580

POLY(p-PHENYLENEVINYLENE)

Vol. 7

POLY(p-PHENYLENEVINYLENE)

Introduction

Poly(p-phenylenevinylene) (PPV) and its derivatives are polymers that have been
widely studied because of their potential applications in optoelectronic devices.
PPV is a conjugated polymer, with a backbone consisting of alternating single
and double bonds. Many conjugated polymers are known and exhibit remark-
ably high electrical conductivities when oxidatively or reductively doped (1) (see
E

LECTRICALLY

A

CTIVE

P

OLYMERS

). Much of the early research on PPV focused on

the relatively disappointing properties of the doped material; however, interest
in this material was reawakened in 1990 when Friend and co-workers in Cam-
bridge discovered that films of undoped PPV could be used as the emitting layer
in organic electroluminescent (EL) devices (2). This discovery stimulated intense
research in the area, including many fundamental studies into the properties of
PPV and its derivatives, as well as extensive academic and industrial interest on
the applications of the materials (see L

IGHT

-E

MITTING

D

IODES

).

This article seeks to provide an overview of the methods by which PPV and

some of its derivatives may be prepared, the physical and electronic properties
of these materials, and the applications that are being explored. The reader who
seeks a deeper and more detailed understanding of this fascinating material is
referred to several excellent and comprehensive reviews that have been published
on the synthesis, properties, and applications of PPV (3–7).

Methods of Preparation

Unsubstituted PPV.

Many methods have been devised to prepare PPV

for fundamental and applied studies. Because of its rigid conjugated backbone,

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

Vol. 7

POLY(p-PHENYLENEVINYLENE)

581

unsubstituted PPV and even short oligomers are insoluble and intractable
materials. An important consideration in all preparative routes that seek to pre-
pare high molecular weight material, therefore, is the solubility of the growing
polymer chain. One of the most successful approaches to high molecular weight
(10,000–100,000) PPV is the Wessling or sulfonium precursor route, which pro-
ceeds via a soluble precursor polymer that is subsequently thermally converted
to fully conjugated PPV. Other methods are available but generally produce low
molecular weight material.

Sulfonium Precursor Route.

In this route, polymerization of the bis sulfo-

nium salt 1 with base yields a soluble polyelectrolyte 2 (8,9). This intermediate
may then be purified, processed, and finally thermally converted to PPV. Both
the nature of the sulfide used in the sulfonium salt and the counterion affect
the conditions required in the preparation, as well as the molecular weight and
structure of the resulting polymer (9,10). When dodecylbenzenesulfonate is used
as a counterion, conversion to PPV is achieved in 3 min at only 115

◦

C (11). A

modified sulfonium precursor route has also been developed, in which the soluble
methoxy-substituted polymer 3 is converted to PPV in the presence of HCl gas
(12).

X

X

S

S

+

S

+

NaOH

MeOH/H

2

O

S

+

n

∆

vacuum

or

H

2

/N

2

MeOH

OMe

HCl(g)

X

= Cl, Br

PPV

2

1

n

3

n

2X

−

The mechanism of the polymerization has been the subject of some debate in

the literature, with both radical and anionic mechanisms proposed. The presence
of oxygen during the polymerization results in lower molecular weight polymer
consistent with a radical mechanism (8). A p-xylylene or p-benzoquinodimethane
intermediate is postulated and has been observed spectroscopically. Other studies
have suggested that this intermediate polymerizes via an anionic mechanism (13,
14). Despite these conflicting results, the polymerization is typically carried out
in the absence of oxygen, and the thermal conversion step done either in vacuum
or under an inert or forming gas atmosphere.

Other Routes.

Other synthetic routes to PPV include the Wittig reaction

(eq. 1) (15), Pd-catalyzed Heck reaction (eq. 2) (16), and McMurry coupling (eq. 3)
(17). Reaction of

α,α



-dichloro-p-xylene with potassium tert-butoxide yields PPV

582

POLY(p-PHENYLENEVINYLENE)

Vol. 7

(eq. 4); this procedure was discovered by Gilch and Wheelwright and is referred
to as the Gilch route (18). All of these methods yield PPV directly from soluble
monomers, and thus produce primarily low molecular weight oligomers.

Ph

3

+

P

P

+

Ph

3

2Cl

−

+

H

O

H

O

Base

n

(1)

Br

n

Pd(OCOCH

3

)

2

DMF

(2)

n

O

H

O

H

TiCl

4

/Zn

(3)

ClCH

2

n

CH

2

Cl

KO-t-Bu

(4)

Electropolymerization is a convenient method for the preparation of insolu-

ble conjugated polymers such as PPV because it yields thin films directly on an
electrode surface. Such a film may then be directly utilized in an application re-
quiring a conducting contact, such as electroluminescence. Several methods have
been reported for the electrochemical preparation of PPV films. Electrochemical
reduction of 4,5,6 has been shown to yield PPV films on metal and indium tin ox-
ide (ITO) electrodes (19–21). Another direct route to thin-films is chemical vapor
deposition (CVD) from precursors such as 7 or 8 (22,23).

Ph

3

+

P

P

+

Ph

3

2Br

−

Br

2

HC

CHBr

2

CCl

3

Cl

3

C

4

5

6

ClH

2

C

CH

2

Cl

Cl

Cl

8

7

Ring-opening metathesis polymerization (ROMP) has been used to prepare

unsubstituted PPV via a soluble precursor. Substituted paracyclophan-1-ene (9)
(24) and norbornadiene derivatives (10) (25) have been used as monomers, with

Vol. 7

POLY(p-PHENYLENEVINYLENE)

583

Mo(NAr)(CHC(CH

3

)

2

C

6

H

5

)[OCCH

3

(CF

3

)

2

]

2

(Ar

= 2,6-diisopropylphenyl) used as

the catalyst in the polymerization. This route proceeds via a living polymerization
and consequently yields a relatively narrow molecular weight distribution.

OSi(Bu)Me

2

Me

2

(Bu)SiO

n

1. Bu

4

NF

2. HCl

n

9

H

3

COCO

OCOCH

3

n

∆

OCOCH

3

OCOCH

3

10

[Mo]

[Mo]

Substituted PPV.

In order to increase the solubility and processibility of

PPV in the conjugated form, substituents such as alkoxy or phenyl groups have
been added to the backbone structure. In addition to enhancing the solubility, these
substituents also change the electronic properties of the polymer via both inductive
and conjugative effects. An additional benefit of soluble derivatives of PPV is
that techniques for polymer characterization that require soluble material may
be brought to bear on, and provide direct information about, molecular weights
and higher-order structure.

Sulfonium Precursor Route.

The Wessling route has also been used to pro-

duce soluble derivatives from monomers containing solubilizing substituents on
the phenyl ring. For example, dialkoxy-substituted monomers yield 11, which is
soluble in organic solvents such as chloroform and chlorobenzene (26), as well
as poly[2-(2-ethylhexyl)oxy-5-methoxy-p-phenylene)vinylene], or MEH–PPV (12)
(eq. 5) (27). The branched side chains in MEH–PPV improve the solubility of this
derivative over unbranched analogs, and this polymer is one of the most popular
for use in electroluminescence applications.

RO

S

+

S

+

OR

′

2 Cl

−

Base

RO

OR

′

n

11 R, R

′ = hexyl

12 R = methyl, R

′ = 2-ethylhexyl (MEH-PPV)

(5)

Other Routes.

Substituted PPV derivatives have also been prepared by

several other routes. In cases where the resulting polymers are soluble, these
methods are often successful in preparing high molecular weight material. The
Gilch route has been used to prepare phenyl and crown ether substituted polymers

584

POLY(p-PHENYLENEVINYLENE)

Vol. 7

(13 and 14) (eqs. 5 and 6) (28,29), and McMurry coupling has been used to yield
a dihexyl-substituted polymer (15) that is soluble in a range of organic solvents
(eq. 7) (30).

Br

Ph

Br

Ph

n

18-crown-6

13

KO-t-Bu

(6)

O

O

O

O

O

O

Br

Br

DMF

O

O

O

O

O

O

n

14

KO-t-Bu

(7)

H

O

C

6

H

13

H

O

C

6

H

13

C

6

H

13

C

6

H

13

n

Zn/Cu

DME

TiCl

3

15

(8)

Copolymers.

A number of studies have focused on the preparation of

copolymers containing fully conjugated backbones, as well as those containing
both conjugated and nonconjugated blocks. This work has been motivated by the
desire to prepare materials having a range of electronic properties that can easily
be tuned by variations in the proportion and nature of the monomers used in the
copolymer synthesis. In addition, local variations in the

π–π

∗

energy gap can be

introduced in this way, which has been shown to result in dramatic improvements
in the performance of these copolymers in electroluminescent devices.

S

+

S

+

1

S

+

S

+

MeO

OR

2Cl

−

2Cl

−

+

1. NaOH

2. HCl(aq)
3. Dialysis

S

+

OMe

MeO

OR

OMe

n

m

−o

o

Cl

−

vacuum

220

°C

MeO

OR

a

+c

b

+d

220

°C, HCl

MeO

OR

MeO OMe

OR

OMe

d

c

b

a

17

16 R = Me, 2-methylpentyl, 2-ethylhexyl

Vol. 7

POLY(p-PHENYLENEVINYLENE)

585

The sulfonium precursor route has been used to prepare copolymers by us-

ing various proportions of different monomers in the synthesis. This method has
yielded both partially conjugated (16) and fully conjugated polymers (17) (31,32),
as well as copolymers containing other aromatic groups in the backbone in addi-
tion to phenylenevinylene moieties such as in copolymer 18 (33).

R

R

S

x

y

18 R = H, OMe Cl

The Wittig reaction has also been successfully used to prepare soluble copoly-

mers with substituents such as alkoxy groups on the backbone (19) (34), as well as
copolymers containing flexible, nonconjugated spacers (20 and 21) (35,36). Heck
chemistry has been applied to prepare copolymers such as 22 (37).

R

MeO

OMe

OR

′

CF

3

R

F

3

C

O(CH

2

)

6

O

R

n

20 R = OC

8

H

17

19 R = H, OR

′; R′ = (CH

2

)

15

CH

3

,

(CH

2

)

11

CH

3

, (CH

2

)

7

CH

3

O

(C

2

H

4

O)

n

n

OR

RO

n

22 R = C

n

H

2n

+1

(n = 4

−16)

21

Composites and Blends.

The mechanical and optoelectronic properties

of PPV and its derivatives may be optimized for specific applications through the
use of polymer blends (qv) and Composite materials (qv). Blends of MEH–PPV
and polyethylene have been used to significantly enhance the degree to which the
conjugated polymer chains are aligned upon stretching (38). The density of entan-
glements in gels of polyethylene is much lower than in spin-cast polymer solutions,
and this effect remains after removal of the solvent. This allows tensile drawing of
such blends to large draw ratios (

>200), which causes the conjugated polymers to

align to a degree normally expected in single crystals. Charge transport through
blends can also be controlled by using a host polymer with specific properties. Blue
emitters have been prepared from blends of poly(p-phenylphenylenevinylene)
(PPPV) in poly(9-vinylcarbazole) (PVK), a hole-transporting polymer (28). In ad-
dition to enhancing the processibility of the PPPV, the PVK blue-shifts the electro-
luminescence, enhances hole-transport, and increases the probability of radiative
recombination because of the dilution effect.

586

POLY(p-PHENYLENEVINYLENE)

Vol. 7

Composites of PPV with silica (SiO

2

) and vanadium oxide (V

2

O

5

) have been

prepared for use in nonlinear optical applications (39–41). These composites are
prepared via sol–gel processing methods, and allow the superior nonlinear optical
properties of the conjugated polymers to be combined with the very low optical
losses found in inorganic glasses. In homogeneous nanocomposites of PPV with
SiO

2

, inter- and intrachain disorder in the polymer chains is disrupted when the

silica interparticle separation is comparable to the coherence distance in the poly-
mer. This influences the absorption and emission spectra, and results in reduced
electronic conductivity (42). Composites of PPV with gold-coated silica nanopar-
ticles showed enhanced stability to photo-oxidation, an effect attributed to an
electronic interaction of the metal particles with the triplet excitons in PPV (43).
Composite films of insulating SiO

2

, TiO

2

, and Al

2

O

3

nanoparticles with MEH–

PPV were prepared and result in more efficient charge injection and transport
in electroluminescent devices formed from them, as well as enhanced emission
intensities (44,45). Photovoltaic and time-resolved microwave conductivity mea-
surements were also used to study nanocrystalline TiO

2

/PPV composites, showing

that excitons generated in the polymer are dissociated at the polymer/TiO

2

inter-

face, with the electrons transferred to the nanocrystals (46). Indium arsenide-zinc
selenide nanocrystals have been blended with MEH–PPV, giving materials emit-
ting in the near-IR region (1–1.3

µm). External efficiencies of devices made from

these composites were

∼0.5% (47).

Composites of PPV in films of the polyelectrolyte Nafion have been synthe-

sized by electrostatically binding the dicationic monomer to the film, followed
by treatment with base and thermal conversion (48). Ordered nanocomposites
of PPV have been synthesized from mixtures of polymerizable lyotropic liquid
crystals with PPV precursors (49). Photopolymerization of the host results in a
hexagonal architecture, which can be fabricated into fibers and thin-films. A sig-
nificant enhancement in photoluminescence of the composite relative to PPV was
found.

Properties

Absorption and Emission.

Films of PPV show three absorption bands,

with maxima at 6.12, 5.06, and 3.09 eV respectively (50). The two higher-energy
bands are attributed to localized molecular states, while the lower-energy band
is due to a delocalized electronic excitation. The emission spectrum of PPV upon
excitation at 355 nm is resolved into three lines with a spacing of 0.16 eV because
of vibronic coupling (51). Migration of the excited state to the longest conjuga-
tion length segments in the polymer appears to occur prior to radiative decay,
since smearing out of the vibrational fine structure is expected if emission from a
distribution of sites within the polymer occurs. The photoluminescence efficiency
varies between 5 and 25%, depending on the synthetic route used and the conver-
sion conditions (52).

Two descriptions of the excited state have been applied to organic semi-

conductors, the exciton and the band model. The appropriate model depends
on the extent of coupling between sites, with strong coupling yielding uncorre-
lated electrons and holes, while weak coupling favors correlated electron–hole

Vol. 7

POLY(p-PHENYLENEVINYLENE)

587

pairs (excitons). Time-resolved fluorescence and polarized fluorescence experi-
ments suggest that the exciton model is appropriate in PPV (53). Rothberg and
co-workers have examined the relative effects of interchain vs. intrachain excita-
tions in MEH–PPV by comparing excited-state lifetimes and quantum yields in
films to dilute solutions (54). They observed significantly lower quantum yields for
emission in the films, and attributed this to the formation of nonemissive inter-
chain excitons, which are not formed in dilute solution. They have also concluded
that film morphology can play a significant role in the photophysical behavior of
PPV (55). The presence of trace oxygen in the conversion of precursor polymer to
PPV reduces the photoluminescence of the resulting material (56). This was shown
to be correlated to the formation of carbonyl groups in the polymer backbone, and
can be prevented by carrying out the conversion in a reducing atmosphere.

Photoconductivity.

The first measurements of photoconductivity in PPV

were carried out in the early (57). A later study revealed low dark conductivities
for PPV films (

<10

− 15

S/cm), but significant photoconductivity upon irradiation

at 440 nm. Significant conductivity was also found upon irradiation of the film in
the near-IR region, despite insignificant optical absorption in this region. This was
attributed to a charge-transfer mechanism involving trace oxygen (58). Oriented
films of PPV showed enhancements in the photoconductivity with light polarized
parallel to the direction in which the films were stretched (59). Transient photocon-
ductivity measurements have also been used to address the question of the nature
of the charge carriers in MEH–PPV films. The exciton model predicts a strong de-
pendence of photoconductivity on temperature, and this is indeed observed for
films greater than 120 nm in thickness. For thinner samples, both steady-state
and fast time-resolved photoconductivity measurements demonstrate that photo-
conductivity is independent of temperature. These results are inconsistent with
the exciton model (60).

Doping and Electrical Conductivity.

Pristine PPV films are insulators

at room temperature; however, exposure to oxygen causes an increase in conduc-
tivity to 10

− 11

S/cm—an effect attributed to reversible doping, with the oxygen

acting as an electron acceptor (61). Irreversible doping of PPV films with strong
oxidants such as FeCl

3

or H

2

SO

4

produces black films with very high conductivi-

ties relative to the pristine material (62). Films doped with sulfuric acid showed
conductivities of

∼10

4

S/cm that were only weakly temperature-dependent, in-

dicative of metallic behavior. The conductivity of films doped with FeCl

3

was

slightly lower (10

3

S/cm) and decreased with temperature. The doped films are

stable in oxygen, but are moisture-sensitive. Copolymers of PPV derivatives con-
taining electron-donating groups, such as poly(1,4-phenylenevinylene-co-2,3,5,6-
tetramethoxy-1,4-phenylenevinylene) (23), can also be doped with weaker oxi-
dants such as I

2

to give materials with conductivities as high as 7

× 10

− 2

S/cm

(63).

m

n

OMe

MeO

23

MeO

OMe

588

POLY(p-PHENYLENEVINYLENE)

Vol. 7

Microstructure and Liquid Crystallinity.

The degree of structural order

in PPV films depends greatly on the method of preparation. The degree of broaden-
ing in electron diffraction patterns has been used to assess the extent of ordering
in different PPV samples (64). High resolution transmission electron microscopy
revealed crystalline regions approximately 7 nm in size in oriented PPV films,
and these crystallites are retained upon doping the films with H

2

SO

4

(65). Sub-

stituents also have an effect on the structural order of PPV derivatives. Methoxy
substituents allow chains to interlock and form a more ordered three-dimensional
structure, while methyl groups cause the backbone to distort because of repulsive
interactions between the backbone and side groups, resulting in a more disordered
material (66). X-ray diffraction studies on MEH–PPV films cast from different
solvents have shown that the chain orientation is anisotropic, and the aromatic
rings are oriented predominantly parallel to the film plane (67). The crystallinity
of films cast from THF is highest, and the crystalline domains largest, compared
with films prepared from other solvents.

n

24

(CH

2

)

9

O

CN

CH

2

CH

2

n

25

Cl

O

O

O

O

(CH

2

)

10

n

26

Liquid crystallinity has been observed in derivatives of PPV bearing meso-

genic substituents, as well as in copolymers containing phenylenevinylene seg-
ments in the main chain. Copolymers in which some of the phenyl rings have
alkoxy side chains (22) exhibit a nematic liquid crystalline phase, which has
been characterized by polarized microscopy and differential scanning calorimetry
(36). The temperature range between the melting point and the nematic–isotropic
phase transition was found to depend on the length of the alkoxy group. A PPV
derivative (24) bearing the well-known cyanobiphenyl mesogen as a side chain
has both nematic and smectic mesophases (68). This polymer was oriented by
rubbing a film with a Teflon stick, and a significant degree of orientation observed
by polarized UV/vis and IR spectroscopies. Several examples of main-chain liquid
crystalline polymers containing phenylenevinylene moieties bridged by saturated
linkers are known. The thermotropic polymer 25 was prepared using a Wittig pro-
cedure, and was found to melt anisotropically (69). A related main-chain polymer
(26) has a mesophase that exists between 218 and 275

◦

C (70).

Nonlinear Optical Properties.

For many optical signal processing appli-

cations, it is desirable for materials to have large optical nonlinearities and fast
response times. For example, third-order nonlinear optical properties (qv) result
in laser-pulse–induced refractive index changes that occur on the femtosecond

Vol. 7

POLY(p-PHENYLENEVINYLENE)

589

timescale. These changes could potentially be exploited in fast optical switches.
Conjugated polymers are expected to be good candidates for such applications
because of the delocalization of charge in the polymer backbone (71).

n

BuO

O

N

N

N

O

2

N

27

PPV has many of the characteristics desired in an NLO material, including

good transparency, high

π-electron density, and optical quality films that may be

oriented and ordered. PPV has a third-order nonlinear optical susceptibility (

χ

(3)

)

of 7.8

× 10

− 12

esu (72), while

χ

(3)

for a substituted derivative, poly(2,5-dimethoxy-

p-phenylenevinylene), is 5.4

× 10

− 11

esu at 1.85

µm (73). The higher value for the

dimethoxy-substituted derivative may be due to more extended conjugation in
this material. An alternative strategy that has been investigated is to introduce
an NLO-active moiety pendent to a PPV backbone. For example, Disperse Red 1
has been tethered in this way (27), and the resulting polymer has a

χ

(3)

value of

2.5

× 10

− 12

esu (74).

Applications

Photovoltaics.

Heterojunctions between conjugated polymers and films

of electron acceptors behave as rectifying p–n junctions and may be used in pho-
tovoltaic devices. Such junctions have been prepared by vacuum evaporation of
n-type buckminsterfullerene (C

60

) onto spin-cast films of p-type MEH–PPV on

ITO/glass substrates (Fig. 1) (75). These devices behave as rectifiers in the dark,
and pass a photocurrent upon illumination with visible light. The open circuit
voltage (V

oc

) saturates at 0.53 V, with a fill factor of 0.48 and a power conversion ef-

ficiency of 0.04%. Photovoltaic devices made from blends of PPV with a processible
methanofullerene showed power efficiencies of 0.25% under white light (76). The
Cambridge group has also reported photovoltaic devices in which a heterojunction
between bis(phenethylimido)perylene and PPV is sandwiched between ITO and
Al. These devices had a somewhat greater fill factor (0.6) and V

oc

approaching 1 V,

with a quantum yield of 6% (electrons per incident photon) (77). A related approach
involves using a blend of MEH–PPV and bis(phenethylimido)perylene, giving a
fill factor of 0.27 at an open circuit voltage of 0.58 V; however improvements to
these devices are limited by the poor solubility of the bis(phenethylimido)perylene
(78).

590

POLY(p-PHENYLENEVINYLENE)

Vol. 7

䊞

䊝

Au/Al

C

60

MEH–PPV

ITO/glass

Fig. 1.

Schematic of a MEH–PPV-based photovoltaic cell.

Photovoltaic cells have also been constructed from light-emitting electro-

chemical cells (LECs, see below) (79). In these devices, a phase-separated blend
of MEH–PPV and cyano-PPV is used along with a solid electrolyte consisting
of a mixture of polyethylene oxide (PEO) and LiCF

3

SO

3

. Sandwich photovoltaic

cells using Al and ITO as the electrode contacts were doped using a prebias of
3 V, resulting in a V

oc

of 1.0 V and a power conversion efficiency of 0.1%, as-

suming a fill factor of 0.25. The built-in potential is determined by the chemi-
cal potential difference between the p-doped and n-doped layers, rather than the
work function of the electrodes, thus air-stable electrodes can be used in these
cells.

Optical Memory.

The permanent storage of data in films of PPV deriva-

tives has been accomplished by irradiating films of the sulfonium precursor poly-
mers with either a Xe arc lamp or Ar ion laser (488 nm) (80). Subsequent heating
of the films resulted in the formation of colored, conjugated films only in regions
that were not irradiated. Photochemical scission of the polymer chains leaves a
water-soluble residue, which is readily removed by rinsing the heat-treated films
with water. This process may also be used for the lithographic patterning of PPV
films onto substrates.

Light-Emitting Devices.

In the late 1980s Tang and VanSlyke reported

electroluminescent devices that utilized thin films of 8-hydroxyquinoline alu-
minum (Alq

3

) as the emitting material (81). They distinguished these devices

from those based on conventional inorganic semiconductors by calling them “or-
ganic,” despite the fact that the emissive compound is actually an inorganic co-
ordination complex. The devices consisted of a layer of a hole-transporting aro-
matic amine on an ITO electrode, 60 nm of the luminescent Alq

3

and a Mg/Ag

electrode. The devices behaved like rectifiers, and emitted light with a peak in-
tensity at 550 nm, with a forward bias of as little as 2.5 V. In a subsequent pub-
lication, Tang and VanSlyke showed that doping of the Alq

3

layer with other

highly fluorescent molecules, such as coumarin 540, increases the electrolumi-
nescence efficiency and allows tuning of the color from blue-green to orange-red
(82).

Vol. 7

POLY(p-PHENYLENEVINYLENE)

591

N

O

N

O

Al

N

O

Alq

3

In 1990, the discovery that PPV could be utilized as the emitter in an elec-

troluminescent device was reported by Friend and co-workers at the Cavendish
Laboratory in Cambridge (2). The devices consisted of a PPV film, prepared using
the sulfonium precursor route, sandwiched between an ITO and an Al electrode,
and emitted green-yellow light under forward bias of 14 V, with a quantum ef-
ficiency of up to 0.05%. Shortly after this initial publication, Braun and Heeger
demonstrated that MEH–PPV could also be used to fabricate EL devices in which
the polymer was directly cast in the conjugated form from solution. They used
both indium and calcium cathodes, and observed visible light at 4-V forward bias
with the calcium cathode, with an efficiency of 1%.

Device Operation.

Single-layer devices consist of an electroluminescent

layer sandwiched between an electron-injecting cathode (usually a low work func-
tion material such as Ca or Al) and a hole-injecting anode (most commonly the
transparent conductor ITO on glass).

The operation of the device under forward bias may be understood using a

simple band diagram (Fig. 2a). The anode and cathode materials are chosen to
provide low barriers to electron and hole injection by matching the valence and

electrons

Energy

emitted
light

holes

h

+

e

−

recombination z

one

cathode

EL polymer

anode

(a)

(b)

Energy

emitted
light

holes

h

+

h

+

h

+

h

+

e

−

recombination z

one

cathode

EL polymer

anode

ETL

Fig. 2.

Band diagram for (a) single-layer and (b) two-layer polymer EL devices (ETL

=

electron transporting layer).

592

POLY(p-PHENYLENEVINYLENE)

Vol. 7

conduction band energies of the polymer to the electrode work functions. A dis-
advantage of the single-layer device configuration is that charge recombination
often occurs close to the cathode, since most EL polymers are better hole conduc-
tors than they are electron conductors. The metal electrodes are able to quench
excitons in close proximity, thus reducing the EL efficiency.

An approach that has been used successfully to move the emitting zone away

from the electrodes is the construction of two-layer cells in which recombination
occurs at the interface between the two organic layers (Fig. 2b). Here, the material
in the layer adjacent to the cathode is selected to have a high electron mobility
but a lower hole mobility than the EL polymer that is located adjacent to the
anode. Thus, electrons and holes are readily injected into the adjacent layers
contacting the respective electrodes and accumulate at the interface between the
two layers. A number of materials have been exploited for use as the electron-
transporting layer (ETL), including 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-
oxadiazole (butyl PBD) 28 (83). Two-layer devices constructed with PPV as the
emitting layer and butyl PBD dispersed in poly(methylmethacrylate) (PMMA) as
the ETL showed a tenfold improvement in efficiency relative to analogous single-
layer devices constructed using only PPV. Polymers containing oxadiazole moieties
pendent to the backbone, as well as in the main chain, have been synthesized and
tested as ETLs, and also act to improve external quantum efficiencies in two-layer
devices (84). The EL efficiency was found to be temperature-independent in these
devices, suggesting that charge injection from both electrodes is well-balanced.
Multiwalled carbon nanotubes have been used as the ETL in devices, leading to
an increase in brightness (85).

N

N

O

C(CH

3

)

3

28

Hole injection from the anode can be improved via the use of a hole-

transporting layer, which functions by improving hole injection into the elec-
troluminescent layer and maximizing recombination of injected charges. Op-
timally, these materials should possess high glass-transition temperatures
(T

g

) and low ionization potentials. A large variety of triaryl amines have

been investigated for use as hole-transport materials, including N,N



-diphenyl-

N,N



-di-m-tolyl-1,1



-biphenyl-4,4



-diamine (TPD) (29) (86), N,N



-1-naphthyl-N,N



-

diphenyl-1,1



-biphenyl-4,4



-diamine (NPD) 30 (87), 1,3,5-tris(2



-anthracyl-4



-

methoxyphenylamino)benzene (31) (88), and the atropisomeric cis- and trans-5,11-
dihydro-5,11-di-1-napthylindolo[3,2-b]carbazole (32) (89).

Vol. 7

POLY(p-PHENYLENEVINYLENE)

593

N

N

29

N

N

30

N

OCH

3

N

N

OCH

3

H

3

CO

N

N

31

trans-32

The brightness of EL polymer based devices varies depending on the de-

vice configuration and polymer used. Uniax Corporation has reported ITO/PANI–
CSA/MEH–PPV/Ca (PANI–CSA

= polyaniline–camphor sulfonic acid) devices

with a brightness as high as 10,000 cd/m

2

for bias voltages

>5 V, over twice the

brightness of a fluorescent lamp (90). Device brightness can be increased by using
short drive pulses, which permit high current operation (91). Luminous efficien-
cies as high as 0.8 cd/A could be achieved in this way using current densities
of 1 kA/cm

2

in single-layer PPV-based devices. Poly(3,4-ethylenedioxythiophene)

(PEDOT) doped with polystyrene sulfonic acid (PSS) has also been investigated
as a polymeric anode, and has been shown to result in increases in efficiency,
brightness, and lifetime for devices based on MEH–PPV (92). When PPV is ther-
mally converted with a PEDOT/PSS overlayer a large surface area interface re-
sults because of interpenetration of the two layers, possibly contributing to the
improvement in quantum efficiency measured for these devices (93).

An application for which polymer-based EL devices may be uniquely suited is

as flexible emitting materials. Such devices were first reported in 1992 by workers
at Uniax, using a thin layer of PANI–CSA on flexible poly(ethylene terephthalate)
(PET) as the anode, an MEH–PPV film as the emitter and vacuum-deposited
Ca as the cathode (94). The PANI–CSA film is light green in color; however the
absorption is low in the region of the spectrum where MEH–PPV emits (500–
700 nm), so emission is readily observed through the anode.

Doped silicon has also been used as a hole-injecting electrode material (95).

Devices consisted of an MEH–PPV film on a thin SiO

2

layer atop a heavily p- or

594

POLY(p-PHENYLENEVINYLENE)

Vol. 7

n-doped Si substrate. The top contact was a semitransparent Ca or Au electrode,
and light emission was observed through this layer. The insulating SiO

2

layer

allows the electrode Fermi level to be matched to the conduction band of the
polymer, thus permitting the devices to operate in reverse bias with a high work
function cathode material such as Au. The Cambridge group has also investigated
the effect of separating the polymer layer from the Al cathode with a thin SiO

2

layer. This results in an increase in PL quantum efficiency since the emission zone
is brought away from the metal interface (96).

O

O

O

O

m

n

33

Circularly polarized electroluminescence (CPEL) has been obtained from a

single-layer device in which the emitter is a PPV derivative with chiral pendent
groups (33) (97). The CPEL is due to the inherent chirality of the polymer, since
polymers containing racemic side groups did not show any effect; however aggre-
gation of the polymer chains in the solid state is believed to enhance the magnitude
of the effect.

The external efficiencies of electroluminescent devices based on PPV deriva-

tives are typically between 0.2 and 4% (4), comparable to the efficiencies of inor-
ganic EL devices. In addition to the use of electron- and hole-transporting layers
to enhance efficiencies, other strategies have also been used. Son and co-workers
reported polymers in which cis linkages were intentionally introduced into the
PPV backbone (98). These defects prevent the polymer chains from packing effi-
ciently in the solid state, thus producing amorphous PPV films. Both single- and
two-layer devices prepared from this material had significantly higher efficiencies
than comparable devices fabricated using PPV synthesized in the conventional
fashion (high trans content). This improvement in efficiency may result from bet-
ter chain separation in the amorphous material.

Degradation of polymer EL devices leading to limited lifetimes is a serious

problem, particularly for commercial applications that typically demand lifetimes
of

>10,000 h. The low work function metals used as cathode materials (Al, Ca, In)

are all air- and moisture-sensitive to varying degrees. Encapsulation to prevent
intrusion of air and water is necessary for practical devices. The polymer layer has
also been shown to be susceptible to degradation by singlet oxygen, which can form
from oxygen impurities in the film by energy transfer from a nonradiative exciton
in the polymer (99). Strict removal of oxygen in the film preparation process thus

Vol. 7

POLY(p-PHENYLENEVINYLENE)

595

results in longer device lifetimes. Oxidation of MEH–PPV has been shown to
result in reduced fluorescence because of the formation of carbonyl groups, along
with reduced carrier mobility as a consequence of chain scission. Localized shorts
also form in these devices, which are initially isolated by melting of the cathode;
however these eventually coalesce, causing device failure (100).

Although most studies have been carried out on PPV prepared using solution

processing, EL devices have also been prepared from PPV prepared by chemical
vapor deposition (101). The advantage of this approach is that many of the side
reactions that typically occur during solution processing are eliminated, and this
method is also compatible with existing methods for processing of inorganic LEDs.
Single-layer devices prepared in this way had turn-on voltages as low as 4.5 V,
with a brightness of 20 cd/m

3

.

m

n

O

2

CCH

3

34

Modifications of the PPV structure have also been demonstrated to have an

effect on the performance of EL devices (102). A PPV copolymer bearing acetate
side groups (34), which disrupt the conjugation in the backbone, has been used
to fabricate encapsulated single-layer devices with Al cathodes and ITO anodes,
which operated continuously in air without degradation for 12,000 h. The devices
were encapsulated by gluing a glass slide over the top of the device with epoxy
resin.

For many display applications it is useful to pattern the emitting area of a

device. Several successful approaches have been developed in this vein, including
inkjet printing and solvent-assisted micromolding. In the inkjet approach, a con-
ventional printer was modified to spray a solution of PEDOT onto defined areas
of an ITO electrode (103). The whole area was then coated with an MEH–PPV
film and a Ca cathode deposited to complete the device. Emission occurs only in
the areas defined by the PEDOT layer because of the enhanced charge injection
from this polymer relative to the ITO layer. Typical pixel sizes ranged from 180
to 400

µm, and the density of the dots could be used to control the brightness

of emission from a given area. Solvent-assisted micromolding involves wetting a
polydimethylsiloxane mold with methanol and allowing a solution of PPV precur-
sor polymer to wick into the recessed regions of the mold (104). The mold is initially
prepared by casting and curing a prepolymer against a lithographically prepared
relief structure. When the methanol evaporates, the mold is removed, leaving the
PPV precursor polymer patterned on the surface. Curing yields PPV, and devices
are completed by deposition of Ca or Al. Emitting features as small as 800 nm
can be prepared using this method. Inkjet patterning has also been used to pro-
duce a three-color (red–green–blue) display, using PPV derivatives and molecular
dopants to achieve the various colors (105).

Color Range and Tuning.

Variations in polymer structure allow tuning

of the emission color over the entire visible spectrum. Polymers in which the

596

POLY(p-PHENYLENEVINYLENE)

Vol. 7

conjugation is broken because of the presence of the methoxy groups (as in
16) show blue-shifted electroluminescence relative to unsubstituted PPV, with
an emission maximum at 508 nm (106). Blue emission was also obtained from
a copolymer containing short methoxy-substituted phenylenevinylene segments
(35) (107). Single-layer devices containing this polymer sandwiched between Al
and ITO have a maximum in the EL spectrum at 465 nm. A PPV derivative with
Si groups in the backbone (36) emits blue light in both single- and two-layer de-
vice configurations (108). Green light was obtained from multilayer devices with
poly(2-methyloctylsilyl-1,4-phenylenevinylene) (DMOS-PPV) (37) as the emitter
combined with an electron-conducting and hole-blocking layer of either butyl PBD
or a polymeric oxadiazole derivative (109). A low bandgap, cyano-substituted
thienylene phenylenevinylene copolymer 38 was shown to emit in the near-IR
region. The maximum in the EL spectrum is at 740 nm, and emission tails down
to 1000 nm (110).

O(CH

2

)

x

O

H

3

CO

H

3

CO

OCH

3

OCH

3

n

Si

n

Me

Me

OC

4

H

9

C

4

H

9

O

35

36

37

38

39

Me

2

Si

n

S

CN

OC

6

H

13

OC

6

H

13

CN

C

12

H

25

O(CH

2

)

6

O

n

n

Several groups have developed polymer LEDs that emit light over a broad

wavelength range, resulting in white light. Photoluminescence from polymer 39
arises from both the pendent diphenylanthracene units and the polymer backbone,
thus covering much of the visible spectrum (111). However, electroluminescence
from single-layer devices containing this polymer is significantly red-shifted rel-
ative to the photoluminescence, emitting green–red light apparently only from
the main chain. Another approach that has produced white electroluminescence
involves CdSe/PPV multilayer structures (112). These structures were prepared
by alternate absorption of PPV precursor polymer and negatively charged CdSe

Vol. 7

POLY(p-PHENYLENEVINYLENE)

597

Mg

PDT

F-PPP

PPV

ITO/glass

(b)

Mg/In

ROPPV-8/Alq

3

ITO/glass

(a)

䊝

䊞

䊝

䊞

Fig. 3.

Schematics of tunable color light-emitting devices.

particles onto an ITO substrate. In these devices, electroluminescence arises exclu-
sively through charge recombination on the CdSe particles, and has a maximum
intensity at 650 nm. The PPV is believed to act only to transport charge, thus
significantly reducing the turn-on voltage in these devices compared with those
using insulating poly(allylaminehydrochloride) as the host.

Polymer-based light-emitting devices in which the emission color may be

tuned by varying the bias voltage have been fabricated. In one approach, the emit-
ting layer consists of a mixture of poly(2,5-dioctyl-p-phenylenevinylene) (ROPPV-
8) and Alq

3

sandwiched between a Mg/In cathode and an ITO anode (Fig. 3a) (113).

The electroluminescence from this device varies from orange to greenish yellow as
the bias voltage is increased from 15 to 22 V. Increasing the voltage increases the
proportion of the emitted light arising from the Alq

3

. Devices in which ROPPV-8

and Alq

3

are in separate layers emit only from the polymer layer regardless of ap-

plied voltage. Another approach uses a three-layer device, consisting of PPV, per-
fluoropropylated poly(p-phenylene) (F-PPP), and poly(3-dodecylthiophene) (PDT)
(Fig. 3b) (114). The F-PPP acts as an electron-blocking layer, confining emission
only to the side of the device to which a negative bias is applied. Thus, green light
is emitted when the negative bias is on the PPV side, and red light is emitted when
it is applied to the PDT side. Doping a PPV derivative with a dinuclear ruthenium
complex, that acts both as electron-transfer mediator and triplet emitter, gives a
material that exhibits reversible, voltage-dependent switching from red to green
emission. Under forward bias, the excited state of the ruthenium complex is pop-
ulated, giving red light, while under reverse bias, the lowest singlet state of the
polymer is generated, giving green emission (115).

Self-Assembled Devices.

Rubner and co-workers at MIT have developed

a self-assembly approach to constructing PPV-containing heterostructures for
use in EL devices (116). The heterostructures are built up by alternately dip-
ping a substrate in solutions of the sulfonium PPV precursor polymer and an-
ionic polyelectrolytes such as the sodium salts of poly(styrenesulfonate) (PSS) or
poly(methacrylic acid) (PMA). The layers are then heated to convert the PPV to
the fully conjugated form. This method allows for very precise control of thickness,

598

POLY(p-PHENYLENEVINYLENE)

Vol. 7

as well as manipulation of the nature of the surface (cationic or anionic) in con-
tact with the anode and cathode. Device efficiency is very sensitive to the chemical
nature of the interface, and the PMA/Al interface consistently yielded efficiencies
greater by a factor of 2 relative to devices with a PPV/Al interface.

Light-Emitting Electrochemical Cells.

Recently, Heeger and co-

workers have developed light-emitting devices in which a blend of a semicon-
ducting, luminescent polymer with a solid-state electrolyte is used in the emitting
layer (117). These devices function as solid-state electrochemical cells, and charge
injection results in the formation of a p–i–n junction through electrochemical dop-
ing. Many of the requirements in conventional polymer LEDs, such as a match
between the electrode work function and the polymer valence and conduction
bands, are circumvented in such light-emitting electrochemical cells (LECs).

Polymer LECs have been demonstrated to function in a dynamic-junction

mode, in which ions move under the external bias to create the junction. These
devices degrade when the drive voltage is beyond the window of electrochemical
stability. When polyethylene oxide (PEO) is used as the ion-transport medium,
the devices can operate in a frozen-junction mode at 100 K. In this mode, response
times are comparable to those of polymer LEDs. An improvement on this ap-
proach allows the frozen junction to function at room temperature (118). In these
single-layer devices a PPV derivative such as poly(2-butyl-5-

{2-ethyl(hexyl)}-1,2-

phenylenevinylene) was blended with an electrolyte containing a crown ether and
lithium triflate. The devices were activated by applying a small voltage while heat-
ing to 60–80

◦

C, creating the p–i–n junction, and functioned at room temperature

with a turn-on voltage of 2.1 V and an external EL efficiency of 2–3%.

Stimulated Emission and Lasers.

In 1996, Friend and co-workers

demonstrated optically driven lasing from thin films of PPV sandwiched between a
distributed Bragg reflector and an Ag mirror (119). Excitation was with a Nd:YAG
laser, frequency-tripled to 355 nm. Emission occurs in three cavity modes, equally
distributed at low excitation energy; but the 545-nm emission dominates at higher
excitation energy, indicating that the device is lasing. The same year the Santa
Barbara group reported laser emission from solutions and films of MEH–PPV and
TiO

2

nanocrystals. The nanoparticles multiply scatter photons in the composite

material, resulting in the scattering length exceeding the gain length (120). Stim-
ulated emission has also been observed from crystalline films of an oligomer of
phenylene vinylene substituted with octyloxy groups (40) (121).

O

O

40

Vol. 7

POLY(p-PHENYLENEVINYLENE)

599

Future Considerations

The last 10 years have seen an explosion of research interest in PPV and its deriva-
tives, driven by the potential of these materials as emitters in EL devices. Com-
panies such as Philips, Uniax (DuPont Displays), Cambridge Display Technology,
and Covion are engaged in bringing low information content display applications
(cell phone and alphanumeric displays) based on organic emitters to market, and
in late 1999 Pioneer Electronics had already begun selling a monochrome display
for use in vehicles. Higher-information content applications, such as computer and
television displays may be developed in the near future.

Although many of the problems that are of concern for optoelectronic appli-

cations of organic materials have been overcome, there are still many challenges
ahead in the field. Many of the polymers that have been made recently have not
yet been investigated in detail, and more work is needed to optimize these ma-
terials for specific applications. Fundamental questions also remain, such as the
exact nature of the charge carriers in PPV films, and further studies on the pho-
tophysical behavior of these polymers are required.

Perhaps the most exciting prospect that has arisen from the intense research

activity on PPV is the realization that organic materials and polymers can be used
in real world optoelectronic applications. This will hopefully stimulate efforts to
make and study more novel materials and to find new applications for them. In
particular, any integration of conventional and molecular electronics will require
many advances. The success of PPV and other organic EL materials has demon-
strated that research in this field is a worthwhile enterprise, and the development
of entirely new classes of materials with as yet unknown properties may be on the
horizon.

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M

ICHAEL

O. W

OLF

The University of British Columbia