WO2007088006A1

WO2007088006A1 - Modified polyvinylcarbazole matrix materials for use in organic light-emitting diodes with phosphorescence emitters

Info

Publication number: WO2007088006A1
Application number: PCT/EP2007/000641
Authority: WO
Inventors: Silvia Janietz; Armin Wedel; Hartmut Krüger; Manuel Thesen
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2006-01-31
Filing date: 2007-01-25
Publication date: 2007-08-09
Also published as: DE102006004435B4; EP1979436A1; DE102006004435A1

Abstract

The invention relates to new polymeric light-emitting diodes (PLED) which are able to generate light in the different spectral ranges with high efficiency. The structure of such a PLED includes a transparent electrode as the anode, a metal electrode as the cathode, an organic light-emitting layer system, and, if desired, electron and hole injection layers.

Description

FRAUNHOFER SOCIETY ... E.V. 069PCT 1859

Modified polyvinylcarbazole matrix materials for use in organic light emitting diodes with phosphorescent emitters

The invention relates to novel polymeric light emitting diodes (PLEDs) which can produce light in the different spectral regions with high efficiency. The structure of such a PLED includes a transparent electrode as an anode, a metal electrode as a cathode, an organic light-emitting layer system and optionally electron and hole injection layers.

It is known that PLEDs can be built on the basis of phosphorescence emitters, so-called triplet emitters. The efficiency is thereby improved by the addition of suitable so-called hole and electron transport materials (Lamansky, Lam et al, J Appl Phys, 2002: 92 (3), 1570; X. Gong, SH Lim, JC Ostrovsky, D. Moses, CJ. Bardeen, GC Bazan, J Appl Phys, 2004: 95 (3), 948).

As triplet emitter systems, on the one hand, organic molecules are processed to thin layers by vapor deposition in a high vacuum (Pfeiffer, M.R. Forrest, K. Leo, M. E. Thomson, Adv Materials, 2002: 14, 22, 1633). On the other hand, processing into thin films based on solutions is also possible (K.M. Vaeth, C.W. Tang, J Appl Phys, 2002: 92 (7), 3447). For the latter, preferably higher molecular weight organic materials (polymers (X. Yang, D. Neher, D. Hertel, K. Däubler, Adv Materials, 16, 2, 2004, 161), dendrimers (S. -C. Lo, AH Male, JP Markham, SW Magennis, PL Burn, OV Salata, IDW Samuel, Adv Materials, 2002: 14, 975) or at least oligomers). In this case, printing processes can be used for the structured application of the organic layer. The solvent is then evaporated.

The most widely used matrix for solvent-based triplet emitters is the polyvinylcarbazole (PVK). The addition of the triplet emitter (eg transition metal complexes based on Ir, Pt, etc.) makes it possible to build up organic light-emitting elements. The efficiency of such elements can be significantly improved by the addition of electron transport materials, such as 2- (4-biphenylyl) -5- (4-tert-butylphenyl) -1,3,4-oxadiazoles (TBPO). In addition to the matrix function, the PVK also handles the hole transport, but with a HOMO energy level of -5.9 eV has a value which is poorly adapted to the work function of the indium tin oxide / polyethylenedioxothiophene electrode (ITO / PEDOT electrode) , The large energy barrier for the injection of holes into the PVC leads to higher threshold voltages. To improve tion hole injection are therefore often a variety of triarylamines, z. N, N, N ', N'-tetrakis (4-methylphenyl) benzidine (TPD). Efficiencies of up to 38 cd / A have been determined for this system (XH Yang, D. Neher, Appl Phys Lett, 2004: 84, 2476).

These very complex, phosphorescent systems consisting of PVK, low-molecular electron and hole transport materials, as well as the transition metal complex are then separated from the solution together. During processing, the miscibility and solubility limits of these multicomponent mixtures are clearly negatively noticeable. Furthermore, the PVK-based triplet emitter systems have so far only inadequate long-term stability, so that their technical application does not seem appropriate so far.

It was therefore an object of the present invention to reduce the number of components required for highly efficient systems based on PVK (electron and hole transport materials), to improve the hole injection from the ITO / PEDOT electrode into the PVK matrix and the long-term stability of the PVK -based systems.

The object is achieved by the generic polymeric light-emitting diode comprising an organic, light-emitting layer system, a transparent electrode as the anode and a metal electrode as

Cathode with the characterizing features of claim 1 solved. The further dependent claims show advantageous developments.

According to the invention, a polymeric light-emitting diode is provided in which the organic light emitting layer system 3, 6-dialkyl-substituted polyvinylcarbazole (PVK) of the general formula I.

Formula I

with R = Ci-Ci ₈ linear or branched alkyl and / or C ₃ -C ₈ cycloalkyl. The 3 and 6 positions of the carbazole ring, which are responsible for the lack of chemical and electrochemical stability of the PVK, are blocked.

Advantageously, the polymeric light-emitting system contains poly (3, 6-di-tert-butyl-N-vinylcarbazole) (DTB-PVK). Its synthesis can preferably be carried out according to FIG. 1. Starting from carbazole, a Friedel-Crafts alkylation is carried out with tert-butyl chloride. The resulting 3,6-di-tert-butylcarbazole is N-alkylated with β-chloroethyl p-toluenesulfonate. The isolated N-chloroethyl-3,6-di-tert-butylcarbazole is subjected to dehydrohalogenation to give the desired 3,6-di-tert-butyl-N-vinylcarbazole. The polymerization of the substituted N-vinylcarbazole can optionally be cationic (with BF ₃ etherate) or free-radical (AIBN).

The transparent electrode as anode advantageously contains at least one conductive polymer. The conductive polymer of the anode preferably contains Polyethylendioxothiophen / Polystyrensulfon- acid (PEDOT / PSS, Baytron ^®).

In a further advantageous embodiment of the

Invention contains the transparent electrode of the anode indium tin oxide (ITO). Due to the significantly increased HOMO energy level value (about -5.0 to -5.6 eV) of the 3, 6-disubstituted polyvinylcarbazole of the general formula 1

with R = Ci-Ci ₈ linear or branched alkyl and / or C ₃ -Ci ₈ Cycloalkyl is thus a much better adaptation to the ITO / PEDOT PSS electrode (-5,3 eV) permitted, so that the injection of holes is much easier.

The cathode contained in the polymeric light-emitting diode provided according to the invention advantageously contains at least individual metals, alloys of at least two metals and / or ceramic materials.

Furthermore, it is advantageous if further additives selected from the group of electron transport materials, Lochinj ektionsmaterialien and / or phosphorescence emitters may be included in the organic, light-emitting layer system. Particularly suitable electron transport materials are compounds selected from the class of oxadiazoles, triazoles, quinoxalines, quinols, phenanthenes and / or bathocuproins; Very particular preference is given to 2- (4-biphenyl) -5- (4-tert-butylphenyl) -1,3,4-oxadiazole (TBPO). The additional use of hole induction materials is possible but not required.

As a phosphorescence emitter, in a preferred embodiment, organic metal complexes are included. The central atoms can be selected from the elements Re, Ir, Pt, Eu and / or Ru. The choice of a suitable ligand allows a selective adjustment of the emitted color. Lamansky et al, J Appl Phys, 2002: 92 (3), 1570, X. Gong, SH Lim, JC Ostrowski, D. Moses, CJ. Bardeen, are preferred. GC Bazan, J Appl Phys, 2004: 95 (3), 948, M. Pfeiffer, SR Forrest, K. Leo, ME Thomson, Adv Materials, 2002: 14, 22, 1633). Of the listed metal central atoms, in particular the complexes of iridium permit very high phosphorescence yields because of their lower triplet lifetime in the range of a few μs compared to platinum complexes (50 μs).

The improved adaptation of the 3, 6-disubstituted polyvinylcarbazole of the general formula I and the ITO / PEDOT PSS electrode surprisingly made it possible to dispense with additional hole transport materials, as required in the PVK system, and to simplify the construction of the phosphorescent PLED system.

Based on the following examples and figures of the inventive subject matter will be explained in more detail, without limiting this to the specific embodiments shown here.

Fig. 1 shows the synthetic scheme for the preparation of DTB-PVK starting from carbazole

Fig. 2 shows the cyclic voltammogram of PVK

Fig. 3 shows the cyclic voltammogram of DTB-PVK

Fig. 4 shows the brightness-voltage characteristics of the diode systems based on PVK and DTB-PVK

Fig. 5 shows the efficiency of the diode systems based on PVK and DTB PVK

Fig. 6 shows the efficiency of the DTB PVK diode system as a function of additional hole material

Fig. 7 shows the brightness degradation of a PVK and DTB PVK diode system.

used abbreviations

mppy 5-methyl-2-phenylpyridinyl

TBPO 2- (4-biphenylyl) -5- (4-tert-butylphenyl) -

1,3,4-oxadiazoles

The DTB-PVK was electrochemically analyzed in comparison with unmodified PVK. Surprisingly, cyclovoltammetric analysis revealed that the HOMO energy level value of poly (3, 6-di-tert-butyl-N-vinylcarbazole (-5.4 eV) was significantly higher unsubstituted PVK (-5.9 eV) (Figs. 2 and 3).

A polymeric light-emitting diode based on DTB-PVK shows a lower threshold voltage than the PVK system even without additional hole transport material. This becomes clear in a direct comparison of two DTB-PVK and PVK-based polymer light-emitting diodes which were prepared under the same conditions (constant electron transport material content). These properties were detected with two concentrations of both electron conductor and triplet emitter. In Fig. 4 and 5, the Elektroluminezenzverhalten the PVK and the DTB PVK system is shown. Clearly visible is the up to 4 Volt lower threshold voltage of the DTB-PVK system with a maximum efficiency of 33 cd / A @ 12 V.

The concentration of the triplet emitter (Ir (mppy) ₃ ) was chosen to be 3% by weight, the proportion of the electron transport material (TBPO) was 14% by weight. A further increase in brightness and reduction of the operating voltage can be observed at higher concentrations of the electron transport material and the triplet emitter. With 20% by weight of electron transport material and 8% by weight of triplet emitter (Ir (mppy) ₃ ), the threshold voltage of the DTB-PVK further drops to 3.5 V and maximum brightnesses of 5800 cd / m ² at 14 V are achieved. The maximum efficiency was determined on this system at 9 V at 34 cd / A.

The PVK system without hole transport material has a threshold voltage of approx. 8-9 Volt and an efficiency of approx. 17 cd / A at 18 V. equal system with 20% by weight electron conductor and 8% by weight triplet emitter (Ir (mppy) ₃ ) not more than 19 cd / A at 6 V or 3300 cd / m ² at 10 V. Higher voltages lead to a faster degradation in PVK the electroluminescence.

It has been shown that additional hole transport material further decreases the efficiency since there is now an excess of holes which disturb the charge carrier balance (Figure 6).

All DTB-PVK systems show a much slower decrease in brightness as a function of time at a given starting brightness (FIG. 7).

example 1

Synthesis of 3, 6-di-tert-butyl-carbazole

In a 500 ml three-necked flask, 20.02 g of carbazole and 17.65 g of AlCl _{3 are} suspended under nitrogen in 200 ml of dichloromethane. The suspension is cooled to 0 ⁰ C and added dropwise over 90 min 26 ml of tert-butyl chloride. It is then warmed to room temperature and stirred overnight. Thereafter, the reaction mixture is diluted with 400 ml of dichloromethane and shaken out successively twice each with 100 ml of 1 molar HCl and 3 molar NaCl solution. Thereafter, the solvent is removed by rotary evaporation and isolated 32.32 g of crude product. The isomer mixture is purified by recrystallization from methanol. There are obtained 7.69 g of pure 3, 6-di-tert-butyl-carbazole.

Example 2

Synthesis of N-2-chloroethyl-3,6-di-tert-butylcarbazole

7 g of 3, 6-di-tert-butyl-carbazole are dissolved under nitrogen atmosphere in 400 ml of acetone. Subsequently, 20 g of β-chloroethyl p-toluenesulfonate and a solution of 10 g of NaOH in 7.5 ml of water are added and heated to 60 ⁰ C with stirring. After 10 hours, another 10 g of β-chloroethyl p-toluenesulfonate and 4 g of NaOH in 2.5 ml of water are added. After a further 10 h, the mixture is cooled and the precipitate is filtered off. The solution is concentrated. The crude product is purified by column chromatography on silica gel with hexane / ethyl acetate. In this process, unreacted educt is recovered. It will be 2.88 g pure Product isolated.

Example 3

Synthesis of N-vinyl-3,6-di-tert-butylcarbazole

4 g of N-2-chloroethyl-3,6-di-tert-butylcarbazole are dissolved under nitrogen atmosphere in 125 ml of dried ethanol and added dropwise to 35 ml of a 25% methanolic KOH solution and heated to reflux. After 20 h, the reaction is stopped and the reaction product is added to 800 ml of water. The organics are extracted with CHCl ₃ . Subsequently, the pure substance (2.37 g) is isolated by column chromatography over alumina with hexane / toluene.

Example 4

Polymerization of N-vinyl-3,6-di-tert-butyl-carbazole

2.18 g of N-vinyl-3,6-di-tert-butylcarbazole are dissolved under inert gas atmosphere in 71 ml of dichloromethane and cooled to -78 ⁰ C. Thereafter, the cationic polymerization is initiated by addition of 9 ul boron trifluoride etherate. After 30 min, it is quenched by the addition of 1 ml of methanol. The polymer solution is concentrated and the polymer is precipitated in methanol. The

Purification takes place by reprecipitation from THF / methanol and extraction with methanol. There are obtained 1.6 g of poly (N-vinyl-3,6-di-tert-butylcarbazole) with M _n = 282,000 g / mol and M _w = 737,000 g / mol. Example 5

Preparation of a PhPLED structure 1

The ITO substrates were cleaned and treated with an oxygen plasma for 10 min. Immediately thereafter, the substrates were introduced into a glove box. There, the application of the PEDOT / PSS layer (Baytron ^® ) and an annealing at 110 ⁰ C, which has a dry film thickness of about 60 nm result. 3 wt% triplet emitter complex Ir (mppy) ₃ (ADS066GE, American Dye Source) and 14 wt% electron transport material (TBPO) (Aldrich) were added to the corresponding chlorobenzene solution of DTB-PVK (1.5 wt%). In addition, the DTB-PVK hole transport material (TPD) (ST16.3, sensor) was added to one device. The spin-coating process of the finished solutions has been optimized so that layer thicknesses of approx. 80 nm are achieved. The solvent was evaporated at 110 ⁰ C for 60 min. Subsequently, the vapor deposition of the cathode structure was carried out. Here, first a thin LiF layer (3 nm) and then calcium (30 nm) and silver (100 nm) were evaporated.

Example 6

Preparation of a PhPLED structure 2

The ITO substrates were cleaned and treated with an oxygen plasma for 10 min. Immediately thereafter, the substrates were introduced into a glove box. There, the application of the PEDOTrPSS layer (Baytron ^® ) and an annealing at 110 ⁰ C, which resulted in a dry film thickness of about 60 nm Has. 8 wt% triplet emitter complex Ir (mppy) ₃ (ADSO66GE, American Dye Source) and 20 wt% electron transport material (TBPO) (Aldrich) were added to the corresponding chlorobenzene solution of DTB-PVK (1.7 wt%). The spin-coating process of the finished solutions has been optimized so that layer thicknesses of approx. 80 nm are achieved. The solvent was evaporated at 110 ⁰ C for 60 min. Subsequently, the vapor deposition of the cathode structure was carried out. Here, first a thin CsF layer (4 nm) and then calcium (30 nm) and silver (100 nm) were vapor-deposited.

Example 7

Comparative Example to PhPLED Structure 1

The ITO substrates were cleaned and treated with an oxygen plasma for 10 minutes. Immediately thereafter, the substrates were introduced into a glove box. There, the application of the PEDOT: PSS layer (Baytron ^® ) and an annealing at 110 ⁰ C, which has a dry film thickness of about 60 nm result. 3 wt% triplet emitter complex Ir (mppy) ₃ (ADS066GE, American Dye Source) and 14 wt% electron transport material (TBPO) (Aldrich) were added to the corresponding chlorobenzene solution of the PVK (2.2 wt%). The spin-coating process of the finished solutions has been optimized so that layer thicknesses of approx. 80 nm are achieved. The solvent was added

110 ⁰ C evaporated for 60 min. Subsequently, the vapor deposition of the cathode structure was carried out. Here, first a thin LiF layer (3 nm) and then calcium (30 nm) and silver (100 nm) were evaporated. Example 8

Comparative Example of the PhPLED Structure 2

The ITO substrates were cleaned and treated with an oxygen plasma for 10 min. Immediately thereafter, the substrates were introduced into a glove box. There, the application of the PEDOT: PSS layer (Baytron ^® ) and an annealing at 110 ⁰ C, which has a dry film thickness of about 60 nm result. 8 wt% triplet emitter complex Ir (mppy) ₃ (ADS066GE, American Dye Source) and 20 wt% electron transport material (TBPO) (Aldrich) were added to the corresponding chlorobenzene solution of the PVK (2.2 wt%). The spin-coating process of the finished solutions has been optimized so that layer thicknesses of approx. 80 nm are achieved. The solvent was evaporated at 110 ⁰ C for 60 min. Subsequently, the vapor deposition of the cathode structure was carried out. Here, first a thin CsF layer (4 nm) and then calcium (30 nm) and silver (100 nm) were vapor-deposited.

Claims

FRAUNHOFER-GESELLSCHAFT ... eV 069PCT 1859Patentansprüche

1. A polymer light-emitting diode containing an organic, light-emitting layer system, a transparent electrode as an anode and a metal electrode as a cathode, wherein the organic light-emitting layer system 3, 6-disubstituted polyvinylcarbazole of the general formula I

with R = Ci-Ci ₈ linear or branched alkyl and / or C ₃ -C ₈ cycloalkyl.

2. Polymer light-emitting diode according to claim I _7, characterized in that the organic light-emitting layer system poly (3, 6-di-tert-butyl-N-vinylcarbazole).

3. Polymer light-emitting diode according to one or more of claims 1 to 2, characterized in that the transparent anode at least contains a conductive polymer.

4. Polymer light-emitting diode according to one or more of claims 1 to 3, characterized in that the transparent anode polyethylene dioxythiophene / polystyrene sulfonic acid (PE-DOT / PSS) contains.

5. Polymer light-emitting diode according to one or more of claims 1 to 4, characterized in that the transparent anode contains indium tin oxide (ITO).

6. Polymer light-emitting diode according to one or more of claims 1 to 5, characterized in that the cathode contains individual metals and / or alloys of at least two metals.

7. Polymer light-emitting diode according to one or more of claims 1 to 6, characterized in that further additives, selected from the group of electron transport materials, hole injection materials and / or phosphorescence emitters may be contained in the organic, light-emitting layer system.

8. Polymer light-emitting diode according to claim 7, characterized in that the electron transport materials compounds selected from the class of oxadiazoles, triazoles, quinoxalines, quinolines, phenanthrene and / or batho- cuproine included.

9. Polymer light-emitting diode according to one or more of claims 7 or 8, characterized in that the electron transport materials lien 2- (4-biphenyl) -5- (4-tert-butylphenyl) - 1, 3, 4-oxadiazole ,

10. Polymer light-emitting diode according to one of claims 7 to 9, characterized in that as phosphorescence organic metal complexes with central atoms selected from the elements Re, Ir, Pt, Eu and / or Ru may be contained.

11. Polymer light-emitting diode according to one or more of claims 1 to 10, characterized in that the energy level of the organic light-emitting layer system used is between -5.0 and -5.6 eV, preferably between -5.3 and -5.5 eV.