WO2000036004A1

WO2000036004A1 - Organic salt-containing light-emitting polymer device

Info

Publication number: WO2000036004A1
Application number: PCT/AU1999/001102
Authority: WO
Inventors: Liming Dai; Liming Dong
Original assignee: Securency Pty Ltd
Priority date: 1998-12-11
Filing date: 1999-12-13
Publication date: 2000-06-22
Also published as: AUPP765598A0

Abstract

A polymeric composition is provided that is capable of electroluminescence. The composition is a conjugated polymer and a tetrabutylammonium p-xylene sulfonate. The compositions are useful as security devices in bank notes.

Description

ORGANIC SALT-CONTAINING LIGHT-EMITTING POLYMER DEVICE

Technical Field

This invention relates to polymer electroluminescence devices which can be used as security devices in banknotes and other materials and which can emit light under certain conditions.

Background of the Invention

Conjugated polymers have attracted a great deal of interest for the use in electronic and optical devices. For example, these polymers have been successfully used in field-effect transistors, light- emitting diodes, and polymer grid triodes, where they play an active role in regulating performance. Among them, the conjugated polymer-based light-emitting diodes (LEDs) reported, for the first time, in the early 1990s are of special interest for developing novel display technologies. To construct the light-emitting device, a transparent substrate (eg. glass) coated with indium tin oxide (ITO) is often used as the anode. On this substrate a light-emitting polymer may be deposited by, for example, spin-coating as a film of about 100 nm in thickness. The top contact is formed by thermal evaporation of a low work- function metal such as aluminium. Upon application of an electrical voltage onto a LED device, electrons from the low work-function cathode such as aluminium are injected into the lowest unoccupied molecular orbital (LUMO) of the light-emitting layer. This leads to the formation of negatively charged polarons, whereas holes from the high work- function anode (ie ITO) are injected into the highest occupied molecular orbital (HOMO) producing positively charged polarons. These negatively and positively charged polarons migrate under the influence of the applied electric field and combine in the band gap of the light-emitting polymer layer, resulting in the emission of light.

More recently there has been an increased interest in electrochemically- driven light- emitting cells, in which an in situ p-n junction diode is formed by simultaneous '-type and «-type electrochemical doping of electrolyte-containing conjugated polymer films on opposite electrodes. The light emitting electrochemical cell (LEC) combines the novel electrochemical properties of conjugated polymers with the ionic conductivity of polymeric electrolytes. An example of such a composition is disclosed in US Patent No. 5,682,043 assigned to Uniax Corporation. A solid-state polymer electrolyte such as poly(ethylene oxide) (PEO):Li⁺ complex is normally used as an ionic conductor and each of the examples of that patent require the presence of such polymeric electrolytes. The presence of PEO assists with conductivity. The performance of LEC devices depend on the miscibility of the polymer electrolytes with the conjugated luminescent polymers, which has severely limited the performance of the LEC devices in terms of response time, efficiency, and lifetime. The presence of the polyelectrolyte can also lead to phase separation problems. Summary of the Invention

The present invention provides a polymeric composition capable of electroluminescence comprising an organic conjugated polymer and an organic salt of the following structure:

where,

R is an alkyl, alkenyl, alkyne group or substituted alkyl, alkenyl, alkyne group, preferably lower alkyl, alkenyl, alkyne;

R' is an aromatic or substituted aromatic group.

Preferably the organic conjugated polymer is selected from the group consisting of poly(2-methoxy,5-cyclohexanemethoxy-p-phenylene vinylene)

Poly(2-methoxy,5-(2'-ethylhexyloxyl)-p-phenylene vinylene); random copolymer of 2-methoxy,5-cyclohexanemethoxy-p-phenylene vinylene and 2-methoxy,5-(2'- ethylhexyloxyl)-p-phenylene vinylene; random copolymer of 2-methoxy,5-(2'- ethylhexyloxyl)-p-phenylene vinylene and 2,5-diethoxyl-p-phenylene vinylene; random copolymer of 2-methoxy,5,-cyclohexanemethoxy-p-phenylene vinylene and 2,5-diethoxyl-p-pheneylene vinylene; random copolymer of 2-methoxy,5-(2'- ethylhexyloxyl)-p-phenylene vinylene, 2-methoxy,5-cyclohexanemethoxy-p- phenylene vinylene and 2,5-diethoxyl-p-phenylene vinylene; and random copolymer of 2-methoxy,5-(2'-ethylhexyloxyl)-p-phenylene vinylene and 2,5- di ethyl -p-phenylene vinylene . Preferably the polymeric composition is formed on a transparent plastic substrate and more preferably on a conductive plastic substrate.

Preferably the organic salt cation is tetra-alkyl ammonium.

Preferably the aromatic or substituted aromatic group is xylene, and more preferably p-xylene

Preferably the composition is substantially free of poly(ethylene oxide)

In an alternative form this invention provides a security document, preferably a bank note, comprising a polymeric composition capable of electroluminescence comprising an organic conjugated polymer and an organic salt of the following structure:

R₄N^+"S0₃R^* where,

R is an alkyl, alkenyl, alkyne group or substituted alkyl, alkenyl, alkyne group, preferably lower alkyl, alkenyl, alkyne; R' is an aromatic or substituted aromatic group.

Brief Description of the Drawings

Figure 1 shows the electroluminescence spectra of the device with 14 mol. % TBAXS. Figure 2 shows the current-light-voltage characteristics of this device.

Figure 3 shows the current-light- voltage characteristics for the device with a hight salt content (28 mol.% TBAXS).

Figure 4 shows the different light-emitting characteristics at forward and reverse bias, respectively, for an ITO/MEH-DM-PPV+TBAXSYWOP- PPV+TBAXS/A1 tow-layer device.

Detailed Description of the Invention Definitions:

In describing the present invention, the following terms will be employed, and are intended to be defined as indicated below. The term "alkyl" as used herein refers to a branched or unbranched saturated hydrocarbon group of 1 to 24 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, «-butyl, isobutyl, t-butyl, octyl, decyl, tetradecyl, hexadecyl, eicosyl, tetracosyl and the like, as well as cycloalkyl groups such as cyclopentyl, cyclohexyl and the like. The term "lower alkyl" intends an alkyl group of 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms. In this specification a mixture of alkyl groups together with a mixture of one or more alkyl, with one or more alkenyl, with one or more alkyne group is embraced.

The term "alkenyl" as used herein refers to a branched or unbranched hydrocarbon group of 2 to 24 carbon atoms containing at least one carbon-carbon double bond, such as ethenyl, «-propenyl, isopropenyl, «-butenyl, isobutenyl, t- butenyl, octenyl, decenyl, tetradecenyl, hexadecenyl, eicosenyl, tetracosenyl and the like. Preferred alkenyl groups herein contain 2 to 12 carbon atoms and 2 to 3 carbon-carbon double bonds. The term "lower alkenyl" intends an alkenyl group of 2 to 6 carbon atoms, preferably 2 to 4 carbon atoms, containing one -C=C- bond.

The term "alkynyl" as used herein refers to a branched or unbranched hydrocarbon group of 2 to 24 carbon atoms containing at least one -C = C- bond, such as ethynyl, n-propynyl, isopropynyl, «-butynyl, isobutynyl, t-butynyl, octynyl, decynyl and the like. Preferred alkynyl groups herein contain 2 to 12 carbon atoms. The term "lower alkynyl" intends an alkynyl group of 2 to 6, preferably 2 to 4, carbon atoms, and one -C≡C- bond. "Optional" or "optionally" means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not. For example, the phrase "optionally substituted lower alkyl" means that the lower alkyl group may or may not be substituted and that the description includes both unsubstituted lower alkyl and lower alkyl where there is substitution.

The term "aromatic" means compounds that have unsaturated cyclic hydrocarbons containing one or more rings.

The organic salts used in this invention may be synthesised according to scheme 1 : R₄NOH + RS0₃H _► R-jN⁺ SO^' + H₂0 where, R represents any alkyl, alkylene or alkynyl groups; an aromatic group, for example

R is benzyl, or substituted benzyl group.

Examples of preferred R and R' include: R = -(CH₂)₃CH₃, -CH₂CH₃, -CH₃

R' = -C₆H₃(CH₃) , -C₆H (CH₃), -C₆H₅

The reaction shown in Scheme 1 can be carried out according to conventional procedures known to those skilled in the art. Typically, the base is titrated by the acid to a pH7 prior to removal of the solvent, resulting in the formation of solid white crystals.

Various soluble conjugated polymers may be used in this invention as the light-emitting material. They include:

1) Poly(2-methoxy,5-cyclohexanemethoxy-p-phenylene vinylene) [MCH-PPV]

2) Poly(2-methoxy,5-(2'-ethylhexyloxyl)-p-phenylene vinylene) [MEH-PPV] 3) Random copolymer of 2-methoxy,5-cyclohexanemethoxy-p-phenylene vinylene and 2-methoxy,5-(2'-ethylhexyloxyl)-p-phenylene vinylene [MCH-

PPV/MEH-PPV]

4) Random copolymer of 2-methoxy,5-(2'-ethylhexyloxyl)-p-phenylene vinylene and 2,5-diethoxyl-p-phenylene vinylene [MEH-PPV/DEO-PPV] 5) Random copolymer of 2-methoxy,5,-cyclohexanemethoxy-p-phenylene vinylene and 2,5-diethoxyl-p-pheneylene vinylene [MCH-PPV/DEO-PPV] 6) Random copolymer of 2-methoxy,5-(2'-ethylhexyloxyl)-p-phenylene vinylene, 2-methoxy,5-cyclohexanemethoxy-p-phenylene vinylene and 2,5- diethoxyl-p-phenylene vinylene [MEH-PPV/MCH-PPV/DEO-PPV] 7) Random copolymer of 2-methoxy,5-(2'-ethylhexyloxyl)-p-phenylene vinylene and 2,5-diethyl-p-phenylene vinylene [MEH-PPV/DE-PPV]

The copolymers with random sequence distributions may be synthesised to obtain the required solubility. In this invention polymer solubility is assessed in either chloroform or tetra hydro form or mixtures of these two solvents. The polymerisation reaction is shown in scheme 2:

Each of the R, R', R" and R'" represents one of the functional groups including methoxy, 2-ethylhexyloxyl, cyclohexanemethoxy, diethyl, and diethoxyl. The general method of preparation is as follows:

A solution of potassium tert-butoxide (1J equivalent) in anhydrous THF was added, under nitrogen, into a solution of the compounds III and IV preferably in anhydrous THF. The resulting mixture was stirred at ambient temperature for 1 hour, and then another 5 equivalent potassium tert-butoxide solution in anhydrous THF was added. After having been stirred for 24 hours, the reaction mixture was poured into an alcohol and stirred. This caused the polymers to precipitate out. Then, the typically red precipitate of polymers was washed with distilled water and reprecipitated from THF/methanol mixture and dried under vacuum at 60°C.

The performances of the light-emitting devices made from the resultant conjugated polymers have been measured. In most cases, a red-orange light emission was observed under an applied bias at both forward and reverse directions. The apparent threshold voltages for appreciable current injection and visible light emission decreased with increase of the organic salt concentration in the active light-emitting layer, as was the response time and the rectification ratio. Under a reverse bias, the electron/photon conversion efficiency was 3 orders of magnitude higher than that of the device under an equivalent forward bias. The maximum external efficiency was estimated to be about 5% photon/electron. The invention is further described by reference to preferred embodiments in the following examples. EXAMPLE 1 Preparation of tetra n-butylammonium p-xylene-2-suIphonate [TBAXS]

To a solution of a n-butylammonium hydroxy (l.Og) in 20 ml water, a solution of p- xylene-2-sulphonic acid was added dropwise until the pH was 7, 10ml of ethanol was then added. After water/ethanol were evaporated, a white crystal solid was obtained.

EXAMPLE 2 Preparation of MEH-PPV/DEO-PPV Solutions of 0.5g (1.5 mmol) of 2,5-bis(chloromethyl)-l-methoxy-4-(2- ethylhexyloxy) benzene and 0.4g (1.5 mmol) of 2,5-bis(chloromethyl)-l,4- bis(ethoxy)benzene in 20ml of anhydrous THF were mixed. To the solution mixture, a solution of 0.389g of the 5% potassium tert-butoxide (1J equivalent) in 20ml of anhydrous THF was added at room temperature with stirring. The reaction mixture was stirred at ambient temperature for 1 hour, and then another solution of 1.85g of the 5% potassium tert-butoxide in 80ml anhydrous THF was added. The mixture was stirred at room temperature for 24 hours. The reaction mixture was then poured into 500ml of methanol with stirring. The resulting red precipitate was washed with distilled water and reprecipitated from THF/methanol and dried under vacuum at 60°C to afford 0.38g product (48% yield). EXAMPLE 3

Fabrication of the Light-Emitting Devices and Performance Measurement The light-emitting devices were fabricated on a transparent ITO coated glass substrate. The mixture solutions of MEH-PPV/DEO-PPV and TBAXS in chloroform at 10, 14, 28, 40 mol. % of TBAXS, respectively, were spin-cast onto the ITO substrates. The thickness of polymer layer was in the range of lOOnm to 600nm (typically, 300nm). The 200 nm Al layer was evaporated onto the polymer film at pressure 5X10^"5 Torr. The active area of the devices was about 6mm².

EXAMPLE 4 Fabrication of Two-Layer Light-Emitting Devices Apart from MEH-PPV and DEO-PPV, we have also synthesised soluble copolymers of MEH-PPV with DEO-PPV, 2-methoxy, 5-cycloheanemethoxy-l,4- phenylene vinylene (CHO-PPV), or 2,5-dimethyl-l,4-phenylene vinylene (DM- PPV), as are PPV copolymers containing Para- and meta- phenylene units (WOP- PPV). It was found that these copolymers exhibit solubilities different from the MEH-PPV homopolymer. In particular, while MEH-PPV is soluble in many organic solvents including chloroform and THF, the copolymer of MEH-PPV and DM-PPV (50:50 mol ratio) was found to be soluble in chloroform, but not in THF. On the other hand, the WOP-PPV was demonstrated to be soluble in THF. We have exploited these properties for the fabrication of two-layer light-emitting devices. In a typical experiment, the copolymer of MEH-DM-PPV was first spin- coated onto ITO glass from its chloroform solution, then a layer of WOP-PPV was formed by spin-casting the polymer solution in THF. Both the emitting layers contain an ion salt (eg tetrabutylammonium p-xylene-2-sulphonate (TBAXS)). The device can have quite different light-emitting characteristics at forward and reverse bias, respectively, if the layer thickness for each of the individual polymer layers is optimised. A lOnm difference in the maximum emitting peak has been observed under the forward and reverse bias, respectively (Figure 4).

Figure 1 shows the electroluminescence spectra of the device with 14 mol. % TBAXS. The emitting peak is about 585 nm which corresponds to a red-orange colour. The observed difference in the peak shape between forward and reverse bias is attributable to the difference in self-absorption of the polymer film between forward and reverse bias. It may be suggested that the light-emitting region in the thin film is different when the device is operated under the forward and reverse bias, respectively. Figure 2 shows the current-light- voltage characteristics of this device. The current-voltage curve is asymmetric about the zero bias, which differs from both the conventional LEDs where the current under reverse bias is negligible, and the reported LECs where the current- voltage curve is symmetric about the zero bias. Although the currents under the forward bias are much larger than that under reverse bias, the emitted light under forward bias is much weaker than that under reverse bias. Therefore, the device efficiency under reverse bias are much higher than that under forward bias (10 times higher). The light emission was detected at forward bias of 5V and reverse bias of -15V. Figure 3 shows the current-light- voltage characteristics for the device with a hight salt content (28 mol.% TBAXS). The overall shape is similar to Figure 2. However, the threshold voltages for visible light emission reduced to 2.7V and - 3.7V for forward and reverse bias, respectively. Figure 4 shows the different light-emitting characteristics at forward and reverse bias, respectively, for an ITO/MEH-DM-PPV+TBAXSYWOP- PPV+TBAXS/A1 tow-layer device.

Since modifications within the spirit and scope of the invention may be readily effected by persons skilled in the art, it is to be understood that the invention is not limited to the particular embodiment described, by way of example, hereinabove.

Claims

Claims:

1. A polymeric composition capable of electroluminescence comprising an organic conjugated polymer and an organic salt of the following structure:

where,

2. A polymeric composition as defined in Claim 1 wherein the organic salt cation is tetra-alkyl ammonium.

3. A polymeric composition as defined in Claim 1 or 2 wherein the aromatic or substituted aromatic group is xylene.

4. A polymeric composition as defined in Claim 3 wherein the aromatic or substituted aromatic group is p-xylene.

5. A polymeric composition as defined in any one of Claims 1 to 4 wherein the organic conjugated polymer is selected from the group consisting of poly(2- methoxy,5-cyclohexanemethoxy-p-phenylene vinylene)

Poly(2-methoxy,5-(2'-ethylhexyloxyl)-p-phenylene vinylene); random copolymer of 2-methoxy,5-cyclohexanemethoxy-p-phenylene vinylene and 2-methoxy,5-(2'- ethylhexyloxyl)-p-phenylene vinylene; random copolymer of 2-methoxy,5-(2'- ethylhexyloxyl)-p-phenylene vinylene and 2,5-diethoxyl-p-phenylene vinylene; random copolymer of 2-methoxy,5,-cyclohexanemethoxy-p-phenylene vinylene and 2,5-diethoxyl-p-pheneylene vinylene; random copolymer of 2-methoxy,5-(2'- ethylhexyloxyl)-p-phenylene vinylene, 2-methoxy , 5 -cyclohexanemethoxy-p- phenylene vinylene and 2,5-diethoxyl-p-phenylene vinylene; and random copolymer of 2-methoxy,5-(2'-ethylhexyloxyl)-p-phenylene vinylene and 2,5- diethyl-p-phenylene vinylene .

6. A polymeric composition as defined in any one of Claims 1 to 5 wherein the composition is formed on a transparent plastic substrate.

7. A polymeric composition as defined in Claim 6 wherein the transparent plastic substrate is conductive.

8. A polymeric composition according to anyone of Claims 1 to 7 wherein the polymeric composition is substantially free of poly (ethyl ene oxide).

9. A security document comprising a polymeric composition capable of electroluminescence comprising an organic conjugated polymer and an organic salt of the following structure:

where,

R' is an aromatic or substituted aromatic group.

10. A security document as defined in Claim 9 wherein the document is a bank note.