close to charged solitons, then the solitons exchange their charge by hopping. This mechanism has been extended to polarons by Bredas et al. [14]. Charged solitons can only move if they are photogenerated, because in that case they are not electrostatically bound to the counter-ion of a dopant.

Roth et al. [9] believe that there is so much disorder in doped polyacetylene ( and in other conductive polymers) that the idealized conjugational defects, such as solitons and polarons, willlose much of their original charaderistic properties. A model with a constant density of localized states around the Fermi level, as in amorphous semiconductors, based on isotropie variabie-range hopping would be more appropriate. Such adoptions would involve asymmetrie instead of spherically symmetrie wavefundions for the localized states to account for the chain structure of the polymer, a relation between the localization length and the conjugation length (defect concentration, chain ends), a relation between the density of states at the Fermi level and the doping level, and some structure in the density of states function close to the Fermi level (e.g. a minimum or pseudogap caused by the Coulomb interaction). This model of unspecified hopping is not in contradiction to intersoliton or interpolaron hopping, but it allows for the possibility that some, or even many, of the localized states in the gap are due to defects other than solitons and polarons.

2.2 The poly-LED

The simplest polymer LED consists of a metal electrode deposited on top of a polymer film, which is spincoated on a glass substrate covered with a transparent electrode, such as indium-tin-oxide (ITO). Figure 2.7 displays the device geometry [3].

The principle of the device operation of a polymer LED is to inject charges from the electrades into the polymer, where the opposite charge carriers recombine and decay radiatively.

Figure 2.8 shows a schematic band diagram of a single polymer layer EL diode under forward bias. D.E!j is the energy difference between the high work function contact, of work function <1> 1 , and the PPV valenee band. The holes need to surmount this energy difference to be injected. D.Eb is the energy difference between the low work function contact, of work function <1> 2 , and the polymer conduction band and forms the barrier for the electron injection. In order to obtain high device efficiency, it is important that both cantacts form junctions with ( almost) the same harrier heights to allow each charge carrier

8 Photochemical Stability of Polymers used in -LEDs

indium/tin OXIde

poly(p-phenylenevinylene)

~ n

alummium. magnesium or calcium

/ ____ .,.. E.rtemal

Circuit

glass substrate

Figure 2. 7: Schematic structure of a polymer LED formed with a single layer of conjugated polymer

to recombine with an opposite charge carrier. Thus, to get good electron and hole injection 6.E§ and 6.Elf should be low.

Figure 2.8: Schematic band diagram of a single PPV layer EL diode under forward bias. èl.E(f is the energy difference between the high work function contact, of work function <I>1, and the PPV valenee band. èl.E~ is the energy difference between the low work function contact, of work function <I>2, and the polymer conduction band. V is the applied voltage.

The ITO contact is the positive hole-injecting contact and is chosen because of its high work function combined with being transparent for light emission. For the top electrode, which is the negative electron-injecting contact, a metal with a low work function (e.g. thin aluminium) is chosen. In these devices, the injected positive and negative charges move through the conjugated polymer under the influence of the applied electric field. The charges either annihilate one another to form a triplet or a singlet exiton, of which only the singlet may decay radiatively, or they pass through the conjugated polymer layer to the electrode of opposite charge.

For the single polymer layer sandwich structure in fig 2.8 the current is likely to be carried predominantly by holes, because 6.Elf is smaller than 6.E§. These holes, therefore have a high probability of crossing the electroluminescence (EL) layer without recombining with an oppositely charged carrier, thus reducing the device efficiency.

The first Polymer LED was reported by the Cambridge group [15], who used a solutionprocessable precursor polymer, which can be used for spin-coating thin films (50-150 nm)

Chapter 2. Theory 9

on suitable substrates. This precursor PPV neecis a thermal conversion step totransfarm it into PPV, suitable to be used in an electraluminescent device. The Cambridge group used ITO as the hole injecting contact and thin aluminium as the electron injecting electrode and reported a quanturn efficiency of 0.05%.

From measurements on the LED with PPV sandwiched between ITO as high work fundion contact and aluminium as low work fundion contact, the ITO appears to be the more efficient carrier injector. This is the reason why Braun and Heeger [2] suggested to use calcium instead of aluminium for the negative electron-injecting contact because of its lower work function. The quanturn efficiency improved to 1.0%. The use of calcium has the disadvantage that it is highly susceptible to atmospheric degradation and therefore neecis to be encapsulated, which is difficult and presents problems for device stability.

Braun et al. [2] also reported the use of poly(2-methoxy,5-(2'-ethyl-hexoxy )-1,4-phenylenevinylene) or MEH-PPV. MEH-PPV consists of a PPV backbone with poplar sidegroups attached to it. Compared to the unsubstituted PPV it has the advantage of being soluble in the conjugated form in organic solvents and it does not need the thermal conversion step.

The Cambridge group designed a new processible polymer, poly( cyanoterephthalylidene )s or CN-PPV in which the valenee and conduction band lie at lower energies than those of PPV. The electron injection harrier is now lowered, with the consequence that the use of highly unstable calcium with its low work function as electron injector is not longer necessary.

Although the electron-injection harrier is lowered when the CN-PPV is used instead of PPV, the hole-injection harrier at the junction of the ITO contact with the CN-PPV layer is raised. This has led to a double layer structure with PPV as a hole-transporting layer tagether with CN-PPV as the light emission layer. This device had an efficiency of 4% [16].

Similar internal efficiencies were obtained with both calcium and aluminium cantacts as electron injecting electrode deposited on the CN-PPV layer [16]. In a single layer device with PPV, at least a tenfold reduction in efficiency is found in changing the electron injecting contact from calcium to aluminium, as was outlined previously. These results indicate that electron injection is improved by the use of CN-PPV in a double layer structure, with the effect that device efficiency is now determined by processes at the ITO electrode.

The 1r, highest occupied molecular orbital (HOMO) and the 1r*, lowest unoccupied molecular orbital (LUMO) levels of CN-PPV are lowered by 0.6 and 0.9 eV respectively, compared with those of PPV. These offsets are large enough to present a significant harrier at the heterojunction to both electrans and holes at room temperature, leading to charge confinement. A schematic energy level diagram is shown in figure 2.9 [3].

By confining charges at the interface, a large field is established which promotes tunnelling through the harriers. Tunnelling across one or other harrier will allow electron-hole capture and electroluminescence. Tunnelling across the lower of the two harriers, which should be that for holes is expected [3]. This clarifies why CN-PPV is the light-emitting layer. .

The introduetion of the PPV layer causes a significant reduction in the field (drive

10 Photochemical 8tability of Polymers used i~ LEDs

Ca

DPV CN-PPV

Figure 2.9: Schematic energy-level diagramfora bilayer device under forward bias. The positions of the Fermi energies for the electrode metals with respect to the 1r,

highest occupied molecular orbital (HOMO) and the 1r*, lowest unoccupied molecular orbital (LUMO) levels are indicated. The differences in electron affinity (óEA) and ionization potential (óiP) between PPV and CN-PPV are shown.

voltage/tatal polymer layer thickness) required to drive the device, which indicates that charge injection really is improved in the bilayer structure [16].

2.3 Organic luminescence

Luminescence can take on a variety of forms such as photoluminescence, cathodoluminescence, radioluminescence, thermoluminescence and electroluminescence. These different farms are concerned with the excitation processes which occur prior to the emission of luminescence. The emission processes are unaffected by these different forms of excitation unless secondary effects such as radiation damage or chemical decomposition occur. Photoluminescence, the excitation of luminescence by ultraviolet or visible radiation, is the most useful process for studying the fundamental behaviour of luminescent materials because it allows the greatest control over the excitation conditions. Photoluminescence will therefore be used to predict the behaviour of the materials in case of electroluminescence.

What is the di:fference between organic and inorganic luminescence? The answer lies in the molecular and atomie structure of these materials. Inorganic materials are held tagether by ionic or covalent bands between individual atoms, and thus can be regarcled as "atomie" solids. Organic materials are held tagether by van der Waals farces between molecules and are therefore molecular solids. The consequence of this molecular nature is that the luminescence processes in organic materials are associated with the excited states of molecules, whereas in inorganic materials the luminescence spectra are associated with either defects or impurities in the atomie lattice or with the excited states of the isolated atom or ion.

The electronic properties of an organic solid are governed by the electronic transitions which can take place in the molecule. These electronic transitionsin turn are characterized by the spectroscopie properties of the system. A molecule in its ground electronic state will normally contain two electrans of equal and opposite spin in each filled molecular orbital. An electronic state of this configuration is called a ground singlet state. When a molecule

Chapter 2. Theory 11

in its ground singlet state absorbs a photon, one of the electrans in the highest filled 1r

molecular orbital is raised to the lowest unfilled 1r molecular orbital. Usually, the spin of the elevated electron is still opposite to the spin of the remairring electron and the new electronic state is called an excited singlet state. If the spin of the elevated electron is reversed and now parallel to the remairring electron, then the new electronic state is called an excited triplet.

Once an electronic transition has taken place the molecule strives to get rid of the excess energy andreturn to its ground electronic state. The Jablonski diagram of figure 2.10 shows the main processes responsible for the dissipation of the excess energy [17]. These processes compete with each other and the relative magnitude of the rate constants determines the contribution made by a particular pathway. Vibrational Relaxation is the conversion of

ISC ISC -

lf" \ r.

Thermol oCtlvOtlon

~

Figure 2.10/~ablonski diagram showing some of the radiative and non-radiative processes available to molecules (VR=vibrational relaxation; IC=internal conversion; ISC=intersystem crossing)

the excess vibrational energy into kinetic energy. It is such a rapid process that emission almast exclusively occurs from the v' = 0 level.

In a radiative transition an exited species passes from a higher excited state to a lower one with theemission of a photon. Fluorescence is caused by a radiative transition between states of the same multiplicity, usually S1 --+ Sa, and it is a rapid process. Phosphorescence is the result of a transition between states of different multiplicity, typically T1 --+ Sa. The process, being spin forbidden, is a much slower process.

Radiationless transitions occur between isoenergetic ( or degenerate) vibrational-rotational levels of different electronic states. Sirree there is no change in the total energy of the system, no photon is emitted. Internal Conversion is a radiationless transition between isoenergetic statesof the same multiplicity. Such transitions between upper states are extremely rapid, accounting for the negligible emission from upper states. Internal conversion from the first excited singlet state (S1 --+ Sa) is so much slower that fluorescence can compete. lntersystem Crossing is a radiationless transition between states of different multiplicity. The intersystem crossing S1 --+ T1 , which is competitive with fluorescence, is the process by which the triplet states are normally populated.

In order togainsome feeling for the relative magnitude of the rate constants for possible

12 Photochemical Stability of Polymers used i~ LEDs

transitions, we will give a summary of experimental data on large molecules in dense media and try to explain these experimentallaws.

The basic data emerging from many measurements [17] is:

• Emission almast invariable occurs from S1 or T1 , independent of the state which is initially excited (Kasha's Rule).

• The quanturn yield of fiuorescence, <I> f < 1.

• <Ph does notdepend on which state (S1 , S2 , S3 etc.) is first excited (Vavilov's Law). This implies that very rapid internal conversion occurs between upper excited singlet states.

• The rates of radiationless transitions fall very rapidly with increase in the energy difference between the v = 0 levels of the states concerned. They conform to an "energy gap law".

Radiationless transitions can be visualized by thinking of the intersection of potential energy surfaces. A molecule on the potential energy surface corresponding to state 1 "crosses" at the point of intersection (X) with the potential energy surface associated with state 2, as depicted in figure 2.11 [17]. At the internuclear separation corresponding to the

State 1 State 2

C( \D E {

·~ jL ~ ~\ I

z "-...__/

Internuclear disrance

Figure 2.11: The Franck-Condon principle and radiationless transitions

intersection, state 1 and state 2 have the same energy and the same internuclear distance. A molecule in the level AX of state 1, when it arrives at X, has merely to change the quanturn numbers of one of its electrans ( to rearrange the motion of one of its electrous) to be in state 2 in the level BX. Since X is a turning point in the vibrations A-X-A and B-X-B, where the kinetic energy is zero and the system is momentarily at rest, more time is available at this point than at any other for the system to cross from one state to the other. The process becomes irreversible if the molecule having arrived in the level BX undergoes rapid vibrational relaxation.

Now consider the possibility of a radiationless transition from level CE. If the transition occurs when the molecule is at the turning point E, then virtually instantaneously there must either be a change in the nuclear coordinates to arrive at points D or F in state 2, or a change in kinetic energy ( equal to EZ) if the internuclear separation remains constant.

Chapter 2. Theory 13

Similar arguments apply to transition at ( say) point D. Such rapid changes are forbidden by the Franck-Condon principle, which applies equally to radiative and non-radiative processes.

From quanturn mechanical treatments we know that the rates of absorption and spontaneous emission, and therefore intensities, of electronic transitions from the initial to the :final state are proportional to the square of the transition moment (T.M.), see eq. 2.1.

T.M. =< wi I tt I w 1 > (2.1)

where W is the total wavefunction (nuclear and electronic) for the system and tt is the dipole moment operator. This integral cannot be evaluated, because we do not know the exact form of the wavefunctions of any molecule. In order to make progress the BornOppenheimer approximation is introduced [17].

Because of the mass difference, nuclear motion is very sluggish in comparison with electronic motion, so that the electrans may be thought of as moving in the potential field of the static nuclei. The Born-Oppenheimer approximation factorizes the total wavefunction into a nuclear ( vibrational) wavefunction () and an electronic wavefunction 'Ij;. Thus W = ()·'Ij;. The electronic wavefunction, 'Ij;, is assumed to be the product of one-electron wavefunctions ( orbitals), cp. The fin al approximation is that these orbitals can in turn be factorized into a product of space and spin orbitals ( cp = <p · S), which yields:

(2.2)

The :first term is the overlap integral of the wavefunctions for nuclear vibrations, which is a quanturn mechanical formulation of the Franck-Condon principle. The second term is a spin overlap integral, and its value depends on the initial and :final spin states of the promoted electron. The third term is called the electronic transition moment, and its value depends on the symmetries and amount of overlap of the initial and :final spatial orbitals.

If ()1 and ()2 are vibrational wavefunctions associated with two electronic states, 1 and 2, then the probability of crossing between the states is proportional to J ()1 ()2drn. Compare the situation of :figure 2.12, where the zero-point levels of the two states have approximately the same energy, with that of :figure 2.13, where there is a considerable gap between the zero-point levels. It is apparent that the vibrational overlap integral at the crossing point

State 1

... V'1 V' 1 ~ .. c w

v,o v'o

Figure 2.12: Large vibrational overlap at the crossing point of approximately degenerate electronic states.


Slole 2

Figure 2.13: Vibrational overlap with a large energy difference between the states.

in figure 2.12 is of significant dimensions, but because of the rapidly oscillating character of the v = n function in figure 2.13, the positive contributions to the vibrational overlap integral are largely cancelled by the negative contributions, so that the integral is very small.

There is an inverse correlation between the rates of non-radiative transitions invalving in the lowest states of similar molecules and the difference in energy between the v = 0 levels of the states involved. In other words, the smaller the energy gap the bigger the rate. This correlation can be understood by noting that as the gap increases the radiationless transition from a given level of state 1 will be to an increasingly high vibrationallevel of state 2, with reduced vibrational overlap and a correspondingly reduced rate constant.

The energy gap law can provide a simple rationalization of Kasha's Rule and Vavilov's Law. Since upper excited states are densely packed, i.e. since the energy gaps between them are small, internal conversions between them will be very rapid, so that the internal conversion rate constant is much larger than the rate constant for fiuorescence and therefore their fiuorescence will not be observed. However, the energy difference between 50 and 51

or T1 is much larger, and radiationless depopulation of 51 or T1 will in many cases be unable to quench emission from these states.

Chapter 3

Photochemical stability

Photochemical degradation seems to be a property inherent to organic materials, when a polymer absorbs light it has a finite probability to be irreversibly converted into other molecules. In everyday life we witness this effect as the bleaching of dyed fabrics in sunlight. Generally speaking it is a one photon absorption process, that generates directly or indirectly a reactive excited state. This exited state reacts with other polymer molecules or, most likely, with impurities like oxygen, resulting in the destructien of the molecule.

3.1 Quanturn yield for bleaching

In order to compare the stability of different polymers we need to find a measure for the stability. If a well defined method to determine the photochemical stability is derived, we are able to predict a lifetime for the polymer when it is used as emissive layer in an electraluminescent device as well. Photoluminescence and absorption measurements are used to determine the stability.

Under given conditions like temperature and impurity concentration, the photo-oxidation process can be described by a quanturn yield for bleaching, 1, being the inverse of the average number of photons a molecule can absorb before being destructed[18].

An elegant method to measure this quanturn yield for bleaching was proposed by Boyarskii et al. [19]. This method consists of irradiating the material and monitoring the increase of the transmission of this bleaching beam as more and more molecules are destructed. This process can be analytically modelled [18] and the experimental result fitted to the theory, the only adjustable parameter being I·

We consider the irradiation with a Gaussian laser beam along the z-axis of a polymer film of thickness d. The local absorption coe:fficient is determined by the local concentratien of intact molecules N(z, r, t).

a(z, r, t) =u N(z, r, t) (3.1)

where u is the absorption cross-section. N(z, r, t) decreases in time through photochemical bleaching as given by Eq. 3.2.

8N(z, r, t) ( ) ( ) at = -1a z,r,t I z,r,t (3.2)

where 1 is the quanturn yield for bleaching, whereas the decrease in excitation intensity along the propagation direction, z, is given by Eq. 3.3.

8I(z,r,t) ( ) ( ) az = -a z, r, t I z, r, t (3.3)

15

'


In the above Eqs. it is assumed that the molecules resulting from the degradation process do not absorb at the laser wavelength, as will be shown in Section 4.2.3. The boundary conditions are:

I(z = O,r,t) = Iaexp(-r2 /r5) = Io(r) (3.4)

where r 0 is the Gaussian beam waist and

N(z, r, t = 0) = No (3.5)

This set of coupled partial differential equations can be solved by substituting

w(z, t) = 1t I(z, t')dt' (3.6)

which yields 1:

(3.7)

Solving w through standard integration and reeavering I(z, t) through partial differentiatien yields:

I(z r t) = Io(r) ' ' 1 + ( eoNoz _ 1 )e-u"Ylo(r)t (3.8)

The time dependenee of the transmission through the film is given by:

_ 27r j 0

00

rl(z = d, r, t)dr _ T00 { Toeu"Ylot + T00 - T0 } T(t)- roo Too- -ln

27r Jo rl(z = O,r,t)dr trylot T00

(3.9)

where Too is the transmission after infinitely long bleaching and T0 = T00 e-a-Nd is the initial transmission.

1 For a more complete derivation see appendix A.

Chaptèr 3. Photochemical stability 17

3.2 Experimental setup

Figure 3.1 shows the experimental setup. A Kr laser, which can produce light of different wavelengths, is used. The laser beam is guided by mirrors via an attenuation filter to a vacuum chamber, where the sample is placed. The transmitted and refl.ected light intensities are measured by the transmission and reflection diode, respectively. In order to monitor the bleaching intensity of the laser beam, 10% of the power is defl.ected, measured by the reference photodiode and used as reference signal. The signals from the fotodiodes are measured with picoampèremeters and the data is stored by a computer.

turbo pump

Transmission diode Raferenee diode

Kr laser

Figure 3.1: The experimental setup for measuring the transmitted and reflected signa] of the Kr laser. The EL signal, PL signa] and PL spectrum of the polymer layer can be monitored toa. The turbomolecular pump is used to measure these signals for with different ( oxygen) pressures.

The photoluminescence signalis focussed on the PL diode. The fiber, which is connected to a spectrometer is used to measure the PL spectrum of the sample. In order to monitor the EL and PL signal at the same time, the laser light is chopped and a lock-in amplifier is used -to seperate the PL signal from the approximately 1000 times higher EL signal. The Xe lamp can be used as alternative light source.

The vacuum chamber is connected toa turbo pump and can he pumped toa pressure below 10-6 mbar. The valves are used fortheinlet of oxygen, nitrogen or air.

Chapter 4

Results & discussion

4.1 Determination of '"Y

The dashed line in Figure 4.2 shows the measured transmission versus time for a 300 nm thick, 88% conjugated OC1 C10 PPV film on a glass substrate. lrradiation was performed in air using the 482 nm line of a Kr ion laser. The chemical structure of OC1 C10 PPV is given in figure 4.1.

O~H3 n = 0.88 ; m = 0.1

Figure 4.1: Cbemical structure of 88% conjugated OC1 C10

The transmission in Figure 4.2 increases strongly in the first 200 s, which is caused by the photobleaching of the polymer, and saturates for longer bleaching times. The reflection is almost constant in time (R = 0.11).

If we consider the transmission for longer bleaching times and add the constant reflecti"on, we conclude that almost 10% of the impinging laser light is not transmitted, nor reflected. This 10% is due to absorption and diffuse reflection. The number of photons that is still being absorbed for longer bleaching times is not due to absorption by the bleached polymer, but to the absorption by non-bleached morromers at the edges of the laser beam. At the edges of the Gaussian profile of the laser beam, the power density is lower and therefore the bleaching of the polymer will take longer time 1

.

In order to determine the quanturn yield for bleaching, /, the experimental result is fitted to theory. We consider the result of the analytical derivation of the time dependenee of the transmission through the film:

T(t) = Too ln { Toerrylot + Too- Ta} rJ'floi Too

( 4.1)

1 Fora complete explanation for the shape of the transmission curve versus bleaching time of Figure 4.2, see appendix A.

19


1.0 ---r----------------,

0.8

0.6 c 0 ëii (/)

·~ 0.4 c ctS

~ 0.2

0.0

·········· Measured data - Fitledcurve

Y= 1.14 a·•

0 200 400 600 800 1 000 1200 1400

Time (s)

Figure 4.2: Measured and fitted transmission curve versus irradiation time for a 300 nm thick, 88% conjugated OC1 C1o PPV film on a glass substrate. The dasbed line shows the measured transmission and the solid line represents the fitted curve, according to Eq. 4.1. Irradiation was performed in air using the 482 nm line of a Kr ion laser. The quanturn yield for bleaching is 1.1 1 o-4 .

where T0 is the initial transmission, T 00 is the transmission after infinitely long bleaching, 0' is the absorption cross-section and Io is the intensity of the laserbeam impinging on the polymer film.

T0 and Too can be measured directly. We use Eq. 3.1 todetermine 0'. Befare bleaching (at timet = 0) 0' = a 0 / N0 • The initial absorption coefficient, a 0 , is polymer and wavelength dependent. By measuring T0 for different film thicknesses a 0 can be calculated using

To = Too exp( -aod). (4.2)

Figure 4.3 shows the initial transmission versus the thickness of the polymer layer for different wavelengths. The circles represent the measured transmission for different OC1 C10

PPV layer thicknesses, measured with À = 482 nm. The squares show the measured data with À = 568 nm. The solid lines in Figure 4.3 are fitted to the measured data points. The initial absorption coefficient for À = 482 nm is 9.1 · 104 cm-1 and for À = 568 nm, a 0

= 1.6 · 104cm-1. The initial concentration of intact monomers, N0 , of the polymer can be

calculated from the density, p, and the mass of a monomer, M. The most intensely stuclied polymer is 88% conjugated alkoxy-PPV, or OC1 C10 . Ta

bie 4.1 displays the used material characteristics to determine 1's for OC1 C10 . The absorption is given for À= 482 nm.

The laser beam has a Gaussian profile as is described in Eq. 3.4. In order to obtain a value for 10 [photonsfcm2 s] we measure the total power of the laser beam, which is related to I~ [joulefcm2s] and the Gaussian beam waist, r0 •

( 4.3)

Chapter 4. Results & discussion 21

1e+O

1e·1

• • À.=482nm

1e-3 • À.•568nm •

1e-4

200 400 600 800 1 000 1200

Thickness layer (nm)

Figure 4.3: The initia] transmission versus the thickness of the polymer layer for different wavelengths. The circles represent the measured transmission for different OC1 C10 PPV layer thicknesses, measured with À = 482 nm. The squares show the measured data with À = 568 nm. The solid lines in ligure 4.3 are fitted to the measured data points. The initia] absorption coeflicient for À = 482 nm is 9.1 · 104cm- 1 and for À= 568 nm, ao = 1.6 · 104cm- 1 .

Table 4.1: Material characteristics for OC1 C10. pis the density of the polymer, Mis the mass of a monomer, No is the initial concentration of intact monomers, a 0 is the initia] absorption coeflicient and (j is the absorption cross-section.

p[gjcm3] M [gjmon.] N0 [mon.jcm3

] a0 [cm 1] o-(cm2

]

oc1 c10 0.71 4.8 10 22 1.47 1021 9.1 104 6.2 10 17

The Gaussian beam waist is measured by scanning a pinhole through the laser beam. The measured data is then fitted to a Gaussian profile, see Figure 4.4. The desired value for 10 [photonsjcm2 s] is now obtained by dividing J~ by the photon energy of the particular wavel€mgth.

The only parameter left to be fitted in Eq. 4.1 is 1- Figure 4.2 shows the measured transmission curve versus irradiation time together with the curve fitted with Eq. 4.1. From figure 4.2 we see that Eq. 4.1 provides an excellent theoretica! frame to adjust the curve to the measured values by fitting only one parameter, /·

The resulting value for the quanturn yield for bleaching for OC1 C10 PPV with a laser wavelength of 482 nm is 1.1 · 10-4 • This means that the average OC1 C10 PPV morromer can absorb, 1/i = 9.1 · 103 , photons before it is bleached. The quantity, /, can be used to predict a lifetime for the polymer when it is used in an electroluminescent device. This is the subject of Section 4.6.

22 Photochemical Stability of Polymers used iil. LEDs

1.2.--------------,

1.0

0.8

.~ i 0.6

.S "i 0.4 .lj

~ g 0.2

0.0

-0.2 Beem walat, r0 • 0.36 nvn

-1.5 -1.0 ..0.5 0.0 0.5 1.0 1.5

beam width (mm)

Figure 4.4: The intensity profile of the laser beam. The datapoints are measured by scanning a pinhole through the laser beam. The solid line is a fit to the data using Eq. 3.4. The fitted beamwaist, r 0 , is 0.36 mm.

4.2 Parameter dependency of 1

In order to know whether the methad to fit a quanturn yield for bleaching is reproducible and can be used with different power densities, the dependenee of the fitted 1 on several parameters, such as power density, excitation wavelength and temperature is concerned.

4.2.1 The bleaching power density dependenee of r In order to know whether 1 depends on the power density, we determined 1 for different attenuation factors of the laser beam, keeping the beamwaist constant.

Figure 4.5 shows the measured transmission vs time and fitted curve for a 300 nm thick layer oei CIO, bleached with a laser beam of 482 nm with different laser intensities. The power density of the laser beam without attenuation filter is 9.5 Wj cm2

, the power density with a 90% attenuation filter is 0.95 Wjcm 2

. Curve (AI) and (A2 ) are measured without filter, while curve (B) is measured with the 90% attenuation filter. The fitted 1's for curve (AI) and (A2 ) without filter are respectively 1.3.10-4 and 1.2.10-4, which shows that the measurements are reproducible within an error of 20% The fitted 1 for curve (B) with filter is 1.1.10-4, from which we conclude that the 1 is not very dependent on the different bleaching power densities. In order to find out whether the 1 will become dependent on the bleaching power density, if a wider range of power densities is used, the 1's for samples with different thicknesses are determined over power density range of almost 3 orders of magnitude.

Figure 4.6 shows the fitted 1's for OCI C10 PPV samples with thicknesses of 300 nm and 100 nm over power density range of from 158 mWjcm2 to 55.2 Wjcm 2

• In Figure 4.6 we see that in the range of 158 mWjcm2 to 20 Wjcm 2 the fitted 1's are almost independent of the power density of the bleaching laser beam.

A different way to alter the power density is to change the beam waist of the laser, while keeping the total power constant. No dependenee of the beam waist for 1 was found.

From this section we conclude that the proposed method to determine a measure for

Chàpter 4. Results & discussion 23

1.0..--------------,

0.8

0.6

0.4

0.2

0.0

A 12 = Bleaching without filter gamma,.J .. 1.3 a-4 rasp. 1.2 &-4

B = Bleachlng with 10 % lilter gamma,. 1.1 e-4

OC1C10; gl = 482 nm; d = 300 nm

0 1 000 2000 3000 4000 5000 6000 7000

nme(s)

Figure 4.5: Transmission VS time of a ocl clO sample of 300 nm thickness. The power density of the laser beam without attenuation filter is 9.5 Wjcm2 , the power density with a 90% attenuation filter is 0.95 Wjcm 2 . Curve (A 1) and (A2 ) are measured without filter, while curve (B) is measured with the 90% attenuation filter. The fitted 1's for curve (A 1 ) and (A 2 ) without filter are 1.3.10-4 and 1.2.10-\respectively. The fitted 1 for curve (B) with filter is 1.1.10-4 .

the stability by fitting the transmission curve versus time to Eq. 4.1 is reproducible, and reliable over a wide range of bleaching power densities, within a marge of approximately 20%. Because the stability of different polymers, in terms of 1, differs several orders of magnitude2 , an error of 20% is relatively small if we want to cernpare the different polymers.

The fact that the determined quanturn yield for bleaching is nearly independent of the bleaching power density over a wide range, provides a fast method to determine the photochemical stability. If the 1 would he dependent on the bleaching power density it could not serve as a method to predict the lifetime of the polymer subjected to relatively low bleaching power densities, such as sunlight.

4.2.2 Wavelength dependency of '"Y

In the previous sections, the 1 is determined for only one laser wavelength ("\ = 482 nm). If 1 is very dependent on the wavelengthof the excitation source, a fair comparison of the stability with other polymers, with a slightly different absorption spectrum 3 , would be impossible.

In order to determine the bleaching wavelength dependency of I' the oei elO sample is bleached with 3 different laser wavelengths. Figure 4. 7 shows the absorption spectrum of 88% conjugated oei e10. Bleaching experiments were performed using wavelengths of 355, 482 and 568 nm, which corresponds to absorption in the high-energy region, the centre of the peak and the low-energy tail, respectively ( see Figure 4. 7). For these three wavelengths

2The 1's of different polymers are determined in section 4.5. 3see section 4.5.


1e·3 .,--------------.,

• >- 1e-4

A d = 300 nm • d=100nm

•

1e·5 +-----..----,----~

0.1 10

Power density (W/cm2)

100

Figure 4.6: 1 vs power density of the bleaching laser beam; dis the thickness of the OC1 C1o PPV film. The circles refer to a layer thickness of 100 nm and the triangles represent a layer thickness of 300 nm.

the initial absorption coefficient and /, determined from a fit to the measured data, are given in Table 4.2. The quanturn yield for bleaching, 1, is fitted, for the different laser

........ ·· Absorption spectrum

2.09+5

1.59+5

1.09+5

5.09+4

0.09+0 ............ ~....._.~....._"~......._.~ ........... ~·-"' ...... ·= ................ 100 200 300 400 500 600 700

wav919nght (nm)

Figure 4. 7: The absorption spectrum of 88% conjugated OC1 C10 .

wavelengths with their absorption coefficient, to the transmission curves (see table 4.2). From Table 4.2 we conclude that the quanturn yield for bleaching for OC1 C10 is independent of the laser wavelength. This can be explained by the fact that the different excitation wavelengths all are situated in the same absorption peak in the case of OC1 C10 PPV. The vibrational relaxation to the state with lowest vibrational energy is a very fast process, which makes it difficult for another process, such as degradation, to compete with.


Table 4.2: Absorption coefflcients and 1 's for different laser wavelengtbs

À=355 nm À=482 nm À=568 nm ao ( cm-1

) 4.1 104 9.1 104 1.55 104

I 1.5 10-4 1.1 10-4 1.2 10-4

4.2.3 The influence of bleaching on the photoluminescence

PL spectra were measured while the polymer was being bleached, in order todetermine the influence of bleaching on the PL spectrum. Each PL spectrum in Figure 4.8 corresponds to a different bleaching time. The more morromers are bleached monomers, the lower the intensity of the PL spectra. The inset of Figure 4.8 shows the normalized PL spectra of the highest and the lowest intensity. It is clear from the normalized PL spectra of the intact

400

1.0 350

,0.8 . 0.6

300 1 0.4

1 0.2 Jk 250 0.0 ..

.l!l 200 400 600 800 1000 c:

::s wavelength (nm) 0 0

150

100

50

0

500 600 700 800 900

wavelength (nm)

Figure 4.8: PL spectra of OC1 C10 PPV with different bleaching times; the inset shows the highest and lowest PL signa] normalized.

and the bleached polymer that the photoluminescence spectrum of OC1 C10 PPV does not change significantly by bleaching.

If we measure the total PL signal and number of absorbed photons, we can calculate the PL efficiency. The total PL signal and PL efficiency of OC1 C10 PPV versus bleaching time are depicted in Figure 4.9. From Figure 4.9 it is clear that the PL efficiency is almost independent of the amount of bleached monomers.

We can explain these results by looking at the excited states that move along the polymer chain. According to Bässler[21], the energy difference between the fluorescence and absorption peak (Stokes shift) can be explained by a self-localized and a non-localized effect. The polymer is visualized as an array of chain segments, each with an effective conjugation length, Lef f, much less than the tot al chain length. In polymers, rapid energy transfer is likely to occur along a chain segment or between adjacent chain segments. The


- PL effic1ency -4- Plsignal

0 200 400 600 800 1 000 1200 1400

time (s)

Figure 4.9: The PL signa] and efficiency VS bleaching time of ocl Clo PPV.

excitations execute a random walk among the chain segments relaxing in energy, until they reach a segment from which they cannot escape within their lifetime. This is the non-localized effect, which explains the Stokes shift.

The energy transfer from the lowest energy segments is blocked on energetic grounds, because there are very few nearby sites of lower energy to transfer to. The only mechanism to reduce its energy without energy transport is the formation of self-localized 'polaronexciton' stateé.

The distance that the excited state can travel along the polymer chain is dependent on the level of disorder and conjugation length. From the result that the bleached and un-bleached polymer give PL spectra without an energy shift we conclude that the distanee an excited state can travel is shorter than the distance between consecutive bleached monomers.

The bleached morromers seem not to reduce the distance an excited state can travel, because if the travelling excited state encounters a harrier wall, formed by a distartion due to bleaching, it cannot pass andreduce its energy, but will decay resulting in a blue shifted PL spectrum.

If the bleached morromers are quenching sites for the photoluminescence, the PL efficiency would decreases during the bleaching process, when more excited states encounter a quenching site, formed due to bleaching.

We found that the PL efficiency for OC1 C10 PPV is constant u pon bleaching. The condusion that can be drawn is that a bleached morromer does not act as an active quenching site of excited states5 . This means that an excited state does not "notice" the preserree of bleached morromers and the PL spectrum and efficiency will therefore be the same. Yan [20] on the other hand, suggested that their decreasing PL efficiency can be explained by exciton diffusion to quenching centres, formed by the bleaching process.

4 See section 2.1. 5See also section 4.4.

Cha:pter 4. Results & discussion 27

4.2.4 Temperature dependenee of 1

The temperature dependenee of the quanturn yield for bleaching is important, because in an electraluminescent device, which dissipates power, the temperature rises. If Î is increasing strongly with temperature it has an important influence on the lifetime.

The empirica! variation of the rate constant can be expressed in the Arrhenius form:

Î = Aexp( -Ea/kT) ( 4.4)

where Ea is the activation energy. If we have a value for Îl and T1 we can predict the 12

for a certain temperature, T2 , by: (4.5)

Figure 4.10 shows the measured relation between 1 and temperature of a OC1 C10 PPV sample with a film thickness of 100 nm, tagether with the curve of Eq. 4.5. From Figure 4.10

1e-3

1e·4

300

• measured data

- Y,:Y,T11T2

350

Temperature (K)

400

Figure 4.10: 1 versus temperature.

we can see that the 1 of OC1 C10 PPV for an elevated temperature of 80° C is approximately 4 times higher than the Î for room temperature. This makes clear that it is important to achieve a high EL efficiency in order to reduce heating of the device by dissipating power.

4.3 0 2 concentration dependency of ry

The measurements presented in the previous sections were performed in air. It is well known that most photochemical degradation reactions are influenced by the preserree of oxygen. This section is therefore focussed on the question of the degradation reaction of OC1 C10 PPV is governed by oxygen as well.

In order to test if the preserree of oxygen affects the bleaching rates of OC1 C10 PPV, encapsulated samples were measured. The encapsulation exists of a transparent epoxy layer covered with a glass top-substrate onto the polymer layer, which itself if spincoated


on a glass bottom-substrate. Two different encapsulated samples were measured. The polymer layer of the first sample is prepared, spincoated and encapsulated in a nitrogen environment. The other sample is encapsulated in air after being exposed to air for 24 hours.

The transmission versus irradiation time for these two samples is depicted in Figure 4.11. The solid line is the measured curve for the sample, which has been exposed to air. It is evident from figure 4.11 that the fitted curve does not match the measured data very well.

The inset of Figure 4.11 shows the transmission curve of the sample, which has not been exposed to air, versus time tagether with the theoretica! fit, with 1 = 1.2 · 10-7 . As can be seen from the inset of Figure 4.11 the fitted curve matches the measured data of the non-exposed sample very well. The fact that the curve of the sample, which has been

.~ ~

0.65

0.60

Ë 0.55

~ ,....

0.50

0.45

7 I

I I

i::::0 1 0.48

.. 0.46 1f

o; 15000 30000

-~--------_______ , --/'.",....... Encapaulated ample no aJr

r Y= 1.210"7

- measureddata -· Fitteeldata

0.40 ...J...._-,-----,----..,-----,----,----,----j

0 5000 1 0000 15000 20000 25000 30000

Time (s)

Figure 4.11: The transmission vs irradiation time curves of an encapsulated samples; one that has been exposed to air (upper curve) and one that was nat (lower curve) The fitted curve of the encapsulated sample with air does not match very well. The inset shows the enlarged curve and fit of the encapsulated sample without air.

exposed to air, cannot be fit properly with one 1, gives rise to the idea that the 1 changes during bleaching. This problem can be solved by splitting the curve in two and fitting each part separately. Figure 4.12 shows the measured transmission versus time curve of the sample, exposed to air, tagether with the two separate fits. The fit of the first part of the transmission curve gives: 11 = 1.1 · 10-6

; the fit of the second part gives: 12 = 9.7 · 10-8•

These results indicate that the sample has absorbed oxygen, while being exposed to air for 24 hours. The first part of the curve must therefore be fitted with a ten times higher 1, due to the preserree of oxygen, compared to the latter part of the curve where the available oxygen is almost used.

Figure 4.11 shows that there is a dependency of 1 on the amount of oxygen. We therefore performed a series of measurements at well defined oxygen pressures. The polymer sample is placed in a vacuum chamber, which is pumped to a pressure below 5 · 10-6 mbar and filled with pure oxygen to a certain pressure. The 1 is determined for oxygen pressures


0.70

0.68

0.66

0.64

0.62

0.60

0.58

0.56

0.54

0.52

0.50

. ·· ' I

! I I J

.... .•J>" . ·· , .

& ..... ··'*'··_. ......... ......

• measurad data

-. '" 1; "f-1.1 10~. to0-5000 s ········· tn 2; "f-9.7 1o4

• ;,.sooo s

0 5000 10000 15000 20000 25000 30000

Time (s)

Figure 4.12: The transmission vs irradiation time curves, fitted separately, of an encapsulated sample being exposed to air. /l = 1.1·10-6 ; 12 = 9.7 ·10-8

between 0.4 and 2500 mbar. Figure 4.13 shows the 1's of OC1 C10 PPV versus the oxygen pressure. The thickness

of the polyrner film is 360 nrn and the laser wavelength is 482 nrn.

10

:1.,... =482 nm, prep OC1C1., d.:l60 nm •"' J • • • •

• .. 0 • :;::

• 0.1 I

• • •

0.01

0.1 10 100 1000 10000

Pressure 0 2 (mbar)

Figure 4.13: 1 of OC1 C1o PPV versus the oxygen pressure. The thickness of the polymer film is 360 nm and the laser wavelength is 482 nm

The 1 at an oxygen pressure of 200 mbar is approxirnately 1 · 10-4, which is consistent

with the gamma at atrnospheric air pressure (assurning 20% oxygen content of air). The 1 is also rneasured with a constant nitrogen flow, which contains approxirnately 5 pprn oxygen (5 10-3 mbar partial oxygen pressure), resulting in a 1 of 2.3 · 10-7

, which is consistent with the value found by extrapolating Figure 4.13.

It rnay be striking that the quanturn yield for bleaching is not proportional to the oxygen pressure, but can be fitted with by Eq. 4.6.

( 4.6)


where Po2 is the oxygen pressure and A is a constant. An explanation may be found in the relation between the absorbed amount of 0 2 molecules in the polymer layer and the 0 2

pressure outside the polymer. The oxidation rate, and hence the stability of the polymer, will be more directly dependent on the concentratien of the absorbed 0 2 molecules in the polymer.

When we irradiate a sample in the absence of oxygen, the quanturn yield for bleaching is much lower and, surprisingly, it is possible to increase the PL efficiency, while keeping the transmission constant.

Figure 4.14 shows the PL signal and transmission versus irradiation time (,\=482 nm) of an encapsulated OC1 C10 PPV sample. The number of absorbed photons is constant,

2.0 -r--------------,

:j 1.5 ~ ~

'" c

~ ~ 1.0 c 0 ·;;;

" ·~ c ~ 0.5

-- Plsignal Encapsulated prop 1

- Transmission

---

0 1 000 2000 3000 4000 5000 6000 7000 8000

Time (s)

Figure 4.14: Increasing PL signaland constant transmission in time of an encapsulated sample upon irradiation.

while the emitted PL signal increases upon irradiation, which leads to the increase of the PL efficiency. A possible explanation for this effect is that the irradiation destructs the quenching sites, where the excitations decay non-radiatively, without the fotodegradation of the absorbing monomers. This can only he seen by irradiating an encapsulated sample without the preserree of oxygen, because otherwise the relatively small increase in PL signal will vanish in the strong decrease caused by bleaching.


4.4 The photochemical oxidation reaction

So far, we have discussed changes in the optical absorption and photoluminescence during bleaching of OC1 C10 PPV. We have shown that the quanturn yield for bleaching can be determined from the transmission versus irradiation time curve. In addition, it was shown that the unbleached morromers stilllumines with the same efficiency and that the photoluminescence spectrum is unchanged. In the previous section, the important influence of the preserree of oxygen on the stability is shown. In this section we will further investigate the photodegradation of OC1 C10 PPV by consiclering the chemical changes in the polymer during bleaching. Finally, we will propose a model for the degradation mechanism.

In order to notice the chemical changes in OC1 C10 PPV, Fourier Transform Infra Red Spectroscopy (FTIR) is used6 . Silicon is used as substrate, because it is transparent in the infra red. The spectra are measured in transmission in the wavelength area of 4000 -650 cm-\ on PPV samples irradiated for different duration. Figure 4.15 shows the infra red absorption spectra of spots irradiated with 1.27, 270 and 907 J j cm2 respectively. The laser wavelength is 482 nm.

Each peak of the infra red absorption spectrum corresponds to a vibrational mode of an atomie bond. The characteristic absorption bands in the infra red are listed in Table 4.3.

Table 4.3: Characteristic infra red absorption bands, their increase or decrease upon irradiation is indicated.

3000 - 2800 cm - 1 aliphatic C-H stretch constant 1800 - 1600 cm-1 C=O stretch mcrease 1000 - 1300 cm-1 C-0 stretch decrease

3059 cm-1 C=C C-H stretch decrease 968 cm 1 C=C C-H deformation decrease

Most evident from Figure 4.15 is the increase of the C=O stretch peaks (1800-1600 cm-1). Figure 4.16 shows the C=O stretch and the C=C C-H stretch absorption peak at 3059 cm-1 as a function of the irradiation close.

The data indicate there is a correlation between these two absorption peaks. From Figure 4.15 and 4.16 we conclude that photo-oxidation leads to the destruction of the vinyl double bond and to the formation of carbonyl groups. An exact degradation mechanism of polymers is in general very complicated and diffi.cult to proof, and will therefore not be proposed here.

6The FTIR measurements were carried out by J. J ansen.


, Ul 0

Ul , 0

"' ., 0 ..

u z,._

"'"' <Do :§· (11 ID ....

0

;_

0 . 095 JJO~ OS

;;; zo. oe JJ0396 0

Ul , 0

.• 10 0 ..

u z,._

"'"' "'a :§· Ul ID ....

"' u

0

;;j 67.5 JJ039B 0

Ul , 0

10 m 0

z,.. "'"' ~~ Ul m

"'"' 0

.. L{<·~"""t ... (· H .,.~

C-o ....!i

00

Figure 4.15: Infra red absorption spectra of OC1 C1o PPV samples, irradiated with 1.27, 270 and 907 J / cm2 respectively.


0.01 5

D.OI 2

D.OO l

0.000 _,6

I' I I ~ I I 1111 IOC V SI F I I I 1111 OCICin PPV 1 otnc 1em se 1e stn t lelt - op SI •o octemsc•es n1 Ie I ICIO-PP op. t.OOS

fl - ~ ---- D

.•

0.00

I\ ' "' },~ 2

""" i"-I

' _..

' /d

0/ /

I

O.ftO.I

~ 0.00:

D-......... t-- - ·-- -

vu 0.000

2 .0 0 4 0 Rellc:hllug (lnA.sK)

Figure 4.16: The C=O stretch and the C=C C-H stretch absorption peaks as a function of the irradiation dose. Note that lmA ·sec corresponds toa dose of13.4 J j cm2.

-0

We compare our experimental results regarding photo-oxidation with those published earlier. Cumpston [22] and Yan [20] both report the formation a carbonyl group and the destruction vinyl double bond, which is in agreement with our results. There is a striking difference between our res1,1lts and those from Yan [20]. In his experiments, which were performed on pure PPV, the carbonyl group acts as an effective luminescence quencher. The quenching results in a decreasein the PL efficiency. The fact that in our measurements the PL efficiency is constant upon bleaching, shows that the effect of the carbonyl groups is different for our samples7

.

4.4.1 Photochemical degradation reaction mechanism

In this subsectien a degradation mechanism is proposed, based on a similar mechanism of the degradation of a PPV derivative stuclied by Cumpston [22].

Interaction between a molecule in its excited state and a molecule in its ground state can lead to deactivation of the electronically excited state and the generation of the excited state of the other molecule. This phenomenon is generally known as quenching [23], see Eq. 4.7

l'vf* + Q---+ M + Q*. (4.7)

The reverse process, in which a molecule in the ground state is raised to its excited state by energy transfer from another excited state molecule, is known as sensitization, see Eq. 4.8

M +Sens* ---+ M* +Sens. (4.8)

Molecular oxygen is an important triplet-state quencher. lts ground state is a triplet state and it can interact with many triplet excited states of other molecules by a transfer of energy, which results in the production of a low-energy excited singlet state of molecular oxygen, termed singlet oxygen.

In the previous sections we showed that the degradation reaction of PPV is an oxidation reaction, activated by the absorption of light. Cumpston et al. [22] stuclied the photooxidation reaction of poly(2,5-bis( cholestanoxy)-1,4-phenylene vinylene (BCHA-PPV). He

7See Figure 4.9 and the discussion in Sectien 4.2.3.


observed the formation of an ester and volatile aldehyde species. Infra red reflection absorption spectroscopy also showed the formation of a carbonyl group and the attenuation of the peak associated with the vinyl double bond.

Excited-state singlet oxygen was suggested to be, at least partially, responsible for the photo-oxidation by attacking the vinyl double bond in the PPV backbone of the polymer8 .

Singlet oxygen can be created by an energy transfer process from the excited triplet state of the polymer, which in turn is produced by inter system crossing from the excited singlet state. Figure 4.17 shows the energy levels for BCHA-PPV and 0 2 . It is clear that the BCHA-PPV triplet level at 1.55 eV [24] is sufficiently energetic and long-lived to produce 1 .6.9 0 2 which has an energy of 0.98 eV. Because of the similarity of our infra red results

BCHA-PPV Singlet poi<Jron exciton

(radiative)

{ = 0( t 0 -6 - 10 -9) s

Triplel rol;~ron exciton (non·radialive)

----- 119

+ L63 eV

----- 1 óg 0.98 eV

_____ 3rg-

Figure 4.17: Energy levels for BCHA-PPV and molecular oxygen. The polymer valenee band and the ground state level of oxygen are fixed at 0. 0 e V to offer a relative comparison of the energy levels.

and those on BCHA-PPV by Cumpston [22] and the fact that both polymers have the same PPV backbone, we believe that also for our material the oxidation reaction is caused by the sensitization of singlet oxygen.

8 A complete reaction mechanism is discussed in [22].


4.5 Cyano PPV s; the relation between colour and stability

Cyano PPVs (CN-PPV) have the same conjugated PPV backbone as OC1C10 PPV but with a nitrogen insteadof a hydragen atom attached toa carbon atom of the vinyl bond. By varying the side chains attached to the phenyl ring, the bandgap can be altered. Although this is presently not well understood, it is generally believed that the structure of the side chains determines the amount of steric hindrance and thereby the conjugation length and the photoluminescence energy.

It is possible to produce CN-polymers with luminescence peaks varying from 400 to 750 nm. This means that the emitted colour can be varied over the hole of the visible spectrum, which is useful for future applications such as flat, large-area full-colour displays.

Figure 4.18 shows the quanturn yield for bleaching as a function of photoluminescence energy for a set of cyano PPVs with different sidechains. It is clear from Figure 4.18 that the shorter the emitted wavelengthof the cyano PPV, the higher the 1 and hence the lower the stability.

• 1e-3

• Cyano PPV's

•

•• "' • ~ 1e-4 • •

C!)

• gl1-=355nm

1e-5

• •

2 3

EPL (eV)

Figure 4.18: 'Y as a function of photoluminescence energy. Note that 1 is plottedon a logarithmic scale.

Only a qualitative explanation can be given for the results shown in figure 4.18. Again we assume that the degradation reaction is due to oxidation by singlet oxygen, which is produced by an energy transfer reaction of the excited PPV triplet state, see Section 4.4.1. Figure 4.19 shows the ground state and excited singlet and triplet states of a blue and a red luminescent CN-PPV.

Here it is assumed that the midgap states of polymers with different bandgaps are situated around the same absolute energy level. The first excited singlet state of the blue emitting polymer is therefore at a higher energy level than that of the red-emitting polymer (see Figure 4.19). If the energy difference of the first excited singlet and the first excited triplet state is of the same order of magnitude for both the blue and the red emitting


3

2

~ i;; 0 a; c: w

-1

-2

-3

s,

s,

s.

Blue

T,

s,

T, s,

Midgap level

s.

Red

T,

T,

Figure 4.19: The ground state and excited singlet and tripletstatesof a blue and red luminescent CN-PPV.

polyrner, their vibrational overlap, and therefore the population of the triplet states, will be cornparable.

The triplet state rnay act as a sensitizer for the formation of singlet oxygen. The triplet state of the blue ernitting CN-PPV will be a more effective sensitizer, than the triplet state of the red ernitting CN-PPV, due to its higher energy level. The more singlet oxygen per absorbed photon is forrned, the faster the degradation reaction will be. This implies that the blue ernitting CN-PPVs will be less stabie than the red ones, as is indeed observed in Figure 4.18.

Consicier the absorption spectrum of the Cyano-PPV (KN-73) in Figure 4.20, which displays the different absorption peaks. The 355 nrn laser wavelength is in the top of the high energy absorption peak, whereas the 482 nrn wavelength is in the low energy absorption peak. The quanturn yields for bleaching are: /355nm = 7.4 · 10-6 and /482nm = 2.6 · 10-7

. The lower energy absorption peak corresponds to the absorption peak as is seen in OC1 C10 (Figure 4. 7), the higher energy absorption peak in the spectrum of KN-73 corresponds to another absorption site of the molecule, possibly forrned by the electronnegative cyano group. This rneans that an excitation in the high energy peak has a 28

Absorbance 2.0

1,8

1.6

1,4

1,2

1.0

0.8

0,6

0,4

0.2

"' 400 500 700 800 Wavelengtil (nm)

Figure 4.20: The absorption spectrum of cyano-PPV (KN-73).


times higher chance of breaking a monomer, than the low energy excitation. Absorption in the high energy peak leads to a singlet state with higher energy. Inter

system crossing from the high energetic singlet state leads to a triplet state with higher energy, which is a more effective singlet oxygen sensitiser than the lower energy triplet state. The higher energy singlet and triplet states are denoted in Figure 4.19 by 52 and T2 respectively.

This means that excitations in the high energy band lead to more reactive singlet oxygen atoms, which in turn leads tofaster degradation of the polymer.

4.6 Polymer degradation in an EL device

The previous sections showed that fitting a 1 from the transmission versus irradiation time curve, provides an excellent method to determine the stability and lifetime under irradiation for different PPV s used in electraluminescent devices.

When the polymer is used as light-emitting layer in an electraluminescent device, the injected electrous and holes recombine to form excited states, which may decay radiatively. The polymer, when used in an electroluminescent, will also be subjective to degradation. The excited state formed by electron-hole recombination may act as a sensitizer as well.

If we want to measure the polymer degradation of an electraluminescent device under operation, it is impossible to measure the variation of transmission through the polymer layer, because the calcium electrode is opaque. To solve this problem, the result of Section 4.2.3 can be used. We observed that the PL efficiency remains constant upon bleaching and that the bleached morromers do not act as active PL quenchers. For this reason it is possible to use the PL signal as a measure for the amount of bleached monomers.

We performed an experiment in which we irradiate a device with a laser beam, while the device was operated. In order to separate the PL signal from the EL signal, the laser was chopped and a lock-in amplifier was used 9 • Figure 4.21 shows the PL signal, EL signal and diode çurrent versus time of a 2x2 mm electraluminescent device, operating at 6 V. It is evident from Figure 4.21 that the EL signal decreases and the current through the LED increases, while the PL signal is constant. It seems that the polymer has not degraded at all, at the end of the LEDs lifetime. Apparently, there are some other mechanisms, that cause the degradation of the device.

It is believed that at the moment, the short operating lifetime is mainly due to the oxidation and ablation of the metal contacts. This results in black, non-emissive, spots. The current densities in these spots can be high, which will heat the device locally. When the temperature is raised, the destructive mechanisms are accelerated.

When the problems with the metal cantacts are solved, the main problem is the stability of the polymer. In order to predict the lifetime of the polymer used in an electraluminescent device, the results of the photoluminescence measurements can be used. This will be outlined below.

9See the experimental setup in Sectien 3.1.


I

I 0.8 i

ë 11

~

a o.s ~ Q) '

! J Ljj 0.4 i

ë! I 0.2 ~

I o.o I

~----~--~--~--~----~--~

0 5000 1 0000 15000 20000 25000 30000 35000

Time (s)

Figure 4.21: PL signa], EL signa] and diode current versus time of a 2x2 mm electraluminescent device, operating at 6 V.

We will try to answer the following question: what is the predicted lifetime of an unencapsulated electraluminescent device with a brightness comparable to a computer screen (100 cd/m2) with an area of 1 cm2

, if we define the lifetime as the time at which 50% of the morromers are destructed?

First, we need to calculate the number of emitted photons that correspond to a brightness of 100 cd/m2 . Because each photon of a different wavelength has a different intensity perception for the eye, the speetral distribution of the emitter must be known. From the number of emitted photons one can calculate the number of electron-hole recombinations needed to produce these photons. Each electron-hole recombination can be compared with the absorption of a photon, because the excited state that is formed through recombination of an electron and a hole is similar to one that is the result of the absorption of a photon. We know the number of photons a monoroer can absorb before it is destructed, so we can calculate the EL lifetime of the polymer.

_If an emitter has a speetral distribution 5(>.) then the luminous flux (lm), <Pv, is given by:

f 5(.-\)V(.\)d,\ <Pv = 683 J 5(.\)d,\ <Pe ( 4.9)

where V(,\) is the photopic response of the human eye and <Pe is the radiant power or flux (W).

For a Lambertian emitter the angular distribution of the luminous intensity (cd), lv, is given by:

( 4.10)

where 0 is the angle from the normal of the emitter. If the emitter has surface area A, the luminanee ( cd/m2) is:

L _ Io V- A (4.11)


For Lambertian emission the luminous flux and the luminanee are related by:

(4.12)

For OC1 C10 PPV with an emission spectrum peaked at 590 nm Eq. 4.13 can be used as an approximation.

f S(.\)V(.\)d.\ ~ 0 6 f S(.\)d.\ .

( 4.13)

Now we can calculate the radiant power for a OC1 C10-PPV LED with an emitting area of 1 cm2 and brightness of 100 cdjm2• If we divide the radiant power by the energy of a photon, Ephoton, with a wavelengthof 590 nm, the result is the number of emitted photons per second, Jphoton:

Lv1rA ( ) Jphoton = 4.14

0.6Ephoton683

For our device 2.3 · 1014 photons are emitted per second. The light that is emitted through the front surface determines the luminanee of a LED.

However 50 % of the internal generated light is waveguided and leaves the device at the edges. The number of internal generated photons persecondis thus 2Jphoton· OC1 C10-PPV has a PL efficiency of approximately 15%, this means that 15% of the excited states decays radiatively. The number of excitations (electron-hole recombinations) per second, Jeh, is:

J _ 2Jphoton

eh - 0.15 ( 4.15)

Equation. 4.15 leads to 3 ·1015 electron-hole recombinations per second. The time, tzife,

it will take to destruct 50 % of the morromers is given by

Ndev tlife = -2 J ·

I eh ( 4.16)

The -number of OC1 C10 monomers, Ndev, in a device with an area of 1 cm2 and film thickness of 100 nm is 1.5 · 1016 • 1 for 0C1 C10 , in air of atmospheric pressure, is 1.1 10-4

monomersfphoton. These values result in a lifetime of only 6 hours. It is evident that a lifetime of approximately 6 hours must be extended to a much

longer lifetime, before the polymer LED will be stable enough to be used in applications. The most obvious solution for fast polymer degradation is to encapsulate the EL devices without the preserree of oxygen. U sing the measured quanturn yield for bleaching of an encapsulated OC1 C10 PPV sample, which has not been exposed to air ( lencap=l.2.10-7

),

we find a lifetime of 5700 hours. Although the predicted lifetime of an encapsulated EL device is encouraging, a critical

note must be made. In Section 4.4.1 we proposed that the degradation is caused by singlet oxygen, which is produced via sensitization of the triplet state of the polymer. In the optical absorption experiments this triplet state is not directly populated, but only via the spin forbidden inter system crossing mechanism. In the lifetime calculation accounts every


electron-hole recombination in the EL device for one absorbed photon in PL measurements. However, when the excited states are created by recombination of injected carriers, it is not obvious that predominantly singlet states should be populated. It is in literature suggested that an electron-hole recombination produces only 25% singlet states and therefore 75% triplet states [25]. This means not only a 75 % reduction of EL efficiency, but will have dramatic consequences for the quanturn yield for bleaching, due to the sensitization of the triplet state for the production of reactive singlet oxygen. This note emphasises the importance of excluding oxygen from the device.

From the above we can conclude that the calculated lifetime of 6 hours for an unencapsulated sample is an optimistic estimate. Thus, it is clear that, if we want to make use of OC1 C10 PPV, we must encapsulate the device to achieve a lifetime that is acceptable for applications.

Chapter 5

Conclusions

The use of conjugated polymers, such as poly(p-phenylene vinylene) or PPV, as emitting layer in electroluminescent devices sure is promising. Their relatively high efficiency and high brightness level, together with the possibility to adjust the polymer to almost every colour in the visible spectrum, form a basis for a range of applications. A flat, large area full colour display is for instanee an application that in future possibly can be made with cheap processible polymers.

The disadvantage of these polymers is that they are often not very stable. The photochemical stability of polymers for electroluminescent devices has therefore been stuclied and the results of this work are described in this report.

In Chapter 3 a model is presented, which is used to determine the quanturn yield for bleaching, 1, from a measurement of the transmission versus irradiation time ( 1 is the probability that a morromer is bleached by absorbing a photon). This is an excellent way to determine the photochemical stability of various polymers. It is shown that 1 is independent of the bleaching power density (see Section 4.2).

1 is also independent of the excitation wavelength for excitation to the various vibrational levels of the same absorption peak. For these different wavelengths 1 = 1.3 · 10-4 ± 0.2 · 10-4 for OC1 C10 PPV.

The decrease in PL signal during bleaching is proportional to the reduction of the number of absorbed photons, leading to constant PL efficiency. The PL spectrum does not change upon bleaching either as can beseen inSection 4.2.3. The bleached morromers seem not to reduce the distance an excited state can travelalong the conjugated polymer chain. If the travelling excited state encounters a harrier ~all, formed by a distortien due to bleaching, it cannot pass andreduce its energy, but will decay resulting in a blue shifted PL spectrum. When the distance an excited state can travel is shorter than the distance between consecutive bleached monomers, the probability that an excited stateencounters a queuehing carbonyl group is low, and the PL efficiency will remain constant.

The temperature dependenee can be depicted in the Arrhenius form. If we determine lt at a certain temperature T1 , then we can predict 12 at temperature T2 • The measured 1 at 80°C is approximately 4 times higher than the 1 at 20°C. This illustrates that it is important to achieve a high EL efficiency since this reduces heating of the device by dissipating power (see Section 4.2.4).

The quanturn yield for bleaching is highly dependent on the oxygen concentratien in the polymer. For an encapsulated sample, which is prepared in a nitrogen environment with approximately 5 ppm oxygen, 1 is 1000 times smaller than fora sample measured in a1r.

41


From Fourier transfarm infra red spectroscopy, we conclude that photo-oxidation leads to the destruction of the vinyl double bond, the most sensitive unit in PPV, and to the formation of carbonyl groups. Although the carbonyl group is reported as PL queueher in literature [22], it does not act as a queuehing site in OC1 C10 PPV (see Section 4.4).

The photodegradation is probably due to the formation of singlet oxygen, which oxidizes the polymer. The absorption of a photon leads to the excitation of the singlet polymer ground state to an excited singlet state (S0 --+ S1 ). The excited singlet state populates the excited triplet state by inter system crossing (S1 --+ T1 ). The excited triplet state of the polymer acts as a sensitizer for the formation of singlet oxygen. This is discussed in Section 4.4.1.

InSection 4.5 the relation between luminescence spectrum and stability of CN-PPVs is concerned. Large differences in stability are found. Î varies from 5.0 · 10-6 to 1.2 · 10-3 as peak of the emitted colour is changed from 1.7 to 2.6 eV. A model to explain this relation is discussed in Section 4.5. This model is based on the height of the absolute energy level of the excited polymer triplet state, the higher its energy level the more effective it will be as a sensitizer for singlet oxygen formation.

When the excitation wavelengthof the laser is shorter and leads to an excitation, which corresponds to a different, higher energy absorption peak of the polymer, the oxidation reaction will also befaster as is seen in the different 1's for cyano polymer KN-73 (1,x=4s2 = 2.6 · 10-7 and Î.\=355 = 7.4 · 10-6

), in Section 4.5. We predict the lifetime of the polymer, from an optimistic point of view, in an electro

luminescent device, with a brightness comparable to a computer screen (100 cd/m2) and

an emitting area of 1 cm2, based on the results of the photoluminescence measurements

(see Section 4.6). The lifetime for an unencapsulated device (in air) is approximately 6 hours, whereas the lifetime for an encapsulated device (without oxygen) is approximately 5700 hours. This result has important consequences for applications based on polymer LEDs. If we want to use OC1 C10 PPV as light emitting layer in an electroluminescent device, this device must be encapsulated to prevent it from oxidation in order to achieve an .fi.Cceptable lifetime.

List of Figures

2.1 Progression from atomie orbitals in two isolated atoms to the coalescence of orbital energy levels into bands . . . . . . . . . . . . . . . . . . . . . 3

2.2 Chemical structure of the most important conjugated polymers 4 2.3 Trans-polyacetylene as a one-dimensional analogue of an alkali metal. 5 2.4 Changes in the electronic density of states and in the electron dispersion

relation at the Peierls transition . . . . . . . . . . . . . . . . . . . . . . . . 5 2.5 (a) A soliton and (b) a polaron in trans-polyacetylene . . . . . . . . . . . . 6 2.6 Light doping charges the already existing neutral solitons, heavy doping

creates new solitons by breaking chemical honds. . . . . . . . . . . . . . . 6 2. 7 Schematic structure of a polymer LED formed with a single layer of conju-

gated polymer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.8 Band diagram of a single PPV layer EL diode . . . . . . . . . . . . . . . . 8 2.9 Schematic energy-level diagram for a bilayer device under forward bias. . . 10 2.10 Jablonski diagram showing some ofthe radiative and non-radiative processes

available to molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.11 The Franck-Condon principle and radiationless transitions . . . . . . . . . 12 2.12 Large vibrational overlap at the crossing point of approximately degenerate

electronic states. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.13 Vibrational overlap with a large energy difference between the states. 14

3.1 The experimental setup . . . . . . . . . . . . . 17

4.1 Chemical structure of 88% conjugated OCI C10 19 4.2 Measured and fitted transmission curve versus irradiation time. 20 4:3 Initial transmission versus layer thickness . . . . ; . . . . . . . . 21 4.4 The intensity profile of the laser beam, fitted to a Gaussian profile. 22 4.5 Transmission VS time of a oei clO sample of 300 nm thickness. 23 4.6 1 vs the power density of the bleaching laser beam 24 4.7 The absorption spectrum of 88% conjugated OCI C10 . . . . . . 24 4.8 PL spectra with different bleaching times . . . . . . . . . . . . 25 4.9 The PL signaland efficiency vs bleaching time of OCI C10 PPV. 26 4.10 1 versus temperature. . . . . . . . . . . . . . . . . . . . . . . . . 27 4.11 The transmission vs irradiation time curves of encapsulated samples. 28 4.12 The transmission vs irradiation time curves, fitted separately ( see text ), of

an encapsulated sample being exposed to air. . 29 4.13 1 of OCI C10 PPV versus the oxygen pressure. . . . . . . . . . . . . . . . . 31

43


4.14 Increasing PL signal and constant transmission in time of an encapsulated sample upon irradiation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.15 Infra red absorption spectra of OC1 C10 PPV samples. . . . . . . . . . . . . 33 4.16 The C=O stretch and the C=C C-H stretch absorption peaks as a function

of the irradiation close. . . . . . . . . . . . . . . . . . 34 4.17 Energy levels for BCHA-PPV and molecular oxygen. . . . . . . . . . . . . 35 4.18 Î as a function of photoluminescence energy. . . . . . . . . . . . . . . . . . 36 4.19 The ground state and excited singlet and triplet states of a blue and red

luminescent CN-PPV. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 4.20 The absorption spectrum of cyano-PPV (KN-73). . . . . . . . . . . . . . . 37 4.21 PL signal, EL signal and diode current versus time of a 2x2 mm electrolu-

minescent device, operating at 6 V. . . . . . . . . . . . . . . . . 39

B.1 Photon absorbance vs beam width at different bleaching times. . 52 B.2 The number of absorbed photons versus time. . . . . . . . . . . 52

Appendix A

Derivation of r

In this sectien we derive the Eq. 4.1. We consider the irradiation with a Gaussian laser beam along the z-axis of a polymer film of thickness d. The local absorption coefficient is determined by the local concentratien of intact molecules N(z, r, t).

a(z, r, t) =a N(z, r, t) (A.l)

where a is the local absorption cross-section. N(z, r, t) decreases in time through photochemical bleaching as given by Eq. A.2.

oN(z, r, t) ( )J( ) at =-,a z,r,t z,r,t (A.2)

where 1 is the quanturn yield for bleaching, whereas the decrease in excitation intensity along the propagation direction, z, is given by Eq. A.3.

ol(z,r,t) ( ) (. ) oz = -a z, r, t I z, r, t (A.3)

In the above Eqs. it is assumed that the molecules resulting from the degradation process do not absorb at the laser wavelength, as is shown in Sectien 4.2.3. The boundary conditions are:

I(z = O,r,t) = J0 exp(-r2 /r~) = l 0 (r),

I(z,r,t = 0) = I0(r)exp(-aN0 z)

whete r 0 is the Gaussian beam waist and

N(z, r, t = 0) =No.

Equation A.2 can be written as:

N(z, r, t) = N0 exp { -1a 1t I(z, r, t')dt'}.

Note that

:t {1t I(z,r,t')dt'} = I(z,r,t)

Substituting N(z, r, t) in Eq. A.3 yields

(A.4)

(A.5)

(A.6)

(A.7)

(A.S)

:z { :t {1t I(z, r, t')dt'}} = ~0 :t { e-~u 1t I(z, r, t')dt'} (A.9)

45


Using

w(z, r, t) = [ I(z, r, t')dt'and! { ~:} = :z { ~~} (A.10)

g1ves

(A.ll)

Thus

(A.12)

U sing the relation

(A.13)

yields: 1 Cn

z = --ln(l + -e"~uw) + C2 (A.14) 1aC1 No

w = J__ ln { No (e-yuC!(z-C2 )- 1)} + c2 (A.15) /0' Cn

When z = 0, I(z condition we find

0, r, t) = l 0 (r) and w(z = 0, r, t) = l 0 (r)t. Using this boundary

Substituting Eq. A.16 in Eq. A.15 gives:

w = 2_ ln { eu-yC1z eu-yl0 t + No e-yuC1z No } 0'/ en Cn

Using I(z, r, t) = Bw(;t·t) and Eq. A.17 we obtain

Using l(z, r, t = 0) = l 0 (r)euNoz gives C1 = _&_ and "'

I( ) Io(r) z, r, t = 1 + ( euN0 z _ 1 )e-Io(r)u-yt

(A.16)

(A.17)

(A.18)

(A.19)

The transmission after infinitely long bleaching is T 00 • In general T 00 < 1, because of refl.ection and scattering losses. The initial transmission is T0 , with T0 = T00 e-uNd, where d is the thickness of the polymer film.

The time dependenee of the transmission through the film is given by:

) _ 211' J0

00rl(z = d,r,t)dr _ t'

T(t - roo Too - -Too 211' Jo rl(z = O,r,t)dr n

(A.20)

r2

u sing x = e-r~ yields

Appendix A. Derivation of 1

00 2

n' = 27r j l 0e- ~5 rdr = 1rr2 !0

0

1 , J r6 Io t = 21r - dx = 2 1 + ( eaNod _ 1 )ea"Ylotx 0

2 T { _1_1 { Toea"Ylot + T oo- To}} 1rr0 1o l n T .

O""( ot oo

Eqn. A.20, A.21 and A.24 yield

T(t)= Too ln{Toea"Ylot+Too-To} O""( lot T oo

which is Eq. 3.9 of Section 3.1.

47

(A.21)

(A.22)

(A.23)

(A.24)

(A.25)

------------------

Appendix B

The transmission vs bleaching time curve

If we consider the transmission vs time curve B of figure 4.5, a relatively fast increase in transmission (for t=0-1000) foliowed by a slow saturational increase (for t=1000-6000) can be seen. In order to understand the time dependenee of the transmission through the sample, the number of absorbed photons versus time is calculated. The absorbed fraction, a(r, t 1), at time t 1 is given in Eq. B.1

( )_I(O,r,t1 )-I(d,r,t1)

a r,t1 - I(O,O,O)- I(d,O,O) (B.1)

where the denominator represents the normalization factoranddis the film thickness. The intensity at z = dis given by1

:

I ( d r t) - --:-:-I (.:.,--0.;__, r...:....., 0~) ---=--:-~ ' ' - 1 + ( euNod _ 1 )e-lo(r)u"'Yt

(B.2)

Figure B.1 shows the absorbed photon intensity versus the beam width for different times. It is clear from figure B.1 that at t = 0 the most photons are absorbed around the middle of the laser beam, while at t = 8 only photons from the edges of the Gaussian profile are absorbed. At the edges of the Gaussian profile the power density is lower and therefore the bleaching of the polymer will take longer time.

Integrating over the beam width leads to the total number of absorbed photons at each time, indicated in figure B.2. Calculating the transmission (1-absorption-re:fl.ection) willlead to the shape of curve B in figure 4.5. The number of photons that is still being absorbed at t = 6000 in curve B of figure 4.5 is not due- to absorption by the bleached polymer, but to the absorption at the edges of the laser beam.

1See appendix A

49


1.2

1.0 1=0

0.8

~ ëii c: 0.6 .2! .5 c: g

0.4 .2 '0 (\)

e 0.2 0 en .0 as

0.0

-0.2

-0.4

-0.4 0.0 0.4 0.8 1.2 1.6 2.0 2.4

beam width (mm)

Figure B.l: Pboton absorbance vs beam width at different bleaching times.

1.0

Ê 0.8 0 .s en c: g .2 0.6 '0 (\)

~ en .0 as Qj 0.4 .0 E " c:

IS {:. 0.2

0

• •

•

•• ••

·· ... " ..

· ...... . 2 4 6 8 10

Time (a.u.)

Figure B.2: The number of absorbed photons versus time.

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