Spectral stability study and molecular modeling of fluorence based conjugated polymers 1

1
CHAPTER 1 GENERAL INTRODUCTION



2
1-1 Scope of thesis

The work of this thesis could be roughly categorized into two parts. The first part
examines spectral and thermal spectral stability in films and aggregation in solutions for
fluorene-based conjugated polymers. The second part reports influence of donor and
acceptor substituents on the electronic characteristics of poly(fluorene-phenylene).

The following sections in this chapter give a description of stability and
processing, as well as optical and electronic properties of conjugated polymers, followed
by a discussion on the development of polymer light-emitting diodes. Polyfluorene
derivatives and light-emitting diodes are also discussed.

Chapter 2 discusses experimental and calculation methods used in this work.
Ultraviolet-visible absorption spectroscopy, photoluminescence spectroscopy and
differential scanning calorimetry are described. It is then followed by a discussion on
semiempirical molecular orbital calculation.

Chapter 3 discusses in details spectral and thermal spectral stability of seven
fluorene-based conjugated polymers in film states. Ultraviolet-visible absorption and
fluorescence spectra of these polymers were presented. Their differential scanning
calorimetry and crystallization analysis results were discussed.

Chapter 4 investigates aggregation behavior of polyfluorene derivatives in
solutions. Five representative polyfluorene derivatives were examined with respect to

their absorption and emission spectra in chloroform/methanol mixtures.



3

Chapter 5 investigates theoretically influence of presence of acceptor or donor
group(s) along poly(9,9-dihexylfluorene-1,4-phenylene) backbone. The present
quantum chemistry calculations report on changes in geometric and electronic
properties of poly(9,9-dihexylfluorene-1,4-phenylene) unit cell induced by substitution
with cyano, methoxy or amino group(s).

1-2 Introduction

1-2-1 Conducting polymers

There is hardly an aspect of our lives that is not touched by synthetic polymers.
The role of polymers in the electronics industry has been traditionally associated with
insulating properties, whether these are for isolating metallic conductors or for use in
photoresist technology. From that starting point it was the pioneering work of
MacDiarmid, Heeger, and Shirakawa et al. that inspired chemists and physicists to
consider the opportunity of using polymers as conductors. The report in 1977 from the
University of Pennsylvania of high conductivity in charge-transfer complexes formed
with a polymer, polyacetylene, which exhibited extended
π
conjugation along the
polymer chain, provoked considerable excitement.
1
Conjugated polymers derive their
conducting properties by having delocalized

π
-electron bonding along the polymer
chain. The
π
(bonding) and
π
* (antibonding) orbitals form delocalized valence and
conduction wavefunctions, which support mobile charge carriers. As the length of the
conjugated sequence is increased, the energy gap between the filled
π
and empty
π
*
states falls, though in the long chain limit the gap remains finite, and takes a value of



4
about 1.5 eV. Polyacetylene has served as the prototypical conjugated polymer; the
simplicity of its structure has allowed theoretical modeling.

This was the first demonstration of metallic behavior within the intramolecular
π

electron system along the polymer chain, and the significance of these results was
quickly picked up by many other groups worldwide.

Conducting polymers have tremendous potential for innovation. After 20 years
of progress, these unusual polymeric materials can now be used as transparent antistatic
coatings, electromagnetic shielding, superconductors, modified electrodes,

electrochromic windows, supercapacitors, transistors, light-emitting diodes, lasers,
conducting photoresists, photovoltaic cells, biosensors, and so forth.
2,3
The significance
of this class of polymers was recently highlighted by the awarding of the 2000 Nobel
Prize in Chemistry to H. Shirakawa, A. G. MacDiarmid, and A. J. Heeger, the three
scientists who pioneered this novel materials field.

Whereas conventional polymers are readily processed in solution or in the melt
and can be cheaply manipulated into desirable forms, this is not in general possible for
conjugated polymers. The delocalized
π
electron system makes the molecular chains
rigid, with resultant high melting points and low solubilities. The dilemma of a
potentially attractive material that cannot be processed is not new in materials science in
general or polymer science in particular. Two well-established lines of attack on such
problems involve either modifying the molecular structure so as to retain the property of
interest while rendering the material processible, or carrying out the processing stages
with a more tractable precursor, which can be converted subsequently to the desired



5
material. Both of these approaches, have been successfully applied to the processing of
most classes of conjugated polymers, and the methods adopted are summarized in
Section 1-3 for the major structural classes.

Conjugated polymers behave as “molecular materials,” and there is a
considerable reorganization of the local
π

electron bonding in the vicinity of extra
charges added to the chains. This results in self-localization of the added charge, to
form, in general, polarons, though for the particular symmetry of the trans isomer of
polyacetylene, these take the form of bond-alternation defects, or solitons. The
theoretical models developed to describe these processes are discussed in Section 1-4,
along with a description of fluorescence from conducting polymers.

1-2-2 Polymer light-emitting diodes

The availability of film-forming conducting polymers in the late 1970s resulted
in attempts to fabricate a range of semiconductor devices, principally two-terminal
diodes formed as sandwich structures with metallic electrodes to either side of a film of
polymer. The first report on polyacetylene formed by the Shirakawa route, of in situ
polymerization of acetylene gas onto the bottom electrode, revealed that Schottky
barriers could be formed against metals with appropriate work functions (here the
polyacetylene was functioning as a p-type semiconductor),
4
However, these early
experiments were constrained by the poor processibility of the polymers then available.

Thin-film electroluminescence (EL), that is the emission of light when excited
by flow of an electric current, in conjugated polymers has provided the other major area



6
of device-related activity for display. The discovery that conjugated polymers could act
as both transport and emissive layers, reported in 1990,
5
has generated a very high level

of interest. Device fabrication can be very straightforward, with a layer (or layers) of
polymer sandwiched between two electrodes, one of which is transparent. This is
illustrated in Figure 1.1 for the case of the first EL diodes.
5
The polymer was prepared as
a thin film, of thickness of order 100 nm, by spin-coating a “precursor” polymer from
solution, using a standard photoresist spin-coater, and subsequently converting the
“precursor” polymer to the semiconducting PPV by heating. Spin-coating from solution
has been demonstrated to be capable of producing highly uniform layer thickness, with a
thickness variation of no more than a few ångströms spread over several cm
2
. This
polymer layer was formed on a glass substrate coated with indium-tin oxide (ITO),
which has a relatively high workfunction and is therefore suitable for use as a
hole-injecting electrode, and the other electrode then formed by thermal evaporation of
the selected low workfunction metal such as Al, Mg or Ca, which are suitable for
injection of electrons, as shown in Figure 1.1. Organic electroluminescent displays
represent an alternative to the well-established display technologies based on
cathode-ray tubes and liquid-crystal displays (LCDs), particularly with respect to
large-area displays for which the existing methods are not well suited. Rapid progress
has since been made, and EL diodes with a wide range of emission colors and with
quantum efficiencies (photons/electron) of several percent are now reported. The
development of polymer light-emitting diodes is discussed in Section 1-5.




7

Figure 1.1 Structure of an electroluminescent diode based on a conjugated polymer.

The polymer film is formed on a glass substrate coated with indium-tin
oxide, and the top electrode is then formed by thermal evaporation.



1-2-3 Polyfluorenes

Polyfluorenes are an important class of electroactive and photoactive materials.
In the last few years this research field has literally exploded because of polyfluorenes’
exceptional electrooptical properties for applications in light-emitting diodes. This is the
only family of conjugated polymers that emit colors spanning the entire visible range
with high efficiency and low operating voltage. The development of this area is
discussed in Section 1-6.

1-3 Chemical structures, stability and processing of conducting polymers

The chemical structures of some common conjugated polymers are shown in
Table 1.1.








8
Table 1.1 Some common conjugated polymers
Polymer Chemical Name Formula Bandgap
a

(eV)
PA trans-polyacetylene 1.5
PDA polydiacetylene 1.7
PPP poly(p-phenylene) 3.0
PPV poly(p-phenylenevinylene) 2.5
RO-PPV poly(2,5-dialkoxy-p-phenyl
enevinylene)

2.2
PT polythiophene 2.0
P3AT poly(3-alkylthiophene) 2.0
PTV poly(2,5-thiophenevinylene) 1.8
PPy polypyrrole 3.1
PAni polyaniline 3.2
n
R
R
n
n
n
RO
OR
n
S
S
n
S
R
S

R
n
n
S
N
H
n
NH
n
a
The band gap is taken as the energy at the maximum slope,
∂α
/
∂
E, of the
spectrum of the optical absorption (
α
).


In order to successfully exploit the properties of conducting polymers in
commercial applications, it is imperative that the candidate materials exhibit good
environmental stability and be amenable to a wide variety of processing techniques.

A polymer with poor environmental stability is essentially unstable in its doped



9
state under normal atmospheric conditions. Compared to polyacetylene,

polyheterocycles (such as polythiophene and polypyrrole) have demonstrated much
better environmental stability
6
. For example, polypyrrole displays only minor changes
in its conductivity state even after exposing in air to temperatures as high as 200 °C for
extended periods of time. In general, the stability of a conducting polymer depends on a
number of factors including its susceptibility and accessibility to external chemical
species, the nature and type of counterion present in the material, the reactivity of its
doped sites to surrounding chains, and the flexibility and conformational states of its
backbone. A great deal of progress has been made towards the development of stable
conducting polymers; however, this issue continues to be of paramount importance to
the successful utilization of these materials in commercial applications.

Many of the initially prepared conducting polymers were formed as intractable,
insoluble films or powders that, once synthesized, could not be further manipulated into
forms with more ordered, controllable structures. In fact, the structural attributes that
give rise to the interesting electrical and optical properties of the conducting polymers,
namely their rigid, planar conjugated backbones, severely limit the ways in which the
polymer can be processed. To overcome these limitations, a number of structurally
modified polymers and novel processing schemes have been developed that allow
substantially more control over the state of the final product. These processing schemes
can be conveniently divided into four categories.

The first category is the manipulation of soluble precursor polymers. This
scheme is based on the synthesis and manipulation of a processible, nonconducting
precursor polymer that, once fabricated into a suitable form using conventional polymer



10

processing techniques (usually by thermal treatment), can be converted into an insoluble
electrically conducting polymer. This route has been successfully utilized to prepare
highly oriented thin films and fibers of polyacetylene
7
, poly(phenylene vinylene)
8
,
poly(thienylene vinylene)
9
and some other similar polymers
10
.

The second processing scheme is the manipulation of soluble conducting
polymer derivatives and copolymers. This is to modify the structure of the polymer in
such a way to improve the processibility without compromising its electrical or optical
properties. For example, it is possible to dramatically modify the processibility of the
polythiophenes without severely compromising the electrical properties. By simply
substituting the hydrogen atom attached to the three position of the thiophene ring with
an alkyl group containing at least four carbons, conjugated polythiophenes that are both
solution and melt processible can be achieved
11
. Meanwhile, the conductivities of the
doped derivatives are also comparable to the parent polymer and generally range from
1-200 S/cm.

The third processing scheme is the in-situ polymerization of conducting
polymers in insulting matrix polymers. This processing scheme focuses on the growth of
insoluble and intractable conjugated polymers within a performed polymer matrix. In
this case, a processible, insulating polymer impregnated with a catalyst system is

fabricated into a desired form such as a thin film or fiber. This activated polymer matrix
is then exposed to the monomer, usually in the form of a gas or vapor, resulting in a
blend typically comprised of an isolated or semi-continuous conjugated polymer phase
dispersed throughout a continuous phase of the host polymer. For example, stretched
aligned blends of polyacetylene/polybutadiene exhibit conductivities at least one order



11
of magnitude larger than that of the unstretched material
12
. This enhancement in
conductivity reflects a higher state of order resulting from the deformation process.

The last processing scheme is the manipulation of conducting polymers via the
Langmuir-Blodgett (LB) technique. This technique relies on the ability of the LB trough
to manipulate surface active molecules into highly ordered thin films with structures and
film thickness that are controllable at the molecular level. The true promise of the LB
processing technique is its unique ability to allow control over the molecular
architecture of conducting polymer thin films.

1-4 Optical and electronic properties of conjugated polymers

1-4-1 Introduction

The electronic structure of conjugated polymers can be conveniently described
in terms of
σ
bonding formed by overlap of sp
2

hybrid orbitals, and
π
bonding formed by
overlap of P
z
orbitals on adjacent carbons. This description then allows a useful
parameterization of the electronic properties in which the contribution of the
σ
electrons
provide the force constant for the carbon–carbon bonds, and in which the
π
electrons are
described using Hückel (tight-binding) methods. This approach has proved to be
particularly important for describing the coupling of the lattice to the electronic
excitations in the
π
electrons caused by photoexcitation from
π
to
π
* or by charge
injection. However, the input parameters in such models need to be chosen empirically,
and the use of more sophisticated quantum chemical calculations has been important in



12
gaining an accurate description of the electronic structure. We briefly review both
approaches here.

It is appreciated that the effect of the electron-lattice and electron-electron
interactions is to cause localization of excited electronic states on the polymer chain.
These are variously described as solitons, polarons, bipolarons, or excitons, depending
on the symmetry of the polymer chain and charge on the excitation, as we discuss in
more detail in Section 1-4-3. The application of models for infinite isolated chains to
measurements made on materials in which the polymer chains comprise relatively short
straight conjugated sections, separated by conformational or chemical defects, requires
some caution, and we discuss later how interchain interactions and disorder may modify
these isolated-chain descriptions.

A model for the electronic properties of an infinite one-dimensional chain of the
polymer trans-polyacetylene was developed by Su, Schrieffer, and Heeger.
13,14
This
model and its refinements aimed at modeling polymers are described in Sections 1-4-2
and 1-4-3.

One unique property of conducting polymers is the efficient photoluminescence
in thin film states. This property makes conducting polymers attractive and most suitable
for the applications as active elements in PLEDs. Section 1-4-4 gives the general
conception of the fluorescence from the conducting polymers. The formation of
excimers and quenching center is also discussed in order to emphasize the importance of
achieving high fluorescence efficiency of the conducting polymers. The basic
conception of band gap is also described.



13
1-4-2 Electronic structure – Ground state

1-4-2a Tight-binding models

The band structure of trans-polyacetylene has been modeled by several
groups
15,16
; the most widely used model was developed by Su, Schrieffer and Heeger
(SSH),
13,14
and involves a tight-binding calculation for a polymer chain with cyclic
boundary conditions, neglecting electron-electron interactions.

1-4-2b Quantum chemical calculations

The usefulness of the empirical Hückel models discussed in Section 1-4-2a
derives from their correspondence to the results obtained from more sophisticated
calculations. A wide variety of computational techniques has been used, ranging from
ab initio calculations to highly parameterized semiempirical methods.
17
The details of
these techniques are described elsewhere.
18


These calculations provide many of the basic parameters that define the
electronic structure relevant to the operation of polymer light-emitting diodes, including
the energy gap between the
π
and
π
* states, the widths of the

π
and
π
* bands, and the
energies of these bands with respect to the vacuum level.







14
1-4-3 Electronic structure – Excited states

1-4-3a SSH model

The fundamental excitations of the Peierls-distorted chain with a half-filled band
are known to be phase kinks, or solitons, in the pattern of the bond alternation. This was
shown for polyacetylene
13-15
to take the form of the bond alternation defects. An
important insight into the nature of these excitations from the work of Su et al.
13,14
is that
the bond alternation defect is not localized at a single carbon site, but is spread over
some 10 to 15 carbon sites. This delocalization is crucial to the energetics of the
stabilization of the soliton and is clearly demonstrated experimentally.

1-4-3b Polymers with a nondegenerate ground state

The charged excitations of a nondegenerate ground state polymer are termed
polarons or bipolarons and represent localized charges on the polymer chain, with an
accompanying local rearrangement of bond alternation. These states may be considered
as being equivalent to a confined soliton pair, and in this model, the two nonbonding
mid-gap soliton states form bonding and antibonding combinations, thus producing two
gap states symmetrically displaced about mid-gap (±
ω

0
). These levels can be occupied
by 0, 1, 2, 3, or 4 electrons, giving a positive bipolaron (bp
2+
), positive polaron (p
+
),
polaron exciton, negative polaron (p
-
), or negative bipolaron (bp
2-
), respectively.






15
1-4-3c Excitons

An exciton can be considered as a bound electron-hole pair, and may be
classified as a Frenkel exciton if the electron-hole pair is located on one molecular unit,
or as a Mott-Wannier exciton if it extends over many molecular units. The intermediate
case, where the exciton extends over a few adjacent molecular units, is termed a
charge-transfer exciton.

If the intermolecular contacts are optimized via a geometrical change following
excitation, such excitons are described as excimers (where the exciton extends over
identical molecular units) or exciplexes (where the exciton extends over two or more
different molecular units).

1-4-4 Fluorescence from conducting polymers

1-4-4a General conception

Figure 1.2 illustrates the scheme of photoluminescence (PL) and
electroluminescence (EL) of the conjugated polymers. Irradiation of a fluorescent
polymer excites an electron from HOMO to LUMO. In a typical conjugated polymer,
two new energy states are generated upon relaxation within the original HOMO –
LUMO energy gap and are each filled with one electron of opposite sign (singlet excited
state). The excited polymer may then relax to the ground state with emission of light at a
longer wavelength than that absorbed. This process is photoluminescence.




16
In a polymer LED, electrons are injected into the LUMO (to form radical anions)
and holes into the HOMO (to form radical cations) of the electroluminescent polymer.
The resulting charges migrate from polymer chain to polymer chain under the influence

of the applied electric field. When a radical anion and a radical cation combine on a
single conjugated segment, singlet and triplet exited states are formed, of which the
singlets can emit light. This process is electroluminescence.

Figure 1.2 Scheme for a) photoluminescence (PL) and b) electroluminescence (EL)
of the conjugated polymers [Kraft, A.; Grimsdale, A. C.; Holmes, A. B.,
Angew. Chem. Int. Ed., 1998, 37, 402.]


1-4-4b Excimers and quenching center

As described above, photoluminescence is believed to be the radiative decay of
singlet excited states. Therefore, photoluminescence efficiency of conjugated polymer is
one of the most important factors for the photonic devices. However, this efficiency may
be limited by quenching of extrinsic or conformational defects and intermolecular
interaction that includes excimer formation
19
. Excimer can be formed when the



17
backbones of neighboring chains are packed very closely. In this case, the
wavefunctions spread out across more than one chain. Usually, the emission from the
excimers is red shifted, spectrally broad, and inefficient
20
. Elimination of the excimer
formation can be achieved by modifying the polymers’ structures in order to get higher
photoluminescence efficiency of the photonic devices.

The nature of quenching sites in polymers is mainly due to the non-radiative
recombination through the structure defects. A small concentration of the defects will
greatly reduce the photoluminescence efficiency of a polymer because excitons can
migrate and find the location of the defects, which have an energy level within the band
gap of the polymer. Taken as a whole, stable and higher photoluminescence efficiencies
of light emitting diodes can be achieved by encapsulating the conjugated polymers
whose side chains will prevent neighboring backbones from forming excimers in an
inert atmosphere and a hermetic packaging.

1-4-4c Band gap

Band gap of the conjugated polymers can be tuned by modifying the polymers’
structures to obtain emission colors of the polymers in the visible and near infrared
regions of spectra. Band gap can be determined by the bond alternation and torsion
angles between rings in the polymers’ backbone. Also, the extent of electron
delocalization is the main factor that may determine the band gap of the conjugated
polymers. By tuning these factors, the band gap of conjugated polymers can be
fluctuated in fine increments from 1.1 eV to 3.3 eV. Polyparaphenylene (PPP) has one of
the largest band gaps (3.0 eV) of all conjugated polymers because its excited state



18
wavefunctions are localized to one repeat unit, while polyisothianaphene (PITN) has the
smallest band gap (1.1 eV) because its wavefunctions are highly delocalized and it has
minimal bond alternation.

1-5 Polymer light-emitting diodes

1-5-1 Conjugated polymer electroluminescence

Conjugated polymers exhibit a number of attractive properties for
electroluminescence. As described in Section 1-2-2, thin films can be formed over large
areas by spin-coating from solution. These films can be highly fluorescent, and different
polymers give emission colors spanning the whole of the visible spectrum.

Electroluminescence in the conjugated polymer poly(p-phenylenevinylene)
(PPV) was first observed in Cambridge in 1989.
5
In the course of investigating its
dielectric properties, a thin film of PPV was sandwiched between aluminum electrodes,
one of which was coated with an interfacial layer of aluminum oxide, and yellow-green
EL was seen through one of the electrodes when a voltage was applied. These early
devices were inefficient, and suffered from problems of nonuniformity and unreliability.
Improvements were made using indium-tin oxide for hole injection, and internal
quantum efficiencies of 0.01% have been achieved, still using aluminum for electron
injection.
21





19
A schematic energy-level diagram for a PPV LED is shown in Figure 1.3.
Polymer LEDs operate by the injection of electrons and holes from negative and positive
electrodes, respectively. Electrons and holes capture one another within the polymer
film and form either singlet or triplet excitons. Of these, the singlet excitons may decay
radiatively, giving out light which is observed through one of the electrodes, which must
be semitransparent.

PPV
I
P
Φ
ITO
∆Ε
h
∆Ε
e
Φ
Al
E
A
vacuum
LUMO
HOMO
ITO
Al

Figure 1.3 Schematic energy-level diagram for an ITO/PPV/Al LED, showing the
ionization potential (IP) and electron affinity (EA) of PPV, the work
functions of ITO and Al (φ
ITO
and φ
Al
), and the barriers to injection of
electrons and holes (∆E
e
and ∆E
h

). [Handbook of Conducting Polymers,
2nd ed.; Skotheim, T.; Reynolds, J. R.; Elsenbaumer, R. L., Eds.; Marcel
Dekker: New York, 1998.]

Barriers exist at both of the electrode and polymer interfaces. The height of the
barrier for hole injection is determined by the difference between the work function of
the anode and the energy level of the highest
π
(valence) band; the height of the barrier
for electron injection is determined by the difference between the work function of the
cathode and the energy level of the lowest
π
* (conduction) band. The barrier height can
be determined in this simple way since most conjugated polymers do not have surface



20
states that pin the Fermi level
22
.

When a positive bias is applied to the LED, the Fermi level of the cathode is
raised relative to that of the anode. Carriers tunnel across the barrier primarily by
Fowler-Nordheim field emission tunneling, and also by thermionic emission if the
barriers are small and the temperature is relatively high. Since the rate of injection due to
Fowler-Nordheim is determined by the electric field strength, it is important to keep a
thinner polymer layer that is around 100-200 nm. In this case, high electric fields can be
obtained at low applied voltages. To optimize the performance of LEDs, it is important
to minimize the barriers for charge injection by choosing the right electrodes whose

work function are well matched to the energy bands of the polymer. Indium tin oxide
(ITO), polyaniline
23
, polypyrrole
24
and poly(3,4-ethylenedioxythiophene) (PEDOT)
25

are the most commonly used anode materials because they have high work functions and
are transparent. Transparency is a very important factor for the anode because it allows
light to escape from the device. Calcium is widely chosen as the cathode material
because of its low work function. But due to its high reactivity, PLEDs must be
hermetically sealed to prevent the degradation when using calcium as cathode. Recently,
Campbell et al. and Cao et al. made a great progress in improving electron injection by
coating the aluminum (which is more stable) with a polar self-assembled monolayer.
This treatment effectively shifts the electrode work function
26,27
. If the electrodes are
well matched to the bands of the polymer, the barrier for charge injection is small and
therefore the current that passes through the LED is not limited by injection.






21
1-5-2 Efficiencies

From the schematic energy-level diagram of the polymer light-emitting device

(Figure 1.3), one can note that once the current has been applied, electrons and holes will
be injected into the polymer, they must encounter each other and recombine radiatively
to give off light. But there are several factors that will determine the efficiency of the
PLEDs.

The first factor is the degree of the carrier balance. If one carrier type is injected
much more efficiently than the other one, the majority carriers traverse the entire
polymer layer without recombining with the minority carrier. This problem can be
solved by either choosing the appropriate electrodes so that both carriers are injected
efficiently or by adding a hole or electron blocking layer so that the blocking layer
creates a barrier at the interface of two polymers that blocks the flow of the majority
carrier. Table 1.2 shows the range of efficiencies achievable with the standard PPV EL
device using an ITO anode and a variety of metals as cathodes, with and without the
electron transporting/hole blocking (ETHB) layer.
28
It is noteworthy that useful
efficiencies are already accessible using metals other than calcium.
Table 1.2 Comparison of efficiencies achieved in the low voltage regime for an
ITO-PPV-Metal EL device with an analogous device containing an extra
PBD ETHB layer

Metal Contact Work Function
(eV)
Efficiency
(PPV)
Efficiency
(PPV+PBD)
Ca 2.9 0.1% 1%
Mg 3.7 0.05% 0.35%
Al 4.2 0.002% 0.06%

Au 5.3 0.00005%




22
As the density of the majority carrier increases at the blocking interface, the
electric field at the minority carrier injecting electrode increases, which will enhance the
minority carrier injection. But some times it is very difficult to spincast multiple layers
of polymers onto the glass substrate because the first layer may be dissolved during the
spincasting of the subsequent layers. Onitsuka et al. found a way to prevent this problem
by depositing the polymers one monolayer at a time using a polyelectrolyte
self-assembly technique in which alternating layers have opposite charge and are
coulombically bound to each other
29
. Ho et al. have demonstrated that depositing a
monolayer as thin as 10-20 Å thickness can sinnificantly improve carrier balance and
enhance the performance of the PLEDs
30
.

The second factor that can determine the efficiency of PLEDs is the
photoluminescence (PL) efficiency. The photoluminescence is the fraction of
photoexcited states that recombine radiatively. Since the radiative lifetime of most
conjugated polymers is less than 1 ns and there are few non-radiative channels for
relaxation, the PL efficiency can be relatively high. Many conjugated polymers have PL
efficiencies higher than 60%
31
. It is generally accepted that when an electron and a hole
are combined, approximately 25% of the electron-hole pair in an LED form singlet and

75% of the pair form triplet states. Since triplet excitations are non-emissive, EL
efficiency can be no greater than 25% of the PL efficiency
32
. Interestingly, Cao et al.
have made LEDs with an EL efficiency of 50% PL efficiency, and demonstrated that all
electron-hole pairs are potentially emissive
33
.

The third factor that determines the luminescence efficiency of LEDs is the
quenching behavior by the metal electrode. This behavior can be caused when electrons



23
and holes recombine too close to one of the metal electrodes
34
. The primary mechanism
for the quenching is interference between the radiation field from virtual image
oscillators in the metal. Metal quenching is a serious problem for extremely thin devices
or for polymers whose electron mobility is so low that the electrons are not able to travel
away form the cathode. The quenching behavior can be avoided by using an
electron-transporting layer that also acts as a hole-blocking layer and thereby moves the
emission region away from the metal region.

1-5-3 Control of color

One of the attractive features of polymer EL is that the color of emission can be
controlled by altering the chemical structure of the polymer. PPV gives emission in the
yellow-green; the emission color can be moved toward the red by the substitution of

electron-donating groups such as alkoxy chains at the 2- and 5- positions on the phenyl
ring.
35
Substituents can also cause changes in energy gap through steric, rather than
electronic effects by disrupting the conjugation along the chain. The substitution of
bulky cholestanoxy groups, for example, has been used to obtain green emission in
soluble polymers.
36
Other polymer systems, not based on PPV, can also be used for EL;
the poly(alkylthiophenes), for example, conveniently give emission in the red region of
the spectrum.
37-40


An alternative strategy to obtain blue emission is the use of completely different
conjugated polymer systems. Blue EL has been reported in poly(p-phenylene) (PPP),
41

poly(alkylfluorene),
42
fluorinated polyquinoline,
43
and PPP-based ladder
copolymers.
44,45
One problem that is found for the larger gap polymers is that although



24

the luminescence from isolated polymer chains (in solution or in solid solution) may be
blue, solid films often show red-shifted emission. This is observed for the PPP-ladder
polymers, for which the dominant emission band is in the yellow part of the spectrum,
46

and it has been established that this is due to formation of aggregates in the solid film,
which support excitons that extend over more than the single chain.
47
The tendency to
form aggregates can be controlled by introduction of disorder in the ladder-PPP
copolymers, which can produce good blue emission.
44,45
Soluble copolymers of PPP and
alkyl- or alkoxy-substituted PPP have also been used to produce blue EL.
48-50


1-5-4 Operating lifetime

The operating lifetime of PLEDs means the time needed for light emission to
degrade to half of the initial value at constant current. Since the novel conjugated
polymers are regarded as the new generation light emitting materials, there is one
question existed: can commercial display products with sufficient lifetime be achieved
with polymers processed from solution? Until recently, there was doubt that the level of
purity required for semiconductor applications could be achieved. However, recent
progress at UNIAX Corporation has demonstrated that high performance PLEDs can be
fabricated with long operating life. At 400-500 Cd/m
2
initial brightness, room
temperature operating lifetime of several thousand hours can be achieved. Accelerated

lifetime studies at 85 °C (initial brightness of 100 Cd/m
2
) have demonstrated in excess of
400 hours to half brightness, indicative of more than 40,000 hours at room temperature
(independent measurements give an acceleration factor of approximately 100 between
85 °C and room temperature). Thus, PLEDs fabricated with materials processed from
solution and spincast onto substrates can meet the requirements for commercial products



25
with operating lifetime in excess of 10,000 hours at display-level brightness.

1-5-5 Towards applications

Polymer LEDs are expected to have commerical applications as backlights for
liquid crystal displays and as the emissive material in alphanumeric displays within the
near future. Extension to a full-colour graphic display (for computer monitors and for
video display) is very attractive. However, this required red, green and blue colours with
appropriate chromaticity, methods for colour patterning, and also new addressing
schemes. Rapid progress is being made with these three problems. Development of full
colour has been reported for polymers
51
. Solution-processing of polymers offers new
methods for colour patterning, among which there is particular interest in ink-jet
printing, to place separated pixels of red-, green- and blue-emitting polymers onto the
prepared substrate. This is being developed by Seiko-Epson and Cambridge Display
Technology
51
, and the use of ink-jet printing has also been reported by other groups

52,53
.
Active-matrix transistor arrays, modified from those at present used for liquid-crystal
displays, can now provide sufficient current-driving capability to meet the requirements
of polymer LEDs. Demonstrator active-matrix polymer displays have been made
51
.

1-6 Polyfluorene derivatives and light-emitting diodes

Poly(9,9-dialkylfluorene)s have recently received a lot of attention that can be
attributed to a major issue: the possibility that they could be used to develop all plastic,
full-color, light-emitting diodes. An important driving force for this research is the
dream of building ultrathin and flexible screens for computers and televisions. The first