The present invention is related to electroluminescent conjugated polymers modified with high electronegative heterocyclic groups suitable for use in the fabrication of polymeric light emitting diodes (PLED).
The research of organic light emitting diodes started from 1963 by Pope et al (L. Chem. Phys. 38 (1963) 2042) using the single crystal of anthracene as the electroluminescent material, which emit blue light under high voltage. Although many scholars carried on the research and improvement (Phys. Rev. Lett. 14 (1965) 229; Sol. State Comm. 32 (1979) 683; Thin Solid Films 94 (1982) 476), the operating voltage was still too high, and the rate of energy conversion was low, hence there was little value for its application.
In the year 1987, Tang et al (Appl. Phys. Lett., 51 (1987) 914) made organic light emitting diodes having a structure of ITO/Diamine/Alq3/Mg:Ag by using evaporation, wherein ITO is a transparent electroconductive indium/tin oxide, Alq3 is tris(8-hydroxyquinoline) aluminum. This device can achieve external quantum efficiency of 1% and brightness of 1000 cd/m2 at 10V, which motivates a fast development in the research of organic light emitting diodes. Two years later, the research group of Carvendish laboratory in the Cambridge University used poly(phenylene vinylene) (hereinafter abbreviated as PPV) as the light emissive material, ITO as a positive electrode and Ca as a negative electrode to obtain a light emitting diode with a structure of ITO/PPV/Ca, which emits yellowish green light. The PPV has an external quantum efficiency 0.05% (Nature, 347 (1993) 539; U.S. Pat. No. 5,247,190 (1993); U.S. Pat. No. 5,425,125 (1995); U.S. Pat. No. 5,401,827 (1995)). The simplest kind of organic light emitting diode device has a single organic emissive layer sandwiched between a transparent electrode (as a positive electrode) and a metal electrode (as a negative electrode). In order to improve the electroluminescent (EL) efficiency of the organic light emitting diode devices, these devices can contain two organic layers, the first layer being a hole transport layer, and the second layer being the organic emissive layer; or the first layer being the organic emissive layer, and the second layer being an electron transport layer. These two layers are then placed between a transparent electrode (as the positive electrode) and a metal electrode (as the negative electrode). Moreover, there is an organic light emitting diode device containing three organic layers, which are arranged in an order of a hole transport layer, organic emissive layer, electron transport layer. These three layers are placed between the transparent positive electrode and the negative metal electrode. The light emitting process is activated by applying a forward bias across the electrodes, wherein, under the drive of the electric field, the hole and electron inject respectively from the positive and negative electrodes after overcoming their energy barrier, and then meet in the organic emissive layer to form an exciton. The exciton then decays from the excited state to the ground state by emitting al photon.
The PPV (poly(arylene vinylene)) due to its excellent electroluminescent property was widely used in the fabrication of light emitting diodes. However, this kind of conjugated polymer is not soluble in solvents and can not be molten by heating, therefore, the Wessling precursor route (U.S. Pat. No. 3,401,152 (1968); U.S. Pat. No. 3,706,677 (1972)) was used for its preparation. In the Wessling precursor route, an elimination reaction is carried out to form a fully conjugated polymer by coating the precursor and heating the resulting layer in vacuum. In order to simplify the fabrication of the devices, a long carbon chain like alkyl or alkoxy is introduced to the side chain of the poly(arylene vinylene). This can improve the polymer solubility (Polym. Preprint, 1 (1990) 505; U.S. Pat. No. 5,408,109 (1995); U.S. Pat. No. 5,679,757 (1997)), allowing it to be soluble in common organic solvents, and at the same time changing its energy gap. Beside that a block co-polymer containing a rigid segment and a flexible segment was first co-polymerized by Karasz (Macromolecules, 26 (1993) 1180; Macromolecules, 26 (1993) 6570) using the Wittig reaction, in which the rigid segment is arylene vinylene and the flexible segment may be alkyl, ether or ester. By controlling the length of the rigid segment one can alter the color of the emissive light. The flexible segment not only can block the conjugation, but also enhance the solubility and film-forming ability of the co-polymers
Currently, the emissive colors of polymeric light emitting diodes (PLED) include blue, green and even infra-red light. The color of light of PLED can be determined by the selection of one single electroluminescent polymeric material (Syn. Met., 71 (1995) 2175; Syn. Met., 71 (1995) 2117; U.S. Pat. No. 5,514,878 (1996)), or by the processing condition of the same electroluminescent polymeric material (Nature, 356 (1992) 47). Blending of two or more electroluminescent polymeric materials can also be used to yield various colors of PLED including the white light (Jpn. J. Appl. Phys., 32 (1993) L921; J. Appl. Phys., 76 (1994) 2419; Nature, 372 (1994) 444).
The common conjugated conducting polymers are p-type materials which can be oxidized easily, hence their transporting rate of hole is faster than that of electron transporting rate. Consequently, these two injected charges can not reach equilibrium, and thus the EL efficiency of the PLED is low.
In order to enhance the EL efficiency of the organic light emitting diode device, an additional electron transport layer (ETL) can be added to obtain a multilayer diode device with an improved quantum efficiency. This electron transport layer can be of (11) a thin film of electron transport material having a small heterocyclic molecule (like 2-(4-biphenylyl)-5-(4-tert-butylhenyl)-1,3,4-oxadiazole, PBD) evaporated onto the light emissive layer (Adv. Mater., 12 (1996) 979; Adv. Mater., 9 (1997) 127); (2) a thin layer formed on the light emissive layer by coating a solution of a blend of the small molecular electron transport material and an inert polymer such as poly(methyl methacrylate) (PMMA) (Appl. Phys. Lett., 61 (1992) 2793; J. Electron. Mater., 7 (1993) 745); (3) a thin layer formed on the light emissive layer by coating a solution of a traditional polymer such as poly(methacrylate) (PMA) having a side chain of a high electronegative heterocyclic moiety (Science, 267 (1 995) 1969); and (4) a thin layer formed on the light emissive layer by coating a solution of a conjugated or non-conjugated polymer having a high electronegative heterocyclic moiety incorporated to the backbone the of (Appl. Phys. Lett., 69 (1996) 881; Adv. Mater., 7 (19 95) 830; Chem. Mater., 7 (1995) 1568; Appl Phys. Lett., (1996) 2346).
Other than the multilayer structure described above, the blends of emissive materials and charge transport materials as a single active layer have also been developed to achieve the goal of improving the performance. There were 1) a direct blend of an electron transport material of a small organic molecule containing a high electronegative heterocyclic moiety and the emissive material (J. Electron. Mater., 5 (1994) 453; Macromolecules, 28 (1995) 1966; Jpn. J. Appl. Phys., 34 (1995) L1237); (2) a traditional polymer grafted with side chains of a high electronegative heterocyclic moiety and an emissive moiety (Macromolecules, 30 (1997) 3553), Syn. Met., 84 (1997) 437; Adv. Mater., 7 (1995) 898); and (3) a conjugated or non-conjugated polymer having a high electronegative heterocyclic moiety incorporated to the backbone thereof (Adv. Mater., 9 (1997) 1174; Polym. Preprint, 39 (1997) 103).
Although the above single and multilayer structures can improve the performance of the light emitting diode devies, they also have the following disadvantages. (1) When the electron transport material of the small organic molecules is evaporated on the emissive layer or is coated thereof after being blended into a polymer matrix, recrystallization and phase separation occur for these small molecules. Especially the joule heat occurred during the operation of the device will accelerate the recrystallization, and thus reduces the device stability. (2) For the multilayer structure having an electron transport layer made of a polymer containing a high electronegative heterocyclic moiety, not only the production process is cumbersome, but the operating voltage of the device will also increase greatly. (3) When the traditional polymer grafted with side chains of a high electronegative heterocyclic moiety and an emissive moiety is used,the resultant polyester possesses a larger energy gap. The device so fabricated will also have a higher operating voltage and is less stable. (4) For the conjugated polymer having a high electronegative heterocyclic moiety incorporated to the backbone thereof, the emissive light color may vary if that the conjugation is blocked by the electronegative heterocyclic moiety due to its strong electronegative property. As a result the emissive color is not easy to be predicted. Moreover, the property of the polymer may also change from a material possessing the original hole transport characteristic (prior to the electronegative heterocyclic moiety being incorporated) to a material having an undesired characteristic of electron conducting/hole blocking (ECHB).
In order to avoid these problems, the high electronegative heterocyclic moiety can be incorporated into a highly electroluminescent poly(arylene vinylene)polymer as a side incorporation of this moiety as a side chain, where the heterocyclic moiety is separated by a divalent group from the main chain, will not alter the emissive light color of the backbone. The resulting device will achieve the balance of the hole and electron injected, and significantly improve its efficiency. Also, the operating voltage of this device is not high. This technique was first reported by the inventors of the present application, xe2x80x9cPoly(p-phenylene vinylene)s Modified with 2,5-Diphenyl-1,3,4-oxadiazole Moieties as EML Materialsxe2x80x9d, International Conference on Organic Electroluminescent Materials, Sep. 14-17, 1996, Rochester, New York, USA.
In the year 1998, Z. Bao et. al. (Chem. Mater., 10 (1998) 1201) used the Heck reaction to synthesize a PPV modified with an oxadiazole side chain.