EP1008164A1

EP1008164A1 - Bilayer polymer electroluminescent device featuring interface electroluminescence

Info

Publication number: EP1008164A1
Application number: EP98911635A
Authority: EP
Inventors: Arthur J. Epstein; Darren D. Gebler; Yunzhang Wang
Original assignee: Ohio State University
Current assignee: Ohio State University
Priority date: 1997-03-12
Filing date: 1998-03-12
Publication date: 2000-06-14
Also published as: AU6554898A; CA2307035A1; JP2000513153A; WO1998040899A1

Abstract

An electroluminescent device includes a first polymeric layer (1) as a hole transport/electron blocking layer and a second polymeric layer (2) as an electron transporting emissive layer, wherein the first layer and the second layer are in electrical contact so as to form an interface being capable of producing an exciplex-like emission upon a current being passed through the interface. The first layer has a greater hole transport capability than the second layer, and the second layer having greater electron transport capability than the first layer.

Description

BILAYER POLYMER ELECTROLUMINESCENT DEVICE FEATURING

INTERFACE ELECTROLUMINESCENCE

Related Application Data

This application claims the benefit of U.S. provisional application No.

60/036,232 filed on March 12, 1997, which is incorporated herein by reference.

Technical Field

This invention relates to light-emitting devices driven by an electric field

and which are commonly referred to as electroluminescent devices.

Background

Conjugated polymer based light-emitting devices have become a topic of great

interest since the report of electroluminescent (EL) properties in poly(phenylene

vinylene) (PPV). A large variety of polymers, copolymers, and their derivatives have

been shown to exhibit EL properties, including a relatively new class: polvpyridines,

and poly(pyridyl vinylene)s. The configurations of these devices may consist of a

simple single layer, bilayers, or blends used to enhance efficiency and tune the

emission wavelength, or multilayers that may allow the device to operated under an

AC applied voltage. In single layer devices, the low efficiency frequently is due to the imbalance of-

electrons and holes. Inserting a hole-transport (electron blocking) or electron-

transport (hole-blocking) layer provides a means to enhance minority carriers and

block the majority carriers and confine them to the emitter layer, which increases the

probability of recombination. Poly(N-vinylcarbazole) (PVK) has been used as a hole

transport layer and occasionally in blends with the emitter polymer. PVK is a well

studied photoconductive polymer which often forms exciplexes with organic

molecules, e.g., dimethyl terephthalate.

Recently, there has been interest in exciplex formation between PVK and

conjugated polymers. Osaheni and Jenekhe have reported PL due to exciplex

formation in bilayers of poly(p-phenylene benzobisoxazole) (PBO) and tris(p-

tolyl)amine, but not EL, although they suggest exciplexes may be important in light-

emitting devices. Even though many groups have studied bilayer and multilayer

devices, EL due to exciplex formation until now has not been reported. For example,

in highly efficient bilayer devices of CN-PPV and PPV and of PPV and 2-(4-

biphenylyl)-5(-4-tert-butyphenyl)-l,3,4-oxadiazole exciplex formation is not

observed.

It is thus an object of the present invention to provide light emission in

polymer-based light-emitting devices that occurs relatively further away from the

operating electrodes which tend to quench luminescence.

It is also an object of the present invention to provide polymeric light-emitting

devices with increased probability of electron-hole recombination, and thereby

increased attendant efficiency. Another object of the present invention is to provide polymeric light-emitting-

devices which provide enhanced protection of the emission from environmental

degradation, such as that due to exposure to oxygen.

Finally, it is also an object of the present invention to provide for polymeric

light-emitting devices that may optionally be used to produce laser light.

In view of the present disclosure and the practice of the present invention,

other advantages of the present invention may become apparent.

Summary Of The Invention

In general terms, the present invention includes an electroluminescent device

comprising (a) a first polymeric layer adapted to act as a hole transport/electron

blocking layer; and (b) a second polymeric layer adapted to act as electron

transporting emissive layer, wherein the first polymeric layer and the second

polymeric layer are in electrical contact so as to form an interface, the interface being

capable of producing an exciplex-like emission upon a current being passed through

the interface.

An exciplex is a transient donor-acceptor complex between the excited state of

the donor and ground state of the acceptor.

The first polymeric layer has a greater hole transport capability than the

second polymeric layer, and the second polymeric layer having greater electron

transport capability than the first polymeric layer.

The Hole Transport/Electron Blocking Layer

Referring to Figure 1 , the hole transport/electron blocking layer 5 is shown in

electrical contact with electrode 1 and electron transporting layer 4. This layer may comprise any appropriate polymer, copolymer or oligomer, or derivative thereof. An

example of the polymeric hole transport/electron blocking layer may be any

carbazole-coating polymer, such as poly(vinyl carbazole) (PVK).

Figure 2 shows the chemical structure of the repeating units of polymers that

may be used in accordance with one embodiment of the present invention, structure

(b) being that of a repeating unit of poly(vinyl carbazole) (PVK).

The Electron Transporting Layer

Also shown in Figure 1 is the electron transporting layer 4 shown in electrical

contact with electrode 3 and hole transport/electron blocking layer 5.

The polymeric layer acting as the electron transporting emissive layer may

comprise any appropriate polymer, copolymer or oligomer, or substituted or wrapped

derivatives thereof, such as poly(pyridyl vinylene phenylene vinylenes),

poly(dithienylene phenylenes), such as shown in the examples below. This layer may

comprise, for instance, a poly(pyridyl vinylene phenylene vinylene) (PPy VPV)

polymer, copolymer or oligomer, or derivative thereof, such as shown in structure (a)

of Figure 2, being that of a repeating unit of a poly(pyridyl vinylene phenylene

vinylene) (PPy VPV) derivative (i.e., PPyVP(COOC₁₂H₂₅)₂V).

Figure 7(a), 7(b) and 7(c) show other examples of copolymers and their

derivatives that may be used as the electron transporting emissive layer.

The electron- withdrawing nature of the side groups, such as those in the

copolymer PPyVP(COOC₁₂FI₂₅)₂V, render the copolymer more resistive to oxidation

than the unsubstituted copolymer, and are thus preferred. The copolymers are soluble in common organic solvents such as ^~

tetrahydrofuran (TFIF), xylene, and chloroform.

Hole-Injecting Electrodes

With respect to such alternative materials and referring to Figure 1, the hole-

injecting electrode 1 may be of any appropriate material. Electrodes may be

fashioned from any suitable conductive material including metals, degenerate

semiconductors, and conducting polymers including, but not limited to, a wide variety

of conducting materials such as (1) indium-tin-oxide ("ITO"), (2) metals such as gold,

aluminum, silver, copper, indium and magnesium, (3) alloys such as magnesium-

silver, (4) conducting fibers such as carbon fibers, and (5) highly-conducting organic

polymers such as highly-conducting doped polyaniline, highly-conducting doped

polypyrrole, polyaniline salt (such as PAN-CSA) or other doped pyridyl nitrogen-

containing polymer, such as polypyridylvinylene.

Electron-Injecting Electrodes

With respect to such alternative materials and referring to Figure 1, the

electron-injecting electrode 3 may be of any appropriate material. Electrodes may be

fashioned from any suitable conductive material including metals, degenerate

of conducting materials such as (1) metals such as aluminum, calcium, silver, copper,

indium and magnesium, (2) alloys such as magnesium-silver and lithium-aluminum,

(3) conducting fibers such as carbon fibers, and (4) highly-conducting organic

polymers such as highly-conducting doped polyaniline, highly-conducting doped

containing polymer, such as polypyridylvinylene. In typical applications where the device is used for illumination and_ display, ^"at

least one of the electrodes may be fashioned from a transparent material such as ITO

or a partially transparent material such as highly-conducting doped polyaniline.

Partially transparent electrodes may be used to advantage to filter or clip unwanted

frequencies of light coming from the light-emitting material.

It is noted that it is not necessary that the electrode material be transparent or

even partially transparent. In cases where the electrode materials are opaque to the

emitted light, light emission from the edge of the device may be utilized in, for

example, edge-lighted displays or in coupling applications such as in coupling the

device to an optical fiber.

Substrate

For ease of manufacture and safety purposes, it is often desirable to form

the device on a substrate which also serves to protect and often to insulate (both

physically and electrically) the device during use. The substrate layer may be any

appropriate material, such as glass or clear electrically insulating plastic substrates

which are preferred when the device is used for lighting and display purposes. A DC

driven device is especially suitable for light emission from both sides of the device in

which case electrode materials, as well as any protective substrates that may be used

with one or both electrodes, are at least partially transparent. The substrate layer is

shown in Figure 1 as substrate layer 2.

The Source of Electrical Energy

The devices of the present invention may be operated by any appropriate

source of electrical energy 8 shown in Figure 1. The first electrode and the second electrode are electrically connected to-a

potential difference. For instance, the first electrode can be connected to a positive

potential (anode) while the second electrode is connected to a negative potential (cathode)

The electrodes 1 and 2 are connected to a voltage source 8 by means of

suitable electrical connectors or contacts. Such electrical connectors and contacts are

conventional in the art and may include wire leads, printed circuit connectors, spring

clips, snaps, solder, wrapped posts, conducting glues, etc. It is also to be understood

that the electrical connector or contact can be the electrodes 1 and 3 themselves. That

is, the potential difference from voltage source 8 may be applied directly to the

electrodes in which case electrodes 1 and 3 may become the electrical contact or

connector.

The devices of the present invention may feature a relatively low turn-on and

operating DC voltage of less than about 24 volts, depending upon polymeric

thickness. More preferably, a turn-on and operating voltage of less than about 12, and

even less than about 5 volts may be achieved. Such low voltages make these devices

particularly advantageous for use in toys, as commercial light strips such as found on

airplanes and in theaters, as signs, and as flat panel displays for computer and

television use. Devices of the present invention may be operated with AC current in

which case the device will operate when current is flowing in the forward direction.

Advantages of the devices of the present invention include that the light

emission occurring relatively further away from the electrodes (i.e., such as the Al and

ITO electrodes), which may tend to quench luminescence. Another advantage is that the charge is confined at the polymer/polymer —

interface by the electron blocking nature of the electron blocking layer (i.e., such as

the PVK layer). This leads to an increased probability of electron-hole recombination

due to the density of electrons and holes at the interface.

The devices of the present invention also feature a sequestered

polymer/polymer interface that protects the emission from degradation due to oxygen

that tends to change the vinylene units to carbonyl units which in turn quenches the

luminescence.

The devices of the present invention may also be used to produce electrically

or optically pumped laser light. By using a polymeric layer arrangement of the

present invention, it is possible to concentrate the energy spatially at the interface.

This would allow attainment of lasing at relatively low pump thresholds, whether the

device is electrically or optically pumped. Optically pumped lasing may be attained

by irradiating the interfacing polymer layers with sufficient light intensity at

sufficiently short wavelength to cause photoexcitation of the exciplex. Electrically

pumped lasing may be attained by supplying enough current density to create a

critical density of exciplexes sufficient to cause lasing, which will depend upon

geometrical factors understood in the art.

The foregoing and other advantages of the invention will become apparent

from the following disclosure in which one or more preferred embodiments of the

invention are described in detail and illustrated in the accompanying drawings. It is

contemplated that variations in procedures, processing, structural features,

arrangement of parts, experimental design, ingredients, compositions, compounds, and elements may occur to a person skilled in the art without departing from the scope-

of or sacrificing any of the advantages of the invention.

Brief Description of the Drawings

Figure 1 is a general schematic of a light-emitting device of the present

invention.

Figure 2 shows the chemical structure of the repeating units of polymers that

may be used in accordance with one embodiment of the present invention; structure

(a) being that of a repeating unit of a poly(pyridyl vinylene phenylene vinylene)

(PPy VPV) derivative (i.e., PPyVP(COOC₁₂H₂₅)₂V), and structure (b) being that of a

repeating unit of poly(vinyl carbazole) (PVK).

Figure 3 is a graph showing the photoluminesence spectra of

PPyVP(COOC₁₂H₂₅)₂V at 2.8 eV excitation energy (— ), a bilayer of PVK and

PPyVP(COOC₁₂H₂₅)₂V at 3.6 eV excitation energy (D) and 2.8 eV excitation energy

(0), and PVK at 3.6 eV excitation energy (...), all on quartz substrates, in accordance

with one embodiment of the present invention.

Figure 4 is a graph showing the photoluminesence spectra of a bilayer of PVK

and the copolymer PPyVP(COOC|₂H₂₅)₂V in a light-emitting device in accordance

with one embodiment of the present invention, as a function of both excitation and

emission energy.

Figure 5 shows graphs of the absorption and photoluminesence excitation

(PLE) spectra of (a) a single layer of the copolymer PPyVP(COOC,₂H₂₅)₂V ( ),

(b) a single layer of PVK ( . . . . ), and (c) a bilayer of PVK and the copolymer PPyVP(COOC|₂H₂₅) V ( — ) in a light-emitting device in accordance with one — embodiment of the present invention.

Figure 6 shows graphs of the electroluminesence and photoluminesence

spectra of a light-emitting device in accordance with one embodiment of the present

invention. Also shown are the current density vs. voltage and brightness vs. voltage

data.

Figure 7 shows the chemical structures (a) - (e) of the repeating units of

copolymers that may be used in accordance with several embodiments of the present

invention; structure (a) shows three alternative derivatives according to variations in

moiety R, such that R, = OC,₆H₃₃, C₁₂H₂₅ or COOC₁₂H₂₅, designated structures "ax,"

"ay" and "ex," respectively; structure (b) shows an unsubstituted "wrapped"

copolymer, designated wPPyVPV; structure (c) shows three alternative derivatives of

"wrapped" copolymers according to variations in moiety R, such that R, = OC₁₆H₃₃,

C₁₂H₂₅ or COOC₁₂H₂₅, designated wPPyVPV(ax), wPPyVPV(ay), and

wPPyVPV(cx), respectively . Structure (d) of Figure 7 is designated wPTP and

structure (e) is designated wPDTP.

Figures 8a through 8e are graphs showing the photoluminesence and PLE

spectra of several polymer/copolymer materials that may be used in accordance with

several embodiments of the present invention.

Figure 9 is a graph showing the photoluminesence spectra of several

wPPyVPV(ax)/PVK blends in accordance with several embodiments of the present

invention; graph (a) showing PL film efficiency as a function of wPPyVPV(ax)

content, and graph (b) showing PL relative intensity and normalized PL intensity as a

function of energy. Figures 10 and 1 1 show the PL spectra of single layers of PVK and each of the

copolymers along with the corresponding bilayers. Figure 10 shows (a) the film PL

spectra of PPy VPV (structure ay; solid lines), PVK (♦) and a bilayer of

PVK/PPyVPV (structure ay) (dashed lines); and (b) the film PL spectra of PPy VPV

(structure ex; solid lines), PVK (♦) and a bilayer of PVK/PPy VPV(structure ex)

(dashed lines). Figure 11 shows the PL for single layer films (solid lines), bilayer

films (dashed lines) and PVK( ), for three copolymers wPPyVPV(ax),

wPPyVPV(ay), and wPPyVPV.

Figure 12 shows the EL of two bilayer devices (see caption) along with the

corresponding PL results. The EL and PL of these bilayer devices are substantially

the same, demonstrating that the EL originates from the exciplex states formed at the

interface between the hole- and electron-transporting layers.

Detailed Description of the Preferred Embodiments

In accordance with the foregoing summary of the invention, the following

describes preferred embodiments of the present invention which are presently

considered to be the best mode of the invention.

Example of a Sample Preparation

A conjugated polymer light-emitting device consists of an emitting material

(layer) sandwiched between two electrodes, one of which is preferably transparent.

Indium-tin-oxide was used as the positive transparent electrode (anode) and aluminum

was most often used as the negative electrode (cathode). The ITO-coated glass

(commercially available from Donnelly Applied Films or Delta Technologies Ltd.) was purchased in large sheets 12" x 12". The ITO-coated glass was cut into —

appropriate size pieces (typically 2 cm x 2 cm) by the glass shop.

Each individual substrate then was etched before use. The etching was done

with a solution of 20 % HC1, 5% HN0₃, and 75% distilled water, by volume, heated

to -50 to 60° C. After etching, the ITO substrates were cleaned.

The emitting polymer was then spin-coated onto the clean etched ITO

substrate or on top of a previously spin coated layer of PVK from the appropriate

solvents. PPy was cast from formic acid solution and the copolymers of PPy V and

PPV were cast from tetrahydrofuran (THF), xylene or chloroform.

Solution concentrations of the copolymer were typically s-10 mg/ml. The

powders were weighed on a balance after which the appropriate solvent was added.

The solutions were stirred with a spin bar for at least 1 hour or until the powders were

almost completely dissolved. The solutions were filtered with either a 1 micron or 0.2

micron pore filter and stored in a hood until used. In the class 100 cleanroom, the

films were made by dropping 3 - 5 drops of solution from a pipette on to the ITO

glass substrate. The substrate was then immediately spun at speeds ranging from

1000 to 2000 rpm. Following spin-coating the top electrode (Al or Au) was vacuum

deposited (evaporated) at pressures below -10^" torr. A mask was used to evaporate

the appropriate electrode pattern. To prevent damage due to heating during

evaporation, the substrates were

mounted on a cold water cooled stage during deposition. In addition, evaporation

rates were -0.5-1.4 A/s for the first 100 A of deposition and then increased to -3-5

A/s until the desired thickness was reached (usually 1000 to 2000 A). The device performance was improved by inserting either a hole-transporting —

layer between the anode and emitter or an electron-transporting layer between the

emitter and cathode. The most commonly used hole-transporting layer, poly(9-vinyl

carbazole) (PVK), was cast from THF (10 mg/ml) onto the ITO at 3000 rpms. When

fabricating multilayer devices, the choice of solvents is critical. The second layer

should not dissolve the original layer and the solvents should be compatible enough to

make uniform films.

Each of the copolymers was treated identically although the solution

concentration may have been slightly different. For the PL experiments the Al

electrodes were not evaporated.

After fabrication some devices are annealed at 80° C for 2 hours.

The photoluminesence and electroluminesence results were as follows:

Figure 3 is a graph showing the photoluminesence spectra of

PPyVP(COOC,₂H₂₅)₂V at 3.6 eV excitation energy (D) and 2.8 eV excitation energy

(O), and PVK at 3.6 eV excitation energy (...), all on quartz substrates, in accordance

with one embodiment of the present invention. The PL of single PVK layers excited

at 3.6 eV has a peak emission energy at 3.05 eV, similar to previous reports of the PL

of PVK. The PL for single layer copolymer films excited at 3.1 eV shows an

emission peak at 2.05 eV. The bilayer when excited at an energy less than the

absorption edge of the PVK, but greater than the absorption edge of the copolymer,

shows PL peaked at the same energy as for the copolymer along with a low intensity

tail to the blue side. When the bilayer was excited at energy equivalent to the

excitation energy for the single PVK layer (3.6 eV), the PL emission spectrum contains contributions from both single layers (3.05 eV and 2.05 eV), as well as from-

a completely new species, which may be identified with an exciplex. To the low

energy side of the exciplex PL is a weak shoulder near the PL energy for the single

layer of the copolymer.

Figure 4 is a graph showing the photoluminesence spectra of a bilayer of PVK

and the copolymer PPyVP(COOC₁₂H₂₅)₂V in a light-emitting device in accordance

emission energy. The 3D plot shows three prominent features: a peak due to the

PVK (excitation energy from 3.6 to 4.2 eV, emission energy 2.8 to 3.4 eV), a peak

due to the copolymer (excitation energy from 2.4 to 3.0 eV, emission energy 1.8 to 2.2

eV), and the exciplex peak (excitation energy from 3.6 to 4.2 eV, emission energy 2.2

to 2.8 eV).

At excitation energies above 3.6 eV the PL due to the exciplex and PVK are apparent,

but if the excitation energy is lowered below 3.4 eV these peaks have essentially

disappeared. As the excitation energy is further lowered into the peak absoφtion of

the copolymer, PL from the copolymer strongly predominates (excitation energy 2.6

to 3.0 eV and principal emission energy 1.8 to 2.2 eV). The 3D plot shows three

prominent features: a peak due to the PVK (excitation energy from 3.6 to 4.2 eV,

emission energy 2.8 to 3.4 eV), a peak due to the copolymer (excitation energy from 2.4 to 3.0 eV, emission energy 1.8 to 2.2 eV), and the exciplex peak (excitation

energy from 3.6 to 4.2 eV, emission energy 2.2 to 2.8 eV).

Figure 5 shows graphs of the absorption and photoluminesence excitation

(PLE) spectra of (a) a single layer of the copolymer (b) a

single layer of PVK, and (c) a bilayer of PVK and the copolymer PPyVP(COOCι₂H₂₅)₂V, on quartz substrates, in a light-emitting device in accordancσ

with one embodiment of the present invention. The PLEs were recorded at 2.05, 3.05,

and 2.55 eV, respectively. The copolymer absorption is 5 times less than shown.

The absorption and photoluminescence excitation (PLE) spectra are shown in

Figures 5a and 5b. The onset of the absorption of the single PVK layer is at about 3.5

eV and shows two spectral features at 3.6 and 3.75 eV similar to previous reports.

The PLE of PVK follows the absoφtion showing nearly identical features. The

absorption and PLE of the copolymer peak at 2.95 eV, with the onset at about 2.4 eV.

The absorption of the bilayer is the sum of the single PVK layer absoφtion and the

single copolymer absorption and shows both the copolymer peak at 2.95 eV and the

PVK peaks at 3.6 and 3.75 eV. The PLE of the bilayer is also the sum of the PVK

PLE and the copolymer PLE and shows both the PVK spectral features and the

copolymer peak, although the copolymer peak is shifted to slightly higher energy.

The lack of any new absorption or PLE features in the bilayer film implies that the

new species is not directly accessible from the ground state of the copolymer or PVK.

and is thus consistent with the assignment of an exciplex.

The PL, PLE and absorption were measured on the same films making it

possible to estimate the relative PL quantum efficiencies of the copolymer emission

and the exciplex emission. The copolymer absolute PL efficiency was reported

previously to be 18%. A lower bound on the quantum efficiency of the exciplex was

calculated to be 15-20%, nearly the same as the copolymer efficiency.

Figure 6 shows graphs of current-voltage( — ) and brightness-voltage (G)

characteristics of a typical bilayer light-emitting device. Inset: PL (...) and EL ( — ) of a bilayer light-emitting device, in accordance with one embodiment of the present"

invention.

Bilayer devices were fabricated using ITO as the anode and aluminum as the

cathode. The inset of Figure 6 shows the EL spectrum of a typical device with the PL

spectrum from the same device. The devices can easily be seen in a bright lit room,

appear bright green to the eye, and have internal quantum efficiencies of - 0.1-0.5%.

Although the PL efficiencies are comparable, the EL efficiency of the bilayer

configuration ~ 0.1-0.5% is much greater than for a single layer device which has an

EL efficiency of less than 0.0001%. The similarity between the PL and EL of the

bilayer device demonstrates that the exciplex is responsible for the EL emission.

Figure 6 shows the current density-voltage and brightness-voltage characteristics for a

typical bilayer device. The turn-on voltage of the bilayer devices depends on the

thickness of the polymer layers and in this case is ~ 18 volts, with the brightness

following the current. The generality of this concept has been demonstrated using

several other pyridine-based copolymers. Through the use of polyaniline network

electrodes, the threshold voltage was lowered to below 5 volts while maintaining the

same efficiency.

The increase in efficiency of the bilayer device compared to the single layer

device appears to be due primarily to charge confinement at the PVK/copolymer

interface. The electrons are injected from the Al electrode into the conduction band of

the copolymer, but are confined when they reach the electron blocking PVK. Also,

the holes are injected into the valence band of the PVK and are confined at the

interface. The electron and hole blocking at the interface enhances exciplex emission.

That the electron and holes are unable to easily conduct through both layers leads to a small current density (< or ~ ImA/mm ) and hence a greatly increased efficiency. In-

addition, the buried interface implies that most of the radiative recombination will

occur at the interface and away from the EL quenching electrodes.

A wide range of devices were fabricated with a variety of different emitting

polymers and hole transport layers, including bilayer devices with the following

emitter layers: poly(pyridyl vinylene), PPyVP(R)₂V (R=OC₁₆H₃₃, R=C₁₂H₂₅,

R=COOC₁₂H₂₅), a strapped copolymer PPyVP(R)₂V (with RAH, R=OC₁₆H₃₃,

C|₂H ₅), poly(thienylene phenylene) with a strap, and poly(dithienylene phenylene)

with a strap. The molecular repeat unit of these copolymers is shown in Figure 7,

structures (a) - (e).

Figures 8a through 8e are graphs showing the electroluminesence and

photoluminesence spectra of several polymer/copolymer materials that may be used in

accordance with several embodiments of the present invention.

In Figure 8(a) the polymer referred to as "C₁₂H₂₅" is that shown in Figure 7(a)

where R=C₁₂H₂₅.

In Figure 8(b), the polymer is that shown in Figure 7(a) wherein

R=COOC₁₂H₂₅.

In Figure 8(c), the polymer referred to as "D40" is that shown in Figure 7(c)

wherein R=OCι₆H₃₃.

In Figure 8(d) the polymer referred to as "D41" is that shown in Figure 7(c)

wherein R= C₁₂H₂₅.

In Figure 8(e). the polymer referred to as "Dl 12" is that shown in Figure 7(b).

In each case, the PVK was used as the hole-transporting layer. Blends of these polymers/copolymers with PVK show PL emission due to-

exciplex formation (Figure 9) and are expected to show EL emission from exciplexes.

The generality of using pyridyl containing polymers as the emitting polymer

thus has been shown and it is expected that the other light-emitting polymers such as

polyparaphyenylene (PPP), polyparaphenylene vinylenes (PPV), polythiophenes (PT)

and their derivatives and/or copolymers to behave similarly based on chemical and

electronic similarities. Other hole-transporters such as PPPs, PPVs, polymethyl

methacrylate, polystyrene, polyethylene, poly(ethylene teraphthalate) and blends of

these materials are expected to behave similar to PVK.

Figure 9 is a graph showing the photoluminesence spectra of several

wPPy VPV(ax)/PVK blends in accordance with several embodiments of the present

invention; graph (a) showing PL film efficiency as a function of wPPyVPV(ax)

function of energy.

Figures 10 and 1 1 show the PL spectra of single layers of PVK and each of the

spectra of PPy VPV (structure ay; solid lines), PVK (♦) and a bilayer of

(structure ex; solid lines), PVK (♦) and a bilayer of PVK/PPyVPV(structure ex)

films (dashed lines) and PVK( ), for three copolymers wPPyVPV(ax),

wPPyVPV(ay), and wPPyPV.

Figure 12 shows the EL of two bilayer devices (see caption) along with the

corresponding PL results. The EL and PL of these bilayer devices are substantially the same, demonstrating that the EL originates from the exciplex states formed at the —

interface between the hole- and electron-transporting layers.

In summary, heteroj unctions of PVK and PPyVP polymers show a strong

photoluminescence and electroluminescence feature due to exciplex emission at the

interface. The absoφtion and PLE spectra and have shown that the exciplex is not

directly accessible from the ground state. The exciplex is also the primary species of

electroluminescence emission in the bilayer devices. The efficiency of the bilayer

devices is greatly enhanced over single layer devices due to charge confinement and

exciplex formation and emission at interface.

Thus, the bilayer devices of the described embodiment of the present invention

use PVK as the hole transport layer and a derivative of the copolymer PPy VPV as the

emitter layer. Absorption, photoluminesence and electroluminesence results are

consistent with emission due to exiplex formation between the PVK and PPy VPV

copolymer. The PL and EL of bilayer films are dramatically different from that of the

single layer.

The preferred embodiments herein disclosed are not intended to be exhaustive

or to unnecessarily limit the scope of the invention. The preferred embodiments were

chosen and described in order to explain the principles of the present invention so that

others skilled in the art may practice the invention. Having shown and described

preferred embodiments of the present invention, it will be within the ability of one of

ordinary skill in the art to make alterations or modifications to the present invention,

such as through the substitution of equivalent materials or structural arrangements, so

as to be able to practice the present invention without departing from its spirit as reflected in the appended claims. It is the intention, therefore, to limit the inventiorr

only as indicated by the scope of the claims.

Claims

What is claimed is: ΓÇö

1. An electroluminescent device comprising:

(a) a first polymeric layer adapted to act as a hole transport/electron blocking

layer; and

(b) a second polymeric layer adapted to act as an electron transporting

emissive layer,

said first polymeric layer and said second polymeric layer being in electrical

contact so as to form an interface, said interface capable of producing an

exciplex light emission upon a current being passed through said interface.

2. An electroluminescent device according to claim 1 wherein said first polymeric

layer comprises a carbazole-containing polymer.

3. An electroluminescent device according to claim 1 wherein said first polymeric

layer comprises a poly(vinyl carbazole).

4. An electroluminescent device according to claim 1 wherein said second polymeric

layer is selected from the group consisting of a poly (pyridyl vinylene phenylene

vinylene), poly (thienyl phenylene) and poly (dithienyl phenylene) polymers,

copolymers and oligomers, and substituted and wrapped derivatives thereof.

5. An electroluminescent device according to claim 1 additionally comprising a

source of direct electric current adapted to provide a flow of electrons through said

second polymeric layer to said interface.

6. An electroluminescent device comprising:

layer; and (b) a second polymeric layer adapted to act as an electron transporting ΓÇö

emissive layer,

said first polymeric layer having greater hole transport capability than said

second polymeric layer, and said second polymeric layer having greater electron

transport capability than said first polymeric layer,

said first polymeric layer and said second polymeric layer being in electrical

contact so as to form an interface, said interface capable of producing an

exciplex light emission upon a current being passed through said interface.

7. An electroluminescent device according to claim 6 wherein said first polymeric

layer comprises a carbazole-containing polymer

8. An electroluminescent device according to claim 6 wherein said first polymeric

layer comprises a poly(vinyl carbazole).

9. An electroluminescent device according to claim 4 wherein said second polymeric

vinylene), poly (thienyl phenylene) and poly (dithienyl phenylene) polymers,

copolymers and oligomers, and substituted and wrapped derivatives thereof.

10. An electroluminescent device according to claim 6 additionally comprising a

second polymeric layer to said interface.

1 1. A laser light generating device comprising:

layer; and

(b) a second polymeric layer adapted to act as an electron transporting

emissive layer, said first polymeric layer and said second polymeric layer being in electrical

contact so as to form an interface, said interface capable of producing optically

pumped laser light upon being irradiated with light so as to create a critical

density of exciplex light emission at said interface; and

(c) a source of light of sufficient intensity and of sufficiently short wavelength

o as to be capable of irradiating said first and second polymeric layers with light so

s to create a critical density of exciplex light emission at said interface.

12. A laser light generating device comprising:

layer; and

(b) a second polymeric layer adapted to act as an electron transporting

emissive layer,

said first polymeric layer and said second polymeric layer being in electrical

contact so as to form an interface, said interface capable of producing

electrically pumped laser light upon being supplied with a sufficient amount of current

so as to create a critical density of exciplex light emission at said interface; and

(c) a source of electrical current adapted to supply said sufficient amount of

current.