14 Polymers for Organic Light Emitting Devices/Diodes (OLEDs) O. Nuyken, E. Bacher, M. Rojahn, V. Wiederhirn and R. Weberskirch Technische Universita ¨ tMu ¨ nchen, Garching, Germany K. Meerholz Universita ¨ tZuKo ¨ ln, Ko ¨ ln, Germany I. INTRODUCTION Facing the 21st century, the development of new techniques that are able to display data faster, more detailed and in mobile applications, is one of the pro spering scientific fie lds. One approach for lightweight, flexible, power-efficient full-color displays are organic light emitting diodes (OLEDs). Such devices with their low driving voltage, bright color and high repetition rate (e.g. for video-application) are ideal for usage in miniature displays as well as in large area screen [1–3]. The basic principle of these devices are electroluminescent ‘semiconducting’ organic materials packed between two electrodes. After charge injection from the electrodes into the organic layer and charge migration within this layer, electrons and deficient electrons (so called ‘holes’) can recombine to form an excited singlet state. Light emission of the latter is then a result of relaxation processes [4–6]. To achieve high electroluminescence efficiencies, the materials have to fulfill several specific requirements including low injection barriers at the interface between electrodes and organic material, balanced electron- and hole-density and mobility and high lumines- cence efficiency. Furthermore, the recombination zone should be located away from the metal cathode to prevent annihilation of the exited state. Since no material known to date is able to meet all these criteria, modern OLEDs consist — besides the transparent substrate (e.g., glass, PET), anode (most commonly indium tin oxide, ITO) and metal cathode (e.g., Mg–Ag-alloy) — of several organic layers for charge injection, transport and/or emission [7,8] (the principal set-up is shown in Scheme 1). ð1Þ Copyright 2005 by Marcel Dekker. All Rights Reserved. In such multilayer diodes, each layer can be separately optimized concerning injection barriers, charge mobility and density and quantum efficiency. Much of the motivation for studying organic materials stems from the potential to tailor desirable optoelectronic properties and process characteristics by manipulation of the primary chemical structure. Objecting optimal charge transport, recombination probability and light emission and consequently a maximum external efficiency of the device, various substances have been developed, modified and tested in the last few years. For hole transport/electron blocking layers, triarylamine- and pyrazoline-structures (see Scheme 2) were found to be most promising [9–11]. ð2Þ For electron transport/hole blocking purposes, a wide variety of electron-deficient moieties are well known, e.g., 1,3,4-oxadiazoles [12], 1,2,4-triazoles [13], 1,3-oxazoles, pyridines and quinoxalines [14] (see Scheme 3). Materials with conjugated p-electron system (e.g., styrylarylenes, arylenes, stilbenes, oligo- and poly(thiophene)s — see Scheme 3) are widely used as co mbined charge transport and luminescence layers as well [12,15]. Basic structures of electron transport=hole blocking materials and oligomeric and polymeric mater ials for charge transport and luminescence ð3Þ Two basic principles are commonly used for the preparation of OLEDs: the sublimation method, in which the organic layers are prepared by vapor deposition results in well-defined layers of excellent purity but tolerates only low molecular mass molecules with high temperature stability [16]. The less expensive preparation out of solution, requires soluble substances or precursors [17] and is therefore widely used in combination Copyright 2005 by Marcel Dekker. All Rights Reserved. with polymers because of their homogeneity, good layer-building-properties and long- term form stability resulting in a long device lifetime. The goal of this article is to describe the scope and limitations of synthetic routes that have been used to produce suitable oligomers and polymers for LED application. The polymers in this article will be discussed on the basis of their backbone structure and the synthetic strategy of their formation and are divided into completely p-conjugated polymers, non-conjugated polymers and polymers with defined segmentation (see Structure 4). ð4Þ II. n-CONJUGATED POLYMERS Since the discovery of electrically conductive polymers by Heeger, MacDiarmid and Shirakawa et al. in 1977 [18] — resulting in the Nobel Prize in Chemistry 2000 [19] — p-conjugated systems have a major role in the field of so called ‘plastic electronics’. Key property of these polymers is the conjugated double bond along the polymeric backbone, allowing charge migration afte r injection via electrodes. A. Poly(p-phenylene-vinylene)s (PPV) The first polymers used for light emitting diodes — discovered by Friend and Holmes et al. in 1990 [20] — and still the most common ones used in recent devices, are completely p-conjugated poly( p-phenylene-vinylene)s. These polymers — which can be used in single layer devices as both charge-transport and green emitting materials — will be discussed on the synthetic strategy of their formation. 1. Precursor Routes Unsubstituted poly(phenylene-vinylene)s (PPVs) are insoluble in any known solvent. To improve solubility and with that processability unsubstituted PPVs were first synthe- sized using precursor routes like the so called Wessling- (or sulfonium-) route [21–24]. Accordingly, the condensation is performed with solubilized monomers, and a soluble polymeric intermediate is formed. The latter is converted to PPV in a final reaction step, that is preferentially carried out in the solid state, allowing the formation of homogeneous PPV films or layers. Following this route, a soluble precursor polymer with excellent film forming pro perties is obtained by base induced polyreaction of p-xylylene-a,a 0 - bisdialkylsulfonium salts. After spin coating, the precursor polymer is converted by polymer analogous heat induced elimination to the corresponding PPVs (Scheme 5). ð5Þ Copyright 2005 by Marcel Dekker. All Rights Reserved. In general, any functionalized poly( p-xylylene) with leaving group in the a-position to the aromatic moieties can be used as precursor, as long as they fulfill the basic requirements of OLED-techniques (i.e., solubility, transparency, excellent film forming properties, good thermal stability after processing, etc.). Commonly used as leaving groups beside the sulfonium group are halogens [25,26] (so called ‘Gilch-procedure’), hydrohalogenides [27], alkoxides [28] and alkylsulfinyles (known as ‘Vanderzane- procedure’) [29]. To avoid unwanted side reactions and damages of other device-layers during thermal conversion (e.g., by ox idation or reaction with volatile corrosive elimination products), organic-solvent soluble PPV derivatives such as poly(2-methoxy-5-(2 0 -ethylhexyloxy)-p- phenylenvinylene (MEH-PPV) or poly 2,5-dihexyloxy-p-phenylenevinylene (DH-PPV) (Scheme 6) have been developed. These materials can be spin-coated from solution after the conversion step. Another advantage of these PPV-derivatives is the possibility to modify the electronic properties of the film with different substitution patterns. Therefore all kind of organic substituents have been introduced into the aromatic system to alter the structure of the aromatic buildin g block, including alkoxy-, alkyl-, cholestanoxy and silicium containing groups [30–35] (Scheme 6). ð6Þ A precursor route not involving heteroatoms in the precursor polymers has also been developed. It is based on the oxidation of soluble poly( p-xylylene)s to corresponding PPVs by using stoichiometrical amounts of 2,3-dichloro-5,6-dicyano-1,4-benzochinone (DDQ) (Scheme 7) but is restricted so far to a-phenyl-substituted poly( p-xylylene)s [36]. ð7Þ Beside spin-coating-based preparation techniques, the so-called chemi cal-vapor- deposition-route (CVD) has gained considerable attention as a solvent free preparation process. Following this route, the starting materials are pyrolized after vaporization, followed by CVD and polymerization of the monomers on the substrate. Finally, the Copyright 2005 by Marcel Dekker. All Rights Reserved. halogeno-functionalized poly( p-xylylene) is converted to PPV by polymer-analogous thermoconversion (Scheme 8) [25,37,38]. ð8Þ 2. Polycondensation and C–C-Coupling Routes Some drawbacks of the precursor routes mentioned above have been overcome by the use of polycondensation- and C–C-bond-coupling reactions. To produce soluble PPV-, poly(thiophene)-, or poly(pyrrol) derivatives for spin coating preparation, various types of transition metal catalyzed react ions, such as the Heck-, Suzuki-, and Sonogashira- reaction, Wittig- and Wittig–Horner-type coupling reactions, or the McMurry- and Knoevenagel-condensation have been utilized. A typical example of the Pd catalyzed Heck reaction of 1,4-dibromo-2-phenylbenzol with ethylene to obtain the poly(phenylphenylene vinylene) [39] is depicted in Scheme 9. A common drawback of this reaction-type is the insufficient regioselectivity, resulting in 1,1 diarylation of the product (>1%, depending on the substituents) [40]. ð9Þ In order to avoid this problem, the Suzuki coupling is used as well to obtain various substituted PPVs. Therefore an aromatic diboronic acid or ester and dibromoalkylene are reacted in the presence of a Pd catalyst as depicted in Scheme 10 [41]. ð10Þ Cyano derivatives of PPV with high oxidation potential are commonly synthesized by Knoevenagel condensation of substituted terephthaldehyde with Copyright 2005 by Marcel Dekker. All Rights Reserved. benzene-1,4-diacetonitriles yielding an alternating copolymer type pro duct (see Scheme 11) [42] ð11Þ Schlu ¨ ter et al. described the synthesis of soluble PPV derivatives from substituted aromatic dialdehydes via McMurry-type polycondensation reaction. With this low valent titanium catalyzed reaction (see Scheme 12), the obtained pro ducts are characterized by a double bond cis/trans ratio of about 0.4 and an average degree of polymerization of about 30 [43]. ð12Þ Phenylic substituents at the vinyl ene positions — increasing both solubi lity of the polymer and stability of the double bond — can be achieved by reductive dehalogenation polycondensation of 1,4-bis(phenyldichlormethyl)benzene derivatives with chromium(II)- acetate as reducing agent [44] (see Scheme 13). ð13Þ A further route leading to unsubstituted PPV was published by Grubbs et al. [45], utilizing ring-opening olefin metathesis reaction as shown in Scheme 14. Starting from Copyright 2005 by Marcel Dekker. All Rights Reserved. bicyclic monomers with bicyclo(2.2.2)octadiene skeleton, the ring-opening metathesis polymerization (ROMP) is performed with Schrock-type molybdenum carbene catalysts. The obtained, well defined, nonconjugated soluble precursors, containing carboxylic ester functions, are then thermally converted to the conjugated PPV. ð14Þ The Wittig reaction (see Scheme 15) is also a commonly used method for yielding PPV derivatives from arylene bisphosphonium salts and bisbenzaldehydes. Since only products of moderate molecular weight are obtained, more interest in this reaction is given in the field of spacer segregated poly( p-phenylene vinylene)s with defined conjuga- tion length (see III.A) [46]. An improvement concerning the degree of polymerization is obtained by the Horner modification of the Wittig procedure (‘Wittig–Horner reaction’). Following this route, the bisphosphonium salt is replaced by bisphosphonates or aromatic bisphosphine oxide monomers [47]. ð15Þ Due to the side chain induced twist within the main chain the effective conjugation length is notably effected in soluble PPVs. A strategy to overcome this problem and to develop more rigid conjugated systems has been presented by Davey and co-workers in 1995 who prepared poly(phenylene-ethynylene)-type polymers according to the following scheme (Scheme 16) [48]. ð16Þ Copyright 2005 by Marcel Dekker. All Rights Reserved. 3. Other Poly(phenylene-vinylene)s Oligo- and poly(m-phenylene-vinylene) derivatives are not accessible via the polymeriza- tion approach analogue to the Wessling- or Gilch-route. Accordingly, other methods as the reductive dehalogenation polycondensation or the Wittig-type reaction as shown in II.A.1 and II.A.2. are used for their formation. Despite their increased solubility, the 1,3-phenylene-units within the poly(m-phenylene-vinylene)s act as conjugation barriers so that their usage in OLED techniques is very limited. Oligomers of (o-phenylene-vinylene)s can be obtained using various C–C-coupling and polycondensatio n methods. For higher oligomers and polymers, the Stille-type coupling of 1,2-diiodobenzene or 1,2-bis(2-iodostyryl)benzene with bis(tri-n-butylstannyl)- ethylene was introduced by Mu ¨ llen et al. [49] (see Scheme 17). ð17Þ The o-phenylene-vinylene-struct ure represent an intermediate case between the p- and m-derivatives, allowing an extended p-conjugation and simultaneously disturbing it by the non-planar geometry between the vinylene units. Utilizing three different alkyle chains leads to the PPV-copolymer ‘‘Super Yel low’’ — commercially available from Corion Organic Semiconductors GmbH — which shows the best efficiency and lifetime of PPV-derivatives upto now (see Scheme 18): ð18Þ B. Heteroaromatic Systems Heteroaromatic systems, such as the widely used poly(thiophene)s can be obtained by simple oxidat ive polymerization of the soluble monomers or oligomers either by electrochemical means or oxidizing agent such as FeCl 3 [50,51]. This common route is also used to synthesize a variety of mono- and dialkyl-, -alkoxy-, and -alkylsulfonic acid substituted and therefore soluble poly(thiophene)s [52–57 ] (Scheme 19) and can also be utilized to obtain poly(pyrrole)s. The disadvantage of this polymerization methods however is the regiorandom structure of the polyme ric product with non-reproducible properties. Copyright 2005 by Marcel Dekker. All Rights Reserved. ð19Þ For better defined poly(thiophene) structures a variety of organometallic mediated synthesis have been introduced. Most widely employed are Grignard-type organo- magnesium compounds in addition to a nickel catalyst. Highly regioregular head-to-tail 3-alkylpoly(thiophene)s are obtained following the synthetic route of McCul lough et al. (see Scheme 20). ð20Þ Polymers — prepared via the polymerization of 2-bromomagnesio-5-bromo-3- alkylthiophenes — exhibit enhanced conductivity and optical properties when compared with regiorandom materials [58,59]. Another approach to regioregular alkylpoly(thio- phene)s is the usage of zinc instead of magnesium in nickel- or palladium catalyzed polymerizations [60,61]. Due to the improvements, these synthetic methods are by far the most valuable synthetic routes to these materials. In contrast, the regioselective synthesis of substituted poly(pyrrole)s was not reported to date. Heterocyclic, electron deficient conjugated syst ems like poly(1,3,4-oxadiazole)s, poly(1,3-oxazole)s and poly(1,2,4-triazole)s are applied in organic light emitting diodes as electron transport and hole blocking layers. The synthetic strategies for their formation are as manifold as the structures themselves, reaching from polymerization of functional monomers to polymer analogue formation of the conjugated system (e.g., by ring closure dehydration, dehalogenation, etc.). For further details is referred to the reviews of Schmidt et al. [14] and Feast et al. [62]. C. Light Emitting Polymers (LEPs) Based on Polyfluorenes A second important class of p-conjugated polymers are polyfluorenes, which were obtained the first time by oxidative polymerization of 9-alkyl- and 9,9-dialkylfluorenes with ferric chloride [63]. These polymers showed low molecular weight and some degree of branching and non-conjugated linkages through positions other than 2 and 7. A very success ful way to improve regiospecificity and to minimize branching was the synthesis through transition-metal-catalyzed reactions of monomeric 2,7-dihalogenated fluorenes. The palladium-catalyzed synthesis of mixed biphenyles from phenylboronic acid and aryl bromide discovered by Suzuki et al. [64] tolerates a large variety of functional groups and the presence of water. This method can also be used to prepare perfectly alternating copolymers. Copyright 2005 by Marcel Dekker. All Rights Reserved. 1. Polyfluorene-Homopolymers Polyfluorenes with alkyl substituents at C9 are soluble in conventional organic solvents such as aromatic hydrocarbons, chlorinated hydrocarbons and tetrahydrofuran, which made them useful to prepare thin films for OLEDs. As a consequence many efforts have been undertaken to synthesize a large number of high-m olecular-weight, 9-mono-, or disubstituted very pure fluorene-based polymers. ð21Þ 9,9-Disubstituted 2,7-bis-1,3,2-dioxaborolanylfluorene is allowed to react with a variety of dibromoarenes in the presence of a catalytic amount of (triphenylphosphine) palladium (Scheme 21). The improved process yields high-molecular-weight polymers with a low polydispersity (<2) in less than 24 h reaction time, whereas the conventional Suzuki coupling process can take up to 72 h and more to deliver polymers of modest molecular weights. Optimized LEDs based on these polymers, made by improved Suzuki poly- fluorene chemistry, exhibited light emission exceeding 10,000 cd/m 2 with a peak efficiency of 22 lm/W [3]. 2. Polyfluorene Copolymers The described synthesis of polyfluoren homopolymers allows also the design of alternating copolymers. Instead of the 2,7-dibromofluorene a variety of dibromoarenes can be used in the Pd-catalyzed C–C coupling polymerization reaction. An important group of comonomers are tertiary aromatic amines, which have been known as excellent hole- transport materials and have found many applications as photoconductors and in LEDs. The resulting alternating copolymers are all blue emitters, exc ellent film formers and show high hole mobilities [65]. These materials can be used as emitters as well as hole transporters in LED devices. This alternating copolymer concept has been extended to other conjugated mono- mers as shown in Scheme (22). All synthesized copolymers [66] are of high molecular weight, are highly photoluminescent and their emissive colours can be qualitatively correlated to the extent of delocalization in the comonomers. For example the thiophene copolymer emits bluish green light, but the bithiophene copolymer emits yellow light. Copyright 2005 by Marcel Dekker. All Rights Reserved. [...]... The soluble precursor polymer is then aromatized thermally to PPP via elimination of two molecules of acetic acid per structural unit The polymerization of the monomer, however, does not proceed as a uniform 1,4-polymerization: beside the Copyright 2005 by Marcel Dekker All Rights Reserved regular 1,4-linkages about 10% of 1,2-linkages are formed as a result of a 1,2polymerization of the monomer ð30Þ... conformation in spite of the introduction of substituents One of the first examples was the synthesis of polyfluorenes via oxidative coupling of fluorene derivatives as described above [63] Another possibility to reach this aim is the preparation of ‘stepladder’ PPPs Monomers like the 2,7-dibromo-4,9-dialkyl-4,5,9,10-tetrahydropyrenes (Scheme 27) represent suitable starting monomers for the realization of such ‘stepladder’... voltage and improves the stability of the device Further improvement is obtained by reducing the length of the alkyl linker A further interesting concept is the synthesis of a polymer having a fully conjugated backbone and pendant side-chain chromophores, combining electron-transport, holetransport and light emitting properties in a single polymeric material Hybrid polymers of this type with oxadiazole side... (Scheme 41) The polymers prepared via the Heck reaction showed a molecular weight Mn ¼ 28,500 Da with a polydispersity of 3.65 whereas the Stille coupling resulted in lower molecular weight polymers of 8100 Da with polydispersities of 1.67 The polymer with a PPV-like backbone indicated yellow-orange light emission whereas polymers containing a thiophene group emit red-orange light Both polymers showed... photovoltaic effects Segmented conjugated polymers have the advantages over fully conjugated ones that their electronic properties are independent of the degree of polymerization and can be easily tuned by varying the substituents or the conjugation length 1 Conjugated Main-chain Polymers with Twisted Conformation Since the p-overlap is a function of the cosine of the twist angle of adjacent aromatic units two... by changes in the polymer geometry or topology A twisted conformation of the polymer backbone was achieved by introduction of alkyl or alkoxy substituents in 2,5 positions along the PPP backbone causing twist angles of 60–80  [104] A second powerful approach are meta linkages in PPV (Scheme 42) that led also to the interruption of conjugation due to the non-coplanar arrangement of adjacent conjugated... properties can be tailored by the proper selection of the chromophore unit whereas the physical properties can be adjusted by the non-chromophoric part Burn et al [112] introduced the notion of isolated chromophores in 1992 by selectively eliminating one of two leaving groups of the precursor polymer to give a conjugated–non-conjugated polymer The design of such polymers with controlled chromophore length... functionalization varied from 4–44% [98] Anionic polymerization of styrene bearing TPD-like side chains has also been reported [99] (Scheme 37) A wide range of hole transporting polymers were prepared with high Tgs ranging from 132–151  C to increase thermal and long-term stability of the device ð37Þ R1 ¼ F R2 ¼ CH3 The ring-opening metathesis polymerization of side chain functionalized norbornene monomers... ladder polymer to minimize the mutual distorsion of adjacent main chain phenylene units The complete flattening of the conjugated p-system by bridging all the phenylene subunits should then lead to maximum conjugative interaction As with the PTHP systems, alkyl or alkoxy side chain should lead to soluble polymers This idea was realized first in 1991 with the first synthesis of a soluble, conjugated ladder polymer. .. precursor polymer was closed to give a double stranded ladder polymer (Scheme 28) In the synthesis Copyright 2005 by Marcel Dekker All Rights Reserved of this LPPP, the precursor polymer is initially prepared by Suzuki aryl–aryl coupling of an aromatic diboronic acid and an aromatic dibromoketone ð28Þ The cyclization to structurally defined, soluble LPPP takes place in a two-step sequence, consisting of a . efficiency of 22 lm/W [3]. 2. Polyfluorene Copolymers The described synthesis of polyfluoren homopolymers allows also the design of alternating copolymers. Instead of the 2,7-dibromofluorene a variety of. ratio of about 0.4 and an average degree of polymerization of about 30 [43]. ð12Þ Phenylic substituents at the vinyl ene positions — increasing both solubi lity of the polymer and stability of the. referred to the reviews of Schmidt et al. [14] and Feast et al. [62]. C. Light Emitting Polymers (LEPs) Based on Polyfluorenes A second important class of p-conjugated polymers are polyfluorenes,