Polymer International Polym Int 55:473–490 (2006)

ReviewOptimization of opto-electronic propertyand device efficiency of polyfluorenes bytuning structure and morphologyPeng Chen,1,2 Guizhong Yang,1,2 Tianxi Liu,1,2∗ Tingcheng Li,2 Min Wang2 andWei Huang2

1Key Laboratory of Molecular Engineering of Polymers, Ministry of Education, Department of Macromolecular Science, Fudan University,220 Handan Road, Shanghai 200433, China2Laboratory of Advanced Materials, Fudan University, 220 Handan Road, Shanghai 200433, PR China

Abstract: Polyfluorene-based oligomers and polymers (PFs) have been studied intensively as active materialsfor organic optoelectronic devices. In this review, the optimization of the opto-electronic property and deviceefficiency of polyfluorenes in the field of light-emitting diodes (LEDs) and photovoltaic cells (PVs) by tuningstructure and morphology are summarized in terms of two typical modification techniques: copolymerization andblending. The relationships between molecular structures, thin film morphologies, opto-electronic properties anddevice efficiencies are discussed, and some recent progress in LEDs and PVs is simultaneously reviewed. Afterthe introduction, the basic knowledge of molecular structures and properties of polyfluorene homopolymers ispresented as a background for a better understanding of their great potential for opto-electronic applications.Immediately after this, three different opinions on the origin of low-energy emission band at 520–540 nm inpolyfluorene-based LEDs are addressed. Rod–coil block copolymers and alternative copolymers are focusedon in the next section, which are a vivid embodiment of controlling supramolecular structures and tailoringmolecular structures, respectively. In particular, various supramolecular architectures induced by altering coilblocks are carefully discussed. Recent work that shows great improvement in opto-electronic properties or deviceperformance by blending or doping is also addressed. Additionally, the progress of understanding concerning themechanisms of exciton dynamics is briefly referred to. 2006 Society of Chemical Industry

Keywords: polyfluorenes; copolymers; blends; structures and morphologies; polymer light-emitting diodes;photovoltaic cells

INTRODUCTIONThe rapid development of organic electronics hasdemonstrated that π-conjugated polymers are verypromising materials for the fabrication of low-cost,yet high-quality novel electronic devices, such aspolymer LEDs (PLEDs) (ultrathin, flexible, full-colordisplays) and PVs, due to their easy processabilityand semiconducting properties.1–3 Typically, thePLEDs comprise a thin film of light-emitting materialsandwiched between a conducting oxide anode anda metal cathode. Under an applied bias, electronsand holes are injected into the emissive layer, wherethey recombine to form excitons and emit light uponradioactive decay.3 In PLED devices, charge injection,charge transport and light emission may be fulfilled inonly one active layer.4 The rapid development of suchdevices has made it possible for the commercialization

of electronic products such as mobile phones, digitalcameras, as well as full-color TV displays.1–4 PVshave an overall architecture similar to that of PLEDs,but the mechanism of operation is opposite to thatof PLEDs. In PVs, photoabsorption leads to theformation of excitons – bound electron–hole pairs.Under an electrical field, the charges (electrons andholes) are eventually collected at the anode andcathode, respectively, through energy transfer, excitondissociation and charge transport.5,6

In past years, a great variety of conjugatedpolymers, for instance, polyacteylene, polyani-line (PANI), polypyrrole, poly(p-phenylenevinylene)(PPV), polyfluorene, polythiophene and poly(p-phenylene) (PPP), have been developed, andtheir chemical and physical properties have beenwell characterized.2,3,7–13 Among these polymers,

∗ Correspondence to: Tianxi Liu, Key Laboratory of Molecular Engineering of Polymers, Ministry of Education, Department of Macromolecular Science, FudanUniversity, 220 Handan Road, Shanghai 200433, ChinaE-mail: txliu@fudan.edu.cnContract/grant sponsor: National Natural Science Foundation of China; contract/grant number: 50403012Contract/grant sponsor: Program for New Century Excellent Talents (NCET) in UniversityContract/grant sponsor: Shanghai Rising-Star Program; contract/grant number: 04QMX1403(Received 22 August 2005; revised version received 4 October 2005; accepted 24 October 2005)DOI: 10.1002/pi.1970

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polyfluorene-based oligomers and polymers have beenspecially focused upon since the first synthesis ofsoluble poly(2,7-fluorene)s via ferric chloride oxida-tive polymerization in 1989.14,15 With the discoveryof Suzuki- and Yamamoto-type synthesis routes (asshown in Fig. 1),16,17 the development of these typesof materials has been greatly accelerated.3,10,13,18–24

Figure 2 shows some typical chemical structures ofreported PFs. In addition to the fluorene-basedpolymers, fluorene-incorporated amorphous molec-ular materials have already been developed for organicLEDs (OLEDs). With the traits of structural rigid-ity and sterical bulkiness of the fluorene moiety, theglass transition temperature (Tg) of the resulting amor-phous molecular materials can be distinctly improvedso that thermally stable amorphous glass above roomtemperature is realised. So far, fluorene-incorporatedamorphous molecular materials with uniform amor-phous films have been broadly reported to functionin specialized roles such as hole transporting, elec-tron transport, hole blocking and emission in OLEDs.Moreover, it is expected that increasing the electricalconductivity of such materials can lead to a decreasein bulk resistivity in organic PVs except for enhancinghole injection from the indium–tin oxide (ITO) elec-trode in OLEDs. Detailed examples can be found in arecent publication by Shirota.25

Extensive attention to PFs in both academic andindustrial fields is driven by the inherent beauty of theirunique molecular structures as well as their appeal-ing properties. The next section will give a relatively

Figure 1. Schematics of synthetic methods of PFs: (A) Yamamotoroute; (B) Suzuki route (R, -alkyl).

complete yet concise picture of the structures andproperties of polyfluorene homopolymers. For appli-cations in PLEDs, PF is an especially good blue lightemitter; however, they generally show a greenish emis-sive band; thus many strategies have been proposedto tune the opto-electronic properties, for instancecopolymerization, blending or doping, bulky side-group and end-group attachment, oligomer approach,dendronization, spiro-substitution, and even hydrogenbonding.18–24,26–32 On the other hand, the efficienciesof PVs are not satisfied owing to the weak abilityof charge collection and transport. It is anticipatedthat the outstanding stability and transport propertiesof PFs will lead to optimized blends of photovoltaicsystems; that is to say, PF-based blends offer an inter-esting way to improve the efficiencies of PVs.33

In principle, macroscopically photophysical prop-erties and device efficiencies of PFs are determinednot only by the individual molecule, but also by theensemble of molecules. In conjugated materials, themolecular structures play a crucial role in the highoccupied molecular orbital (HOMO) and low unoccu-pied molecular orbital (LUMO) band gap, oxidationand reduction potentials. Besides, they strongly affectthe supramolecular architectures, which lead to spe-cific morphologies in the solid state, and the resultingmicroscopic structures impact the properties in reverseby virtue of the complicated local molecular ordersand conformations. Studies by Kim, Muccini, Miller,Tolbert and Schwartz and their coworkers have pro-vided a profile of molecular structures.34,35–38 On theother hand, the study on supramolecular architecturesformed by self-assembly or other methods is an attrac-tive approach to comprehend the impacts of inter-chain interactions and exciton dynamics in conjugatedmaterials. Supramolecular nanostructures may haveelectronic properties that are intermediate betweenmolecular aggregates and bulk polymers. For exam-ple, studies on supramolecular assemblies reveal thatexciton–exciton bimolecular annihilation dominatesthe exciton dynamics at high fluence.39–41 Besides, theordering of organized supramolecular structures at dif-ferent length scales can determine device efficiencies.The early work by Stupp, Hadziioannou, Hempenius,Muller, Jenekhe, Yu and their coworkers revealedthe roles of supramolecular architectures of rod–coilconjugated block copolymers in their opto-electronicproperties and device efficiencies.42–48 In conclusion,the optimization of device performance of PLEDsand PVs can be achieved with recourse to the opti-mization of structures (both molecular structures andsupramolecular structures) and morphologies.

UNIQUE STRUCTURES AND APPEALINGPROPERTIES: WHY ARE POLYFLUORENESFASCINATING?The chemical and thermal stabilities of PFs havebeen proved to be superior to those of PPV. Thedecomposition temperatures of PFs may exceed

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Figure 2. The chemical structures of typical PFs.

400 ◦C.34,49 Due to the tetrahedral σ -bonding atthe C-9 atom, the substituents tend to extend outof the backbone plane and be locally perpendicularto the chain-axis. Therefore, substitution at the C-9position does not greatly increase the steric interactionsbetween or cause distortion in the conjugatedbackbone itself, even though the interaction canlead to significant twisting of the main chain in PFderivatives. Furthermore, the C-9 atom provides afacile site for tuning the solubility, opto-electronicproperties and interchain interactions. Generally, theside-group at the 9-position can orient in two manners:perpendicular to the plane with a ‘T’-shaped fluoreneunit, or parallel to the plane with a ‘Y’-shaped fluoreneunit.50 The rigid planarized biphenyl structure bridgedby a methylene group in the fluorene monomer unitleads to a wide band-gap with blue emission, so that

PFs possess the unique ability to emit color spanningthe entire visible spectrum with high efficiency andlow operating voltage. The photoluminescence (PL)efficiency may excess 80% in dilute solution and 50%in thin film.51,52

Poly(9,9-dioctylfluorene) (PFO), a typical memberof the PF family, has been well characterized.It displays rich phase morphologies, and a slightconformational change in PFO may dramaticallyinfluence its photophysical properties. In addition tothe normal glassy α-phase, a second phase (i.e., theso-called β-phase), which is formed as a result ofthe n-alkyl crystallization of the side chains, has beenidentified.53–56 The polymer chains are consideredto be more planar, and with longer conjugationlength in the β-phase than in the α-phase.57,58 Theconjugation length in the β-phase is much longer

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than that in the ladder-type poly(p-phenylene) (LPPP)and may even extend along the entire chain, sothat individual chains may even ultimately behavelike polydiacetylene quantum wires.59 Moreover, theπ-electron delocalization in the β-phase may giverise to enhancement by orders of magnitude inphotophysical stability in terms of both increasedlifetime and reduced spectral meander.59 However,the PL quantum yield would be reduced becausethe population of polaron and triplet excitonsin the β-phase determined by the fraction ofplanarized chains is significantly larger than inother phase morphologies.58,60–63 The β-phase hasdemonstrated a narrow, well-resolved absorption peakat 2.84 eV (437 nm) and an associated vibronicstructure superimposed upon the bulk absorptionwith a reduced Stokes shift.54,58,60,64 Recently,the structural parameters of the α-phase weremeasured to be orthorhombic (a = 2.56 nm, b =2.34 nm, c = 3.32 nm, space group P212121), witheight chains in the unit cell (each of four fluorenerepeats).65,66 However, at temperatures lower than130 ◦C, some shifts were identified from 2.34 to2.38 nm together with a slight increase of theb-axis, and a lower symmetry along this axis aswell as a preferred orientation of the a-axis alongfilm normal were identified. Hence, a previouslyunspecified modification of the crystalline phase (α-phase), α′-phase, was suggested.66 The photoexcitedemission spectra in four different phases remainhighly similar except for minor red shifts in theemission maximum. In addition, PFO belongs totypical hairy-rod material consisting of a rigid or semi-rigid backbone in which a dense set of side-chains isbonded in a comb-shaped manner.67 Semi-crystallinePFO can display the nematic phase between 160 ◦Cand 300 ◦C.68–71 Due to the distinct thermotropicliquid-crystalline characteristics with chain-axis orderparameters, 0.8 < P2(cos θ) < 0.9, PFO melts into abirefringent fluid state at 170 ◦C, becomes isotropicat 270–280 ◦C, and shows a reversible transitionbetween liquid-crystalline and isotropic phases. Thisinteresting property allows for fabrication of polarizedlight-emitting devices and solid-state lasers.57,72–75

As shown in Fig. 3, PFO displays an absorptionpeak at ca 380 nm (3.25 eV) with a relatively narrowlinewidth (full width at half maximum) of 0.6 eV,which is characteristic of most conjugated polymers,and relies on the distribution of π-stacking lengthto some extent.49,53 The absorption onset definesan optical gap of 2.95 eV (425 nm). The Stokesshift is less than 50 meV, which is similar to thatof highly ordered LPPP.49,76 In PL spectrum, PFOshows a well-resolved structure with peaks at 420,448 and 472 nm, assigned to the 0–0, 0–1, 0–2singlet intrachain excitons, respectively, with 0–0(π∗ –π) transition being the most intense.49 In contrastto the absorption, the emission spectra display awell-resolved vibronic progression with an energeticspacing of 180 meV, which indicates the stretching

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mode of C=C–C=C substructures of the chains.43,64

When casting into the film from the dilute solution,there is a red shift of the peaks by 15–20 nmin the condensed state, with a broader absorptionof 100–250 meV on account of conformationalchanges and even formation of aggregation orexcimers.23,24,53,64 One drawback of PFO is that thehuman eye is not sensitive to its emissive bands. Thus,polyindenofluorene containing three coplanar rings isproposed, which shows a PL emission maximum ataround 430 nm.77–80 HOMO (equivalent to the valueof the ionization potential) and LUMO (equivalentto the value of the electron affinity) energy levelsof PFO in the range of 5.6–5.8 and 2.12–2.6 eV,respectively, are reported.80–82 In addition, PFOis unipolar with hole mobility much greater thanelectron mobility.83 Time-of-flight measurementsreveal that PFO has non-dispersive hole transportwith a hole mobility of 10−4 –10−3 cm2 V−1 s−1,and dispersive electron transport with electronmobility of 10−3 cm2 V−1 s−1.82,84,85 However, thevalue can be further improved, for example, viamonodomain alignment on a rubbed alignment layeror novel friction-transfer technique,57,86 although thealignment extent is subject to the molecular weight.87

Very high mobility, greater than 0.1 cm2 V−1 s−1,has been achieved.88 PFO still exhibits high carriermobilities on non-ohmic electrode contact.82 Inaddition, PFO is considered to be acidic as itsderivatives clearly show a pKa value lower than that ofwater; and after base doping an electrical conductivityof 10−2 –10−3 S cm−1 was reported in air at roomtemperature.89–91

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EXCITON DYNAMICS: WHAT IS THE ORIGIN OFTHE GREEN EMISSION BAND?As mentioned above, PFs can emit pure blue light.However, a green emission band at ca 535 nm(2.2–2.4 eV) and even fluorescence self-quenching areoften observed, which may result in color impurityand short device life-time.92–99 The low-energyemission band is more intense in electroluminescence(EL) spectra than in PL spectra,95,100 and can beenhanced upon annealing or passage of current.101,102

Initially, the excimers are considered to be responsiblefor such phenomena.34,94–97,103–110 The excimersare a kind of excited-state complex formed bythe interaction of an excited chromophore with adissociative ground-state chromophore. They exhibittemperature and concentration dependences dueto the interchain interactions. Huang et al. furtherproved a Tg dependence for the formation ofexcimers.103 Figure 4 depicts the fluorescence spectraof poly(9,9-dihexylfluorene) (PDHF) films thermallytreated in various manners. The additional emissionpeak at about 535 nm becomes much stronger thanthat of pristine film after annealing, whether inan atmosphere of nitrogen or air. This low-energyemission band can be quenched in a dry ice/methanolbath, but remains identical with original film inchlorobenzene solution.

Nevertheless, there remain some doubts whencomparing the molecular structure of PFs to thatof polycarbazoles. X-ray analysis has revealed thatsubstituents at the N position of poly[2,7-(N-alkylcarbozole)]s are parallel to the plane of therepeat unit and should not inhibit strong interchaininteractions, while remote substituents at the C-9position of PFs should limit interchain interactions.However, poly[2,7-(N-alkylcarbozole)]s actually donot show any evidence of aggregate or excimer

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formation.89 Also, even for the highly conjugated pla-nar β-phase conformation, no low-energy emissionwas observed before oxidative degradation.63 Surpris-ingly, the 535 nm emission band does not appear inthe freshly prepared films of PFO via the Suzuki route,but is only observed after significant oxidation.111 Ithas therefore been proposed that the 535 nm emis-sion band originates from fluorenone keto defectsdue to oxidation at the methane bridge.53,64,112–114

The fluorenone moieties can trap singlet excitonsand reduce PL efficiency. Some controlled modelcompounds containing fluorenone were synthesizedto investigate the role of fluorenone on the greenemission peak.115–117 It must be noted that the syn-thetic strategy can directly impact the characteristics ofthe final molecular structures of model systems.115,118

The Yamoto aryl-aryl route facilitates the occurrenceof the side-reaction and thus cannot obtain well-defined molecular structures, whereas the Suzuki routedoes not have such problems. Recent work based onSuzuki coupling polymerization has been reported byJenekhe and coworkers.115 By investigating four statis-tical copolymers containing 1%, 3%, 5% and 10%fluorenone and fluorene-fluorenone-fluorene trimermodel compound, the experiments indicate that thecontroversial green emission band at 535 nm originatesfrom the fluorenone defects in single-chain polyfluo-renes instead of intermolecular aggregates or excimers.Jenekhe et al. further proposed that the increasedintensity of the green emission with the increase ofintermolecular interactions in solution or solid statewas due to the increased energy transfer from fluorenesegments to the fluorenone moieties. Moreover, somemethods of stabilizing the blue emission of polyfluo-renes, such as dendronization and blending, were alsosuggested to impede the oxidation process that formsthe fluorenone defects. Moses et al. showed that theoxidation of PFs is strongly catalyzed in the presenceof calcium.119 Additionally, crosslinking sites play animportant role in augmenting the green emission asthey enhance excitation energy migration or energytransfer to the defects by hindering the twisting ofchain segments.120

Despite this evidence, the presence of fluorenonedefects alone may not be sufficient to producethe green band emission. Just as suggested byBradley et al., even though the fluorenone defectsare concluded to be necessary for the green bandemission, they are considered not to be sufficientalone. The interchain/intersegment interactions areproposed to be required for the appearance of thegreen band, and the low-energy peak is attributable toemission from fluorenone-based excimers rather thanfrom localized fluorenone π –π∗ transitions.118 Mostrecently, the third interpretation on the undesirablepeak has been suggested. Lazzaroni et al. proposedthat the organized structures lead to significantlong-wavelength emission, while non-organized bulkysubstituents retain pure blue emission, by comparingdifferently substituted PFs.121 It was thought that

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the microscopic morphology of the films plays animportant role in the luminescence properties. Almostat the same time, Kallitsis et al. suggested that the535 nm emission peak cannot be attributed to anyother chemical changes such as the formation ofketo defects but could safely be attributed to thepresence of the flexible segments and consequently totheir influence on the organization of the copolymers;namely, the microscopic morphology should beresponsible for the 535 nm greenish peak in theinvestigations of PF-based block copolymers.122

Hitherto, the problem is still debated. For moredetailed studies on the keto defects, it is suggested thatthe reader refers to the work by Scherf and List.53,64

For a better picture of the evolution of studies onthe 535 nm emission band, the introductory sectionof the publication by Sims et al. is recommended.Understanding the nature of the electrical excitationand excimer dynamics will favor the confirmationof the true origin of the low-energy emissivepeak. Principal dynamic processes including energytransfer, charge transport and separation, chargeinjection and recombination, exciton formation anddissociation play significant roles in the characteristicsof conjugated polymers. Recently, Bredas et al.built up a molecular picture of energy-transfer andcharge-transfer processes in π-conjugated oligomersand polymers.6 Gadermaier and Lanzani also painteda comprehensive picture of the elementary processes ofexciton dynamics in conjugated polymers by ultrafasttechniques.123 However, there are still a number ofproblems in photophysics to be well understood.

COPOLYMERIZATION: TUNING THEOPTO-ELECTRONIC PROPERTIES BYMANIPULATING THE STRUCTURESAs is very usual in polymer chemistry, the copolymer-ization technique is an effective synthesis strategy tomodify short- or long-range organization in materialsand consequently brings about some novel propertiesto materials.89 Copolymers are divided into several cat-egories, and here our interests are only focused on twoof them: rod–coil block copolymers and alternatingcopolymers.

Rod–coil block copolymers: controlling thesupramolecular architecturesAs pointed out in the European Materials ResearchSociety Symposium on Supramolecular Approachesto Organic Electronics and Nanotechnology (held inStrasbourg, France, 24–28 May 2004), one ofthe approaches to develop electronic devices isto prepare large domains (several hundreds ofnanometers) by supramolecular organization mainlythrough intensive non-covalent interplay.124 Becauseof the stiff asymmetry between two blocks, rod–coilblock copolymers provide a very powerful andfascinating strategy to construct novel supramoleculararchitectures with well-defined morphology and

functions at the nanometer level. As the conjugatedblocks are incorporated into the copolymers asthe rod segment, the supramolecular structures arereported to impact the photophysical propertiesof conjugated molecules due to the flexible coilsegment. For instance, fast exciton diffusion on chiralsupramolecular assemblies was shown to lead toexciton trapping and luminescence depolarization.125

Moreover, it is relatively convenient to investigate therelationship between structures and properties as thesupramolecular structures can be manipulated by thevariation of molecular weight, composition, processingconditions, and so on.

The concept of rod–coil block polymer witha donor (D) and an acceptor (A) segment wasproposed for PVs. By covalently linking the donorand acceptor in the polymer chain, the covalentbond enables a predefined control over characteristiclength between donor and acceptor and therebythe extent of phase separation.126 Moreover, therod–coil copolymer can self-assemble to form well-ordered micro/nanostructures through microphaseseparation. Thus, the D-A rod–coil copolymerfacilitates the construction of the interpenetratingand bicontinuous network of D-A hetero-interfacein several nanometers, as it combines the merits ofD-A heterojunctions (HJ) and rod–coil copolymer.Apparently, the efficiency of PVs is much improvedusing D-A rod–coil block copolymers. This has beenwell illustrated by the work of Hadziioannou andcoworkers.127–131 However, this strategy has not yetbeen proven to be applicable to a wide range ofsemiconducting polymers.

In PLEDs, the most appealing virtue of conju-gated polymers over organic small molecules is theease of solution processing, which enables the fabri-cation of simple device structures at low cost. Wateris the cheapest and most common solvent; therefore,water-soluble rod–coil conjugated polymers are ofspecial interest. Lu et al. reported conjugated-acidicand conjugated-ionic block copolymers, the two coilsegments of which are based on poly(methacrylic acid)and poly[(2-(dimethylamino)ethyl methacrylate)],respectively.132,133 Aggregates in aqueous solutionswere demonstrated in both cases. Moreover, excimerformation within PF aggregates was confirmed inconjugated-ionic block copolymers, which was sup-ported by an additional emission band at longer wave-length (500–600 nm) reinforced by strong hydropho-bic interactions. It was also shown that the increasein ionic strength of the solution leads to furtherdecrease in quantum efficiency. Additionally, water-soluble rod–coil conjugated polymers are also suitablefor biosensors as their supramolecular aggregationbehaviors can be changed under different conditions.

The first observed supramolecular architecturefor PF-based rod–coil copolymers was fibrillarmorphology.24,50,134–136 The originally studied sam-ple was prepared with the attachment of coilpoly(ethylene oxide) (PEO) block by living anion

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polymerization.134 The absorption maximum of thereported block copolymer located at 377 nm wasslightly bathochromically shifted and an additionalpeak appeared at 428 nm. The fluorescence spectrumdisplayed better resolution than that of PF homopoly-mers; the intensities of the maxima at 437, 460 and495 nm were enhanced and the peak at 420 nm wasdiminished. These spectral data demonstrated theincreased ordering of the luminescent rods in theblock copolymer and the influences of the coil blockson the opto-electronic properties of the rods. Similarfibrillar structures were also observed in other PF-based rod–coil copolymers coupled with different coilsegments such as polystyrene (PS), which were synthe-sized by using end-capping reagents in the Yamamotopolymerization.24,134 The absorption and emissionproperties of these copolymers are very similar tothose of the homopolymers. Figures 5 and 6 show thestudied chemical structures and corresponding fib-rillar morphologies from the solutions, respectively.However, the origin and determination of fibrillarmorphology were not understood at that time.

Most recently, more detailed information concern-ing fibrillar morphology has been obtained usingtapping-mode atomic force microscopy (AFM).136,137

The AFM image of the copolymer with fPEO = 0.09(i.e., the average volume ratio of PEO in thecopolymer) revealed that the width and height of thefibrils appeared to be constant at about 11 ± 1 nm and1.0 ± 0.1 nm, respectively, indicating that the fibrilsresemble ribbon more than cylinder. Moreover, theAFM experiments implied that the formation of fibrilsis driven by the π-stacking supramolecular interac-tions between the conjugated segments in the coreof the ribbons. The formation of fibrillar morphol-ogy is expected to be a result of the subtle interplaybetween π –π interactions of PF segments and interac-tions of PEO segments with the substrate. Combinedwith the molecular modeling calculations, a sketch

Figure 5. The chemical structures of PF-based diblock and triblockcopolymers reported in reference 136.

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Figure 7. Sketch of the stacking of PF-block–PEO chains intonanoribbons. The conjugated chains are π-stacking parallel to eachother, edge-on over the substrate, with the PEO chains alternating oneach side of the ribbon axis. (a) Top view, with the conjugatedsegments perpendicular to the view; PF chains are in black, PEO ingrey. b) View in perspective. For the sake of clarity, the octylsubstituents are not represented. Reproduced by permission ofWiley-VCH Verlag GmbH.137

for the proposed stacking of the chains is shown inFig. 7. This stacking configuration can minimize sterichindrances between the conjugated backbones. Thesefibrillar morphologies can only be attained when the

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average volume ratio of PEO is lower than 0.3, beyondwhich untextured polymer aggregates are formed upondeposition. In addition, the solvent evaporation kinet-ics and crystallographic symmetry of the substrate canalso affect the orientation of fibrillar supramolecularstructures.

Thanks to the creative work of synthetic chemists,more functional materials with novel and well-definedsupramolecular architectures can be realized in the lab.

One of the striking examples is exemplified by atomtransfer radical polymerization (ATRP) methodology.The ATRP method not only offers more facilities tosynthesize rod–coil copolymers under a facile reactioncondition with a wide range of comonomers,138–140

but also allows for a linear increase in the molecularweight with conversion and quite low polydispersitydue to the activation–deactivation step, which is veryimportant as the segment with a longer conjugation

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length surrounded by short conjugation segmentscould act as a trap for both holes and electrons. Galvinet al. have shown that an increase of almost two ordersof magnitude is realized in device performance bycontrolling polydispersity.141

Using ATRP techniques, a novel PF-based sample(OFPHM, as shown in Fig. 8A), was prepared from2-hydroxyethyl methacrylate (HEMA) with a lowpolydispersity value of 1.33.122 Novel island, string andhoneycomb morphologies, which were different fromthe foregoing fibrillar morphology, were accordinglyobserved. Figure 8(B) shows the AFM image on micasubstrates as a function of concentration. By replacingthe HEMA coil part with PS to produce another novelcopolymer (OFPS, as shown in Fig. 8A), honeycomb-like structures were still obtained, indicating that thecoil segments do not disturb the ability to self-organizebut only affect the size of the structure formed.Microphase separation was also supported by thermalanalysis (e.g., differential scanning calorimetry). It wasworth noting that OFPHM in both solvents (THF,EtOH 95%) displayed an emission peak at 535 nm,indicating the occurrence of aggregates or excimers,while OFPS can emit pure blue light. As mentionedabove, the emission behavior can only be attributed tothe organized supramolecular structure. Interestingly,the emission characteristics of OFPHM in THF didnot change from solution to the solid state, in contrastwith a red shift of 10 nm in 95% EtOH solution.

Similar behavior was also observed in other novelrod–coil di/tri-block copolymers based on terfluorenesegment coupling with PS and poly(tert-butyl acrylate)(PtBA), respectively, by the ATRP method.142 Blockcopolymers with molecular weights up to ca 21 000and polydispersity indices less than 1.5 were obtainedin most cases. Phase separation occurred, but noformation of ground-state aggregates or excimers wasobserved in the solid state, even after annealing athigh temperatures. There seemed to be almost nodifference when changing from the solution to thesolid state, mainly because of the well-diluted PFchromophores in the matrices. The flexible segmentswere demonstrated to inhibit aggregate or excimerformation and do not affect the luminescent propertiesof the PF chromophores, so that the rod–coilcopolymers can emit pure blue light. As far asthese analyses were concerned, the coil blocks playa significant role in the opto-electronic propertiesof the rod–coil copolymers, further supporting thatconstructing rod–coil block copolymer is a vigorousmodification technique to avoid the 535 nm greenemission peak and spectral shift.

More novel morphologies were observed by study-ing PF-based block copolymers coupled with othertypes of segments. Figure 9(A) shows the globu-lar or spherical morphology observed from hybridblock copolymers comprised of helical polypep-tide sequences (poly(γ -benzyl-L-glutamate), PBLG)and rigid PF sequences at a trifluoroacetic acid(TFA)/chloroform (CHCl3) ratio of 30/70 (v/v).143

(B)

PBLGPHF

nm

(A)

0 250 5000

250

500

Figure 9. (A) AFM amplitude image of PF-b–PBLG block copolymercast from TFA/CHCl3 (30/70, v/v) and then annealed in the solventvapor. (B) Possible packing scheme of the block copolymer in thinfilm. Reproduced by permission of the American Chemical Society.143

The morphology is thought to be produced byπ-stacking of PF blocks and the repulsive interactionsof protonated PBLG blocks. However, parallel fib-rillar textures appeared when changing TFA/CHCl3from 30/70 to 3/97 (v/v) (as shown in Fig. 10A).These self-assembled nanostructures were quite dif-ferent from those of conventional rod–coil blockcopolymers containing π-conjugated polymer blocks.Figures 9(B) and 10(B) further depict their possibleself-organization patterns, respectively, to elucidatethe morphological differences. It was proved that thepresence of the polypeptide blocks did not impedecharge injection, charge transport or photophysicalprocesses in EL devices. In addition, branched cylin-drical morphology (‘nanoworms’) was also observedfor PF-b-PANI block copolymers, as shown in Fig. 11by microscopy.144,145

Alternating copolymers: tailoring molecularstructuresFor applications in LEDs, one major problem is theimbalance of the injection and transport of holesand electrons. Most semiconducting polymers havea higher hole mobility than electron mobility dueto low quantum efficiency and degradation as aresult of Joule heating. One approach to solve suchproblems is to exploit the alternating copolymers byincorporating comonomer with high-electron-affinity

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(A)

(B)

Figure 10. (A) AFM amplitude image of PF-b–PBLG block copolymercast from TFA/CHCl3 (3/97, v/v) and then annealed in the solventvapor. (B) Possible packing scheme of the block copolymer in thinfilm. Reproduced by permission of the American Chemical Society.143

Figure 11. TEM image of a spin-coated film (with thickness of ca100 nm) of the PF-b–PANI block copolymer (with permission fromreference 144).

moieties. Just as Aviram and Ratner suggested,146

one D-A alternating molecule should behave as anelementary diode, the effect of which is associated withthe existence of a low-barrier charge-separated D+A−state.146,147 The copolymer with alternating donor and

acceptor units can lower the barrier for the balancedD-A charge injection. In addition, the D-A polar HJdestroys the electron hole symmetry and disturbs theformation of long-lived charge-transfer intermediates.Therefore, the formation of the emissive singlet excitonis accelerated. Huang et al. proposed the conceptof p–n HJ alternating structure, in which typical p-dopable and n-dopable segments are incorporatedinto one backbone of conjugated polymer in orderto adjust the HOMO and LUMO levels of theresulting conjugated polymers, thus matching theFermi energy levels of the electrodes.148–153 Thedesigned polymers usually possess good chargeinjection and charge transport properties for both holesand electrons. Consequently, the approach avoidsthe technical problems originating from the highreactivity of metals with low work functions as thecathodes as well as the introduction of additionalelectron-transporting/injecting layers when fabricatingdevices.

Electronic and physical properties, redox behaviorand emission color of the alternating copolymers canbe fine tuned by tailoring the molecular structures,including attaching different functional groups ontothe comonomers, changing the steric configurationin n-dope type segments or by controlling the repeatnumber of comonomers in one alternating unit. Huanget al. investigated a series of PF-based copolymersalternated with various structural units.154 Figure 12shows the studied compounds and synthetic route.Most of these copolymers exhibit partial reversibilityin both n-doping and p-doping processes. Theexperiments revealed that modifications on the mainchain and related side chain lead to the tuning ofHOMO and LUMO energy levels in the range of0.4–0.5 eV. The attachment of electron-withdrawingester groups leads to an obvious blue shift in theabsorption spectrum, thus enhancing the displacementof the equilibrium geometry of the polymer at theexcited states from its ground state but stronglyreducing the fluorescence quantum yields, while theattachment of electron-donating alkoxy groups on thephenylene ring causes a red shift. The introductionof carbazole species also causes a spectral blue shiftwith increased HOMO and decreased LUMO energylevels. In addition, the heterocyclic comonomers canoffer some protection against electrically inducedoxidation,155 projecting a possible increase in deviceefficiency.

It has been documented that tailoring the molecularstructures of alternating copolymers are a kind ofrobust molecular design to tune the color acrossthe entire spectrum. It is facile to achieve thegoal of full-color display only by easily modify-ing the comonomers. Successful color tuning relieson effective energy transfer from the high-band-gap fluorene units to the low-energy n-type moi-eties. Therefore, a variety of aromatic heterocyclescontaining N, S or O atoms with narrow bandgaps was easily introduced, for example, containing

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O O

OO

B B

H13C6 C6H13

C6H13

H13C6

H13C6

H21C10 C10H21

H13C6OH13C6

C6H13

C6H13 OC6H13

11

+ Br BrAr

Ar

Ar

1-10 P1-10

=

1 2 3 4

CN

5

8 9 10

6 7

S

COOEt

EtOOC

N N

Figure 12. Chemical structures and synthetic route for PF-based alternating copolymers. Reagents and conditions: (PPh3)4Pd(0) (1.0 mol %)toluene/2 M K2CO3 (3:2), reflux. Reproduced by permission of the American Chemical Society.154

benzothiadiazole, pyridine, thiophene, oxadiazole orphenothiazine.155–170 The pure red emission (CIEcoordinate values x = 0.66, y = 0.33) was shownfor alternating copolymers containing thiophene andcyano groups, which is almost identical to the standardred (x = 0.66, y = 0.34) designated by the NationalTelevision System Committee.167 Organosilicon andorganotin units with aromatic or aliphatic groups werealso incorporated into π-conjugated systems to pro-duce novel alternating copolymers. Such π-conjugatedsystems displayed blue color with relatively high lumi-nescence efficiencies at a low operating voltage of lessthan 10 V, due to the participation of the d-orbitalsof silicon or tin atoms and the decrease in LUMOlevel.169,170 Additionally, oligofluorene-type moietieshave been incorporated as part of the ligand frameworkof iridium tris-complexes to avoid crystallization.171,172

There has been great success in fabricating high-efficiency LEDs with alternating copolymers contain-ing fluorene moieties. For instance, a high externalquantum efficiency (EQE) of 2.54% with luminousefficiency of 1.45 cd A−1 was reported for saturated redpoly emitter using alternating copolymers containingbenzothiadiazole.168

In essence, the approach of alternating copolymerscan be attributed to conjugation confinement, whichlimits π-stacking along the backbone by insertinginert non-conjugated moieties into the polymer chain.

Although shortening of the conjugated segments mayhinder energy transfer to lower-energy-gap chro-mophores originating from some defects and thusenhance the EL capacities because of lesser non-radiative processes and the reduced interchain inter-actions, a larger ratio of interruption of π-conjugationmay lead to a decrease in carrier mobility. More-over, the incorporation of flexible units into the rigidbackbone will, in turn, impose constraints on molec-ular size, reduce the extent of rigidity and coplanarityin molecular geometry and affect the microscopicallymolecular order of the polymer. As a result, it becomesmore difficult to form morphologically stable and uni-form amorphous thin films. Ultimately, higher turn-onand operating voltages are usually a result. Therefore,there should exist an optimal structure for the choiceof fabrication of LED devices.2,173

The incorporation of fluorene derivatives intopolyesters produces another type of alternatingcopolymers,174–177 which also shows promising opticaland electrical properties for LED applications. Underthe right conditions, this type of alternating copolymerdoes not exhibit any excimer formation or variationafter thermal treatment. It belongs to the reversiblep–n dopable polymers.174 Durocher et al. synthesizeda series of fluorene-based polyesters, and proved thatthe oligofluorene units are well isolated in the polyesterchain, with almost no changes in the conformations

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and spectral properties of polyesters.175–177 Theluminescence intensity of polyesters remains intense,and the fluorescence spectra show better vibronicresolution and sharper peak but significantly red-shifted bands, indicating a narrower distribution ofconformers in the first excited singlet state. The lengthof the oligomer chains and addition of carbonyl groupsdo not significantly affect molecular conformation ofthe ground state.

BLENDING (DOPING): IMPROVING DEVICEEFFICIENCY BY OPTIMIZING MORPHOLOGIESA significant achievement of OLED developmentwould be to break through the statistical limit of25% with reference to spin-orbit coupling with aheavy metal atom and subsequently triplet exci-ton harvesting by doping the phosphorescent dyesinto semiconducting host materials.178–181 An EQEof 15.4 ± 0.2% and luminous power efficiency of40 ± 2 lm W−1 were achieved by doping phosphores-cence dyes.182 Internal quantum efficiencies of 32%and a peak luminance of 100 000 cd m−2 for tris(2-phenylpyridine) iridium in dicarbazole biphenyl werealso demonstrated.183,184 Moreover, doping fluores-cent or phosphorescent dyes into conjugated polymerhosts has been demonstrated to be very useful totune color and emit full color as an alternativeapproach for designing alternating copolymers.76,184

Berggren et al. have shown that emission color canvary as a function of applied voltage by blendingpolymers with different emission and charge trans-port characteristics.33,185 In such host–guest systems,the energy of excitons formed in the host is trans-ferred to the luminescent dyes by a combination ofForster and Dexter energy transfer. Figure 13 sketchesthe energy transfer pathways between dyes and hostmaterials.186,187 Figure 13(a) shows the singlet-to-singlet transfer responsible for fluorescence in mostdoped organic EL devices, and Forster transfer domi-nates at low fluorescent dye concentrations because ofits long-range nature. Figure 13(b) depicts the possiblyslower, triplet–triplet Dexter transfer from the host.Figure 13(c) presents singlet–singlet transfer betweenthe host and the phosphor, and Forster transfer isstill the dominant process in this case. Figure 13(d)represents direct Dexter transfer between triplet statesin the host and phosphor dopants. However, the dop-ing method may lead to energy loss due to transferto low-lying triplet state or triplet–triplet annihila-tion, and the efficiency may be limited by phaseseparation and aggregation of dopants even at lowblending concentrations.188 Besides, the utilization ofpolymers as host materials for LED applications isstill a challenge; thus non-doped phosphorescent hostemitters have been developed to avoid the dopingprocess.189

On the other hand, the history of the develop-ment of PLEDs has seen that improvement in ELefficiencies has successfully been made by blending

Energy transfer to fluorescent dyes

Energy transfer to phosphorescent dyes

(a)

(b)

(c)

(d)

host singlet

host singlet

host triplet dye triplet

dye triplet

ISC

host triplet dye triplet

dye singlet

dye singlet

energy transferhost guest

Figure 13. Proposed energy transfer mechanisms in films doped witha fluorescene dye and films doped with a phosphorescent dye. Foreach molecule the figure shows the ground-state energy level S0, theexcited singlet level S1, and the excited-state triplet level T1.Reproduced with permission from Baldo MA, O’ Brien DF, ThompsonME and Forrest SR, Phys Rev B 60: 14422 (1999). Copyright 1999by the American Physical Societ. http://prb.aps.org/.186

conjugated polymers. The earlier work of Nishino,Salaneck, Shim, Sainova, and Inganas and theircoworkers should be highlighted.2 There are tworeasons that together account for the improvementin device efficiencies. One is that dispersive emitterswithin the polymer matrix alleviate the formation ofaggregation or excimers due to dilution. The other isthat efficient Forster energy transfer from the donorto the acceptor, as well as spatial confinement ofthe excitons in the acceptor polymers, leads to adecrease in exciton quenching and excimeric emissionvia a non-radiative process. Moreover, electron holerecombination preferentially takes place at the domainboundaries of two phases.190 Ho et al. demonstratedthat devices fabricated from an active layer consist-ing of PFO blended with poly(dioctylfluorene-alt-benzothiadiazole) (F8BT) have an external forwardefficiency of 6.0% at a luminance of 1600 cd m−2 at5 V, exhibiting nearly balanced injection, near-perfectrecombination and greatly reduced pre-turn-on leak-age currents.191 Yang et al. suggested that the PLand EL efficiency of MEH-PPV-based PLEDs couldbe dramatically enhanced by blending with PFO,and their results showed that EL efficiency is sig-nificantly increased from about 0.5 cd A−1 for purepoly[2,5-bis(cheolestranoxy)-1,4-phenylene vinylene](BCHA) to nearly 3.0 cd A−1 for the blend of BCHA

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and PFO, and the absence of interchain species wasrevealed in the blends.192 Jenekhe et al. reported thatthe spectrally stable blue EL is obtained for LEDsfabricated from binary blends of PFO with eitherpoly(vinyldiphenylquinoline) (PVQ) or PS as a resultof higher thermal stability of the blends on accountof higher Tg of both PVQ and PS relative to thatof PFO.193 Meanwhile, brightness and EL efficiencywere improved by a factor of 5–14 compared to PFOhomopolymer devices.

The PF-based blends open a new approach tounderstanding some principal issues from LED appli-cations, including the basic processes of excitondynamics. Regarding the usually observed black spots(non-emissive areas) in the period of device operationthat lead to complete device degradation within a fewhours, Kim et al. investigated PLEDs fabricated fromF8BT blended with poly(9,9′-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine (TFB) as the emissivelayer by using in situ micro-Raman spectroscopy, andproposed that the black spots are not responsible forthe reduction in EL efficiency with device operation,but only effectively cause a reduction of the deviceactive areas and the concomitant loss of total lumi-nance output.194 Concerning the fact that the singletgeneration fractions of conjugated polymers do notobey to the limit of 25%,178,195 Greenham et al. stud-ied blends of PFO and F8BT, and thought that thesinglet:triplet ratio relies on the chemical structuresof the polymers. Moreover, the singlet yield could beunexpectedly high; that is to say, triplet formation isnot an obstacle to high-efficiency device operation.196

Regarding the difficulties of charge injections, Grosset al. showed that doping π-conjugated polymer layersin PF-based systems is conducive to improving theinjection of holes in OLEDs, and the barrier to holeinjection can be continuously reduced by increasingthe doping level of conjugated polymers.197 In respectof energy transfer in blends, Byun et al. blended aseries of polyfluorenes with poly(N-vinylcarbazole),and demonstrated that the D–A distance, whichaffects the efficiency of energy transfer, was alteredby the side-chain length of fluorene rather than thespectral overlap integral.198 For example, blends con-taining PFs with longer and bulkier substitutions atC-9 atoms have a greater distance between donor andacceptor and thus the rate and efficiency of energytransfer decrease. Referring to device structure andperformance, Morteani et al. reported that productionof high-efficiency LEDs with green emission at 2.1 Vand a luminance of 100 cd m−2 was achieved fromde-mixed blends of PFs by fabricating ‘distributed HJ’structures when investigating the properties of two dif-ferent blend systems: blends of TFB and F8BT, andblends of poly(9,9′-dioctylfluorene-co-bis-N,N ′-(4-butylphenyl)-bis-N,N ′-phenyl-1,4-phenyldiamine(PFB) and F8BT.199 The ‘reverse photo-inducedcharge transfer’ state was proposed to explain the verylow operating voltage, where electron hole capture isa barrier-free process to form the exciplex, which has

significant charge transfer character and is lower thanthe charge-separated state in energy.

Beyond all doubt, device performance is verysensitive to blend morphology. Spherical or cylindricalmorphologies are believed to be favorable for LEDapplications.122 Vertically and laterally segregatedstructures have also been observed in PF-based blends.However, it has been proved that the external quantumefficiencies of LEDs made with vertically segregatedstructures are more efficient than those of laterallysegregated LEDs, between 25% and 100%.200 Recentstudies reporting on spin-cast thin-film morphologyof blends can support this. X-ray photoelectronspectroscopy (XPS) and AFM experiments furtherrevealed that lateral phase-separated domains are notpure at the submicron length scale, and vertical phasesegregation at the nanoscale occurs in blend films.190

Processing conditions also have some influence ondevice efficiency. For instance, a pre-heated devicewas shown to exhibit an increase in efficiency while apost-heated device did not.201

Conjugated polymer blends are used not only inLEDs but also in PVs. For applications in PVs,blends consisting of donor and acceptor polymersare commonly employed to increase efficiency. Aninterpenetrating network of spatially distributed D–AHJs are constructed.202 Because of the large surfaceareas of dispersed interfaces between phase-separatedcomponents, exciton dissociation into polarons iseffectively promoted. As a matter of fact, higherdevice efficiency can be achieved by increasing diverseinterfacial areas between mesoscale phases. Besides,the charges generated close to the phase-separatedinterfaces are more readily collected owing to theease in percolating into their preferred phases. Friendet al. further studied a device involving a blend ofPFB and F8BT, and suggested that effective chargecollection only occurs for charges generated close tothe corresponding electrode,203 and charge transportrather than charge generation is the limiting factor indevice performance. Also, owing to the direct pathsfrom cathode to anode within either separated phase,the open-circuit voltage of a blended layer is lower thanthat of a bilayer device comprising distinct layers ofdonor and acceptor materials.203,204 A major problemwith blends is that a possible partial discontinuity ofthe donor and/or acceptor phases may occur. Besides,increased energy level disorder in both phases maylead to an increase in charge trap density and areduction in electron and/or hole mobility. Therefore,a degradation in overall device performance could beexpected.126,205

The efficiency of photovoltaic blends stronglyrelies on both blend morphology and compositionalmicrostructure like that of LEDs. For example, bicon-tinuous structures are considered to be pivotal forhigh-efficiency PVs since they are crucial for thetransport of holes and electrons to the respective elec-trodes without recombination.205,206 With regard tophase-separated morphology of the blend, Stevenson

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found that micro-scale phases penetrate substantiallythrough the film and contain significant volumes ofboth constituents.207 Lidzey et al. further investigatedthe structures of the blend of PFO and F8BT byscanning near-field optical microscopy, scanning forcemicroscopy and nuclear reaction analysis, and pro-posed that one of the polymers may preferentiallywet the surface and form a partially crystallized wet-ting layer.206 However, enhanced residual emissionstill may come from the regions beneath the contin-uous surface layer and a relative depletion in theconcentration of F8BT acceptor molecules at theedge of PFO-rich domains. Additionally, a hierar-chy of micro- and nanoscale morphology probablyemerges.187 In addition, the preparation method ofthe blend film, for example the solvent, solvent evap-oration rate, substrate temperature and saturation ofthe ambient atmosphere during film preparation, allplay important roles in controlling phase-separatedmorphologies and ultimately PV performance.33,208

For instance, PVs using PFB/F8BT prepared fromxylene solution show an EQE of merely 1.8%, whereasdevices from chloroform solution exhibit an EQE of4% at 3.2 eV.33

As mentioned above, the dimension of phaseseparation should be controlled in the range ofthe exciton diffusion length equal to a few tensof nanometers. Thus, the overall layer thicknessshould not greatly exceed the penetration depthof the incident light.206 Moreover, an increase inPV efficiency occurs together with a decrease infeature size. For a columnar, segregated or three-dimensional interpenetrating network structure in theblend film, a linear or super-linear dependence ofPV efficiency on length scale might be expected.187

However, the blends in the conventional way aredifficult to realize, although the characteristic phase-separated length scales vary with conditions such asthe solvent, substrate temperature or saturation of theatmosphere over the transforming film. Luckily, themicro-emulsion method was successfully applied inpolymer blends to precisely manipulate the dimensionof phase separation from 30 to 500 nm.206,207,209 Thesolvent used in the micro-emulsion process does notaffect the efficiency of PVs, indicating that not thekinetics of particle formation but rather the particlesize and composition control the device performance.Two kinds of blends prepared in this way were studied(as shown in Fig. 14). An EQE of ca 4% is attainedfor a device made from polymer blend nanoparticlescontaining PFB:F8BT at a weight ratio of 1:2 ineach individual nanosphere, which is among thehighest values reported so far for PVs. Further studiesrevealed the influence of the layer composition on PVefficiency.209 In addition, the micro-emulsion methodhas already been applied to the fabrication of OLEDsto improve their opto-electronic characteristics byproducing semiconducting polymer nanospheres.210

Moreover, PF-based polymer blends also show greatpromise in other fields. The maximum luminance

Blend of twodifferent dispersions

Two different polymers inindividual particles

Figure 14. Strategies to prepare binary polymer blends usingpolymer nanospheres. Phase-separated structures at the nanometerscale can be prepared either by coating a layer from a dispersioncontaining nanoparticles of two different polymers, or by usingdispersions that contain both polymers in each individualnanoparticle. Reproduced by permission from Macmillan PublishersLtd: Nature Mater 2: 408 (2003).205

of white polymer light-emitting diodes comprised ofpolyfluorene doped with a ‘twistancene’ can exceed20 000 cd m−2. A maximum luminous efficiencyof 3.55 cd A−1 at 4228 cd m−2 was achieved witha maximum power efficiency of 1.6 lm W−1 at310 cd m−2, while device emission color is not afunction of bias currents.211 Polymer blends areattractive for lasers as they can form four-level systems,thereby impeding self-absorption and reducing thethreshold for amplified spontaneous emission due tothe highly efficient host–guest Forster energy transfer.However, Edman demonstrated that the ‘defectpolymer’ exhibits more advantages than polymerblends in light of PF-based studies for opticallypumped lasers.212

CONCLUSIONS AND OUTLOOKThere is no doubt that PFs are one of themost promising materials for opto-electronic devices,especially PLEDs and PVs. As demonstrated by thisreview, their opto-electronic properties and deviceperformance can be greatly enhanced by subtlycopolymerizing or blending with other molecules. Amass of novel organic materials has been created, andthe great improvement benefits from the optimizationof molecular structures, supramolecular orders andmorphologies. For PLED applications, color purityand stability in long-term operations remain achallenge, while efficiency has met the requirements ofcommercialization. Solving such problems correlateswith progress on knowing the origin of the unwantedgreenish emission band. Understanding the electronicproperties of polyfluorenes and mechanisms of excitondynamics is momentous for LED development. Inthe long term, as Kohler et al. predicted,35 thefocus of LED development will be to optimizethe out-coupling of light from the device insteadof the emission efficiency. On the other hand, forapplications of PFs in PVs, it is crucial to effectivelymanipulate the length from donors to acceptorsin the nanometer regime of the exciton diffusionrange. Hence, the concepts of ultra-fast photo-inducedenergy and electron transfer between donor andacceptor units, D-A bulk HJ and polymer blends of

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donor and acceptor have been proposed to promoteefficient exciton dissociation and balanced transport.Recently, Forrest et al. proved that efficiency couldbe much increased by designing a smart devicestructure. They demonstrated a hybrid planar-mixedmolecular HJ photovoltaic cell consisting of a mixedlayer of donor and acceptor molecules sandwichedbetween homogeneous layers of donor and acceptormaterials,213 and then the maximum power conversionefficiency of (5.0 ± 0.3) % was first realized.

Although much progress has been made in the fieldsof PLEDs and PVs, the relationship between theirstructure, morphology and photophysical propertieshas not been quite clear, and the different hierarchies oforganization from molecules to devices remain a chal-lenge. As a result, nanoscience and nanotechnology arelogically merged and consequently facilitate in-depthstudies. The development of synthetic methodology,especially controlled radical polymerization methods,opens new avenues to precisely control structures andmorphologies. For electronic devices comprised ofπ-conjugated polymers, the role of inter-/intra-chainelectronic coupling has not yet been well understood.Thus, one of the elementary but key subjects is toform a picture of quantum mechanical processes ofexciton dynamics. Nevertheless, valuable productioncan successfully proceed under the guidance of suchprocesses. For instance, Klimov et al. showed a sim-ple but elegant, high-efficiency device structure byexploiting the non-radiative Forster energy transferprocess.214,215 Their significant work can lead to anew generation of ultra-high-efficiency light sourcesranging from nano-optics to city blocks. In a word, theongoing efforts on materials designing, device archi-tecture and processing technologies will advance thedevelopment of polyfluorenes in organic electronicsand ultimately boost the commercialization of devicessuch as LEDs and PVs.

ACKNOWLEDGEMENTSThis work was financially supported by the NationalNatural Science Foundation of China under Grant50403012, the ‘Program for New Century ExcellentTalents (NCET) in University’, and the ‘ShanghaiRising-Star Program’ under grant 04QMX1403.

REFERENCES1 Friend RH, Gymer RW, Holmes AB, Burroughes JH, Marks

R, Taliani C, et al., Nature 397:121 (1999).2 Kraft A, Grimsdale AC and Holmes AB, Angew Chem Int Ed

37:402 (1998).3 Bernius MT, Inbasekaran M, O’Brien J and Wu W, Adv Mater

12:1737 (2000).4 Holder E, Langeveld BMW and Schubert US, Adv Mater

17:1109 (2005).5 Ouali L, Krasnikov VV, Stalmach U and Hadziioannou G,

Adv Mater 11:1515 (1999).6 Bredas JL, Beljonne D, Coropceanu V and Cornil J, Chem Rev

104:4971 (2004).7 Tolshima N and Hara S, Prog Polym Sci 20:155 (1995).

8 Roncali J, Chem Rev 97:173 (1997).9 Chan HSO and Ng SC, Prog Polym Sci 23:167 (1998).

10 Kim DY, Cho HN and Kim CY, Prog Polym Sci 25:1089(2000).

11 Mitschke U and Bauerle P, J Mater Chem 10:1471 (2000).12 Heeger AJ, Angew Chem Int Ed 40:2591 (2001).13 Akcelrud L, Prog Polym Sci 28:875 (2003).14 Fukuda M, Sawaka K and Yoshino K, Jpn J Appl Phys 28:433

(1989).15 Fukuda M, Sawaka K and Yoshino K, J Polym Sci Polym Chem

Ed 31:2465 (1993).16 Miyaura N, Yanagi T and Suzuki A, Synth Commun 11:513

(1981).17 Yamamoto T, Morita A, Muyazaki Y, Maruyama T,

Wakayama H, Zhou ZH, et al., Macromolecules 25:1214(1992).

18 Lu S, Fan QL, Xiao Y, Chua SJ and Huang W, Thin SolidFilms 417:215 (2002).

19 Pei J, Yu WL, Huang W and Heeger AJ, Chem Commun114:1631 (2000).

20 Zeng G, Chua SJ and Huang W, Thin Solid Films 417:194(2002).

21 Liu B, Yu WL, Lai YH and Huang W, Macromolecules 35:4975(2002).

22 Ling QD, Kang ET, Neoh KG and Huang W, Macromolecules36:6995 (2003).

23 Liu B, Yua WL, Pei J, Liu SY, Lai YH and Huang W,Macromolecules 34:7932 (2001).

24 Becker S, Ego C, Andrew C, List EJW, Marsitzky D,Pogantsch A, et al., Synth Met 125:73 (2002).

25 Shirota Y, J Mater Chem 15:75 (2005) and references citedtherein.

26 Halls JJM, Walsh CA, Greenham NC, Marseglia EA, FriendRH, Moratti SC, et al., Nature 376:498 (1995).

27 Ego C, Grimsdale AC, Uckert F, Yu G, Srdanov G andMuller K, Adv Mater 14:809 (2002).

28 Elghayoury A, Schenning APHJ, van Hal PA, van Duren JKJ,Janssen RAJ and Meijer EW, Angew Chem Int Ed 40:3660(2001).

29 Pogantsch A, Wenzl FP, List EJW, Leising G, Grimsdale ACand Muller K, Adv Mater 14:1061 (2002).

30 Geng Y, Trajkovaka A, Katsis D, Ou JJ, Culligan SW andChen SH, J Am Chem Soc 124:837 (2002).

31 Katsis D, Geng YH, Ou JJ, Culligan SW, Trajkovska A,Chen SH, et al., Chem Mater 14:1332 (2002).

32 Wu W, Inbasekaran M, Hudack M, Welsh D, Yu W, Cheng Y,et al., Microelectron 35:343 (2004).

33 Arias AC, MacKenzie JD, Stevenson R, Halls JJM,Inbasekaran M, Woo EP, et al., Macromolecules 34:6005(2001).

34 Kreyenschimdt M, Klaerner G, Fuhrer T, Ashenhurst J,Karg S, Chen W, et al., Macromolecules 31:1099 (1998).

35 Kohler A, Wilson JS and Friend RH, Adv Mater 14:701(2002).

36 Nguyen TQ, Wu J, Tolbert SH and Schwartz BJ, Adv Mater13:609 (2001).

37 Kim J and Swager TM, Nature 411:1030 (2001).38 Muccini M, Murgia M, Biscarini F and Taliani C, Adv Mater

13:355 (2001).39 Shimizu M, Suto S, Yamamoto A and Goto T, Phys Rev B

64:115 417 (2001).40 Vacar D, Maniloff ES, McBranch DW and Heeger AJ, Phys

Rev B 56:4573 (1997).41 Nguyen TQ, Martini IB, Liu J and Schwartz BJ, J Phys Chem

B 104:237 (2000).42 Hempenius MA, Langeveld-Voss BMW, van Haare JAEH,

Janssen RAJ, Sheiko SS, Spatz JP, et al., J Am Chem Soc120:2798 (1998).

43 Francke V, Rader HJ, Geerts Y and Muller K, Macromol RapidCommun 19:275 (1998).

44 De Boer B, Stalmach U, Nijland H and Hadziioannou G, AdvMater 12:1581 (2000).

Polym Int 55:473–490 (2006) 487DOI: 10.1002/pi

P Chen et al.

45 Tew GN, Pralle MU and Stupp SI, J Am Chem Soc 121:9852(1999).

46 Jenekhe SA and Chen XL, J Phys Chem B 104:6332 (2000).47 Wang H, Wang HH, Urban VS, Littrell KC, Thiyagarajan P

and Yu L, J Am Chem Soc 122:6855 (2000).48 Lee M, Cho BK and Zin WC, Chem Rev 101:3869 (2001).49 Neher D, Macromol Rapid Commun 22:1365 (2001).50 Leclre Ph, Hennebicq E, Calderone A, Brocorens P, Grims-

dale AC, Muller K, et al., Prog Polym Sci 28:55 (2003).51 Sainova D, Miteva T, Nothofer HG, Scherf U, Glowacki I,

Ulanski J, et al., Appl Phys Lett 76:1810 (2000).52 Grice AW, Bradley DDC, Bernius MT, Inbasekaran M, Wu

WW and Woo EP, Appl Phys Lett 73:629 (1998).53 Scherf U and List EJW, Adv Mater 14:477 (2002).54 Grell M, Bradley DDC, Long X, Chamberlain T,

Inbasekaran M, Woo EP, et al., Acta Polym 49:439 (1998).55 Franco I and Tretiak S, J Am Chem Soc 126:12 130 (2004).56 Ariu M, Lidzey DG, Lavrentiev M, Bradley DCC, Jandke M

and Strohriegl P, Synth Met 116:217 (2001).57 Grell M and Bradley DCC, Adv Mater 11:895 (1999).58 Khan LTA, Sreearunothai P, Herz LM, Banach MJ and

Ohler AK, Phys Rev B 69:085 201(2004).59 Becker K and Lupton JM, J Am Chem Soc 127:7306 (2005).60 Cadby AJ, Lane PA, Mellor H, Martin SJ, Grell M, Giebler C,

et al., Phys Rev B 62:15 604 (2000).61 List EJW, Kim CH, Naik AK, Scherf U, Leising G, Graup-

ner W, et al., Phys Rev B 64:155 204 (2001).62 List EJW, Kim CH, Shinar J, Pogantsch A, Leising G and

Graupner W, Appl Phys Lett 76:2083 (2000).63 Ariu M, Lidzey DG, Sims M, Cadby AJ, Lane PA and

Bradley DCC, J Phys Condens Mater 14:9975 (2002).64 Gamerith S, Gadermaier C, Scherf U and List EJW, Phys Stat

Sol 201:1132 (2004).65 Chen SH, Chou HL, Su AC and Chen SA, Macromolecules

37:6833 (2004).66 Chen SH, Su AC and Su CH, Macromolecules 38:379 (2005).67 Knaapila M, Kisko K, Lyons BP, Stepanyan R, Foreman JP,

Seeck OH, et al., J Phys Chem B 108:10 711 (2004).68 Grell M, Bradley DDC, Inbasekaran M and Woo EP, Adv

Mater 9:798 (1997).69 Teetsov JM and Fox A, J Mater Chem 9:2117 (1999).70 Blondin P, Bouchard J, Beaupre S, Belletete M, Durocher G

and Leclerc M, Macromolecules 33:5874 (2000).71 Kawana S, Durrell M, Lu J, Macdonald JE, Grell M, Bradley

DDC, et al., Polymer 43:1907 (2002).72 Geng Y, Chen ACA, Ou JJ, Chen SH, Klubek K, Vaeth KM,

et al., Chem Mater 15:4352 (2003).73 Grell M, Knoll W, Lupo D, Meisel A, Miteva T, Neher D,

et al., Adv Mater 11:671 (1999).74 Long X, Grell M, Malinovski A, Bradley DDC, Inbasekaran M

and Woo EP, Opt Mater 9:70 (1998).75 Miteva T, Meisel A, Knoll W, Nothofer H, Scherf U, Muller

DC, et al., Adv Mater 13:577 (2001).76 Lane PA (ed), Organic Light-Emitting Devices: A Survey.

Springer, New York, Ch 10, p 26 (2004).77 Marsitzky D, Scott JC, Chen J, Lee VY, Miller RD, Setayesh S,

et al., Adv Mater 13:1096 (2001).78 Grimsdale AC, Leclere Ph, Lazzaroni R, Mackenzie JD, Mur-

phy C, Setayesh S, et al., Adv Funct Mater 12:729 (2002).79 Setayesh S, Marsitzky D and Muller K, Macromolecules

33:2016 (2000).80 Liao LS, Fung MK, Lee CS, Lee ST, Inbasekaran M, Woo EP,

et al., Appl Phys Lett 76:3582 (2000).81 Janietz S, Bradley DDC, Grell M, Giebler C, Inbasekaran M

and Woo EP, Appl Phys Lett 73:2453 (1998).82 Babel A and Jenekhe SA, Macromolecules 36:7759 (2003).83 Campbell AJ, Bradley DDC, Virgili T, Lidzey DG and

Antoniadis H, Appl Phys Lett 79:3872(2001).84 Redecker M, Bradley DDC, Inbasekaran M and Woo EP, Appl

Phys Lett 73:1565 (1998).85 Campbell AJ, Bradley DDC and Antoniadis H, Appl Phys Lett

79:2133 (2001).

86 Misaki M, Ueda Y, Nagamatsu S, Yoshida Y, Tanigaki N andYase K, Macromolecules 37:6926 (2004).

87 Banach MJ, Friend RH and Sirringhaus H, Macromolecules36:2838 (2003).

88 Meng H, Bao Z, Lovinger AJ, Wang BC and Mujsce AM,J Am Chem Soc 123:9214 (2001).

89 Leclerc MJ, J Polym Sci Polym Chem 39:2867 (2001).90 Ranger M and Leclerc M, Macromolecules 32:3306 (1999).91 Ranger M, Rondeau D and Leclerc M, Macromolecules 30:7686

(1997).92 Jenekhe SA and Osaheni JA, Science 265:765 (1994).93 Cimrova V, Scherf U and Neher D, Appl Phys Lett 69:608

(1996).94 Lee JI, Klamer G and Miller RD, Chem Mater 11:1083 (1999).95 Weinfurtner KH, Fujikawa H, Tokito S and Taga Y, Appl Phys

Lett 76:2502 (2000).96 Yua WL, Cao Y, Pei J, Huang W and Heeger AJ, Appl Phys

Lett 75:3270 (1999).97 Jacob J, Oldridge L, Zhang J, Gal M, List EJW, Grims-

dale AC, et al., Curr Appl Phys 4:339 (2004).98 Pei Q and Yang Y, J Am Chem Soc 118:7416 (1996).99 Palsson LO, Wang C, Monkman AP, Bryce MR, Rumbles G

and Samuel IDW, Synth Met 119:627 (2001).100 Ucker F, Tak YH, Muller K and Bassler H, Adv Mater 12:905

(2000).101 Prieto I, Teetsov J, Fox MA, Bout DAV and Bard AJ, J Phys

Chem A 105:520 (2001).102 Pei J, Liu XL, Chen ZK, Zhang XH, Lai YH and Huang W,

Macromolecules 36:323 (2003).103 Zeng G, Yu WL, Chua SJ and Huang W, Macromolecules

35:6907 (2002).104 Yu WL, Pei J, Huang W and Heeger AJ, Adv Mater 12:828

(2000).105 Bliznyuk VN, Carter SA, Scott JC, Klarner G, Miller RD and

Miller DC, Macromolecules 32:361 (1999).106 Herz ML and Phillips RT, Phys Rev B 61:13 691 (2000).107 Teetsov J and Fox MA, J Mater Chem 9:2117 (1999).108 Tetsov J and Bout DAV, J Phys Chem B 104:9378 (2000).109 Silva C, Russell DM, Dhoot AS, Herz LM, Daniel C, Green-

ham NC, et al., J Phys Condens Matter 14:9803 (2002).110 Lemmer U, Hem S, Mahrt RF, Scherf U, Hopmeier M,

Siegner U, et al., Chem Phys Lett 240:373 (1995).111 Ariu M, Sims M, Rahn MD, Hill J, Fox AM, Lidzy DG, et al.,

Phys Rev B 67:195 (2003).112 Gaal M, List ELW and Scherf U, Macromolecules 36:236

(2003).113 Romaner L, Pogantsch A, Freitas PSD, Scherf U, Gaal M,

Zojer E, et al., Adv Funct Mater 13:597 (2003).114 List EJW, Guentner R, de Freitas PS and Scherf U, Adv Mater

14:374 (2002).115 Kulkarni AP, Kong X and Jenekhe SA, J Phys Chem B

108:8689 (2004).116 Zojer E, Pogantsch A, Hennebicq E, Beljonne D, Bredas JL,

Freitas PSD, et al., J Chem Phys 117:6794 (2002).117 Pannozo S, Vial JC, Kervalla Y and Stephan O, J Appl Phys

92:3495 (2002).118 Sims M, Bradley DDC, Ariu M, Koeberg M, Asimakis A,

Grell M, et al., Adv Funct Mater 14:765 (2004).119 Gong X, Iyer PK, Moses D, Bazan GC, Heeger AJ and

Xiao SS, Adv Funct Mater 13:325 (2003).120 Zhao W, Cao T and White JM, Adv Funct Mater 8:783 (2004).121 Surin M, Hennebicq E, Ego C, Marsitzky D, Grimsdale AC,

Muller K, et al., Chem Mater 16:994 (2004).122 Chochos CL, Tsolakis PK, Gregorious VG and Kallitsis JK,

Macromolecules 37:2502 (2004).123 Gadermaier C and Lanzani G, J Phys Condens Matter 14:9785

(2002).124 Van der Auwaraer M and de Schryver FC, Nature Mater 3:507

(2004).125 Daniel C, Herz LM, Silva C, Hoeben FJM, Jonkheijm P,

Schenning APHJ, et al., Phys Rev B 68:235 212 (2003).

488 Polym Int 55:473–490 (2006)DOI: 10.1002/pi

Optimization of opto-electronic property of polyfluorenes

126 Neuteboom EE, Meskers SCJ, van Hal PA, van Duren JKJ,Meijer EW, Janssen RAJ, et al., J Am Chem Soc 125:8625(2003).

127 de Boer B, Stalmach U, van Hutten PF, Melzer C, Kras-nikov VV and Hadziioannou G, Polymer 42:9097 (2001).

128 Stalmach U, de Boer B, Videlot C, van Hutten PF andHadziioannou G, J Am Chem Soc 122:5464 (2000).

129 van der Veen MH, de Boer B, Stalmach U, van de Wetering KIand Hadziioannou G, Macromolecules 37:3673 (2004).

130 de Boer B, Stalmach U, Melzer C and Hadziioannou G, SynthMet 121:1541 (2001).

131 Stalmach U, de Boer B, Post AD, van Hutten PF andHadziioannou G, Angew Chem Int Ed 40:428 (2001).

132 Lu S, Fan QL, Chua SJ and Huang W, Macromolecules 36:304(2003).

133 Lu S, Fan QL, Liu SY, Chua SJ and Huang W, Macromolecules5:9875 (2002).

134 Marsitzky D, Klapper M and Muller K, Macromolecules32:8685 (1999).

135 Leclere Ph, Calderone A, Marsitzky D, Francke V, Geerts Y,Muller K, et al., Adv Mater 12:1042 (2000).

136 Leclere Ph, Surin M, Jonkheijm P, Henze O, SchenningAPHJ, Biscarini F, et al., Eur Polym J 40:885 (2004).

137 Surin M, Marsitzky D, Grimsdale AC, Muller K, Lazzaroni Rand Leclere P, Adv Funct Mater 14:708 (2004).

138 Matyjaszewski K and Xia J, Chem Rev 101:2921 (2001).139 Davis KA and Matyjaszewski K, Adv Polym Sci 159:1 (2002).140 Goto A and Fukuda T, Prog Polym Sci 29:329 (2004).141 Menon A, Dong H, Niazimbetova ZI, Rothberg LJ and

Galvin ME, Chem Mater 14:3668 (2002).142 Tsolakis PK and Kallitsis JK, Chem Eur J 9:936 (2003).143 Kong X and Jenekhe SA, Macromolecules 37:8180 (2004).144 Guntner R, Asawapirom U, Forster M, Schmitt C, Stiller B,

Tiersch B, et al., Thin Solid Films 417:1 (2002).145 Schmitt C, Nothofer HG, Falcou A and Scherf U, Macromol

Rapid Commun 22:624 (2001).146 Aviram A and Ratner MA, Chem Phys Lett 29:277 (1974).147 Karabunarliev S and Bittner E, J Phys Chem B 108:10 219

(2004).148 Wang LH, Kang ET and Huang W, Polymer 42:3949 (2001).149 Wang LH, Chen ZK, Xiao Y, Kang ET and Huang W,

Macromol Rapid Commun 21:897 (2000).150 Lu YW, Meng H, Pei J and Huang W, J Am Chem Soc

120:11 808 (1998).151 Yu WL, Meng H, Pei J, Huang W, Li YF and Heeger AJ,

Macromolecules 31:4838 (1998).152 Meng H, Chen ZK and Huang W, J Phys Chem B 103:6429

(1999).153 Huang W, Meng H, Yu WL, Pei J, Chen ZK and Lai YH,

Macromolecules 32:118 (1999).154 Liu B, Yu WL, Lai YH and Huang W, Chem Mater 13:1984

(2001).155 Sonar P, Zhang J, Grimsdale AC, Muller K, Surin M, Lazza-

roni R, et al., Macromolecules 37:709 (2004).156 Charas A, Barbagallo N, Morgado J and Alcacer L, Synth Met

122:23 (2001).157 Larmat F, Reynolds JR, Reinhardt BA, Brott L and Clar-

son LSJJ, J Polym Sci Polym Chem 35:3627 (1997).158 Huang W, Yu WL, Meng H, Pei J and Li SFY, Chem Mater

10:3340 (1998).159 Liu B, Yu WL, Lai YH and Huang W, Macromolecules 33:8945

(2000).160 Liu B, Niu YH, Yu WL, Cao Y and Huang W, Synth Met

129:129 (2002).161 Sung HH and Lin HC, Macromolecules 37:7945 (2004).162 Zhan X, Liu Y, Wu X, Wang S and Zhu D, Macromolecules

35:529 (2002).163 Kim JH, Park JH and Lee H, Chem Mater 15:3414 (2003).164 Kong X, Kulkarni AP and Jenekhe SA, Macromolecules 36:8992

(2003).165 Hwang DH, Kim SK, Park MJ, Lee JH, Koo BW, Kang IN,

et al., Chem Mater 16:1298 (2004).

166 Lu HF, Chan HSDO and Ng SC, Macromolecules 36:1543(2003).

167 Cho NS, Hwang DH, Jung BJ, Lim E, Lee J and Shim HK,Macromolecules 37:5265 (2004).

168 Hou Q, Zhou Q, Zhong Y, Yang W, Yang R and Cao Y,Macromolecules 37:6299 (2004).

169 Yoshino Y, Nichihara Y, Ootake R, Fujii A, Ozaki M, YoshinoK, et al., J Appl Phys 90:6061 (2001).

170 Baek NS, Kim HK, Chae EH, Kim BH and Lee JH, Macro-molecules 35:9282 (2002).

171 Gong X, Ostrowski JC, Bazan GC, Moses D and Heeger AJ,Appl Phys Lett 81:3711 (2002).

172 Ostrowski JC, Matthew R Robinson Heeger AJ and Bazan GC,Chem Commun 6:784 (2002).

173 Chao TC, Lin YT, Yang CY, Hung TS, Chou HC, Wu CC,et al., Adv Mater 17:992 (2005).

174 Beaupre S, Ranger M and Leclerc M, Macromol Rapid Com-mun 21:1013 (2000).

175 Belletete M, Ranger M, Beaupre S, Leclerc M and DurocherG, Chem Phys Lett 16:101(2000).

176 Lelletete BM, Morin JF, Beaupre S, Ranger M, Leclerc M andDurocher G, Macromolecules 34:2288 (2001).

177 Tirapattur S, Belletete M, Drolet N, Leclerc M and DurocherG, Macromolecules 35:8889 (2002).

178 Wohlgenannt M, Tandon K, Mazumdar S, Ramasesha S andVardeny ZV, Nature 409:494 (2001).

179 Lamansky S, Djurovich P, Murphy D, Abdel RF, Lee HE,Adachi C, et al., J Am Chem Soc 123:4304 (2001).

180 Xie HZ, Liu MW, Wang OY, Zhang XH, Lee CS, Hung LS,et al., Adv Mater 13:1245 (2001).

181 Buckley AR, Rahn MD, Hill J, Gonzalez JC, Fox AM andBradley DDC, Chem Phys Lett 339:331 (2001).

182 Adachi C, Baldo MA, Forrest SR and Thompson ME, ApplPhys Lett 77:904 (2000).

183 Baldo MA, Lamansky S, Burrows PE, Thompson ME andForrest SR, Appl Phys Lett 75:4 (1999).

184 Lane PA, Palilis LC, O’Brien DF, Giebeler C, Cadby AJ,Lidzey DG, et al., Phys Rev B 63:235 206 (2001).

185 Berggren M, Inganas O, Gustafsson G, Rasmusson J, Ander-son MR, Hjertberg T, et al., Nature 372:444 (1994).

186 Baldo MA, O’Brien DF, Thompson ME and Forrest SR, PhysRev B 60:14 422 (1999).

187 Baldo MA, Thompson ME and Forrest SR, Nature 403:750(2000).

188 Sandee AJ, Williams CK, Evans NR, Davies JE, Boothby CE,Kohler A, et al., J Am Chem Soc 126:7041 (2004).

189 Song YH, Yeh SJ, Chen CT, Chi Y, Liu CS, Yu JK, et al., AdvFunc Mater 14:1221 (2004).

190 Kim JS, Ho PKH, Murphy CE and Friend RH, Macromolecules37:2861 (2004).

191 Ho PKH, Kim JS, Burroughes JH, Becker H, Li SFY,Brown TM, et al., Nature 404:481 (2000).

192 He GF, Liu J, Li YF and Yang Y, Appl Phys Lett 80:1891(2002).

193 Kulkarni AP and Jenekhe SA, Macromolecule 36:5285 (2003).194 Kim JS, Ho PKH, Murphy CE, Baynes N and Friend RH,

Adv Mater 14:206 (2002).195 Cao Y, Parker ID, Yu G, Zhang C and Heeger AJ, Nature

397:414 (1999).196 Dhoot AS and Greenham NC, Adv Mater 14:1834 (2002).197 Gross M, Muller DC, Nothofer HG, Scherf U, Neher D,

Brauchle C, et al., Nature 40:5661 (2000).198 Byun HY, Chung IJ, Shim HK and Kim CY, Macromolecules

37:945 (2004).199 Morteani AC, Dhoot AS, Kim JS, Silva C, Greenham NC,

Murphy C, et al., Adv Mater 15:1708 (2003).200 Corcoran N, Arias AC, Kim JS, MacKenzie JD and Friend

RH, Appl Phys Lett 82:299 (2003).201 Hansel H, Muller DC, Gross M, Meerholz K and Krausch G,

Macromolecules 36:4932 (2003).202 Brabec CJ, Sariciftci NS and Hummelen JC, Adv Funct Mater

11:15 (2001).

Polym Int 55:473–490 (2006) 489DOI: 10.1002/pi

P Chen et al.

203 Snaith HJ, Greenham NC and Friend RH, Adv Mater 16:1640(2004).

204 Snaith HJ, Arias AC, Morteani AC, Silva C and Friend RH,Nano Lett 2:1353 (2002).

205 Kietzke T, Neher D, Landfester K, Montenegro R, Guntner Rand Scherf U, Nature Mater 2:408 (2003).

206 Chappell J, Lidzey DG, Jukes PC, Higgins AM, Thomp-son RL, O’Connor S, et al., Nature Mater 2:616(2003).

207 Stevenson R, Arias AC, Ramsdale C, MacKenzie JD andRichards D, Appl Phys Lett 79:2178 (2001).

208 Halls JJM, Arias AC, MacKenzie JD, Wu WS, Inbasekaran M,Woo EP, et al., Adv Mater 12:498 (2000).

209 Kietzke T, Neher D, Kumke M, Montenegro R, Landfester Kand Scherf U, Macromolecules 37:4882 (2004).

210 Piok T, Gamerith S, Gadermaier C, Plank H, Wenzl FP,Patil S, et al., Adv Mater 15:800 (2003).

211 Xu QF, Duong HM, Wudl F and Yang Y, Appl Phys Lett85:3357 (2004).

212 Vehse M, Liu B, Edman L, Bazan GC and Heeger AJ, AdvMater 16:1001 (2004).

213 Xue J, Rand BP, Uchida S and Forrest SR, Adv Mater 17:66(2005).

214 Achermann M, Petruska MA, Kos S, Smith DL, Koleske DDand Klimov VI, Nature 429:642 (2004).

215 Stockman M, Nature Mater 3:423 (2004).

490 Polym Int 55:473–490 (2006)DOI: 10.1002/pi