Definitions

• Multi-block copolymer based tunable light emitting diode polymers suitable therefor and oligomers.
• the present invention is directed to a tunable light emitting diode (LED) based upon multi-block copolymers, to novel thiophene multi-block copolymers, to thiophene oligomers suitable for preparing said polymers, to processes for preparing said oligomers and said polymers and to the use of said polymers in opto-electronics.
• LED light emitting diode
• LED devices are made of inorganic semiconductors e.g. GaAs, GaP etc. which cover almost the whole spectral region. It would be advantageous to have an organic material with electroluminescence in the blue region, as the organic materials are usually more easy to process.
• LED has certain advantages, especially from the viewpoint of processability.
• the active polymer or prepolymer can be cast from solution on a substrate which makes it possible to fabricate large-area devices.
• Conjugated polymers can cover the whole spectral region by chemical tuning of the wavelength of the emission by choice of the polymer and control of the conjugation length of the polymer.
• Another promising feature is the additional use of a conducting polymer as the hole-injecting electrode resulting in a fully flexible LED.
• Suitable candidates for applications in stable optoelectronic devices are conjugated polymers.
• Polymers can be processed relatively easily and especially large area structures are feasible.
• the first encouraging results were reported by Burroughes et al (Nature, Z, 477 (1992)), Braun et al. (Appl. Phys. Lett. 5fl, 1982(1992)) and Grem et al. (Adv. Mater.4., 36 (1992)), using poly(p-phenylene vinylene)s and poly(p-phenylene) as the electroluminescent layer.
• the basic element giving rise to injection luminescence is that of a p-n junction diode operated under forward bias as is illustrated in fig. l.
• the electrons recombine with holes and give rise to bounded excitons, which radiatively decay to photons.
• each injected electron takes part in the radiative recombination but in practice this is not the case.
• the quantum efficiency of a device made of an inorganic semiconductor emitting in the visible region lies in the range 0.05 to 4%.
• the wavelength of the emitted photon is determined by the energy band gap (ideal case) .
• GaAs has a band gap of 1.43 eV.
• the energy gap has to be larger than 2 eV.
• For a blue LED a band gap of 3.4 eV is required.
• Inorganic semiconductors like SiC with these large band gaps tend to have high resistivities next to fabrication problems because of high melting temperatures and structural stability.
• LED' s can be made of organic thin films by using a multilayer structure. An emitting layer is combined with or sandwiched between a hole and an electron injecting layer.
• a LED emitting bright blue light was achieved, thus with a luminance of 700 cd/m2 at a DC drive voltage of 10 v. Unfortunately the stability of the cell is not good yet.
• LEDs can be made with conjugated polymers. Their main advantage over non-polymeric (in)organic semiconductors is the possibility of processing to form large area structures. The structure of such a LED is shown in figures 2 and 3.
• a substrate, usually glass is covered with the transparent electrode, e.g. indium/tin oxide (ITO) functioning as the hole injecting cathode.
• ITO indium/tin oxide
• the emitting layer or a prepolymer is spincoated on top of this layer and covered with a top electrode e.g. Al or Ca, the electron injecting anode.
• the choice of the electrodes is important. Metals with a low work function give higher efficiencies. Disadvantage of these electrodes is the oxidative instability.
• PET polymeric substrate polyethyleneterephthalate
• transparent processable polyaniline as the hole injecting electrode, G. Gustafsson et al., Nature, 357 477 (1992) even fabricated a fully flexible LED.
• the electroluminescent polymer used by the above mentioned scientists is poly(p- phenylene vinylene) or a soluble alkoxy derivative thereof.
• the electroluminescence spectra of these materials are very similar to their photoluminescence spectra.
• the photoluminescence of PPVs is assigned to radiative combination of the singlet polaron exciton (also called neutral bipolaron) formed by intrachain excitation.
• the electroluminescence is assigned to the same excited state and is generated by recombinations of holes and electrons injected from opposite sides of the structure.
• the charge carriers are probably polarons.
• the quantum yield of photoluminescence of PPV is about 4%, the non- radiative processes limit the efficiency of LEDs. This is caused by migration of the excited states to defect sites which act as non-radiative recombination centres.
• the quantum yields for electroluminescence of the conjugated/nonconjugated polymers were strongly enhanced.
• the non-conjugate part acting as a trap for the excitons, preventing the migration to quenching sites.
• LED being tunable, stable and easy to manufacture at low cost.
• said electroluminescing material comprises an electroluminescing material, electrodes and optionally carrier material and/or reflecting material, said electroluminescing material comprising at least one block copolymer consisting of at least two types of blocks, active blocks, sandwiched between non-active blocks, said active blocks being a ⁇ -conjugated block of at least 2 and at most 16 monomeric units, said ⁇ -conjugated block having a substantially uniform blocklength throughout the copolymer, and said non-active block having no ⁇ -conjugation.
• the invention is based thereon that it has been found to be possible to tune the wavelength of the emitted light by sandwiching ⁇ -conjugated blocks having substantially uniform block length between blocks having no ⁇ -conjugation, whereby the length of the ⁇ -conjugated blocks mainly determines the wavelength of the emitted light.
• this definition of sandwiching ⁇ -conjugated blocks between blocks having no ⁇ -conjugation is intended to include the situation that both blocks have ⁇ -conjugation, whereby there is a large difference in band gaps between the blocks, resulting therein that there is no ⁇ -conjugation between the blocks.
• these materials have the advantage that they can be processed with deep UV- photolithography, since the polysilanes are deep-UV photoresists.
• the stability of the above materials under environmental conditions and under the influence of optical and electric fields used, is comparable to that of PPV, the first polymer material used for electroluminescence device applications.
• the mechanical properties of the oligomer blocks are poor, the mechanical properties of the multi-blocks made out of these oligomers generally are excellent and easy to control by adjusting the number of blocks. Further, the solubility as well as some of the electrical and optical properties are controllable by the substituents on the non-active blocks and the ⁇ -conjugated blocks.
• the above approach gives ample flexibility to tune not only the mechanical, but also the optical and electrical properties by tuning the length of the oligomers and the number of blocks in the multi-block copolymers and the chemical nature of the side-groups.
• Suitable ⁇ -conjugated blocks can be based upon all types of components that result in ⁇ -conjugation after polymerization to short blocks. Examples thereof are i.a. thiophenes, suitably substituted thiophenes, vinylene, arylene, vinylene-arylene, thiophene-vinylene and thiophene- arylene.
• a preferred group of oligomers to be used for the present invention are the, optionally substituted, oligo ⁇ thiophenes.
• Suitable oligomers are based upon the various components given in the formula sheet (Fig. 4) .
• a block copolymer based upon a mixture two or three different blocks. In that case each of the components will have substantially the same block length.
• the ⁇ -conjugated blocks can be prepared from the components that give said ⁇ -conjugated blocks upon polymerization.
• Essential is the factor that the block length is substantially uniform.
• various methods in which to obtain said substantially uniform block length are all based thereon that first an oligomer is obtained having a substantially uniform block length. This can for example be accomplished by oligomerising the monomers in a controlled manner to give rise to a product that is already relatively uniform, optionally followed by purification to obtain the required uniform block length.
• Another approach is to control the reaction in such way, that the components can only react to give a well-defined product.
• An example of this latter approach is to provide a starting material R having two reactive groups, which reactive groups each react with one other reactant, ⁇ , to give a product S-R-S that possesses the required block length and ⁇ - conjugation (optionally after further treatment, for example removing or adding substituents) .
• the non-active blocks that can be sandwiched between the ⁇ -conjugated blocks can have any composition, provided that they do not provide ⁇ -conjugation and that they can be sandwiched between the ⁇ -conjugated blocks.
• examples thereof are oligo-organo-silanes, substituted silicium blocks and oligo styrene and derivatives thereof.
• the choice of the intermediate group has influence on the properties of the multiblock copolymer.
• Preferred groups are i.a. ⁇ , ⁇ -unsaturated organic compounds like vinyl and alkylene compounds, oligovinylenes and derivatives thereof, germanium compounds, silicium compounds and carbon compounds.
• Suitable materials for said blocks are based on silicium, germanium or on carbon-carbon oligomers. Of the latter group, especially the styrenic materials, like oligostyrene, and oligomers of styrene derivatives, as well as vinyl compounds such as vinylcarbazole are Suitable.
• the silicium, carbon or germanium based materials are preferably of the type.
• X denotes Si, Ge, C, SiO, or CO
• R 7 and R 8 being identical or different, each denoting a lineair or branched alkyl substituent having 1-6 carbon atoms or a phenyl, optionally substituted with one or more alkyl and/or alkoxy groups.
• the non-active, or ⁇ -conjugated blocks do not have such a strong influence on the wavelength of the LED, although the choice of the blocks and the length thereof certainly influences the behaviour of the LED.
• Suitable block lengths vary between 2 and 8, said block length being based upon the number of atoms in the ⁇ -conjugated block that become part of the polymer chain.
• the silicon-based blocks have the advantage that they can be processed with deep UV-photolithography, since the polysilanes are deep-UV photoresists.
• the blocks based upon silicium and germaniumn act also as intrinsic hole transport material, improving thus the quantum efficiency. It is thus possible to prepare a three block copolymer, wherein two types of non- ⁇ -conjugated blocks are present, namely one type having hole and another having electron transport properties.
• non- ⁇ -conjugated blocks are either directly available chemicals, like vinyl or vinylene oligomers, or can be prepared separately or in situ, i.e. during the assembly of the block copolymer.
• new semi ⁇ conducting organic and/or organic-inorganic block copolymers can be used, (some of which are shown in figure 4) said polymers being obtainable by making various combinations for the optionally alkylated or alkoxylated active blocks of
• Oligothiophenes, oligovinylenes, oligophenylenes and oligo(p-phenylenevinylene)s ( ⁇ -conjugated blocks) with oligosilanes ( ⁇ -conjugated blocks) , oligosiloxanes, oligovinypyridine, oligostyrene non-active blocks in order to obtain a heterostructure based on the principal of the selfassembly of the block copolymers.
• the active block will be sandwiched between the non- active blocks [(-A x -B y -)] z , wherein A x and B y respectively denote the ⁇ -conjugated and the non ⁇ -conjugated blocks, x and y being the respective block-lengths and z being the number of -A x -B y - blocks in the polymer.
• the values of x, y and z are preferably such that the molecular weight of the polymer will be between 2500 and 500,000. Lower values may lead to problems in the processing to films, whereas higher values do not provide additional advantages in terms of polymer properties and may lead to processing difficulties, due to the high molecular weight.
• the proposed block copolymers provide various chemical and physicochemical tuning capabilities and improve the properties that have been described previously, concerning the tunable LEDs.
• the wave length of the emitted light is tuned to a value between 400 and 850.
• the block length preferably varies between 2 and 16 units, each unit comprising two double and two single C-C bonds. ith shorter block lengths the emitted light is more in the blue/green area, whereas larger block lengths lead to a more reddish colour.
• the ⁇ -conjugated blocks are either directly available chemicals, like vinyl or vinylene oligomers, or can be prepared separately or in situ, i.e. during the assembly of the block copolymer.
• the blocks can be assembled into one polymer by known techniques, for example by reaction between the blocks, either directly into one copolymer or in two or more steps.
• One of the further objects of the present invention is to provide a novel class of multiblock copolymers containing short blocks of thiophene and derivatives thereof, which multiblock copolymers are suitable for use in opto- electronics and more in particular in LED's, as described herein.
• a further object is to provide a novel class of multiblock copolymers containing short blocks of thiophene and derivatives thereof, which multiblock copolymers are easy to process, for example by spin coating, into a thin layer on a substrate.
• This block copolymer consists of at least two groups of blocks, thiophene blocks A, sandwiched between no -thiophene blocks B, said thiophene blocks A having the formula l,
• Ri-R ⁇ may each be selected from H, optionally branched, lower alkyl, i.e. C 1 -C 15 and optionally branched, lower alkoxy.
• substituents Ri-R ⁇ in the blocks may vary within each block.
• the block A the thiophene block
• the block A will be sandwiched between the non-thiophene blocks in the manner [ (-A x -By-) ] z
• the values of x, y and z are preferably such that the molecular weight of the polymer will be between 2500 and 500,000. Lower value may lead to problems in the processing to films, whereas higher values do not provide additional advantages in terms of polymer properties and may lead to processing difficulties, due to the high molecular weight.
• the number of thiophene units can be selected, as well as the type and number of substituents. It is preferred that at least one of the thiophene rings has to be substituted as otherwise the processability, especially in spin-coating, is insufficient. Preferably at least two of the rings in each block are substituted with alkyl, aryl, alkaryl, aralkyl, alkoxy, aralkoxy and the like, each substituent containing one to 15 carbon atoms. In practising the invention the substituents Ri and R 2 will generally be identical to R 5 and Re, whereas R 3 and R 4 may be different.
• the number of substituents on each ring will not exceed one, that means that in each ring at least one of the R-groups will denote H.
• the selection of the substituents influences the electroluminescent properties of the material to some degree.
• substituents like butyl, octyl and dodecyl, is very important.
• Suitable multiblock copolymers preferably contain 2 to 16 or more thiophene units in each thiophene block. It is to be noted that when using these multiblock copolymers of the present invention in opto-electronics and more in particular in electro-luminescent devices, the length of the blocks has a profound influence on the wave length of the light. Also important is the choice of the various substituents.
• Suitable multiblock copolymers preferably contain 5 or more thiophene units.
• group B has also influence on the properties of the multiblock copolymer.
• preferred groups are i.a. ⁇ , ⁇ -unsaturated organic compounds like styrene and derivatives thereof, germanium compounds and silicium compounds. Suitable materials for said blocks are those described hereinbefore in relation to the LED.
• the blocks can be assembled into one polymer by known techniques, for example by reaction between the blocks, either directly into one copolymer or in two or more steps.
• new LED's are provided based upon novel semi-conducting organic and/or organic- inorganic block copolymers, said polymers being obtainable by making ' various combinations of ⁇ -conjugated and non- ⁇ - conjugated blocks in order to obtain a heterostructure based on the principal of the selfassembling of the block copolymers.
• the materials for the electrodes needed for the LEDs and quantum-well devices are chosen appropriately following the work function of the active multi-block copolymers. These materials are well-known in the art of LED's. Suitable materials are described in the literature and can be selected by a person skilled in the art based upon the actual configuration to be used.
• the LED comprises two layers of electrodes, such as indium-tinoxide and a conducting metal, between which layers the electroluminescent material has been sandwiched.
• the electroluminescent material is preferably spun-coated on the surface of an electrode.
• a thin layer of polymer forming a hole injecting electrode, transparent in the region of emission may be applied to a flexible substrate, for example a polyester.
• This two layer material forms the cathode of the LED, which is placed in contact with the electroluminescent material.
• the anode can advantageously be evaporated at low pressure onto the surface of the electroluminescent material.
• Suitable metals are calcium, indium, aluminium, tin, magnesium and alloys of those materials.
• FIG l the basic element giving rise to injection luminescence, a p-n junction diode operated under forward bias, is illustrated.
• figures 2 and 3 the schematic set-up of an LED is shown and in figure 4 the general structure of some the multi- block copolymers is given.
• Figure 5 gives the spectroscopic characterization of thin films of multiblock copolymers with varying block length.
• Figures 6-9 give some of the reactions that may be used to prepare the block copolymers used in the invention.
• FIG. 10 shows the wave length pattern of the electroluminescence of two different multiblock copolymers.
• Route A Polycondensation of dilithiumsalt of oligothienylene and oligosilanylene.
• Route B Cross coupling of digrignard- bisthienylsilanylene and dibromooligothienylene.
• Ri, R 2 , R 3 , R 4 , R 5 , Re H, C1-C 2 0 linear or branched alkyl or alkoxyalkyl;
• Thin films of the polymers were prepared by spincoating 5-15% solutions of the various polymers in an organic solvent on a glass slide covered with Indium Tin Oxide (ITO) .
• ITO Indium Tin Oxide
• Various metal electrodes (Ca,Al,In,Sn,Mg and alloys of these metals) were evaporated at low pressure (10-6 torr) .
• the films were assembled into an LED.
• the wave length pattern of the electroluminescence of two different multiblock copolymers has been given in Fig. 10.

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• 1993
  • 1993-12-29 EP EP94904345A patent/EP0677208A1/de not_active Withdrawn

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