conjugated rod-coil block copolymers and optoelectronic applications

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ReviewReceived: 7 April 2010 Revised: 1 June 2010 Accepted: 2 June 2010 Published online in Wiley Online Library: 8 October 2010

(wileyonlinelibrary.com) DOI 10.1002/pi.2915

†Conjugated rod–coil block copolymersand optoelectronic applicationsAnne de Cuendias,a‡ Roger C Hiorns,a,b∗ Eric Cloutet,a,b∗ Laurence Vignauc

and Henri Cramaila

Abstract

This review gives a simple introduction to the electronic, optical and structural behaviour of rod–coil block copolymers in whichthe rod block is conjugated. The current understanding of the optical properties of conjugated polymers is discussed, as is theself-assembly characteristics of rod–coil block copolymers, along with their behaviour with respect to organic light-emittingdiode and photovoltaic devices. Poly(p-phenylene), poly(p-phenylenevinylene) and polythiophenes are then used to giveconcrete examples and a short history of the developments of this astounding field.c© 2010 Society of Chemical Industry

Keywords: rod–coil block copolymer; conjugated; self-assembly; optoelectronic; organic light-emitting diode (OLED); photovoltaic;organic solar cell (OSC)

INTRODUCTIONPolymers have undergone an incredible pace of development overthe last century because, among other things, they exhibit modifi-able properties, good processabilities and, for the most part, all at alow cost. A large number are currently being conceived as special-ity materials with high added values. Semiconducting polymers areone of the most fascinating examples. In the 1970s, MacDiarmid,Heeger and Shirakawa demonstrated the high electrical conduc-tivity of doped polyacetylene.1,2 This discovery, recognized by aNobel prize in 2000, has led to a very large research effort aimedat optimizing their electronic and optoelectronic properties.3

π -Conjugated organic compounds are a particularly promisingclass of materials in the field of electro-optics due to theirnonlinear optical behaviour and photoconductivity.4 – 7 They haveundergone an astounding pace of development for use in elec-troluminescent diodes (polymer light-emitting diodes, PLEDs),8,9

photovoltaic cells,10 – 14 stable electronic memories on flexible cir-cuit boards15 and polymer field-effect transistors.16 – 19 Moreover,their modifiable sensitivity to physical, chemical and biologicalperturbations20 makes them suitable for use in gas and humiditysensing,21 bio-sensing, tissue engineering and neurology.19,22

Nevertheless, conjugated macromolecules often exhibit rela-tively high stiffnesses due to their delocalized electronic structuresand low solubility in organic media due to aggregation arisingfrom interchain π –π -orbital interactions.23,24 Various solutionshave been proposed for limiting the aggregation phenomena, forexample by incorporating cumbersome dendritic moieties to en-capsulate the conjugated backbone25 – 29 or by using comonomersto perturb the linear assembly of chains.30 – 33 Conversely, thisstrong tendency towards aggregation can be usefully directed byusing complementary groups that arrange the macromoleculeswith hydrogen bonds, resulting in the formation of supramolecu-lar structures.33 – 38 Another route to directing the organization ofrod-like conjugated polymers is through their covalent linking withcoil-like polymers. The resulting block copolymers form domains of

like-natured polymers that can be organized to give a plethora ofdifferent morphologies in solid and solvent-dispersed states.39,40

The type of morphology attained from rod–coil blockcopolymers depends on the polymers used and the environmentin which they are brought together, and can greatly modify thebehaviour of the bulk material. The widths of the domains are ofthe order of less than ten to several tens of nanometres and canbe influenced by varying the lengths of the polymers. This orderof scale in turn permits control over the electronic, optoelectronicand electro-optical properties by varying rod and coil lengths.41

π -Orbital–π -orbital interactions between rod polymers in onedomain can induce high degrees of order and crystallinity, and

∗ Correspondence to: Roger C Hiorns, CNRS, IPREM (EPCP), Universite de Pau etdes Pays de I’Adour, 2 avenue President Angot, 64053 Pau, France.E-mail: [email protected]

Eric Cloutet, Universite de Bordeaux, Laboratoire de Chimie des PolymeresOrganiques, 16 avenue Pey Berland, ENSCBP, 33607 Pessac Cedex, France.E-mail: [email protected]

a Universite de Bordeaux, Laboratoire de Chimie des Polymeres Organiques, 16avenue Pey Berland, ENSCBP, 33607 Pessac Cedex, France

b CNRS, Laboratoire de Chimie des Polymeres Organiques, 16 avenue Pey Berland,33607 Pessac Cedex, France

c Universite de Bordeaux, IMS, ENSCBP, 33607 Pessac Cedex, France

† Dedication: This review is dedicated to Professor Francois Schue on the eventof his retirement from the position of Editor-in-Chief of Polymer International.For more than 13 years his energetic commitment to the journal enabled itto become truly respected as scientific words should be. As Emeritus Editor-in-Chief, we hope that we shall see Francois for many years to come at futurePolymer International meetings around the world.

‡ Current address: Department of Chemistry, University of Warwick, CoventryCV4 7AL, UK.

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Conjugated rod–coil block copolymers www.soci.org

Roger C. Hiorns He received his PhDfrom the Universities of Kent at Can-terbury and of Montpellier II in 1999.His post-doctorial studies were under-taken at the Universite de Pau underthe direction of Dr B. Francois and DrJ. Francois and then at the ENSCBPin Bordeaux with Professor Henri Cra-mail and Dr Eric Cloutet. He recentlytook up the position of Charge deRecherche (CNRS) at the Universite dePau in France.

Eric Cloutet He received his PhDfrom the University of Bordeaux in1996. His post-doctorial studies wereundertaken at the University of Akron(Ohio, US) under the direction of Prof.Roderic P. Quirk. He joined the CNRS asa Charge de Recherche in 1997 at theUniversite de Paris 13. He has beenworking as a Charge de Recherche(CNRS) at the University of Bordeauxsince 2001.

can have a strong influence on the type of morphology obtained.The asymmetric nature of the highly rigid conjugated block andthe coil block tends to increase the Flory–Huggins parameter (χ )leading to domain formation even with relatively short polymersand oligomers.42 Indeed, a wide variation of morphologies hasbeen demonstrated through the preparation of mushroom-like aggregates,43 honeycomb structures,44 – 47 vesicles48,49 andcylindrical and lamellar aggregates.50 – 52 And it has been shownthat, due to charge confinement within domains, the self-assemblyprocess can be altered by using parameters such as lightand temperature.53 – 55 It is apparent that rod–coil copolymerscontaining conjugated segments can lead to well-defined objectsat the nanometric scale and that this in turn leads to the materialshaving an extraordinary range of applications.

This review aims to give an overview of the main applications ofπ -conjugated systems, highlighting the importance of controllingthe ineluctable aggregation phenomena and concentrating onthe qualities of rod–coil block copolymers. An initial review ofthe basic electronic and optoelectronic properties of conjugatedpolymers along with a brief synopsis of their use and operationin more notable applications is given as this helps to explain thecharacteristics found and sought for with conjugated rod–coilcopolymers.

ARCHETYPAL CONJUGATED POLYMERSThe discovery in 1977 that polyacetylene exhibits a conductivityof about 103 S cm−1 on doping with Br2, I2 or AsF5 initiated atrue interest in this material.2 However, polyacetylene exhibitslow environmental and thermal stabilities and poor solubilitiesthat limit its technical use. Following these observations, manyaromatic conjugated polymers such as poly(p-phenylene) (PPP),56

polythiophene (PT),57 polypyrrole58 and others shown in Fig. 1were studied. Currently, many other analogues are regularlyprepared. Poly(3,4-ethylenedioxythiophene) (PEDOT), marketedby the Bayer company since the 1980s, is one of the most widelyexploited.59 – 63 This polymer shows reasonable conductivity (ca

1 S cm−1), quasi-transparency in films and high stability in theoxidized state.62 – 64

An important development was the discovery of the greenelectroluminescence of undoped poly(p-phenylenevinylene) (PPV)by Friend and colleagues in 1990.65 This breakthrough wasfollowed by the ground-breaking preparation of the first bluePLED based on poly(9,9′-di-n-hexylfluorene) in 1991.66

ELECTRONIC STRUCTURESUndoped polymersThe semiconducting properties arise from the π -delocalizationof single 2pz valence electrons at each carbon atom along thepolymer backbone. The electron is available as only three sp2

electrons are required for skeletal bonding through σ -orbitals. Thewavefunction of each 2pz electron overlaps outside the frameworkof the macromolecule to give a delocalized π -band that stretchesover a segment of the polymer backbone.

It might be expected that this band would result in metal-like behaviour. However, this is evidently not the case as chargecarrier mobilities (µ) of pure conjugated polymers are typically inthe region of 10−8 cm2 V−1 s−1, for example polyvinylcarbazole,67

and then from 10−4 cm2 V−1 s−1 for a zero-field hole mobilityfor poly(3-hexylthiophene) (P3HT) up to 0.1 cm2 V−1 s−1 for high-molecular-weight P3HT in field-effect transistor devices.68 As acomparison, inorganic materials such as polycrystalline siliconexhibit mobilities of the order of 102 cm2 V−1 s−1. Moreover, thesevalues are for holes, and electron mobilities are consistently aroundan order or two less, even when the polymers are designed to actas effective supports for electronic transport.69

The electronic structure depends heavily on the energy levelsof the constituent repeat units. The highest occupied molecularorbital (HOMO) and lowest unoccupied molecular orbital (LUMO)of these units can be thought of as extending to form, respectively,the valence band (VB) and conduction band (CB) of the polymer,as shown in Fig. 2. The energy levels of these bands are thereforealso reliant on the length of the conjugated segment. Each ofthese units carries a HOMO and LUMO, and as more are broughttogether they extend the chain and collectively combine to formthe VB and the CB, respectively. This linear combination results,eventually, in the formation of bands but also in an alternatingstructure of single and double bonds. This so-called Peierls effectstabilizes the chain but also means that the full VB is separatedfrom the upper empty CB by a discrete amount of energy. This gapis what limits the polymers to being semiconductors rather thanfull metallic conductors: the electrons must overcome this barrierto move. The gap is commonly called the band gap (Eg) and canbe large or small, essentially depending on the structure and typeof the polymer. As well as being the energy between two levelsthe band gap can also be defined as the difference between theenergy to pull an electron from the highest point of the CB (i.e.the ionization potential, or IP) and the energy required to inject anelectron into the lowest point of the VB (i.e. the electron affinity,or EA). Eg is generally of the order of 0.8 to 4.0 eV and correlatesextremely well with the energy of visible light. This means thatthe electrons can interact with light and it is this property that isexploited in many optoelectronic applications. More details on thisand the transport of charges in polymers can be found in the workby Jaiswal and Menon70 and others.71 – 74 Evidently there are manyparameters that can further modify the energy levels of the bands,such as in-chain defects, impurities and so on. One particularlyinteresting case is that of chain planarity as this facilities the

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www.soci.org A de Cuendias et al.

n S nn N

H

n

trans-polyacetylene poly(para-phenylene) polythiophenepolypyrrole

npoly(p-phenylene vinylene)

n

poly(p-phenylene ethynylene)

S n

poly(thienylene vinylene)

S

OO

nn

polyfluorene poly[3,4-(ethylenedioxy)thiophene]

Nn

polycarbazoleH

Figure 1. Chemical structures of some conjugated polymers.

π* (LUMO)

π* (LUMO)

π* (LUMO)

π (HOMO)

π (HOMO)

π (HOMO)

VB

CB

Energy

π* (LUMO)

π (HOMO)

Eg

Vacuum

EA

IP

ethylene butadiene octatetraene poly(acetylene)

2 n

Figure 2. Evolution of the molecular orbital diagram (π -levels) with the number of monomer units. (Adapted from Ref. 78).

formation of the bands. However, chain planarity is also extremelysensitive to structural and environmental perturbations and canlead to wide variations in the optoelectronic properties of thepolymers.75 – 77

Using the band model it is possible to class materials with respectto the size of their band gap. Intrinsic semiconductors have bandgaps from around 0 up to 3 eV. As mentioned above, when thepolymers are not exposed to optical, electrical or thermal energy,their VBs are completely full and their CBs are empty. Insulatorshave a band structure similar to semiconductors but their band gapis much greater (>4 eV) and therefore electron transfers betweenbands are limited.

n n

Figure 3. Mesomer structures for trans-polyacetylene.

The majority of conjugated polymers are semiconductors andhave band gaps of the order of those detailed in Table 1.

An upshot of the alternating single and double bond structureis that the conjugation can be represented using either of twomesomeric structures. If they are energetically equivalent, as isthe case for polyacetylene shown in Fig. 3, then the system iscalled degenerate. However, if the two structures are energetically

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Table 1. Representative band gaps of conjugated polymers

Polymer Band gap (eV) Reference

Trans-polyacetylene 1.4–1.5 79,80

Polythiophene 2.0–2.1 81,82

Poly(p-phenylene) 2.7 83

Poly(p-phenylenevinylene) 2.5 84

Polypyrrole 3.2 85

poly[3,4-(ethylenedioxy)thiophene] 1.6 86,87

∆r

Total Energy

S n

S n

aromatic

quinoid

∆E

Figure 4. Schematic representation of the total energy curve for an infinitePT chain as a function of the degree of bond length alternation �r.

unequal, as is the case for the majority of conjugated polymers – forexample PPP, PT and PPV – then the energy levels are termed non-degenerate. In the example of PT shown in Fig. 4, this means thatthere is a ground state based around the aromatic form, and amore excited quinoid state.72

Doped polymersDoping a conjugated polymer effectively results in a material thatcan combine the mechanical properties of polymers (flexibility,elasticity and so on) with the high conductivities more commonlyassociated with metals. The process is based on a charge-transfer redox reaction of electron-withdrawing (p-type doping)or electron-donating (n-type doping) impurities with the polymer.It is mainly carried out by chemical or electrochemical means.Semiconducting polymers, however, do not easily undergo

reversible, controllable doping. In the case of chemical doping witha chemical oxidant (p-type doping) or reductant (n-type doping), aneutral conjugated polymer can be transformed, respectively, intoa poly(cation) or poly(anion), but the presence of a counter ion isrequired to maintain the total electro-neutrality of the system, asillustrated with the following examples:88

p-type doping : (π -polymer)n + 3/

2ny(I2)

→ [(π -polymer)y+ (I3−)y]n (1)

n-type doping : (π -polymer)n + [Na+(C10H8)−•]y

→ [(Na+)y(π -polymer)y−]n + (C10H8)0 (2)

The introduction of charges during the doping process locallymodifies the alternation of single and double bonds, positioningthe charge carriers on the chain. This results in the appearanceof new localized electronic states with holes (radical cations)or electrons in the forbidden band. The quasi-particles thusformed can be classified in two categories, solitons and polarons,depending on whether or not the ground state of the polymer isdegenerate.6

Solitons appear during the doping of the system at a degenerateground state, as shown in Fig. 5. The soliton is associated with adefect separating two parts of the chain and gives rise to aninversion in the order of single and double bonds. The solitonshown is of charge q = ±e and exhibits zero spin, whereas aneutral soliton (q = 0) has a spin (S = 1/2). Solitons ensureelectronic transport through the formation of a soliton band athigh doping levels.

Polarons and bipolarons are positive or negative chargesassociated with a local deformation of a polymer chain that ischanging from the aromatic form to the quinoid form. Polarons arelocalized over a few repeat units and exhibit spin–charge relationsthat differ from those of solitons, as they are simultaneously charge(q = ±e) and spin carriers (S = 1/2). The displacement (coherentor by hopping) of polarons along or between macromoleculescontributes to the bulk electronic transport of the material. Byremoving (or adding) another electron to the existing polaron, anew species, a bipolaron, can be created. In bipolarons, the twocharges are adapted in the same local deformation of the chain. Abipolaron is in effect a dication that has zero spin. An example ofPT p-type doping is shown in Fig. 6.

On reaching high doping levels, polaronic and/or bipolaronicstates can be reached leading to charge defects moving along

VB

CB

VB

CB

VB

CB

E

E

E

+

+

n

n

+

+

+

n

Ground state

Doping state with1 charge

-e- +e-

-e- +e-

+Doping state with

2 charges

Figure 5. Solitons and p-type doping of polyacetylene.


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Figure 6. p-Type doping of PT and the polaronic states.

polymer chains. As a consequence, and depending on thenature of the dopant and the amount used, it is possible toobserve insulator–conductor transitions and, in some cases,insulator–metal transitions. The conductivities of archetypalconjugated polymers in the neutral and doped states arecompared with some reference materials in Fig. 7.

IMPROVING PROCESSIBILITYThe characterization and utilization of the majority of thepolymers shown in Fig. 1 were hampered for many yearsby their extremely poor solubility in common solvents andlimited fusibility. This problem has been resolved for the mostpart by introducing flexible side groups, such as alkyl89 – 91

and poly(ethylene oxide) (PEO)92 chains, or incorporating polarfunctions such as quaternary sulfonate93,94 or ammonium95

groups such as shown in Fig. 8.The radical change in the properties of these polymers arising

from relatively minor modifications to their structure propelled amore general effort to couple conjugated polymers with flexiblecoil-like polymers such as polystyrene (PS),44,96,97 polyisoprene(PI),98 – 100 poly(methyl methacrylate) (PMMA)44 and PEO.97,100

It was expected that this would not only enhance solubilitiesbut also lead to an exponential divergence in the attainableattributes of the materials. Indeed, it is now well apparentthat the resulting rod–coil copolymers based on the structureshown in Fig. 9 have given rise to an extraordinary range ofsupramacromolecular structures displaying micellar, vesicular andlamellar forms, to name but a few. This self-organization of diblockand triblock conjugated copolymers will be detailed later in thisreview.

APPLICATIONS OF SEMICONDUCTINGPOLYMERSOrganic light electroluminescent diodes (OLEDs)Electroluminescence, the non-thermal conversion of electricityinto light, discovered in 1936, remained confined to laboratories

Figure 7. Electronic conductivities of conjugated polymers on dop-ing(Adapted from Ref. 101).

until the 1960s.102 In 1962, Holonyak and Bevaqua described thepreparation of red-emitting inorganic electroluminescent diodesbased on elements such as arsenic, gallium and phosphorus.103

Significant breakthroughs were reported in this area andinorganic diodes are now commonplace in displays, signpostingand communications. They have the advantages of functioning atlow tensions and having very long lifetimes (105 h). OLEDs arrivedin 1963 with the work of Pope et al. on layers of conjugatedaromatic molecule crystals (anthracene) emitting light at very hightensions (100 V).104 In 1990, the electroluminescence of PPV washighlighted by Burroughes et al.65 to give rise to PLEDs.6,8,9,84,105

Due to their flexibility, malleability and relatively low cost of pro-duction, along with the range of colours that are accessible, PLEDsare expected to deliver well in the field of multicoloured displays.


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S

H3C OO

S

C6H13

C6H13C6H13

S

SO3- Na+

S

OO

O

SO3- Na+

O

O

N

N

Br-

Br-

n

m

n n

n

poly(3-hexylthiophene) (P3HT)

poly(9,9'-di-n-hexylfluorene)

poly[3-oligo(ethylene oxide)-4-methylthiophene]

sodium poly(3-butylenesulfonate-thiophene)n

+

+

n

poly{2,5-bis[3-(N,N,N-trimethy lammonium)-1-oxapropyl]-1,4-phenylene-alt-1,4-phenylene} dibromide

sodiumpoly[2-methoxybutylenesulfonate-

(3,4-ethylenedioxythiophene)]

Figure 8. Examples of pendent groups used to improve the handling properties of conjugated polymers.

Figure 9. Representation of rod–coil diblock and triblock copolymers.

White light diodes are of particular interest for lighting and fordisplays where filtration can lead to red, green and blue pixels.106

OLEDs and PLEDs can be schematized as a thin luminescent filmof the active material (approximately 100 nm thick) sandwichedbetween two electrodes (one of which is transparent) andsupported behind a transparent substrate such as glass orplastic (Fig. 10(a)). The transparent electrode is often made usinga thin coating of indium tin oxide (ITO). The cathode, oftenreflective in nature, acts as a backing to the device and is oftenmade using aluminium, calcium, alloys of magnesium–silver andlithium–aluminium and occasionally multilayer structures (suchas Al/LiF or Al/CsF).107

Electroluminescence is a result of numerous processes.Figure 10(b) shows a general overview of the energy bands in-volved. Initially, the injection of charges in the conjugated polymerunder its neutral state leads to the formation of polarons. These po-larons are negative on the cathode side because they correspondto the injection of electrons, and positive at the anode due to theinjection of holes. However, it is improbable that bipolarons are cre-ated as they generally arise with higher levels of doping. The trans-port of charges by carriers can be modelled as intra- and interchainhops. The recombination of polarons of opposing signs results inthe formation of an exciton, which is a neutral but excited state ofan electron coupled to a hole. The most visible process is then therelaxation of this state that is coupled with the emission of light.

Charge injection in the structure requires the crossing of apotential barrier at the electrode–organic material interfaces(non-ohmic contact). Generally, this energy barrier depends onthe relative position of the HOMO and LUMO levels of the material

Figure 10. Representations of (a) a working OLED and (b) the resulting device electronic band structure.


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and on the work function of the electrodes. The work function ofa metal (Wm) corresponds to the energy required to remove oneelectron from the metal, i.e. the difference between its Fermi leveland the vacuum level. ITO has a work function of approximately4.7 to 4.9 eV and is compatible with the role of injecting holesinto the active material.84,108 A complementary layer of PEDOTdoped with poly(styrene sulfonic acid) (PEDOT-blend-PSS) is placedbetween the active layer and the ITO. The ionization potentialof ITO/PEDOT-blend-PSS was estimated by Brown et al. to be5.2 eV.109 In effect the PEDOT-blend-PSS layer facilitates holeinjection by both reducing the energy barrier between the ITOand the active layer and smoothing the ITO surface.110,111

OLEDs are prepared using two different deposition techniquesthat are highly adapted to the nature of the materials used in theactive layers, namely deposition from a solvent or under vacuumconditions. Either way, the deposition process is performed directlyonto the plastic or glass substrate that is precoated with ITO. Thepreparation of the components usually begins with a cleaning ofthe substrate and photo-engraving of the ITO layer. Processing byoxygen or UV–ozone plasma eliminates organic contaminationson the ITO surface and improves its injection properties byincreasing the work function.112

Deposition of the active layer from a solvent presumes that thelayer material is soluble and that the solvent is easily removableafter deposition. In general though the process is technicallysimple and can be performed using spin-coating, blade-spreading(so-called doctor blade) and ink-jet techniques. The latter permitsthe manufacture of flat screens with individually addressablepixels.113 Thicknesses can be readily controlled to the order ofone to more than tens of nanometres. The deposition is generallycarried out under an inert atmosphere free of dust. If multilayerstructures are prepared (such as those with PEDOT-blend-PSS)then the solvent of the following layer must be carefully chosenso as not to dissolve the layer below.114 PEDOT-blend-PSS is watersoluble and this facilitates the process considerably as most activelayers are, conversely, soluble in organic solvents.

The second process, vacuum evaporation, is more appropriateto materials with low molecular weights (generally below1500 g mol−1). The process is somewhat simplified as solvents arenot required and intrinsically avoids dust and impurities, and filmthicknesses can be determined simply by weighing the amountof material deposited; however, this type of system is generallythought less suitable than solvent-based methods for high-rateindustrial production.3,6

In electroluminescent devices the recombination zone shouldbe in the middle of the emissive layer; thus, similarly efficientcharge injection along with close mobilities of electrons andholes in the organic material are necessary. However, this isusually not observed in the simple device architecture of single-layer OLEDs and results in decreased efficiencies through excitonquenching processes in proximity to the electrodes or non-radiative recombination of charges at the electrodes. Theseproblems can be overcome by the incorporation of additionallayers giving rise to multilayer OLEDs. In these devices the emitter issandwiched between hole and/or electron transport and injectionlayers to provide enhanced recombination of electrons and holesin the emissive layer by shifting the active zone roughly to themiddle of the OLED structure. State-of-the-art OLEDs have upto five layers with charge- and exciton-blocking layers beingessential to obtain excellent device performance. It should alsobe mentioned that doping of the transport layers can be used

N

N

N

Al

O

OO

Figure 11. Chemical structure of the molecule Alq3.

for increasing the efficiencies of the corresponding light-emittingdevices.115

Phosphorescent OLEDs based on emissive layers as charge-transporting hosts doped with phosphorescent emitters haveattracted extensive interest due to their highly efficient emission,compared to conventional fluorescent OLEDs, through radia-tive harvesting of both electro-generated singlet and tripletexcitons.116 Today, a wide range of phosphorescent dyes are usedbased on various complexes containing transition metals such asiridium, platinum, osmium, ruthenium, and so on.117 In order toprevent the quenching that can arise at high concentrations ofphosphorescent dyes, they are usually blended into suitable hostmaterials (small molecules or polymers) from which the excitationenergy is transferred to the phosphorescent guest.118

Since Tang and Van Slyke reported on the first effective OLED in1987,119 the electroluminescent component aluminium 4-tris(8-hydroxyquinolate) (Alq3), shown in Fig. 11, continues to be a widelyused emitter. Alq3 has a HOMO–LUMO gap of around 2.6 eV, andcan effectively cover an emission range from the green to the redwith selective doping with other organic molecules. The range ofmaterials, however, has since been greatly expanded and recentreviews detail the electroluminescence characteristics of variousmaterials along with their handling properties.106,120

PPV, the archetypal polymer for PLEDs, emits in the yel-low–green spectral region.65 This polymer does not, however,exhibit a high solubility in common solvents and is thus limitedin its use. A more soluble derivative, which emits in theorange–red region, is poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and is often used, although itsbehaviour is sensitive to variations in its molecular weight andsolvent treatment,121 and it suffers from relatively poor stabilitywith respect to thermal and oxidative degradation.122

PPV-based copolymers have also been explored in orderto vary and improve device emissions.9,120 Fig. 12 shows howthe controlled incorporation of a 2-butyl-5-(2′-ethylhexyl)-1,4-phenylenevinylene123 (BuEH-PPV) comonomer into the MEH-PPVstructure, leading to (BuEH-PPV)x-(MEH-PPV)y , can determine theemission colour of the active layer. Spreitzer et al. demonstratedthe exceptional effectiveness of modifying the side groups ofthe PPV main chain, in this case with phenyl groups.124 Electron-withdrawing cyano groups have been used to increase the electronaffinity of the PPV chain so that their derivatives are now oftenused as n-type materials.80 More recently, Taranekar et al. preparedseveral copolymers based on cyanofluorene and phenyl groupsplaced in ortho, meta and para positions, as shown in Fig. 13. Theyobtained PLED devices of various colours (blue or green accordingto the energy gap) with luminances of 3000 cd m−2 and efficienciesreaching 2.7 cd A−1 for tensions of 5 V.125 Moreover, Gong et al.126

prepared white diodes from mixtures of poly[(9,9-dioctylfluorene)-co-fluorenone] and Ir(HFP)3 (Fig. 13), and obtained luminances ofthe order of 6100 cd m−2 and efficiencies of 3 cd A−1.


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Figure 12. Emission colours of different conjugated (co)polymers, with violet on the left to red on the right. (Adapted from Ref. 129 and 130).

CNC6H13 C6H13CN

OC8H17C8H17

C6H13 C6H13

N

Ir

n

poly[3-(3-(2-cyano-2-(7-methyl-9,9-dioctyl-9H-fluorene-2-yl)vinyl)phenyl)bis(methylacrylonitrile)]

99 1

PFO-F(1%)

3

tris[2,5-bis(9,9-dihexylfluoren-2-yl)pyridine-k2NC3′] d'iridium(III)

Ir(HFP)3

Figure 13. Chemical structure of some polymers used in PLEDs.125,126

Discovered at the end of the 1980s, OLEDs now have a lifetimesufficient to be marketed for applications of non-demandingdurability (ca 10 000 h) such as nomad devices (mobile phones,digital cameras, etc.). After achieving electroluminescent displaysbased on polymers and smaller organic molecules, industry hasaimed for the production of flat screens that can compete withliquid crystal display (LCD) technology. Device-ready propertiesthat will be required from polymers are:

• a minimum luminance of 100 cd m−2;• a relatively low operating voltage of less than 10 V;• a luminous efficiency higher than 10 lm W−1;• a good thermal stability (high glass transition tempera-

ture); and• a good stability towards oxygen and water.

In 1997, Pioneer was the first manufacturer to integratemonochromic OLED displays into car radios with materials fromKodak. Various manufacturers have since produced small displayswith passive matrices for use in mobile phones and automobiledisplays. In 2003, Kodak marketed the first digital camera using anOLED colour screen (2.16 inches, 512×218 pixels, 120 cd m−2). The

long-term prospects for OLED technology are in television devices.Sony, Panasonic, LG Displays and SMD have demonstrated TVprototypes of 25 inches and larger. What has particularly intriguedindustry watchers is the potential created with Sony’s 2007 releaseof the XEL-1, an 11 inch OLED TV.127 Anyone seeing this TVcompared to a thin-film transistor LCD or plasma display panelTV found that it outperformed these technologies by a widemargin in terms of both image quality and form factor (Fig. 14).OLED TVs appear to have the deepest blacks, the highest contrastratio, the fastest response time, the widest viewing angle and thelowest power consumption. But to compete in the TV market,OLEDs have to continue to progress in the following areas of bothtechnology and manufacturing: scaling the backplane, scalingthe OLED deposition and patterning processes, improving theefficiency and lifetime of blue-emitting materials and developingmore efficient material deposition and patterning tools.

Photovoltaic cellsDiscovered by Becquerel in 1839,128 the photovoltaic effect runsin the opposite sense to electroluminescence in that the energy


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Figure 14. Sony’s OLED-based XEL-1 TV showing the 3 mm thick screen.(Reproduced from reference126).

of light is transduced into electricity. The current interest in solarcells lies in their non-polluting production of ‘renewable’ energyand their ease of installation at varying private and public utilityscales. A study by the International Energy Agency showed thatby integrating photovoltaic panels into 40% of roof surfaces and15% of frontages, countries such as Australia, Spain or the USAcould produce from 50 to 60% of their electricity.131 Solar energyaccounts for only about 0.01% of the actual energy used (therest being accounted for by fossil fuels, nuclear and so on). TheIntergovernmental Panel on Climate Change estimated that by2050 world energy requirements will have doubled, but thatto remain within ‘modestly stringent’ limits of CO2 emissions(i.e. that might, but would not necessarily, lead to catastrophicenvironmental consequences), at least one-half of that energywould have to be produced via non-CO2-emitting routes.132

Nuclear fission would not be able to provide this (at least one1 GW station would have to be built every 1.6 days for the next40 years).133 Given that in an hour the Earth receives more energythan required by humanity in a year, solar energy is understandablyone of the most rational routes to supplying humanity’s needs.133

However, even with government subsidies and bank credits thatmean that systems are virtually self-paying in some countries (e.g.a cost of ¤21 500 for an installation of ca 3 kW can be reducedto around ¤13 000),134 private installations remain out of reachfor many, the reason for this being the price of the silicon-basedpanels. It is therefore an aim to use cheaper polymer-based active

layers to reduce the basic material costs so that such installationsbecome more accessible.

The first solar cells prepared in 1954 by Pearson and co-workers135 were based on silicon, and this remains the materialof choice for the current best commercially available devices.Efficiencies for such systems are currently around 19%.136 Allorganic devices that incorporate polymers in their active layerhave seen efficiencies rise from less than 0.1% at the time oftheir discovery,12,137 to greater than 7% in 2010 for laboratory-scale devices.138,139 It is therefore hoped that research willdeliver higher efficiencies and stabilities appropriate to long-termcommercialization in the near future.

The current best polymer-based organic solar cells (pOSCs) aremade up of a composite of two materials, an electron donorand acceptor, sandwiched between an anode and a cathode asshown in Fig. 15. One of the electrodes is transparent so as tolet in light. The protective substrate can be an inert polymer(such as poly(ethylene terephthalate)) or glass. In the dominantphysical process, the polymer absorbs light and an electron isexcited from the valence band to the conduction band. Theresulting electron–hole pair is termed an exciton. If the exciton isperturbed by an interface between the donor and the acceptor inthe composite, the energy can be transferred across the physicalboundary to yield an electron on the acceptor and a hole on thedonor. The cells are hence called ‘bulk heterojunction’ devices, asthere are a multitude of such interfaces throughout the compositelayer, which is often of the order of 100 nm thick. It should be notedthat the exciton needs to meet an interface within around 5–20 nmof its point of formation for the process to work.140 Once formed,the charges percolate towards the electrodes induced throughthe variant work functions of the cathode (often aluminium) andthe anode (e.g. transparent ITO), as shown in Fig. 16.10 – 14,141 – 143

Importantly, the charges need to find the electrodes beforerecombining (a major factor in energy loss). The created potentialdifference in the cell generates a continuous electrical current.

The most widely studied system is that based on P3HT asthe electron donor due to its ease of preparation with predeter-mined molecular weights,145 – 148 the simplicity of its chemicalmodification,149 – 155 its high solubility in organic solvents, itswell-understood electronic behaviour156 and its semi-crystallinitywhich both enhances interfacial interactions with the electronacceptor molecule and facilitates charge transfer through crys-talline domains.157 – 161 The P3HT is generally combined with a

Figure 15. Representation of a typical composite device showing light arriving from below to pass through a transparent electrode, through a smoothingand hole-preferring PEDOT-blend-PSS layer (ca 100 nm thick),144 and then interacting with the composite (ca 100 nm thick), illustrated here with the nowarchetypal mixture of P3HT and 6,6-phenyl C61 butyric acid methyl ester (PCBM), backed by an electron transport interfacial layer (LiF, ca 1.5 nm thick)and the (reflective) cathode.


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C60

C60

C60e-

e-

e-

ITO Al

e-

P3HT

holetransport

electrontransport

photogenerationof charges

hν

Figure 16. Schematic of charge photogeneration in a solar cell.

modified fullerene (C60), the archetypal product being 6,6-phenylC61 butyric acid methyl ester (PCBM),162 in order to increasethe solubility but retain the electronic behaviour. As well as dis-playing excitonic-scale domain formation, the P3HT-blend-PCBMsystem has also been demonstrated to show a positive verticalprofile; that is, a slight tendency of one component to coalescenear the related electrode (i.e. PCBM tends towards the cath-ode) which might reduce effects caused by counter-diodes (andcharge recombination) within the device.163,164 Improvementsbeyond this composite, which has demonstrated efficiencies ofaround 5% after an enormous number of improvments,165,166

have been sought by improving the correlation of the band gapof the polymer with the available solar light (P3HT is particularlylimited in this sense) through the preparation of, for example,the so-called push–pull polymers based on an alternation ofdifferent repeat units of opposing electronic natures,138,167,168

and by modifying the electron acceptor so that its energy levelsare better placed with respect those of the electron-donatingpolymer.169,170 The combination of P3HT-blend-PCBM in a tandemstructure with poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT),while exploiting the electron transfer properties of TiOx layers,permitted efficiencies of around 6.5%.171 Brabec et al.169 give anexcellent synopsis of the development of these polymers, whereasMorana et al.172 give an excellent insight into how the simplemodification of one heteroatom in such structures can developchanges in morphological and hence optoelectronic behaviours.

CONJUGATED ROD–COIL BLOCKCOPOLYMERSGeneral remarks on rod–coil block copolymer self-assemblyIUPAC defines block copolymers by considering that, ‘In theconstituent macromolecules of a block copolymer, adjacentblocks are constitutionally different, i.e. adjacent blocks compriseconstitutional units derived from different species of monomeror from the same species of monomer but with a differentcomposition or sequence distribution of constitutional units.’173

These A and B blocks tend to minimize their contact surfaceby aggregating into domains; however, in contrast to simplemixtures, they cannot separate at a macroscopic scale. They areconstrained to self-organize into domains which are close in size tothe length of each block,174,175 and indeed there are known ratioswhich can account for the variation between the length of thecoil polymer and the width of the domain.176 The transition froma homogeneous whole towards an ordered system into phases,as well as the size and the organization of the phases, can be

explained by taking into account two elements which are theproduct χABN (where N is the total degree of polymerization) andthe size dissymmetry of the two blocks (volume fraction). TheFlory–Huggins parameter (χAB) represents the incompatibilitybetween the two blocks; a positive value of this term indicates arepulsion between the chains, whereas a negative value meansa compatibility between the blocks. The term χABN has a criticalvalue for which the segregation strength is enough to createphase separation between the two blocks. According to themode of segregation, several phases can be obtained, although itshould be noted that such formations are extremely sensitive toenvironmental factors and film thicknesses.177,178 Depending onthe tendency of the blocks to phase segregate, three modes canbe distinguished that are in accordance with the value of χN: aweak aggregation regime (χN ≈ 10), a strong segregation regime(χN > 100) and an intermediate regime. Modelling of copolymerphase diagrams179,180 has been envisaged with the developmentof the general self-consistent field theory (SCFT) by Helfand andTagami.181

Microphase separation has now been researched for quitesome time.39,182 Initiated by the theoretical studies of Meier in1969,183 most work has been into the related non-conjugatedcoil–coil diblock copolymers, and these systems are now generallywell understood. In the 1990s, Matsen and Bates184 proposed atheoretical phase diagram, according to the volume fraction (f ) ofeach component and the product (χN) due to the characteristicsof the block. In the model shown in Fig. 17(a), for a volumefraction equal to 0.5, the critical reduced parameter (χNc) isequal to 10.5; below this value, the system is disorganized. Variousthermodynamically stable microstructures were predicted, namelylamellae, cylinders organized in a hexagonal arrangement, body-centred cubic (QIm3m), close-packed spheres and bicontinuouscubic (gyroid) phase with an Ia3d symmetry (QIa3d), examples ofwhich are shown in Fig. 17(b).

It should be noted, however, that the majority of these systemsdo not account for the high rigidity, the interchain conjugatedorbital interactions and the crystallinity due to the conjugatedblocks. Hence the importance of the earlier work by Semenovand Vasilenko,185 who performed theoretical studies on the bulkbehaviour of rod–coil diblock copolymers in the 1980s. Theytook into account several factors such as the steric interactionsbetween rigid blocks, the stretching of the flexible coil blocks andunfavoured interactions between the coils. They proposed twophases, a nematic phase and a smectic A phase where the rigidparts are directed perpendicular to the lamellar organization of thedomains. In subsequent developments,186,187 they introduced thesmectic C phase, wherein the rigid blocks take up positions parallelto each other, but inclined at an angle to the domain interface. Theyalso established a theoretical phase diagram describing second-order transitions between the nematic phase and the smectic Aphase and between the smectic A and C phases (Fig. 18).

In 1992, Williams and Fredrickson reported non-lamellarstructures of rigid segments organized into finite cylindrical discssurrounded by a corona of flexible coil blocks (Fig. 18(e)).188

These so-called ‘hockey pucks’ were obtained from rod–coilcopolymers with high coil block volume fractions (fcoil > 0.9) andas such considerably furthered the phase diagram for rod–coilblock copolymers. Figure 19 shows the phase diagram that wasdeveloped according to parameters such as the volume fraction ofthe coil block (φ, λ = φ/(1 − φ)), the Flory–Huggins term (χ ) andthe dimensional ratio of the coil and the rod blocks (ν).The micelle


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Figure 17. (a) Theoretical phase diagram for coil–coil diblock copolymer.184 (Reproduced with permission from the ACS.) (b) Schematic of various blockcopolymer morphologies.178 (Reproduced with permission from Elsevier).

(a)(b)

(c) (d)(e)

Figure 18. Self-assembly of copolymers into (a) nematic phase, (b) bilayer smectic A phase, (c) monolayer smectic A phase, (d) monolayer smectic Cphase and (e) ‘hockey pucks’.

structures minimize the stretching constraints of the flexible coilchains.

Williams and Halperin189 reported lamellar, cylindrical andspherical micro-segregated structures in the strong segregationregime (i.e. where the two phases easily separate), and laterMatsen190 studied the influence of the rigidity on the lamellarphases, by modelling the rigid segment with a worm-like chainand by using the SCFT in the weak segregation regime. Thus thevalue of χNc for rod–coil copolymers was reduced to 6.1. Morerecently, Borsali et al.42 revised the value of χNc to equal 8.5.Muller and Schik191 associated these two regimes of segregationby combining the SCFT approach and the Semenov192 theory forstrong segregation. They showed that only morphologies wherethe flexible coil segments are located on the outer (convex) side ofan interface are thermodynamically stable. Moreover, they noticedthat the weak aggregation regime is mainly based on hexagonalphase formation. Further work on the phase diagram madeit possible to highlight more complex morphologies.39,193 – 196

These systems are particularly well discussed, with a profile ofthe underlying mathematical theory, in a paper by Segalman.177

The same author and co-workers also elegantly demonstrated the

control that can be attained over domain sizes of rod–coil blockcopolymers by introducing compatible homopolymers.197

Variations that introduced the concept of mushroom-shapednano-aggregates were proposed by Stupp and co-workers.43 Ofparticular importance in these works, aside from the use ofbiphenyl-based blocks for the rod and PS for the coil, was thevery flexible connector between these two blocks, namely a 1,4-oligoisoprene link, as shown in Fig. 20. This flexibility allowedthe rods to assembly without hindrance from perturbations bythe repellent coil segments. The upshot was the widely splayeddomain of PS coils fixed, via the isoprene units, to an extremelywell-organized domain of rod groups. This is of interest as a resultas it may have particular consequences in a variety of applications,for example photovoltaics wherein the crystallization of the roddomain is of great importance to charge transfer.

General remarks on block copolymer self-assemblyin dispersionsIn solvents that selectively dissolve one block, there is athermodynamic drive towards the formation of self-organizedstructures. This phenomenon of self-assembly in organic solventsand aqueous media can lead to the formation of micelles, vesicles


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Figure 19. Phase diagram for rod–coil diblock copolymers: (I) bilayerlamellae, (II) monolayer lamellae, (III) bilayer ‘hockey pucks’ and (IV) bilayer‘hockey pucks’.188 (Reproduced with permission from the ACS).

and a range of other structures, and has been well describedelsewhere, especially for use in drug delivery systems.198 – 203

The process of self-assembly in dispersions occurs when theentropy that enables the solvation of the blocks (disorder) cannotcounterbalance the energetic cost of the repulsive contactsbetween the insoluble blocks and the solvent. Thus, the insolubleblocks tend to aggregate thus minimizing repulsive contact withthe solvent molecules. Generally, micellization is induced by thepresence of two opposing forces: an attractive interaction betweenthe insoluble blocks leading to aggregation and a repulsiveinteraction between soluble blocks preventing unlimited micellegrowth. The thermodynamic equilibrium is based on: interactionsbetween the chains making up the micelle core; surface tensionsat the interface between the core and the solvent; and interactionsbetween the chains forming the corona. According to the chemicalnature and size of the blocks, and the concentration (temperatureand pH) of the solution, various morphologies can be obtained,going from filled to hollow spheres, via worm-like cylinders andvesicles (Fig. 21).204

Micelle systems are usually defined using the followingparameters:201

• critical micelle concentration and temperature (CMC and CMT,respectively) at which concentration and temperature thecopolymers aggregate;

• morphology;

(a) (b) (c)

Figure 21. Schematic of (a) a spherical micelle (b) a cylinder and (c) avesicle.204 (Reproduced with permission from Elsevier).

• size;• the molecular weight of the micelle (Mm) and the aggregation

or association number (Z) which corresponds to the averagenumber of polymer chains in the micelle, with Z = Mm/Mu, Mu

being the molecular weight of copolymer (or unimer);• the radius of gyration (Rg) and the total hydrodynamic radius

(RH) of the micelle, with the ratio Rg/RH giving information onthe morphology of the micelle; and

• the micelle core radius (Rc) and the thickness of the shell(corona) formed by the soluble blocks.

The strong tendency of conjugated blocks to self-assemble,often due to the interactions between the labile conjugationorbitals, further drives the aggregation process. The review byOlsen and Segalman gives a clear appraisal of this process andhow the phase diagrams of non-conjugated block copolymers canbe modified by this behaviour.205 A wider range of aggregatesbecome available, such as nanowires,206 corkscrews and thelike.207 – 210

Synthesis and self-assemblyThe literature dedicated to the synthesis of partially or entirelyconjugated copolymers is undergoing a prolonged expansion.Hoeben et al.211 gave an excellent general review on thesupramolecular assembly of π -conjugated systems, and Leeet al.212 also described in clear detail the development ofsupramolecular structures from rod–coil block copolymers inwhich one segment is a conjugated block. Naturally, given theextreme rate of development, more recent reviews have profiledconjugated block copolymers with respect to a specific applicationor set of properties, for example the lucid work by Segalman et al.on optoelectronics213 or the excellent work by Darling and otherson photovoltaics.214 – 218 Other reviews are cited in the introductionof this text. In the remainder of this review we will concentrateon some archetypal macromolecules so as to try and illustratethe course of development over the last ten years or so and

Figure 20. Chemical structure of the triblock copolymer (top) and representation of the non-centrosymmetric mushroom-shaped nanostructure (bottom)consisting of 100 triblock molecules.43 (Image reproduced with permission from the AAAS).


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therefore try to give a more general history of the properties ofthese materials.

One particular aspect of rod–coil block copolymers is thehigh degree of immiscibility between rod and coil segments,which permits domain formation even with relatively low molarmasses,219 in contrast to most coil–coil systems. However,in general, due to the lower solubilities of the rod blocks,the synthetic procedures are slightly more cumbersome thanthose for coil–coil systems. Generally, the rod block is firstprepared and then coupled, through condensation reactions withappropriately modified chain-ends, to the coil segment; however,numerous and more elegant examples have arisen where rodmacro-initiators are used to support the polymerization of coilblocks.

Copolymers based on PPPsFrancois and co-workers44,46,96 synthesized PS-block-PPP viaan anionic PS macro-initiation of a cyclohexadiene and asubsequent dehydrogenation step. This albeit complex andtime-consuming method (the polymerization took upward oftwo weeks) gave rise to copolymers that enabled the discoveryof an unusual macroscopic honeycomb morphology using whatis now commonly termed the breath-figure technique in whichsolutions of the polymers in CS2 are dried under wet atmospheres.These morphologies, with holes ranging from several hundrednanometres to several micrometres, opened research areas in cellculture,220,221 optoelectronics222 and printing masks.223 They arethought to result from micelle formation, as shown in Fig. 22. It wasassumed that the cavity pattern originates from the presence ofwater droplets, which at one stage form a two-dimensional uniformarray.224 Water condensation into small droplets simultaneous tothe cooling of the thin film during solvent evaporation, andthe presence of Rayleigh–Benard, Marangoni225 and Benard226

surface instabilities, enables the formation of the honeycombtemplate. On warming to ambient temperature, the expansionand subsequent evaporation of the encapsulated water dropletsblisters the top layer of the polymer film, and these holes can thenbe displaced through the use of shrinkable substrates.227

The review by Leclere et al. gives a detailed account of howthe PPP repeat unit was further developed by the additionof solubilizing groups, and how its optoelectronic propertieswere altered via interpolymer interactions once within blockcopolymers.228 It should be noted that it is the strong tendency ofPPP to crystallize that often drives and determines to a considerableextent the morphologies of the structures encountered, andthat this is common to most rod–coil copolymers containingconjugated segments.

Scheme 1. Synthesis of PPP-block-PS copolymers (THF,tetrahydrofuran).229

As an example, Lazzaroni and co-workers229 investigatedthe microscopic organization in thin films of block copolymersconstituted of an oligo(2,5-diheptyl-p-phenylene) rigid segmentand an oligostyrene sequence. Their synthetic approach towardsthis rod–coil copolymer was based on the condensation of aliving PS chain-end with a benzaldehyde end-functionalized PPPblock that had been prepared by Suzuki coupling (Scheme 1).Then, they studied the deposition of a solution of copolymerin toluene (0.1 mg mL−1) on a mica substrate and observedfibrillar morphologies using AFM (Fig. 23). Similarly to the previousexample, the block copolymers intrinsically assemble into ribbon-like structures, as a result of microphase separation and π -stackingof the rigid conjugated backbones.

The synthetic benefits (the relatively short polymerization times,high control over molecular weight, simplicity of methodology)that have been brought about by the development of chain-growth condensation polymerization techniques230 were laterextended to the preparation of PPP derivatives (Scheme 2). Whileit was found that LiCl was necessary to ensure low dispersity andcontrol, this was an extraordinary step in the development of PPPchemistry.231,232 It would be expected that this chemistry wouldfacilitate the formation of rod–coil block copolymers incorporatingPPP; however, most reports concerning this technique discuss PTsrather than PPP, probably due to the latter’s high intractability,and this review will therefore turn to PT after looking at what manyconsider to be its successful forerunner, PPV.

Copolymers based on PPVsA major driving force in the development of rod–coil copolymersbased on PPV was the need for new architectures for photovoltaicapplications. As mentioned much earlier, the excited electronic

(a)(b) (c)

Polystyrene (PS)

Poly(p-phenylene)(PPP)

aggregate ofPS-PPP b-copolymer

or

Starlikepolystyrene

Figure 22. Microporous films prepared by evaporation of CS2 solutions of PS-block-PPP: (a) schematic cross-section; and images obtained using(b) scanning electron microscopy and (c) atomic force microscopy.46 (Images republished with permission from Nature).


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Figure 23. AFM phase image (vertical side is 3 µm) of a PPP-block-PS thinfilm deposited on mica.229 (Image reproduced with permission from Wiley).

Scheme 2. Route to PPP via chain-growth condensation polymerization(dppe, 1,2-bis(diphenylphosphino)ethane).230,231.

state (termed exciton73) for a typical polymer can exist over adistance of around 5–20 nm.140,233 In order for this hole–electronpair to be completely separated, the exciton needs to meet aninterface between the polymer on which it resides and another ma-terial with advantageously placed LUMO and HOMO. This distance

coincides extremely well with the typical size of block copoly-mer domains.214 – 218 Given that the domain width can often betailored simply by varying the length of the polymer (assumingthat the polymer simply crosses from one side of the domainto the other and is perpendicular to the interfaces), it shouldbe a relatively simple business to pair together two polymersthat act as mutual donor and acceptor of electrons. Note the‘should’, as it turns out that polymers, in general, do not preferelectron transport, but rather hole transport, for reasons whichremain not altogether understood.6,72 Added to which there isthe supposition that the two blocks can be covalently linked, butnot conjugated, as otherwise the resulting band bending couldlead to loss or recombination of the charges. This means that,in the main, chemists have preferred to prepare block copoly-mers made from a p-type block, such as PPV or its more solublederivatives, and for the n-type component they have been con-strained to preparing polymers that carry electron acceptors aspendent, grafted groups attached to commodity polymers suchas PS. One of these rod–coil copolymers at the centre of a consid-erable amount of work was based on an alkoxy-substituted PPV(MEH-PPV) coupled to a fullerene-substituted PS (Scheme 3).234,235

A short alkoxy-substituted PPV (about seven repeat units) withmodified chain-ends was used as macro-initiator of controlledactivation–deactivation radical polymerizations leading to copoly-mers of PS and poly(4-chloromethylstyrene) with predeterminedmolecular weights. The chloromethyl centres were then addedto C60 through atom transfer radical addition reactions to theC60, although the content of C60 had to be limited in order toavoid crosslinking reactions through multiple additions to theC60 sphere. While the system has a limited absorbance with re-spect the solar spectrum due to the nature of the conjugatedblock, it exhibits excellent phase separation in solid-state films.An extension of this system was enabled by the preparation ofan alkyloxylated PPV with an alkoxyamine chain-end appropriateto the polymerization of acrylates.236 A more reproducible routewas found to be via the addition of the alkoxyamine to the alky-loxylated PPV using Grignard chemistry.237 This again permitteda range of coil–commodity polymers to be prepared from the di-alkyloxylated PPV macro-initiator, again including those carryingchlorostyrene groups. In an attempt to reduce crosslinking reac-tions with C60, the chloromethyl groups were converted to azido

Scheme 3. Preparation of a rod–coil block copolymer incorporating a rigid dioctyloxy PPV (3) segment and a C60-decorated PS.234,235 (Reproduced withpermission from Elsevier).


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groups prior to addition to C60. This again resulted in rod–coilblock copolymers carrying pendent C60 moieties on the coil seg-ment. A detailed characterization of the effect of C60 graftingdensity on the PS on the material’s electronic properties was madeand it was found that there is an increase in electron charge mo-bilities with density (up to the 60% grafting performed), althoughthe mobilities remain way below those of PCBM.238 This remains achallenge to overcome in the future developments of this sort ofgraft copolymer.

The morphology of the rod–coil block copolymers based onPS and dialkyloxylated PPV, this time without the presence ofC60, was further investigated and it was determined that theπ -stacking of the PPV results in thermodynamically stable lamellaclustering of the conjugated blocks within matrices of PS.239 Thisresult is an important one as it confirmed that, in many cases,rod–coil block copolymers containing conjugated segments willtend towards taking up lamella-based morphologies. However,this result should be tempered by the knowledge given by thework of Stalmach et al.236 in which the solid-state phase diagramfor poly(2,5-diethylhexyloxy-p-phenylenevinylene)-block-poly(4-vinylpyridine) (PPV-block-P4VP) was studied. It was foundthat, although the lamellar phase does dominate over a widerange of proportions of PPV to P4VP blocks as expected due tothe liquid-crystalline nature of the rod blocks, hexagonal andspherical microphase-separated morphologies are attainable athigh volume ratios of P4VP as it is the force towards macrophaseseparation that dominates (Fig. 24).

Returning to rod–coil copolymers where the coil is modifiedwith C60, the solid-state morphology of an alkyloxylated PPV rodtied to a coil polymer (based on a mix of azido-styrene units andbutyl acrylate groups) was characterized with and without thepresence of C60.241 The structure is shown in Fig. 25. The clearupshot of this study was that the fullerene, even though bondedto the flexible coil chains, still manages to crystallize and determinequite extensively the behaviour of the system. A range of solutionsto circumvent the problems encountered were suggested, one ofthem including the use of more crystalline PTs rather than PPV.PTs will be considered in the next section.

As a comparison to block copolymer-based systems, it is worthmentioning an original system in which two polymers were simplymixed and used as a composite in a bulk heterojunction device.The two, separate, polymers were a dialkyloxy-substituted PPV

Figure 25. Structure of PPV-based rod–coil copolymer with C60 attachedto the coil via a tertiary amine bridge.241.

(MEH-PPV) and an electron acceptor based on a cyano-substitutedPPV.242 – 244 The efficiencies obtained were extremely high for thattime, at ca 2%. The reason for mentioning this is that this value ishigher than that obtained for most current block copolymer-basedsystems. Block copolymers have not attained high efficienciesperhaps for reasons of processing and optimization, especiallyin terms of correlating the absorption spectrum to that of thesolar spectrum, realizing effective interfacial structures betweenthe block copolymer and the electrodes, and perfecting the size,position and type of insulating, covalent link between the twoblocks that will optimize charge transfer through space betweenthe blocks.218 This in effect sums up some of the challenges facedin the development of effective routes to efficient pOSCs basedon block copolymers.

Aside from systems directed at photovoltaics, there has beena wide variety of rod–coil block copolymers incorporating PPVand its derivatives that have been made over the years. Forexample, Yu and colleagues reported coupling of PI,51 PEO245

and poly(propylene oxide)52 (PPO) (Fig. 26(a)) with alkylated PPV.Various volume fractions of each block were studied. Lamellarmorphologies with bilayer lamellar phases were confirmed usingtransmission electron microscopy and small-angle X-ray scattering.

Figure 24. Phase diagram developed for PPV-block-P4VP.240 (Reproduced with permission from the ACS).


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Figure 26. (a) Chemical structures of PPV-block-PI, PPV-block-PEO and PPV-block-PPO diblock copolymers; (b) schematic representation of the self-assembly of block copolymers in aqueous solutions; and (c) fluorescence micrograph of PPV6-block-PPO70 in solution.52 (Reproduced with permissionfrom Wiley).

Figure 27. (a) SEM image of PPV-block-PI41-HMW thin film (after annealing at 160 ◦C) where lamellae are oriented both parallel to the substrate(featureless regions) and out of the plane of the substrate (regions of alternating light and dark stripes); (b) schematic of parallel and out-of-planeorientations.248 (Reproduced with permission from the ACS).

Small-angle neutron scattering characterizations revealed thestrong tendency of these copolymers to self-assemble, inTHF/water dispersions, into long cylindrical micelles with lengthsgreater than 1 µm and oligophenylenevinylene core diametersof ca 8–10 nm. Moreover, increases in water content led to anincrease in these cylinder diameters until they preferred to stackin parallel compact structures as shown in Fig. 26(b). Fluorescencemicroscopy highlighted these fibrous structures aggregating athigh water contents (14%; Fig. 26(c)). PPV-block-PPO copolymerscontaining more than around 13 PPV repeat units presentedreversible liquid-crystal phase transitions confirming their strongpropensity to self-organize.

Segalman and co-workers investigated the synthesis and theself-assembly of PPV-block-PI copolymers to study the effect of theweak phase segregation on the resulting morphologies.246 – 248

The PI block preferentially wetted both the silicon and the airinterface in thin films. Analyses using SEM and grazing-incidencesmall-angle X-ray scattering revealed that thermally annealedfilms self-assembled into lamellar microphases with both paralleland perpendicular orientations relative to the substrate (Fig. 27).

Incommensurability between the film thickness and the naturalperiod of the block copolymer resulted in the formation of two-level island or hole structures in the films, and the transitionbetween islands and holes and the surrounding regions wasmediated by defect nanodomains with perpendicular orientations.

More recently, in a thorough introduction, Ho et al. explain wellthe various competing forces between rod–rod and rod–coil inter-actions, and go on to investigate the boundary between hexagonaland lamellar packing of the PPV and PMMA domains of poly[2,5-di-(2′-ethylhexyloxy)-1,4-phenylenevinylene]-block-PMMA. Whilethey find that the morphologies are strongly tied to the volumeratios of the PPV and PMMA blocks, there is a rod–rod interactionwhich remains unperturbed by the coil blocks due to the strongmicrophase separation.249

Copolymers based on PTsWhile PPV has somewhat fallen to the wayside, there is stillmuch research being performed with PTs, specifically P3HT. Asmentioned above, P3HT was the first conjugated polymer tobe prepared using the extremely effective and ‘controllable’


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Scheme 4. Mechanism for the chain-growth condensation polymerization of a Grignard metallated thiophene, in which the Ni ‘walks’ intramolecularlywith the addition of thiophenes (1).250 (Reproduced with permission from Wiley).

Scheme 5. Synthesis of polyurethane elastomers and triblock copolymers via the Vilsmeier–Hack formylation of P3HT chain-ends.251 (Reproduced withpermission from Wiley).

chain-growth condensation polymerization, the mechanism ofwhich is shown in Scheme 4.250 The upshot of this reaction isthat chain-ends can be easily introduced by the selective use ofGrignard reagents that terminate the active thiophene–Ni(0) –Brsite.149 – 155,251 Once this has been accomplished it is relativelyeasy to react these chain-ends with a variety of other polymersor to use them as centres with which to perform other typesof polymerization. Given that PPV cannot be prepared in asimilar manner, and that P3HT exhibits better processibilitiesand stabilities (as mentioned above), P3HT has become thecurrent standard-bearer for a wide range of studies. However,PTs have a certain history of development, so before looking atthe formation of block copolymers derived from this chemistry,we will look at a few examples of systems prepared withother methods and of interest for the resulting morphologicalstudies.

McCullough and colleagues studied the self-assembly of P3HT-based block copolymers that had PS, poly(methyl acrylate) (PMA)

or polyurethane as the coil block that had been preparedvia zinc intermediates at low temperatures.251 The subsequentmodification of these P3HTs via the Vilsmeier–Hack formylationreaction shown in Scheme 5 made possible the preparation ofP3HT-based atom transfer radical polymerization (ATRP) macro-initiators, hence leading to the block copolymers shown. Nanowiremorphologies were obtained by slow evaporation of diblock andtriblock copolymer solutions in toluene. Although there was a dropin conductivity commensurate with decreasing weight percent ofP3HT in each material, after exposure to I2 vapour, thin filmsshowed relatively high conductivities (e.g. 0.43 S cm−1 for 26%(w/w) P3HT in a PS-block-P3HT-block-PS copolymer) in respect ofthe proportion of P3HT present.

Not strictly speaking a polymeric conjugate, but elegant inits oligomeric precision, Hempenius et al.252 reported a synthesisof well-defined PS-block-oligothiophene-block-PS coil–rod–coilcopolymers. The syntheses of these triblock copolymers werecarried out employing an α-coupling to yield 11 conjugated


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NaCN, DMF

POCl3

Lawesson's reagent

CH2 CH CH2 CH

OH

SR

SR

SR

S

S

S

SCH2 BrH

SCO(CH2)2OC

SCHO

S S S

S

SR

S

CH2 O PS

S S

S

S

ON

S

S

SCO(CH2)2OC

S

SR

S

S

ON

S SR

103

10R =

sec-Bu

30NaH, DMF,THF

N-methylformanilide+

Scheme 6. Synthesis of PS-block-oligothiophene-block-PS copolymers.252.

Figure 28. (a) Chemical structures of chiral (top) and achiral (bottom) oligomers. AFM images of (b) chiral and (c) achiral oligomer deposited on siliconsubstrate (Si/SiOx) from toluene solutions. AFM images of chiral oligomer (d) on graphite and (e) on mica substrate. The scale bar represents 1.0 µm.255

(Reproduced with permission from the ACS).

thiophene rings in the middle block and a low dispersity(-D = Mw/Mn = 1.1) of the PS30 outer blocks. Scheme 6 showshow the 4-hydroxyphenyl-functionalized PS was first modifiedwith an α-bromine terthiophene unit, and subsequently coupledin a double Stetter reaction with a difunctional α-terthiophene toyield a tetraketone, which was then used as the precursor of thetriblock copolymer. AFM revealed the formation of non-sphericalmicelles with egg-like shapes of about 10 to 14 nm in diametercorresponding to aggregates of around 60 block copolymers.The solvatochromic behaviour of PS-block-oligothiophene-block-PS was evaluated in a mixture of o-dichlorobenzene and methanol.In contrast to most PTs which undergo red shifts in similarsituations due to increasing interchain interactions, this PS-block-oligothiophene-block-PS underwent a blue shift in λmax onaddition of methanol (a non-solvent of the system). This wasnoted to be in agreement with the molecular exciton modeldue to the parallel orientation of oligothiophene segments (H-aggregates72).

The synthesis and the self-assembly of diblock or triblockcopolymers constituted of oligothiophenes and PEOs were

studied by Schenning and co-workers.253 – 258 They achievedthe preparation of chiral triblock copolymers as shown inFig. 28(a). On silicon surfaces, the chiral oligomers led to left-handed helical fibre-like structures, in contrast to their achiralanalogues which formed non-chiral fibrils (Figs 28(b) and (c)).Similar coatings of these materials were also realized on apolargraphite surfaces and on a strongly polar surface, i.e. muscovitemica.253 – 258 On graphite substrates the chiral oligomers tend toorganize as fibrils while aligning along a three-fold symmetry(Fig. 28(d)), whereas on mica, the oligomers form monolayers. Inthe latter case, the molecules pack with the conjugated segmentalmost perpendicular to the substrate, since the length of thefully extended oligothiophene is consistent with the measuredthickness (Fig. 28(e)). Furthermore, these chiral molecules wereself-assembled in 2-propanol into spherical nanocapsules.259 Dueto their sexithiophene sequences, the objects exhibited a particularsensitivity to magnetic fields, deforming into oblate spheroids ina reversible process (Fig. 29).

The chain-growth condensation polymerization techniqueshown in Scheme 4 is of considerable importance for the


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Figure 29. SEM images of (a) spherical (no magnetic field) and (b) deformed (B = 20 T) sexithiophene nanocapsules in an organogel; (c) schematic of themagnetically deformed nanocapsules.259 (Images reproduced with permission from the APS).

Scheme 7. Synthetic route to an ATRP macro-initiator based on P3HT and the subsequent formation of P3HT-block-PMA.260 (Reproduced with permissionfrom Elsevier).

preparation of rod–coil block copolymers as it can be used,simply by varying the amount of Ni to monomer, to predeterminethe length of the rod block. As the width of the domains isdirectly related to the length of the rod polymers, the reactionfacilitates the engineering of the size of the domains. And again, asmentioned above, the chemistry is such that the polymerizationcan be terminated with Grignard reagents that carry specificgroups, and therefore block copolymer chemistry is facilitated.This technique has been widely used. Here we give just a fewnotable examples.

Scheme 7 shows the route to P3HT-block-PMA rod–coilcopolymers.260,261 The molecular weights of the P3HT were main-tained constant, while those of the PMA were varied, this beingfacilitated by the chemistry used. In addition, the chemistry en-abled low dispersities (-D < 1.3). The P3HT was about 45 unitslong, while the PMA varied facilely up to ca 57 mol%.

The following examples all look to exploit the domain formationof rod–coil block copolymers for solar cells. As mentioned above,given that the domains can be engineered to be of a length nearto that of the mean pathway of excitons, and that they can alsochannel charges to electrodes, this is seen as a realistic route toefficient devices.

Dante et al. also exploited the chain-end modulability ofP3HT obtained via chain-growth condensation polymerizationby preparing P3HT with groups appropriate to reversible addition-fragmentation chain transfer (RAFT) chemistry to subsequentlyyield the structure shown in Fig. 30(a).262 The authors indicateda fibrillar structure from AFM characterizations and localizedconductivity measurements (using C-AFM) confirmed that on themodulated surface the lighter areas shown in Fig. 30(b) were dueto P3HT and the darker ones to PS and C60.

An extremely interesting use of a commercially available poly(3-hexylthiophene)-block-poly(L-lactide) (P3HT-block-PLL) copoly-mer was recently presented by Botiz and Darling.263 As shownin Fig. 31, they used the P3HT-block-PLL as a template by degrad-ing the PLL block once the nanometre-scale domains had beenformed. This left a regular structure of P3HT that could then befilled with modified C60. While some measure of control over thesystem is still required, for example in solving problems of collaps-ing P3HT walls, it is such an extraordinarily elegant method that itsurely warrants further work.

Another system looking towards photovoltaic applications wasrecently reported by Sary et al.264 Once again the high degreeof phase separation of the P3HT rod from a coil, this time P4VP,


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Figure 30. (a) The structure of a P3HT-block-PS rod–coil copolymer carrying C60. (b) 500 × 500 × 3 nm topographic AFM image of a spin-coated film ofthe same copolymer.262 (Reproduced with permission from Wiley).

Figure 31. Preparation of a template and its filling to give adonor–acceptor structure designed for photovoltaic applications.263 (Re-produced with permission from the ACS).

was exploited, along with the control over molecular weights, toprepare thin films with domains appropriate to exciton captureand charge transfer. The copolymer was loaded with PCBMwhich tended to enter the P4VP domains due to supramolecularinteractions (Fig. 32(a)) without perturbing the block copolymerself-assembly. However, on preparing a photovoltaic device, it wasfound that the efficiency was extremely low, around 0.03%. TheP4VP tended to preferentially wet the PEDOT-blend-PSS surfacethus perturbing the vertical profile of the device. The problemwas rectified by using an inverse structure (Fig. 32(b)), in which

TiOx was used as the bottom surface and an efficiency of around1.2% was obtained. This again demonstrates the sensitivity of theself-assembly of block copolymers to their casting conditions andenvironment.

P3HT was also used as a well-defined macro-initiating rod inrod–coil block copolymers incorporating poly(perylene bisimideacrylate)265 – 268 electron acceptors (Fig. 33). These systems wereprepared with chain lengths appropriate for domain formationsuitable for exciton capture. Annealing permitted an increase indevice efficiency to ca 0.5%, due to better ordering and microphaseseparation.267 Importantly, it was found that the block copolymerscould act as compatibilizers in a blend of P3HT and perylenebisimide used for an OSC active layer. The effect on OSC deviceefficiency was such that an increase in efficiency of around 50%was attained by limiting excessive crystallization of perylene.268

Comparable systems have been prepared, again using P3HT as amacro-initiator, via either ATRP269 or RAFT270 routes, to preparerod–coil copolymers with groups that could be post-modifiedto carry PCBM-like C60 electron acceptor moieties. Again asactive layers in OSCs, it was found that the block copolymers,engineered to form domains near to the scale required for excitoncapture, i.e. around 10 nm, demonstrated better quenching ofP3HT photoluminescence than in simple blends of P3HT andPCBM.269 And similar to the work performed on perylene-carryingpolymers, it was also found that rod–coil block copolymers basedon P3HT and a flexible block carrying PCBM-like C60 moietiescould induce a compatibilizing effect in blends of P3HT andPCBM that resulted in an approximately 35% increase in deviceefficiency.270 This latter system showed an excellent fibrillarnetwork structure.

Finally, we used the chain-end chemistry of P3HT to preparethe multiblock rod–coil copolymers shown in Scheme 8. Conden-sation reactions were performed coupling the P3HT to a novelpolymer that incorporates fullerene as a repeat unit. The dimen-sions of the resulting domains (repeating at around 20 nm) agreedwith what was expected, given that the length of the P3HT couldbe predetermined and the C60 coiled segment contained aroundnine C60 repeat units.271


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Figure 32. (a) Structure of PCBM-carrying P3HT-block-P4VP and (b) the inverted photovoltaic structure into which it was placed.264 (Reproduced withpermission from Wiley).

NiBr (dppp)BrMg O

Oi)

ii) HCl, THFiii) methanol

O

O

BrBr+

CuBr, pyridine

toluene, 115 °C

K2CO3, toluene,18-crown-6,

85 °C

O

O

Br

m

O

O Br

S

C6H13

O O

n

O

O

Br

m

H

p

O

O

S

C6H13

OH

n

HOS

C6H13

n

Br

Scheme 8. Synthetic route to the multiblock rod–coil poly{(1,4-fullerene)-alt-[1,4-dimethylene-2,5-bis(cyclohexylmethyl ether)phenylene]}-block-poly(3-hexylthiophene) (PFDP-block-P3HT).271

CONCLUSIONSDue to their optical, semiconducting and even magnetic proper-ties, it is apparent that rod–coil copolymers containing conjugatedsegments will lend themselves to numerous applications. Thesimplest reason for this is that the block copolymers are of-ten easier to handle and process than the pure conjugatedmaterials for reasons of solubility, melt behaviour and mechanicalstrength.

Many of their more complex behaviours, e.g. optoelectronicbehaviour, depend not only on macromolecular engineering, butalso on the way the polymers aggregate and align themselves inthree dimensions. With the advent of new synthetic routes, mostnotably chain-growth condensation polymerization, it is apparent

that considerable control over the size and distribution of molarmasses in a sample of conjugated polymers can been obtained.This is not to say that there is little left to do – for example, it will beinteresting to see what happens when polymers with dispersitiesequal to 1.0 are reached as they may have different properties withrespect interdomain boundaries. And it is apparent that there areconsiderable areas where understanding is still required, notablyin being able to predetermine how rod–coil block copolymerswill self-assemble under varying conditions (change in solvent,temperature, substrate, etc.), and in determining the relationsbetween the various forces that guide this self-assembly. All thismeans that the field of rod–coil block copolymers will be a veryrich one for many years to come.


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Figure 33. Structure of a rod–coil block copolymer based on P3HT andperylene.267.

ACKNOWLEDGEMENTEmmanuel Ibarboure is gratefully acknowledged for preparingthe image in the graphical abstract which shows the AFM phasetapping-mode characterization of a surface of PFDP-block-P3HTover an area of 500 × 500 nm2.

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conjugated rod-coil block copolymers and optoelectronic applications

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