Polymer electroluminescent device employing emissive ionomer-type polymer

1. Field of the Invention

The present invention relates to an ionomer-type luminescent polymer and polymeric electroluminescent devices employing the same, more specifically, to an ionomer-type luminescent polymer whose heat-stability is improved by the formation of ionic cross-linkage, and electroluminescent devices employing the same as a luminescent layer.

2. Background of the Invention

Most of the luminescent materials used for polymeric electroluminescent devices are not linked to ions. The luminescent materials are, depending on the backbone structure, generally classified into conjugated polymer, non-conjugated luminescent polymer, and conjugated/non-conjugated multi-block copolymer with alternating conjugated and non-conjugated parts. Conjugated luminescent polymers having π-conjugated double bond backbone, includes poly(1,4-phenylenevinylene, PPV) (see: U.S. Pat. No. 5,247,190) and its derivatives, derivatives of poly(para-phenylene) (see: G. Grem et al, Advanced Materials , 4(36), 1992), derivatives of polythiophen(see: U.S. Pat. No. 5,540,999), and derivatives of polyphlorene(see: U.S. Pat. No. 5,876,864). Non-conjugated luminescent polymers have non-luminescent backbone connected to luminescent side chains with fluorescent materials such as anthracene, to give electroluminescence corresponding to a luminescent region attributed by the fluorescent material. On the other hand, luminescent polymers made of conjugated/non-conjugated multi-block have a structure in which the length of the conjugation is made short to create blue-shift of luminescent region.

Most of polymeric electroluminescent devices, however, are proven to be less satisfactory in the senses that they are able a to emit blue light only at more than 10 V, and have short life span. As an alternative, Vera Cimnova et al. has reported water-soluble, ionic luminescent polymers by introducing ionic groups into backbone in the form of electrolytes. However, they still revealed a shortcoming of short life-span, owing to the unstable nature of polymers in the environment of water and high temperature(see: Vera Cimnova at al., Applied Materials , 8, 585, 1996).

Therefore, there are strong reasons for developing and exploring novel luminescent polymers that can emit light at a low voltage, while enjoying long life-span.

The present inventors have made an effort to provide luminescent polymers of long life-span that can emit light at a low voltage, and discovered that ionomer-type luminescent polymers prepared by introducing ionic groups into luminescent polymers can emit light at a low voltage, and have higher stability to water and long life-span.

A primary object of the present invention is, therefore, to provide ionomer-type luminescent polymers.

The other object of the invention is to provide electroluminescent devices employing a luminescent layer of ionomer-type electroluminescent polymer.

FIG. 1 depicts a cross-sectional view of an electroluminescent device or present invention.

1: panel

2: translucent electrode

3: hole carrier layer

4: luminescent layer

5: electron carrier layer

6: metal electrode

An ionomer-type luminescent polymers of present invention is characterized by an improvement in a luminescent polymer in which a backbone having conjugated, non-conjugated, or conjugated/non-conjugated multi-block structure is bound with or without conjugated or non-conjugated side chains, that 0.1 to 15% (mol/mol) ions are introduced into the backbone or side chains of the luminescent polymer to give an ionomer-type luminescent polymer in which the backbone or side chains are connected to ions directly or indirectly through the linker. The ion concentration should not exceed 15% (mol/mol) to make ionomers insoluble to water. Poly(1,4-phenylenevinylene), poly(para-phenylene), polythiophene, and polyfluorene are preferably used for the conjugated backbone while polystyrene (PS) is preferred for the non-conjugated backbone. For linkers, non-conjugated organic luminescent groups are preferred. Ionomers are classified into anionic or cationic ionomers depending on the ions used: In the structure of anionic ionomers, anions such as sulfonyl (SO 3 − ) carboxyl (—CO 2 − ) or metal ion groups, are connected to backbone or side chains first, and then equimolar amount of cations are bonded to these anions. All kinds of cations that can be bonded to anions may be employed, though alkali metals such as Li + , Na + , Cs + , K + , Mg ++ , Zn ++ , and Eu +++ , or organic ions such as N(CH 3 ) 4 + are preferred; and, in the case of cationic ionomers, cations such as N(CH 3 ) 4 + , are connected to backbone or side chains first, and then equimolar amount of anions, such as carboxyl (—CO 2 − ) group, are bonded to these cations.

Ionomer-type luminescent polymers of present invention includes: first, neither backbone nor side chains are conjugated(see: Chemical formula (I)); secondly, backbone has a conjugated structure, and ions are directly connected to the backbone (see: Chemical formula (II)); thirdly, backbone has a conjugated structure with side chains, and ions are connected to these side chains(see: Chemical formula (III)); fourthly, backbone has a conjugate/non-conjugate multi-block structure(see: Chemical formula (IV)), where ions may be connected to either conjugated or non-conjugated part.

wherein,

A represents an anion;

B represents a cation; and,

n represents a natural number from 1 to 30.

Electroluminescent devices can be prepared by employing ionomer-type luminescent polymers of present invention as a luminescent layer(see: FIG. 1 ). As clearly depicted in FIG. 1, an electroluminescent device employing luminescent layer or ionomer-type luminescent polymer, consists of a translucent electrode(2), a hole carrier layer(3), a luminescent layer made of ionomer-type luminescent polymer(4), an electron carrier layer(5), and a metal electrode(6), which are sequentially formed on top of a transparent panel(1), or consists of an metal electrode(6) which are formed on a luminescent layer mace of ionomer-type luminescent polymer(4), which in turn is plated on a transparent electrode(2) above a panel(1), where indium-tin oxide(ITO) is preferably used as the translucent electrode.

Electroluminescent devices of the invention can emit light at a low voltage of about 2 V, and also, depending on the type of ion, may emit photoluminescence(PL) with 480 nm wavelength and electroluminescence(EL) with 490 nm wavelength.

The present invention is further illustrated by the following examples, which should not be taken to limit the scope of the invention.