Conjugated polymer-based photonic nanostructures

Polymer Chemistry REVIEW Conjugated polymer-based photonic nanostructures Cite this: Polym. Chem., 2013, 4, 5181

Deirdre M. O'Carroll,*ab Christopher E. Petoukhoff,a Jesse Kohl,a Binxing Yu,b Catrice M. Cartera and Sarah Goodmanb Conjugated polymer materials are at the forefront of many next-generation organic optoelectronic technologies including organic light-emitting diodes, photovoltaics and lasers. The photophysical properties of these materials can be controlled and optimized through the formation of nanoscaleconfined geometries such as nanoparticles, aggregates, nanofibers, or thin films. In this review, we discuss the photonic characteristics of conjugated polymer-based nanostructured materials and devices with a focus on how excitons and photons can be manipulated and managed though confinement of polymer chains and through interactions with inorganic nanostructures. We include case studies from

Received 7th February 2013 Accepted 11th July 2013

the literature on how internal molecular morphology can be controlled in conjugated polymer thin-film optoelectronics, nanowires and nanofibers and, in turn, how internal morphology affects the photonic properties of these structures. Extrinsic approaches to controlling or modifying the photonic properties

DOI: 10.1039/c3py00198a

of conjugated polymer materials and devices through the addition of inorganic photonic

www.rsc.org/polymers

nanostructures are also discussed.

1

Introduction

Interactions between organic conjugated (i.e., semiconducting) monomers in solids typically induce strong excitation quenching effects due to the formation of non-emissive dimer, trimer and aggregate complexes.1 However, the addition of side chains and end-capping units to organic molecules can reduce intermolecular interactions and, hence, minimize the formation of inefficient excited-state species.2–7 Addition of side chains can also render the molecules soluble in various organic solvents, and can result in altered molecular properties such as liquid crystallinity, modied charge carrier mobility or enhanced luminescence quantum efficiency.2,5–10 Polymerization of organic monomers to form conjugated oligomers or polymers can stabilize their photonic properties and allow excitations to be extended or to migrate along a single oligomer/polymer chain.11–14 As a result, the photonic properties of these extended molecular systems are strongly dependent on chain conformation and alignment, enabling the characteristics of optical transitions to be tailored through molecular processing conditions such as thermal or vapor annealing, physical chain alignment or aggregation, or through nanostructure formation.3,9,15–17 In the rst part of this paper we will focus on some of the fundamental photophysical characteristics of conjugated polymer materials such as exciton formation, migration, transition a

Department of Materials Science and Engineering, Rutgers University, 607 Taylor Rd., Piscataway, NJ 08840, USA. E-mail: [email protected]

b

Department of Chemistry and Chemical Biology, Rutgers University, 610 Taylor Rd., Piscataway, NJ 08840, USA

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dipole-orientation, and exciton dissociation. In the second part of the paper we will discuss a variety of case studies that help to elucidate the photonic and optoelectronic properties of nanostructured assemblies of conjugated polymers such as thinlms, nanowires, nanoparticles, aggregates, crystallites and lamellae. The relationships between processing conditions, internal molecular morphology and photonic functionality in such conjugated polymer-based nanostructures serve to highlight the appeal of conjugated polymer materials for use in photonic and optoelectronic devices. Strategies to modify the photonic properties of conjugated polymers extrinsically by coupling to inorganic or metallic nanostructures, which can modify the local photonic environment of the polymer, will be presented.

2 Photonic properties of conjugated polymers Conjugated polymers are organic molecules in which p-molecular orbitals are delocalized along a backbone which consists of a series of covalently-linked conjugated monomeric repeat units (typically >20; monomer units range from 0.2 to 2 nm in length; see Fig. 1).17–20 The use of such polymers as active materials within electronic and photonic devices has been largely motivated by the extensive tunability of their physical and optical properties by tailoring substituent groups (e.g., side chain length, degree of copolymerization, endcapping),1,21,22 as well as their excellent lm forming properties and their ease of application over large surfaces through relatively low-temperature, economically-viable coating techniques such as spin coating, spray coating, ink-jet printing and

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Polymer Chemistry

Review 2.2

Fig. 1 Molecular structures of some widely-studied conjugated polymer materials: (a) poly(p-phenylene) (PPP) (b) poly(thiophene) (PT), (c) poly(p-phenylenevinylene) (PPV), (d) poly(9,9-dioctylfluorene) (PFO); (e) poly(9,9-dioctylfluoreneco-bithiophene) (F8T2).

roll-to-roll processing.23–26 The addition of suitable side chains to the polymer backbone renders most conjugated polymers soluble in common organic solvents (Fig. 1d and e). Due to the chain-like structure of polymer molecules and the strongly bound nature of electron–hole pairs (i.e., excitons) in conjugated systems, both optical and electrical dipole orientations are oen preferentially polarized along the molecular chain axis.27–29

2.1

Excitons in conjugated polymers

Functionalized conjugated polymers, like many molecular systems, possess strongly-bound Frenkel-like excitons (i.e., electron–hole pairs bound by a strong Coulomb attraction).30 Exciton binding energies for most conjugated organic materials are in the range 0.1–1 eV (one to two order of magnitude greater than most inorganic semiconductors), which enables observation of excitonic transitions at room temperature.30 Due to the physical anisotropy of conjugated polymer molecules when extended, individual molecular sub-units or segments can act like molecular wires with excitons polarized along the polymer chain axis.12 At somewhat larger size scales (2–100 nm), nanostructures of conjugated polymer materials can be fabricated that possess photonic properties that are determined by collective effects arising from organization and orientation of an ensemble of molecules. Quantum connement effects, i.e., size dependent changes in photonic properties, typically associated with inorganic nanostructures that reach a size scale below the Bohr radius of the semiconductor (in the range 2–5 nm for many inorganic semiconductors), are not readily observed in nanostructured conjugated polymer materials since Frenkel-like excitons are already intrinsically quantum conned to the molecular structure.30,31

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Absorption and emission characteristics

In thin lm geometries, conjugated polymers can exhibit large absorption coefficients with values of up to 2.8
105 cm1 for polyuorene derivatives, which are attributed to the promotion of delocalized (conjugated) p-orbital electrons in the ground state (S0) to an excited p* state (S1).32 The polymorphism of many semicrystalline conjugated polymers provides a unique opportunity to study the inuence of phase morphology on the photophysics of these materials without further chemical modication. Various phases of the polymers (e.g., semicrystalline, nematic liquid crystalline, mesomorphic) can exhibit distinct absorption spectra due to changes in the polymer molecular chain conformation and conjugation length.33–42 For example, different phases of poly(9,9-dioctyluorene) (PFO) show distinct red-shis in the absorption edge wavelength arising from additional low-energy absorption shoulders or lowenergy peaks that occur due to transition from one of the more disordered chain conformations (e.g., glassy-phase or N-phase) to one of the more ordered, extended chain conformations (e.g., a-phase, a0 -phase or b-phase).33–46 Red-shis and sharpening of emission peaks are also commonly observed when conjugated polymers undergo a phase transition from a higher energy, disordered chain conformation to a lower energy, more wellordered chain conformation with extended conjugated length.33,34 Therefore, phases with greater degrees in intrachain or interchain order tend to exhibit the most red-shied absorption and emission spectra for a given conjugated polymer material. The photoluminescence (PL) quantum efficiency of conjugated polymer materials can vary substantially from values of just a few percent for polythiophene homopolymers to values approaching 80% for polyuorenes and polyparaphenylenes.33–36 The PL quantum efficiency can be related to the degree of intermolecular interactions in conjugated polymer solutions or thin lms – intermolecular interactions oen lead to bimolecular emitting species, such as excimers or exciplexes, which typically coincide with lower intramolecular singlet exciton yields.37,38 Electroluminescence (EL) quantum efficiency is typically lower than that of PL quantum yield for conjugated polymers due, in part, to spin statistics – following electrical injection of electrons and holes, three triplet excitons are expected to form for every one singlet exciton.47,48 However, it has been shown that EL quantum efficiency can exceed 25% of that of the PL quantum efficiency for certain conjugated polymers due to a larger formation cross-sections for singlet excitons compared to triplet excitons.48 Incorporation of heavy atoms (e.g. Pt) to the conjugated polymer backbone has enabled radiative phosphorescent emission from triplet excitons at room temperature, enabling EL quantum efficiencies signicantly greater than 50% to be attained in polymer light-emitting diodes (PLEDs).49,50 Interchain excimer formation and uorenone or ketone oxidative defects introduced either during polymer synthesis or during thermal, photo- or electrochemical oxidation are also common inefficient excitation decay pathways in conjugated

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Review polymers and can create unwanted effects such as color-impurities and reduced device operational lifetime, besides modication to quantum efficiency and emission lifetime.51–55 Defect emission is oen more apparent upon electrical or electrochemical excitation compared with PL excitation but it has been shown that defect formation may be suppressed by choice of suitable polymer chain end-caps, blending with low-molecularweight hole transport materials or by the addition of buffer layers during device fabrication.52,54–57 Additionally, excitation in inert atmosphere or conjugated polymer material/device passivation using epoxies or hard dielectric layers can suppress oxidative defect formation and improve emission stability. In addition to linear absorption and emission processes in conjugated polymer materials, many can exhibit large optical nonlinearities, which may be used for applications that require, for example, third harmonic generation, three-photon luminescence and stimulated emission or gain processes.32,58–60 In comparison to other nonlinear optical materials such as rareearth ions which offer limited uorescence emission tunability, quench in moderate concentrations and which exhibit quantum efficiencies that are strongly dependent on the phonon energy of the host medium, conjugated polymers can offer high stimulated emission cross sections and quantum efficiencies with minimal self-quenching in the solid state.17,32,60 Laser dyes also offer wide tunability of uorescence emission but, like rare-earth ions, they also exhibit emission quenching at moderate to low concentrations. Therefore, in contrast to other nonlinear optical materials, conjugated polymers offer larger chromophore densities in the solid state and wide spectral tunability which can be important for many photonic and optoelectronic applications.

2.3

Exciton migration, energy transfer and dissociation

Optically and electrically generated excitations in conjugated polymers may be controlled or redirected by creating spatially and energetically separated donor- and acceptor-type electronic states within a material, with donor–acceptor excitation energy exchange being facilitated by, for example, non-radiative processes such as long-range Coulombic (F¨ orster) energy transfer.61 A range of synthetic and processing approaches may be employed to structure polymers in a manner that enables energy migration, relaxation or exchange, including chemical modication of polymer chains with appropriate donor or acceptor moieties (via, for example, co-polymerization, sidechain substitution or end-capping),56,57 blending of a polymer with another material that acts as an excitation donor or acceptor (e.g., a different conjugated polymer or oligomer material, fullerene derivatives, carbon nanotubes, inorganic semiconductor quantum dots, dye molecules) or the formation of aggregated or phase-separated domains within lms (via, for example, spontaneous assembly or solvent and/or thermal treatments).62 Additionally, certain polymorphic polymers can be self-doped due to formation of a fraction of low-energy chain congurations or ordered domains within a matrix of higher energy, disordered polymer chains. For example, planarized chain segments, or segments with extended conjugation

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Polymer Chemistry lengths, with energy gaps smaller than the surrounding host polymer matrix, can facilitate ultra-fast energy migration of excitations from the higher-energy host matrix to the lower energy planarized polymer chain segments.13,14 Studies of excitonic energy transfer in b-phase containing PFO samples have found that only a small fraction of this phase completely dominates the emission characteristics of a sample as a result of ultra-fast (12 at the P3HT absorption band edge) and spectral broadening (more than 250 nm increase) relative to polythiophene/Ag lms without plasmonic nanorod arrays.189 This approach is suitable for large-area optoelectronic applications where light management in ultrathin polymer layers is desired. To date, active layer integrated absorption enhancement factors of 1.2–3 in theory181,182,184,191 and 1.5–2 experimentally178,185,186 have been demonstrated. Overall efficiency enhancement factors have ranged from 1.05–2.75 experimentally (total power conversion efficiency of 8% has been reported by Chou and Ding [ref. 171]).171,175,190,192–194 Although there have been many enhancements in the amount of light absorbed by the active layer and the total power conversion efficiency, in some cases, it is not always clear what the origin of the enhancement is. Wang et al. reported that addition of Au nanoparticles incorporated in the electron transport layer of an inverted CPPV resulted with an improvement in the total efficiency of the device by a factor of 1.05 relative to an identical device lacking the Au nanoparticles, but this efficiency enhancement was a result of improved ll factor; the absorption in the active layer actually decreased by 5%.194 Identication of the role(s) plasmonic nanostructures play(s) in CPPVs is critical to developing more efficient plasmonic-enhanced CPPVs. A recent review of plasmonic-enhanced organic photovoltaics

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Review elaborates the various congurations of plasmonics in organic photovoltaics and methods of going beyond 10% efficiency.195

5

Nanostructured polymer light sources

Proponents for more environmentally friendly lighting options advocate for organic light-emitting diodes, particularly polymer light-emitting diodes (PLEDs) as a low-cost and sustainable alternative to those that incorporate rare-earth or phosphorescent elements.47–50,196–198 A typical PLED employs conjugated polymer thin lm active layers up to 50
thinner than inorganic semiconductor (e.g., GaN) layer stacks in inorganic LED device architectures. However, the short lifetime and the lower efficiency of blue PLEDs as compared to red and green and the overall cost hinders the commercialization of a full-color PLED for general lighting purposes (e.g. street lighting, public buildings, and homes).197–199 Possible approaches to tackling the issues of lifetime and efficiency are: (1) inverted device congurations,134,200 and (2) light management by control of molecular orientation, or addition of inorganic or plasmonic nanostructures to improve light-outcoupling and/or radiative decay rate (see Sections 5.1–5.3 and 6.1 below).200 In this section, we will discuss nanoscale approaches to modifying the light emitting characteristics – such as polarization, direction, efficiency and decay rate – of conjugated polymer thin lm optoelectronic devices and photonic nanostructures (e.g., by controlling chain alignment, conformation, and local environment of the conjugated polymer material). 5.1

Polarized light emission: thin lms and nanobers

In thin conjugated polymer lms, vertically-oriented conjugated polymer chains (and hence, vertically-oriented transition dipoles) favor trapping of emitted light within the lm, while chains oriented in the plane of the thin lm will radiate emitted light in a direction normal to the plane of thin lm. In particular, the molecular chains of many semicrystalline conjugated polymers, may be readily oriented in an anisotropic fashion by thermal annealing of a thin lm of the material on a rubbed substrate or, by friction-transferring the material onto a smooth substrate.201–203 Using such approaches, thin-lm-format conjugated polymer polarized light-emitting devices exhibiting emission dichroic ratios as high as 25 in both photoluminescence and electroluminescence have been fabricated.201–203 Polarized electroluminescent devices such as these are of interest for a host of display applications since they do not require an additional backlight or polarizer, as in the case of liquid-crystal displays, and, therefore, can result in thinner, low-power-consumption devices. Highly polarized light emission has also been observed in a variety of conjugated polymer nanowire and nanober structures that contain preferential orientation of polymer chains along the wire or ber axis.12,94,104,112 For example, Herland et al. attached an emissive conjugated polyelectrolyte to amyloid-like nanoscale brils by incubating a mixture of the polymer and brils for a few minutes.94 The resulting functionalized brils exhibited strong polarization-dependent emission arising from

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Review the alignment of the conjugated polyelectrolyte chains along the long axis of the brils. Emission dichroic ratios as high as 12.4 were measured at particular spots along single brils. Da Como et al. investigated the polarization dependence of emission from extended “wire-like” single molecules of b-phase PFO using low-temperature (5 K) single molecule uorescence spectroscopy. Emission dichroic ratios between 2 and 99 were determined for single molecules depending on the degree of b-phase chain extension.12 Similar effects have been observed in the case of conjugated polymer nanowires: when optical transition dipoles are oriented along the length of a nanowire, light emitted from the polymer chains will tend to radiate in the nanowire radial direction with a high degree of polarization parallel to the nanowire long axis (Fig. 3b)78. For polyuorene nanowires prepared by solution-assisted template wetting emission dichroic ratios of up to 8.9 have been measured and attributed to a degree of axial polymer chain alignment during template pore lling and subsequent directional solvent evaporation.112 Inorganic compound semiconductor nanowires exhibit comparable emission dichroic ratios but the origin of the transition dipole orientation is substantially different and depends on dielectric effects as well as the internal crystallographic orientation.204,205 It is noteworthy that, depending on polymer chain conformation, the optical transition dipole orientation of conjugated polymers can be oriented off-the molecular axis (for example, between 19 and 26.5 for polyuorenes).12,29,40,41,206 Therefore, knowledge of transition dipole orientation is important for understanding and optimizing the photonic properties of aligned conjugated polymer thin lms and nanostructures (Fig. 2) and in determining the limits to polarization/dichroic ratios that may be achieved using such materials.

5.2

Polymer Chemistry molecules oriented 30 off the nanober long axis), PL propagation loss was found to depend strongly on emission wavelength, with negligible loss observed over a propagation distance of >250 mm at wavelengths between 520 and 560 nm and propagation losses of up to 215 cm1 observed at a wavelength of 505 nm.209 Active waveguiding has also been demonstrated in conjugated polymer nanowires fabricated by melt-assisted template wetting, as described earlier.17,111,113 PFO nanowire waveguides fabricated in this way exhibited a semicrystalline internal molecular morphology whereby emitted light propagated and exited the nanowires primarily at the ends (Fig. 3a). Propagation losses on the order of 1000 cm1 were measured from such structures and were attributed to Rayleigh scattering losses arising from internal changes in material density (e.g., due to crystalline domains), nanoscale surface roughness, a distribution in internal transition dipole orientation, re-absorption and substrate effects.111 Improvements in propagation length may be made by reducing the polydispersity of the constituent conjugated polymer material, the use of conjugated oligomer materials and further control of annealing and template pore surface roughness during nanowire fabrication.17,111 5.3 Emission rate enhancement by plasmonic nanostructures Besides control of nanoscale molecular morphology and alignment, inorganic or metallic nanostructures can be employed to modify the local environment (by scattering or an increase in the local density of optical sates) of the polymer molecules facilitating improved light out-coupling or enhanced radiative

Light trapping in thin lms and nanowires

A number of studies have shown that organic nanowires or nanobers with transition dipoles that are oriented radially or off the nanober long axis will tend to result in emission that propagates along the length of the nanowire due to trapping of the emitted light and effective coupling to waveguide modes of the nanober structure.76,77,207–210 In contrast, organic 1-D nanostructures with axial (i.e., on-axis) internal molecular alignment do not exhibited active waveguide behavior. Therefore, in addition to polarization, the directionality of emission from organic nanowires is particularly sensitive to internal molecular alignment and plays an important role in the functionality of conjugated oligomer and polymer nanober waveguides.77,207–210 For example, para-hexaphenyl (p-6P) nanober active waveguides have been formed by self-assembly on cleaved mica substrates via vacuum sublimation with p-6P molecular orientation almost perpendicular to the long axis of the nanobers.207,208,211 Depending on nanober quality, the propagation loss of waveguided emission was found to range from 100 to 3000 cm1. Self-absorption of waveguided emission was proposed as a major source of loss.208,212 In self-assembled thiacyanine dye H-aggregate nanober waveguides (with

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Fig. 8 (a) Monomer P3HT–gold nanorod antenna. (b) A dimer Au–P3HT–Au nanorod antenna. (c) PL lifetime decay curves for a P3HT thin film, nanorod, monomer P3HT–Au nanorod antenna and a dimer Au–P3HT–Au nanorod antenna.110,215

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Polymer Chemistry emission decay rates.213,214 Coupling of conjugated polymer materials to resonant metallic nanostructures is a means to externally modify the photonic properties of conjugated materials by altering the density of optical states in the vicinity of the material. For example, metal–P3HT heterostructure nanoantenna congurations have been shown to cause signicant radiative rate enhancements from P3HT; see Fig. 8.110,215 Electromagnetic simulations of such hybrid nanostructures, with 20 nm of P3HT sandwiched between two gold nanorods with diameters of 60 nm, indicated that radiative decay rate and modied quantum efficiency enhancements of 53 and 25, respectively, could be achieved for a total gold–P3HT–gold nanorod length of 180 nm, at a free-space wavelength of 700 nm.110 In practice, gold–P3HT and gold–P3HT–gold nanoantennas were fabricated using a template-directed sequential electrodeposition process (Fig. 8a and b).110,215 Experimental characterization of the fabricated structures by PL spectral intensity and lifetime measurements were carried out to determine the PL lifetime, sPL, of the P3HT semiconductor upon on coupling to resonant gold nanorods. Values of sPL determined from picosecond-range PL lifetime measurements for the nanoantennas (60 ps to 240 ps depending on nanoantenna length) were signicantly shorter than sPL values for neat P3HT thin lms and nanowires (600  40 ps and 720  115 ps, respectively) and P3HT–gold monomer nanoantenna structures (415  60 ps); (Fig. 8c). Such structures offer a means to improve the radiative properties of deeply sub-wavelength scale conjugated polymer nanostructures and modify their emission quantum efficiency and direction in a unique way.

6 Nanoscale light management for conjugated polymer-based lasers In comparison to other widely-used gain media, such as laser dyes (e.g., rhodamine and cyanide dye J-aggregates), rare-earth (e.g., Er) doped glasses and semiconductor materials such as InGaAsP, conjugated polymer materials offer higher chromopore densities in the solid state and ease of processability.17,211 However, to-date electrically driven lasing has not been realized in conjugated polymer-based lasers due to low charge carrier densities and losses at the metal electrode which increase the threshold gain required for stimulated emission.214,216,217 For example, PFO exhibits a very large stimulated emission cross section (1015 cm2) making it a desirable active laser medium.217,218 Amplied spontaneous emission (ASE; mirrorless lasing) in thin lm asymmetric slab waveguides of PFO has been demonstrated via stripe excitation with a pulsed UV laser to provide the necessary gain lengths required for light amplication. PFO thin lms excited in this way have exhibited large net gains (74 cm1) with very low propagation losses (3.5 cm1).218 In addition, it was found that the ASE threshold can vary depending on the phase of PFO,219,220 and, in aligned thin lms, with the relative orientation of the excitation and collection polarizations.65 Polyuorene-based conjugated polymers have been incorporated into a variety of microcavity geometries, such as distributed feedback,221 microring,222 photonic band-gap223 and random lasing224 structures to

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Review provide resonant optical feedback. One of the lowest lasing thresholds of PFO (3 mJ cm2 under 1 ns optical pumping), and indeed of any other conjugated polymer laser, was reported by Heliotis et al. using a two-dimensional distributed feedback cavity.221 This type of laser exhibited a very high slope efficiency of 7.8%, indicative of the quality of the cavity structure and the high emission efficiency of PFO. More recently, b-phase PFO has also been proposed as a promising candidate for electrically pumped lasing,217 although much work remains to be done to optimize and balance charge carrier injection into high quality b-phase PFO-containing optical microcavities. 6.1

Coupling to surface plasmon polaritons at electrodes

One of the primary challenges with demonstration of viable electrically-driven organic polymer lasers is supplying enough current density for population inversion to be achieved in the material.214,216,217 This requires ultra-low threshold device designs and control of the optical and electronic interaction between the gain material and the metallic electrode. In that regard, the control of polymer chain dipole orientation through processing is necessary to control the extent of coupling to propagating surface plasmon polaritons (SPP) which may be a source of reduction or enhancement of light emission at the metal electrode in optoelectronic devices.225–231 Dipole orientation plays a critical role in the degree of excitation and coupling to SPP modes at optical frequencies which occur between the emitter and the metal electrode surface.232 A parallel optical transition dipole (i.e., oriented in the plane of the electrode lm) is cancelled by its own image on the metal surface, while a perpendicularly polarized dipole (oriented normal to the plane of the electrode) can couple to a propagating surface plasmon mode and is enhanced. Such coupling has been successfully utilized to implement electrically pumped surface plasmon polariton emitters,233 demonstrate enhanced emission and absorption of emitters233–237 and plasmon laser devices.238,239 For conjugated polymer-based laser and light-emitting devices an understanding of this type of coupling could be critical to optimizing device performance and is an important future direction for development of next-generation organic polymer optoelectronic light sources. 6.2

Miniaturized polymer waveguides and laser sources

Conjugated polymer nanowire-based lasers composed of polyuorene materials are the smallest conjugated polymer lasers demonstrated to date.69 The presence of at or cleaved end facets can, in certain cases, lead to back reection of waveguided light into a molecular nanowire.207 In such cases, the nanowire can exhibit Fabry–P´ erot-like cavity effects with resonant modes superimposed on emission spectra from the nanowire ends. Additionally, optically-pumped lasing can be observed from such structures when the nanowire is composed of a conjugated oligomer or polymer material with large stimulated emission cross-sections and when a short pulsed excitation source is employed above a certain excitation energy density.32 Optically-pumped PFO nanowire lasers with diameters of 250 nm and lengths ranging from 6–10 mm are capable This journal is ª The Royal Society of Chemistry 2013

Review of achieving single-mode operation and narrow line-width blue emission.69 Excitation of such structures with short pulsed, low repetition rate laser excitation is necessary to minimize heating of the nanowires and to allow time for population inversion to occur within the material. Further miniaturization of conjugated polymer-based lasers is diffraction-limited, making the smallest possible laser dimension about half of its lasing wavelength divided by the refractive index of the gain material (i.e., 100–200 nm). However, development of coherent light sources that overcome the diffraction limit may be possible using resonant plasmonic nanostructures in a conguration similar to that of a surfaceplasmon-amplication-by-stimulated-emission-of-radiation device (SPASER).238–242 SPASER devices have been previously created using a metal nanoparticle surrounded by a dye-doped silica shell as the gain medium.239 However, the use of a conjugated polymer as the gain medium may be more advantageous than using an inorganic shell with organic dye molecules in that conjugated polymers do not undergo concentration quenching in the solid state,217,218,243–245 and, therefore, a greater density of chromophores could be placed at the interface with the metal nanoparticle resonator. Such structures would provide a unique opportunity to study and control nanoscale conjugated polymer–surface plasmon interactions and may provide new knowledge of how to optimize the interfacial interactions between conjugated polymer gain media and metallic surfaces.

7

Conclusions

This paper has reviewed a number of case studies that highlight the unique photonic properties of organic conjugated polymer materials from the single-molecule level, to nanoscale assemblies – such as nanobers and aggregates – to thin lms for large-area optoelectronic applications. The excitonic nature of conjugated polymer optical transitions results in strong absorption, emission, and non-linear optical properties which can be controlled through molecular ordering and alignment within nanoconned domains or structures. Additionally, the ability to control or redirect excitation energy within the polymer material by doping through (1) chemical modication of polymer chains with appropriate donor or acceptor moieties, (2) blending of a polymer with another material that acts as an excitation donor or acceptor or (3) the formation of aggregated or phase-separated domains within lms has enabled a host of optoelectronic applications. Numerous top-down and bottomup methods for fabricating nanostructured conjugated polymer thin lms and nanostructures with new or enhanced photonic properties compared to the bulk material – such as highlypolarized light emission, efficient exciton dissociation, enhanced radiative decay rate, light trapping – were discussed. Further advancements in understanding of the photophysics of conjugated polymer materials and their interactions with metallic and inorganic materials in various nanoconned formats are key to development of next-generation conjugated polymer nanostructures and devices for use in applications

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Polymer Chemistry such as optical communications and solid-state lighting, photovoltaic and lasing applications.

Acknowledgements The authors gratefully acknowledge support by National Science Foundation Grant no. 0903661 “Nanotechnology for Clean Energy IGERT”, National Science Foundation Grant no. DMR1309459, Corning Inc. graduate fellowships, a Rutgers' Research Council Grant, Rutgers' Institute for Advanced Materials Devices and Nanotechnology and Rutgers' Aresty Undergraduate Research Fellowship Program. The authors thank Gareth Redmond and Gary Cheung for useful discussions.

References 1 A. K¨ ohler, J. S. Wilson and R. H. Friend, Adv. Mater., 2002, 14, 701–707. 2 M. J. Eslamibidgoli and J. B. Lagowski, J. Phys. Chem. A, 2012, 116, 10597–10606. 3 R. Stepanyan, A. Subbotin, M. Knaapila, O. Ikkala and G. Ten Brinke, Macromolecules, 2003, 36, 3758–3763. 4 D. L. Cheung and A. Troisi, Phys. Chem. Chem. Phys., 2009, 11, 2105–2112. 5 J. Kim and T. M. Swager, Nature, 2001, 411, 1030–1034. 6 D. Marsitzky, M. Klapper and K. M¨ ullen, Macromolecules, 1999, 32, 8685–8688. 7 G.-Z. Yang, W.-Z. Wang, M. Wang and T. Liu, J. Phys. Chem. B, 2007, 111, 7747–7755. 8 Y. Geng, S. W. Culligan, A. Trajkovska, J. U. Wallace and S. H. Chen, Chem. Mater., 2003, 15, 542–549. 9 G. Sauv´ e, A. E. Javier, R. Zhang, J. Liu, S. A. Sydlik, T. Kowalewski and R. D. McCullough, J. Mater. Chem., 2010, 20, 3195–3201. 10 B. Friedel, C. R. McNeil and N. C. Greenham, Chem. Mater., 2010, 22, 3389–3398. 11 J. Jo, C. Chi, S. H¨ oger, G. Wegner and D. Y. Yoon, Chem.– Eur. J., 2004, 10, 2681–2688. 12 E. Da Como, K. Becker, J. Feldmann and J. M. Lupton, Nano Lett., 2007, 7, 2993–2998. 13 G. R. Hayes, I. D. W. Samuel and R. T. Phillips, Phys. Rev. B: Condens. Matter Mater. Phys., 1997, 56, 3838–3843. 14 P. Bojarski, A. Synak, L. Kułak and A. Kubicki, J. Fluoresc., 2006, 16, 309–316. 15 T.-Q. Nguyen, I. B. Martini, J. Liu and B. J. Schwartz, J. Phys. Chem. B, 2000, 104, 237–255. 16 J. A. Teetsov and D. A. Vanden Bout, J. Am. Chem. Soc., 2001, 123, 3605–3606. 17 D. O'Carroll, Conjugated Polymer Nanowires: OneDimensional Structures and Devices for Nanophotonics, PhD thesis, 2007. 18 M. Ranger, D. Rondeau and M. Leclerc, Macromolecules, 1997, 30, 7686–7691. 19 H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C. K. Chiang and A. J. Heeger, J. Chem. Soc., Chem. Commun., 1977, 578–580.

Polym. Chem., 2013, 4, 5181–5196 | 5191

Polymer Chemistry 20 M. Leclerc, J. Polym. Sci., Part A: Polym. Chem., 2001, 39, 2867–2873. 21 H. Sirringhaus, N. Tessler and R. H. Friend, Science, 1998, 280, 1741–1744. 22 F. Carpi and D. De Rossi, Opt. Laser Technol., 2006, 38, 292– 305. 23 S.-I. Na, B.-K. Yu, S.-S. Kim, D. Vak, T.-S. Kim, J.-S. Yeo and D.-Y. Kim, Sol. Energy Mater. Sol. Cells, 2010, 94, 1333– 1337. 24 C. D. M¨ uller, A. Falcou, N. Reckefuss, M. Rojahn, V. Wiederhirn, P. Rudati, H. Frohne, O. Nuyken, H. Becker and K. Meerholz, Nature, 2003, 421, 829–833. 25 E. Tekin, E. Holder, D. Kozodaev and U. S. Schubert, Adv. Funct. Mater., 2007, 17, 277–284. 26 M. Jørgensen, K. Norrman, S. A. Gevorgyan, T. Tromholt, B. Andreasen and F. C. Krebs, Adv. Mater., 2012, 24, 580– 612. 27 J. W. Blatchford, S. W. Jessen, L. B. Lin, J. J. Lih, T. L. Gustafson, A. J. Epstein, D. K. Fu, M. J. Marsells, T. M. Swager, A. G. MacDiarmid, S. Yamaguchi and H. Hamaguchi, Phys. Rev. Lett., 1996, 76, 1513–1516. 28 A. K¨ ohler, D. A. dos Santos, D. Beljonne, Z. Shuai, J.-L. Br´ edas, A. B. Holmes, A. Kraus, K. M¨ ullen and R. H. Friend, Nature, 1998, 392, 903–906. 29 T. W. Hagler, K. Pakbaz and A. J. Heeger, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 49, 10968–10975. 30 G. D. Scholes and G. Rumbles, Nat. Mater., 2006, 5, 683– 696. 31 B. D. Fahlman, Materials Chemistry, Springer, 2nd edn, 2011. 32 M. Campoy-Quiles, G. Heliotis, R. Xia, M. Ariu, M. Pintani, P. Etchegoin and D. D. C. Bradley, Adv. Funct. Mater., 2005, 15, 925–933. 33 M. Ariu, D. G. Lidzey, M. Sims, A. J. Cadby, P. A. Lane and D. D. C. Bradley, J. Phys.: Condens. Matter, 2002, 14, 9975– 9986. 34 K. Asada, T. Kobayashi and H. Naito, Jpn. J. Appl. Phys., 2006, 45, L247–L249. 35 N. C. Greenham, I. D. W. Samuel, G. R. Hayes, R. T. Phillips, Y. A. R. R. Kessener, S. C. Moratti, A. B. Holmes and R. H. Friend, Chem. Phys. Lett., 1995, 241, 89. 36 J. Piris, T. E. Dykstra, A. A. Bakulin, P. H. M. van Loosdrecht, W. Knulst, M. Tuan Trinh, J. M. Schins and L. D. A. Siebbeles, J. Phys. Chem. C, 2009, 113, 14500. 37 Y.-S. Huang, J. Gierschner, J. P. Schmidtke, R. H. Friend and D. Beljonne, Phys. Rev. B: Condens. Matter Mater. Phys., 2011, 84, 205311. 38 S. Tirapattur, M. Belletˆ ete, N. Drolet, J. Bouchard, M. Ranger, M. Leclerc and G. Durocher, J. Phys. Chem. B, 2002, 106, 8959–8966. 39 M. Redecker, D. D. C. Bradley, M. Inbasekaran and E. P. Woo, Appl. Phys. Lett., 1999, 74, 1400–1402. 40 M. Grell, D. D. C. Bradley, G. Ungar, J. Hill and K. S. Whitehead, Macromolecules, 1999, 32, 5810–5817. 41 M. Grell, D. D. C. Bradley, X. Long, T. Chamberlain, M. Insbasekaran, E. P. Woo and M. Soliman, Acta Polym., 1998, 49, 439–444.

5192 | Polym. Chem., 2013, 4, 5181–5196

Review 42 E. Da Como, E. Scheler, P. Strohriegl, J. M. Lupton and J. Feldmann, Appl. Phys. A: Mater. Sci. Process., 2009, 95, 61–66. 43 S. H. Chen, H. L. Chou, A. C. Su and S. A. Chen, Macromolecules, 2004, 37, 6833–6838. 44 S.-H. Chen, A.-C. Su and S.-A. Chen, Macromolecules, 2006, 39, 9143. 45 S. H. Chen, A. C. Su, C. H. Su and S. A. Chen, Macromolecules, 2005, 38, 379–385. 46 O. J. Korovyanko and Z. V. Vardeny, Chem. Phys. Lett., 2002, 356, 361–367. 47 J. Wang, A. Chepelianskii, F. Gao and N. C. Greenham, Nat. Commun., 2012, 3, 1191. 48 Z. Shuai, D. Beljonne, R. J. Silbey and J. L. Br´ edas, Phys. Rev. Lett., 2000, 84, 131–134. 49 J. S. Wilson, A. S. Dhoot, A. J. A. B. Seeley, M. S. Khan, A. K¨ ohler and R. H. Friend, Nature, 2001, 413, 828–831. 50 S.-A. Chen, T.-H. Jen and H.-H. Lu, J. Chin. Chem. Soc., 2010, 57, 439–458. 51 J. Teetsov and M. A. Fox, J. Mater. Chem., 1999, 9, 2117– 2122. 52 X. Gong, P. K. Iyer, D. Moses, G. C. Bazan, A. J. Heeger and S. S. Xiao, Adv. Funct. Mater., 2003, 13, 325–330. 53 K. Becker, J. M. Lupton, J. Feldmann, B. S. Nehls, F. Galbrecht, D. Gao and U. Scherf, Adv. Funct. Mater., 2006, 16, 364–370. 54 M. Sims, D. D. C. Bradley, M. Ariu, M. Koeberg, A. Asimakis, M. Grell and D. G. Lidzey, Adv. Funct. Mater., 2004, 14, 765– 781. 55 I. Prieto, J. Teetsov, M. A. Fox, D. A. Vanden Bout and A. J. Bard, J. Phys. Chem. A, 2001, 105, 520–523. 56 D. C. Neher, Macromol. Rapid Commun., 2001, 22, 1365– 1385. 57 D. Sainova, T. Miteva, H. G. Nothofer, U. Scherf, I. Glowacki, J. Ulanski, H. Fujikawa and D. Neher, Appl. Phys. Lett., 2000, 76, 1810–1812. 58 A. John Kiran, D. Udayakumar, K. Chandrasekharan, A. V. Ahdikari, H. D. Shashikala and R. Philip, Opt. Commun., 2007, 271, 236–240. 59 J. Mattu, T. Johansson and G. W. Leach, J. Phys. Chem. C, 2007, 111, 6868–6874. 60 M. C. Gather, K. Meerholz, N. Danz and K. Leosson, Nat. Photonics, 2012, 4, 457. 61 N. J. Turro, Modern Molecular Photochemistry, University Science Books, California, 1991. 62 J. J. M. Halls, A. C. Arias, J. D. MacKenzie, W. Wu, M. Inbasekaran, E. P. Woo and R. H. Friend, Adv. Mater., 2000, 12, 498–502. 63 A. L. T. Khan, P. Sreearunothai, L. M. Herz, M. J. Banach and A. K¨ ohler, Phys. Rev. B: Condens. Matter Mater. Phys., 2004, 69, 085201. 64 M. Ariu, M. Sims, M. D. Rahn, J. Hill, A. M. Fox, D. G. Lidzey, M. Oda, J. Cabanillas-Gonzalez and D. D. C. Bradley, Phys. Rev. B: Condens. Matter Mater. Phys., 2003, 67, 195333. 65 G. Heliotis, R. Xia, K. S. Whitehead, G. A. Turnbull, I. D. W. Samuel and D. D. C. Bradley, Synth. Met., 2003, 139, 727–730.

This journal is ª The Royal Society of Chemistry 2013

Review 66 G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science, 1995, 270, 1789–1791. 67 M. Vehse, B. Liu, L. Edman, G. C. Bazan and A. J. Heeger, Adv. Mater., 2004, 16, 1001–1004. 68 T.-Q. Nguyen, J. Wu, V. Doan, B. J. Schwartz and S. H. Tolbert, Science, 2000, 288, 652–656. 69 D. O'Carroll, I. Lieberwirth and G. Redmond, Nat. Nanotechnol., 2007, 2, 180–184. 70 G. A. O'Brien, A. J. Quinn, D. A. Tanner and G. Redmond, Adv. Mater., 2006, 18, 2379–2383. 71 F. Quochi, F. Cordella, A. Mura, G. Bongiovanni, F. Balzer and H.-G. Rubahn, J. Phys. Chem. B, 2005, 109, 21690– 21693. 72 A. Andreev, F. Quochi, F. Cordella, A. Mura, G. Bongiovanni, H. Sitter, G. Hlawacek, C. Teichert and N. S. Saricici, J. Appl. Phys., 2006, 99, 034305. 73 F. Quochi, F. Cordella, A. Mura, G. Bongiovanni, F. Balzer and H.-G. Rubahn, Appl. Phys. Lett., 2006, 88, 041106. 74 C. R. L. P. N. Jeukens, P. Jonkheijm, F. J. P. Wijnen, J. C. Gielen, P. C. M. Christianen, A. P. H. J. Schenning, E. W. Meijer and J. C. Maan, J. Am. Chem. Soc., 2005, 127, 8280–8281. 75 P. Yan, A. Chowdhury, M. W. Holman and D. M. Adams, J. Phys. Chem. B, 2005, 109, 724–730. 76 J. Brewer and H.-G. Rubahn, Phys. Status Solidi C, 2005, 2, 4058–4061. 77 H. Yanagi and T. Morikawa, Appl. Phys. Lett., 1999, 75, 187– 189. 78 D. O'Carroll, D. Iacopino, A. O'Riordan, P. Lovera, ´. O'Connor, G. A. O'Brien and G. Redmond, Adv. Mater., E 2008, 20, 42–48. 79 M.-S. Kim, J.-S. Kim, J. C. Cho, M. Shtein, L. J. Guo and J. Kim, Appl. Phys. Lett., 2007, 90, 123113. 80 X. He, F. Gao, G. Tu, D. G. Hasko, S. H¨ uttner, N. C. Greenham, U. Steiner, R. H. Friend and W. T. S. Huck, Adv. Funct. Mater., 2011, 21, 139–146. 81 A. Noy, A. E. Miller, J. E. Klare, B. L. Weeks, B. W. Woods and J. J. DeYoreo, Nano Lett., 2002, 2, 109–112. 82 S. A. Harfenist, S. D. Cambron, E. W. Nelson, S. M. Berry, A. W. Isham, M. M. Crain, K. M. Walsh, R. S. Keynton and R. W. Cohn, Nano Lett., 2004, 4, 1931–1937. 83 Z.-M. Huang, Y.-Z. Zhang, M. Kotaki and S. Ramakrishna, Compos. Sci. Technol., 2003, 63, 2223–2253. 84 D. Li, A. Babel, S. A. Jenekhe and Y. Xia, Adv. Mater., 2004, 16, 2062–2066. 85 C. R. Martin, Science, 1994, 266, 1961–1966. 86 J. C. Hulteen and C. R. Martin, J. Mater. Chem., 1997, 7, 1075–1087. 87 M. Steinhart, J. H. Wendorff, A. Greiner, R. B. Wehrspohn, K. Nielsch, J. Schilling, J. Choi and U. G¨ osele, Science, 2002, 296, 1997. 88 K. Shin, H. Xiang, S. I. Moon, T. Kim, T. J. McCarthy and T. P. Russell, Science, 2004, 306, 76. 89 S. Chuangchote, T. Srikhirin and P. Supaphol, Macromol. Rapid Commun., 2007, 28, 651–659. 90 L. Valentini, F. Mengoni, L. Mattiello and J. M. Kenny, Nanotechnology, 2007, 18, 115702.

This journal is ª The Royal Society of Chemistry 2013

Polymer Chemistry 91 M. R. Abidian, D.-H. Kim and D. C. Martin, Adv. Mater., 2006, 18, 405–409. 92 W. U. Huynh, J. J. Dittmer and A. P. Alivisatos, Science, 2002, 295, 2425–2427. 93 Y. Kang, N.-G. Park and D. Kim, Appl. Phys. Lett., 2005, 86, 113101. 94 A. Herland, P. Bjork, P. R. Hania, I. V. Scheblykin and O. Ingan¨ as, Small, 2007, 3, 318–325. 95 E. Zussman, A. Theron and A. L. Yarin, Appl. Phys. Lett., 2003, 82, 973–975. 96 D. Li, Y. Wang and Y. Xia, Nano Lett., 2003, 3, 1167– 1171. 97 J. Liu, Y. Lin, L. Liang, J. A. Voigt, D. L. Huber, Z. R. Tian, E. Coker, B. McKenzie and M. J. Mcdermott, Chem.–Eur. J., 2003, 9, 604–611. 98 S. I. Cho, W. J. Kwon, S.-J. Choi, P. Kim, S.-A. Park, J. Kim, S. J. Son, R. Xiao, S.-H. Kim and S. B. Lee, Adv. Mater., 2005, 17, 171–175. 99 C.-C. Kuo, C.-H. Lin and W.-C. Chen, Macromolecules, 2007, 40, 6959–6966. 100 F. Cacialli, J. S. Wilson, J. J. Michels, C. Daniel, C. Silva, R. H. Friend, N. Severin, P. Samor`ı, J. P. Rabe, M. J. O'Connell, P. N. Taylor and H. L. Anderson, Nat. Mater., 2002, 1, 160–164. 101 M. Steinhart, R. B. Wehrspohn, U. Gosele and J. H. Wendorff, Angew. Chem., Int. Ed., 2004, 43, 1334– 1344. 102 M. Zhang, P. Dobriyal, J.-T. Chen and T. P. Russell, Nano Lett., 2006, 6, 1075–1079. 103 E. R. Zubarev, J. Xu, A. Sayyad and J. D. Gibson, J. Am. Chem. Soc., 2006, 128, 15098–15099. 104 S. Masuo, H. Yoshikawa, H.-G. Nothofer, A. C. Grimsdale, U. Scherf, K. M¨ ullen and H. Masuhara, J. Phys. Chem. B, 2005, 109, 6917–6921. 105 D. H. Kim, J. T. Han, Y. D. Park, Y. Jang, J. H. Cho, M. Hwang and K. Cho, Adv. Mater., 2006, 18, 719–723. 106 M. Surin, E. Hennebicq, C. Ego, D. Marsitzky, A. C. Grimsdale, K. M¨ ullen, J.-L. Br´ edas, R. Lazzaroni and P. Lecl` ere, Chem. Mater., 2004, 16, 994–1001. 107 M. Knaapila, R. Stepanyan, B. P. Lyons, M. Torkkeli and A. P. Monkman, Adv. Funct. Mater., 2006, 16, 599–609. 108 A. P. H. J. Schenning and E. W. Meijer, Chem. Commun., 2005, 3245–3258. 109 A. Babel, D. Li, Y. Xia and S. A. Jenekhe, Macromolecules, 2005, 38, 4705–4711. 110 D. M. O'Carroll, J. Fakonas, D. M. Callahan, M. Schierhorn and H. A. Atwater, Adv. Mater., 2012, 24, OP136–OP142. 111 D. O'Carroll, I. Lieberwirth and G. Redmond, Small, 2007, 3, 1178. 112 D. O'Carroll and G. Redmond, Chem. Mater., 2008, 20, 6501–6508. 113 D. O'Carroll and G. Redmond, Physica E, 2008, 40, 2468– 2473. 114 F. C. Krebs, T. Tromholt and M. Jørgensen, Nanoscale, 2010, 2, 873–886. 115 N. Espinosa, M. H¨ osel, D. Angmo and F. C. Krebs, Energy Environ. Sci., 2012, 5, 5117–5132.

Polym. Chem., 2013, 4, 5181–5196 | 5193

Polymer Chemistry 116 F. Zhang, X. Xu, W. Tang, J. Zhang, Z. Zhuo, J. Wang, J. Wang, Z. Xu and Y. Wang, Sol. Energy Mater. Sol. Cells, 2011, 95, 1785–1799. 117 M. A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop, Prog. Photovoltaics, 2012, 20, 606–614. 118 D. Fyfe, Nat. Photonics, 2009, 3, 453–455. 119 N. S. Saricici, D. Braun, C. Zhang, V. I. Srdanov, A. J. Heeger, G. Stucky and F. Wudl, Appl. Phys. Lett., 1993, 62, 585–587. 120 H. Spanggaard and F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2004, 83, 125–146. 121 D. E. Markov, E. Amsterdam, P. W. M. Blom, A. B. Sieval and J. C. Hummelen, J. Phys. Chem. A, 2005, 109, 5266– 5274. 122 A. Haugeneder, M. Neges, C. Kallinger, W. Spirkl, U. Lemmer, J. Feldmann, U. Scherf, E. Harth, A. G¨ ugel and K. M¨ ullen, Phys. Rev. B: Condens. Matter Mater. Phys., 1999, 59, 15346–15351. 123 P. E. Shaw, A. Ruseckas and I. D. W. Samuel, Adv. Mater., 2008, 20, 3516–3520. 124 H. Hoppe, M. Niggemann, C. Winder, J. Kraut, R. Hiesgen, A. Hinsch, D. Meissner and N. S. Saricici, Adv. Funct. Mater., 2004, 14, 1005–1011. 125 S. E. Shaheen, C. J. Brabec, N. S. Saricici, F. Padinger, T. Fromherz and J. C. Hummelen, Appl. Phys. Lett., 2001, 78, 841–843. 126 Y. Kim, S. Cook, S. M. Tuladhar, S. A. Choulis, J. Nelson, J. R. Durrant, D. D. C. Bradley, M. Giles, I. McCulloch, C.-S. Ha and M. Ree, Nat. Mater., 2006, 5, 197–203. 127 W. Ma, C. Yang, X. Gong, K. Lee and A. J. Heeger, Adv. Funct. Mater., 2005, 15, 1617–1622. 128 G. Li, V. Shrotriya, Y. Yao and Y. Yang, J. Appl. Phys., 2005, 98, 043704. 129 Y. Kim, S. A. Choulis, J. Nelson, D. D. C. Bradley, S. Cook and J. R. Durrant, Appl. Phys. Lett., 2005, 86, 063502. 130 C. H. Woo, B. C. Thompson, B. J. Kim, M. F. Toney and J. M. J. Fr´ echet, J. Am. Chem. Soc., 2008, 130, 16324– 16329. 131 M.-Y. Chiu, U.-S. Jeng, M.-S. Su and K.-H. Wei, Macromolecules, 2010, 43, 428–432. 132 G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery and Y. Yang, Nat. Mater., 2005, 4, 864–868. 133 S. H. Park, A. Roy, S. Beaupr´ e, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee and A. J. Heeger, Nat. Photonics, 2009, 3, 297–302. 134 Z. He, C. Zhong, S. Su, M. Xu, H. Wu and Y. Cao, Nat. Photonics, 2012, 6, 591–595. 135 A. C. Arias, J. D. MacKenzie, R. Stevenson, J. J. M. Halls, M. Inbasekaran, E. P. Woo, D. Richards and R. H. Friend, Macromolecules, 2001, 34, 6005–6013. 136 J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H. Friend, S. C. Moratti and A. B. Holmes, Nature, 1995, 376, 498–500. 137 F. L. Zhang, A. Gadisa, O. Ingan¨ as, M. Svensson and M. R. Andersson, Appl. Phys. Lett., 2004, 84, 3906–3908. 138 M. S. White, D. C. Olson, S. E. Shaheen, N. Kopidakis and D. S. Ginley, Appl. Phys. Lett., 2006, 89, 143517.

5194 | Polym. Chem., 2013, 4, 5181–5196

Review 139 Y. S xahin, S. Alem, R. de Bettignies and J.-M. Nunzi, Thin Solid Films, 2005, 476, 340–343. 140 S. R. Ferreira, P. Lu, Y.-J. Lee, R. J. Davis and J. W. P. Hsu, J. Phys. Chem. C, 2011, 115, 13471–13475. 141 H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K. Bechgaard, B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W. Meiger, P. Herwig and D. M. de Leeuw, Nature, 1999, 401, 685–688. 142 R. J. Kline, M. D. McGehee, E. N. Kadnikova, J. Liu, J. M. J. Fr´ echet and M. F. Toney, Macromolecules, 2005, 38, 3312–3319. 143 R. J. Kline, M. D. McGehee and M. F. Toney, Nat. Mater., 2006, 5, 222–228. 144 R. Zhang, B. Li, M. C. Iovu, M. Jeffries-El, G. Sauv´ e, J. Cooper, S. Jia, S. Tristram-Nagle, D. M. Smilgies, D. N. Lambeth, R. D. McCullough and T. Kowalewski, J. Am. Chem. Soc., 2006, 128, 3480–3481. 145 H. Hlaing, X. Lu, T. Hofmann, K. G. Yager, C. T. Black and B. M. Ocko, ACS Nano, 2011, 5, 7532–7538. 146 M. Aryal, K. Trivedi and W. W. Hu, ACS Nano, 2009, 3, 3085– 3090. 147 J.-T. Chen and C.-S. Hsu, Polym. Chem., 2011, 2, 2707–2722. 148 H. Xin, F. S. Kim and S. A. Jenekhe, J. Am. Chem. Soc., 2008, 130, 5424–5425. 149 H. Xin, G. Ren, F. S. Kim and S. A. Jenekhe, Chem. Mater., 2008, 20, 6199–6207. 150 L. Li, G. Lu and X. Yang, J. Mater. Chem., 2008, 18, 1984– 1990. 151 X. Yang, J. Loos, S. C. Veenstra, W. J. H. Verhees, M. M. Wienk, J. M. Kroon, M. A. J. Michels and R. A. J. Janssen, Nano Lett., 2005, 5, 579–583. 152 H.-S. Wang, L.-H. Lin, S.-Y. Chen, Y.-L. Wang and K.-H. Wei, Nanotechnology, 2009, 20, 075201. 153 H.-S. Wang, S.-Y. Chen, M.-H. Su, Y.-L. Wang and K.-H. Wei, Nanotechnology, 2010, 21, 145203. 154 T. Kietzke, D. Neher, K. Landfester, R. Montenegro, R. G¨ untner and U. Scherf, Nat. Mater., 2003, 2, 408–412. 155 T. Kietzke, D. Neher, M. Kumke, R. Montenegro, K. Landfester and U. Scherf, Macromolecules, 2004, 37, 4882–4890. 156 H. Kiess and W. Rehwald, Sol. Energy Mater. Sol. Cells, 1995, 38, 45–55. 157 E. Bundgaard and F. Krebs, Sol. Energy Mater. Sol. Cells, 2007, 91, 954–985. 158 J. Huang, T. Goh, X. Li, M. Y. Sfeir, E. A. Bielinski, S. Tomasulo, M. L. Lee, N. Hazari and D. A. Taylor, Nat. Photonics, 2013, 7, 479–485. 159 R. P. Wayne, Photochemistry, Butterworth & Co Ltd, New York, NY, 1970. 160 J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer Science+Business Media, LLC, New York, NY, 1999. 161 N. Cooling, K. B. Burke, X. Zhou, S. J. Lind, K. C. Gordon, T. W. Jones, P. C. Dastoor and W. J. Belcher, Sol. Energy Mater. Sol. Cells, 2011, 95, 1767–1774. 162 D. C. Coffey, A. J. Ferguson, N. Kopidakis and G. Rumbles, ACS Nano, 2010, 4, 5437–5445.

This journal is ª The Royal Society of Chemistry 2013

Review 163 S. Honda, H. Ohkita, H. Benten and S. Ito, Adv. Energy Mater., 2011, 1, 588–598. 164 W. Belcher, K. Wagner and P. Dastoor, Sol. Energy Mater. Sol. Cells, 2007, 91, 447–452. 165 M. Campoy-Quiles, Y. Kanai, A. El-Basaty, H. Sakai and H. Murata, Org. Electron., 2009, 10, 1120–1132. 166 L. Yang, H. Zhou, S. C. Price and W. You, J. Am. Chem. Soc., 2012, 134, 5432–5435. 167 M. Koppe, H.-J. Egelhaaf, G. Dennler, M. C. Scharber, C. J. Brabec, P. Schilinsky and C. N. Hoth, Adv. Funct. Mater., 2010, 20, 338–346. 168 D. Gupta, D. Kabra, N. Kolishetti, S. Ramakrishnan and K. S. Narayan, Adv. Funct. Mater., 2007, 17, 226– 232. 169 R. C. Mulherin, S. Jung, S. Huettner, K. Johnson, P. Kohn, M. Sommer, S. Allard, U. Scherf and N. C. Greenham, Nano Lett., 2011, 11, 4846–4851. 170 C.-H. Chen, C.-H. Hsieh, M. Dubosc, Y.-J. Cheng and C.-S. Hsu, Macromolecules, 2010, 43, 697–708. 171 S. Y. Chou and W. Ding, Opt. Express, 2013, 21, A60–A76. 172 S. S. Kim, J. Na, D. Y. Kim and Y. C. Nah, Appl. Phys. Lett., 2008, 93(7), 073307. 173 N. C. Lindquist, W. A. Luhman, S. H. Oh and R. J. Holmes, Appl. Phys. Lett., 2008, 93(12), 123308. 174 R. A. Pala, J. White, E. Barnard, J. Liu and M. L. Brongersma, Adv. Mater., 2009, 21, 3504. 175 A. J. Morfa, K. L. Rowlen, T. H. Reilly III, M. J. Romero and J. Van De Lagemaat, Appl. Phys. Lett., 2008, 92, 013504. 176 S.-B. Rim, S. Zhao, S. R. Scully, M. D. McGehee and P. Peumans, Appl. Phys. Lett., 2007, 91, 243501. 177 D.-H. Ko, J. R. Tumbleston, L. Zhang, S. Williams, J. M. DeSimone, R. Lopez and E. T. Samulski, Nano Lett., 2009, 9, 2742–2746. 178 W. Bai, Q. Gan, G. Song, L. Chen, Z. Kafa and F. Bartoli, Opt. Express, 2010, 18, A620–A630. 179 T. D. Heidel, J. K. Mapel, M. Singh, K. Celebi and M. A. Baldo, Appl. Phys. Lett., 2007, 91, 093506. 180 M.-G. Kang, T. Xu, H. J. Park, X. Luo and L. J. Guo, Adv. Mater., 2010, 22, 4378–4383. 181 M. A. Sefunc, A. K. Okyay and H. V. Demir, Opt. Express, 2011, 19, 14200–14209. 182 H. Shen, P. Bienstman and B. Maes, J. Appl. Phys., 2009, 106, 073109. 183 B. Wu, X. Liu, T. Z. Oo, G. Xing, N. Mathews and T. C. Sum, Plasmonics, 2012, 7, 677–684. 184 S. Vedraine, P. Torchio, D. Duch´ e, F. Flory, J.-J. Simon, J. Le Rouzo and L. Escoubas, Sol. Energy Mater. Sol. Cells, 2011, 95, S57–S64. 185 D. M. O'Carroll, A. X. Collopy, V. E. Ferry and H. A. Atwater, in 25th European Photovoltaic Solar Energy Conference and Exhibition, 2010, pp. 834–837. 186 D. Duche, P. Torchio, L. Escoubas, F. Monestier, J.-J. Simon, F. Flory and G. Mathian, Sol. Energy Mater. Sol. Cells, 2009, 93, 1377–1382. 187 A. Abass, H. Shen, P. Bienstman and B. Maes, J. Appl. Phys., 2011, 109, 023111.

This journal is ª The Royal Society of Chemistry 2013

Polymer Chemistry 188 J.-L. Wu, F.-C. Chen, Y.-S. Hsiao, F.-C. Chien, P. Chen, C.-H. Kuo, M. H. Huang and C.-S. Hsu, ACS Nano, 2011, 5, 959–967. 189 B. Yu, S. Goodman, A. Abdelaziz and D. M. O'Carroll, Appl. Phys. Lett., 2012, 101, 151106. 190 J. Yang, J. You, C.-C. Chen, W.-C. Hsu, H. Tan, X. W. Zhang, Z. Hong and Y. Yang, ACS Nano, 2011, 5, 6210–6217. 191 B. Zeng, Q. Gan, Z. H. Kafa and F. J. Bartoli, J. Appl. Phys., 2013, 113, 063109. 192 C.-S. Kao, F.-C. Chen, C.-W. Liao, M. H. Huang and C.-S. Hsu, Appl. Phys. Lett., 2012, 101, 193902. 193 D. C. Lim, K.-D. Kim, S.-Y. Park, E. M. Hong, H. O. Seo, J. H. Lim, K. H. Lee, Y. Jeong, C. Song, E. Lee, Y. D. Kim and S. Cho, Energy Environ. Sci., 2012, 5, 9803–9807. 194 J. Wang, Y.-J. Lee, A. S. Chadha, J. Yi, M. L. Jespersen, J. J. Kelley, H. M. Nguyen, M. Nimmo, A. V. Malko, R. A. Vaia, W. Zhou and J. W. P. Hsu, J. Phys. Chem. C, 2013, 117, 85–91. 195 Q. Gan, F. J. Bartoli and Z. H. Kafa, Adv. Mater., 2013, 25, 2385–2396. 196 I. D. Rees, K. L. Robinson, A. B. Holmes, C. R. Towns and R. O'Dell, MRS Bull., 2002, 451–455. 197 S. A. Choulis, V. E. Choong, M. K. Mathai and F. So, Appl. Phys. Lett., 2005, 87, 113503. 198 Y.-S. Tyan, J. Photonics Energy, 2011, 1, 011009. 199 H. Yan, Q. Huang, J. Cui, J. G. C. Veinot, M. M. Kern and T. J. Marks, Adv. Mater., 2003, 10, 835–838. 200 X. H. Li, W. E. Sha, W. C. H. Choy, D. D. S. Fung and F. X. Xie, J. Phys. Chem. C, 2012, 116, 7200–7206. 201 M. Grell, D. D. C. Bradley, M. Inbasekaran and E. P. Woo, Adv. Mater., 1997, 9, 798–802. 202 M. Misaki, Y. Ueda, S. Nagamatsu, Y. Yoshida, N. Tanigaki and K. Yase, Macromolecules, 2004, 37, 6926–6931. 203 K. S. Whitehead, M. Grell, D. D. C. Bradley, M. Jandke and P. Strohriegl, Appl. Phys. Lett., 2000, 76, 2946– 2948. 204 J. Wang, M. S. Gudiksen, X. Duan, Y. Cui and C. M. Lieber, Science, 2001, 293, 1455–1457. 205 J. C. Johnson, H. Yan, P. Yang and R. J. Saykally, J. Phys. Chem. B, 2003, 107, 8816–8823. 206 H.-M. Liem, P. Etchegoin, K. S. Whitehead and D. D. C. Bradley, Adv. Funct. Mater., 2003, 13, 66–72. 207 H. Yanagi, T. Ohara and T. Morikawa, Adv. Mater., 2001, 13, 1452–1455. 208 F. Balzer, V. G. Bordo, A. C. Simonsen and H.-G. Rubahn, Phys. Rev. B: Condens. Matter Mater. Phys., 2003, 67, 115408. 209 K. Takazawa, Y. Kitahama, Y. Kimura and G. Kido, Nano Lett., 2005, 5, 1293–1296. 210 A. N. Lebedenko, G. Y. Guralchuk, A. V. Sorokin, S. L. Yemova and Y. V. Malyukin, J. Phys. Chem. B, 2006, 110, 17772–17775. 211 F. Balzer and H.-G. Rubahn, Adv. Funct. Mater., 2005, 15, 17–24. 212 V. S. Volkov, S. I. Bozhevolnyi, V. G. Bordo and H.-G. Rubahn, J. Microsc., 2004, 215, 241–244. 213 S. A. Choulis, M. K. Mathai and V.-E. Choong, Appl. Phys. Lett., 2006, 88, 213503.

Polym. Chem., 2013, 4, 5181–5196 | 5195

Polymer Chemistry 214 R. Br¨ uckner, A. A. Zakhidov, R. Scholz, M. Sudzius, S. I. Hintschich, H. Fr¨ ob, V. G. Lyssenko and K. Leo, Nat. Photonics, 2012, 6, 322–326. 215 D. M. O'Carroll, C. E. Hofmann and H. A. Atwater, Adv. Mater., 2010, 22, 1223–1227. 216 B. H. Wallikewitz, M. de la Rosa, J. H.-W. M. Kremer, D. Hertel and K. Meerholz, Adv. Mater., 2010, 22, 531– 534. 217 C. Rothe, F. Galbrecht, U. Scherf and A. Monkman, Adv. Mater., 2006, 18, 2137–2140. 218 R. Xia, G. Heliotis, Y. Hou and D. D. C. Bradley, Org. Electron., 2003, 4, 165–177. ¨ 219 M. N. Shkunov, R. Osterbacka, A. Fujii, K. Yoshino and Z. V. Vardeny, Appl. Phys. Lett., 1999, 74, 1648– 1650. 220 G. Ryu, R. Xia and D. D. C. Bradley, J. Phys.: Condens. Matter, 2007, 19, 056205. 221 G. Heliotis, R. Xia, G. A. Turnbull, P. Andrew, W. L. Barnes, I. D. W. Samuel and D. D. C. Bradley, Adv. Funct. Mater., 2004, 14, 91–97. 222 S. V. Frolov, M. Shkunov, Z. V. Vardeny and K. Yoshino, Phys. Rev. B: Condens. Matter Mater. Phys., 1997, 56, R4363–R4366. 223 S. Riechel, C. Kallinger, U. Lemmer, J. Feldmann, A. Gombert, V. Wittwer and U. Scherf, Appl. Phys. Lett., 2000, 77, 2310–2312. 224 R. C. Polson, A. Chipouline and Z. V. Vardeny, Adv. Mater., 2001, 13, 760–764. 225 N. Sudarkin and P. A. Demkovich, Sov. Phys. Tech. Phys., 1989, 34, 764. 226 J. Bellessa, C. Bonnand, J. C. Plenet and J. Mugnier, Phys. Rev. Lett., 2004, 93, 036404. 227 M. Ambati, S. H. Nam, E. Ulin-Avila, D. A. Genov, G. Bartal and X. Zhang, Nano Lett., 2008, 8, 3998. 228 A. De Luca, M. P. Grzelczk, I. Pastoriza-Santos, L. M. IizMarza, M. La deda, M. Striccoli and G. Strangi, ACS Nano, 2011, 5, 5823.

5196 | Polym. Chem., 2013, 4, 5181–5196

Review 229 M. A. Noginov, G. Zhu, M. Bahoura, J. Adegoke, C. E. Small, B. A. Ritzo, V. P. Drachev and V. M. Shalaev, Opt. Lett., 2006, 31, 3022. 230 I. De Lion and P. Berini, Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 78, 161401. 231 G. C. des Francs, P. Bramant, J. Grandidier, A. Bouhelier, J.-C. Weeber and A. Dereux, Opt. Express, 2010, 18, 16327. 232 W. L. Barnes, J. Mod. Opt., 1998, 45, 661. 233 R. J. Walters, R. V. A. van Loon, I. Brunets, J. Schmitz and A. Polman, Nat. Mater., 2010, 9, 21. 234 J. N. Farahani, D. W. Pohl, H. J. Eisler and B. Hecht, Phys. Rev. Lett., 2005, 95, 017402. 235 P. Anger, P. Bharadwaj and L. Novotny, Phys. Rev. Lett., 2006, 96, 113002. 236 K. R. Catchpole and A. Polman, Plasmonic solar cells, Opt. Express, 2008, 16, 21793–21800. 237 Y. Wang, T. Yang, M. T. Tuominen and M. Achermann, Phys. Rev. Lett., 2009, 102, 163001. 238 D. J. Bergman and M. I. Stockman, Phys. Rev. Lett., 2003, 90, 027402. 239 M. A. Noginov, G. Zhu, A. M. Belgrave, R. Bakker, V. M. Shalaev, E. E. Narimanov, S. Stout, E. Herz, T. Suteewon and U. Wiesner, Nature, 2009, 460, 1110–1112. 240 S. R. Das, IEEE Spectrum, 2009, 46, 14–16. 241 M. I. Stockman, J. Opt., 2010, 12, 024004. 242 M. T. Hill, Y.-S. Oei, B. Smalbrugge, Y. Zhu, T. de Vries, P. J. van Veldhoven, F. W. M. van Otten, T. J. Eijkemans, J. P. Turkiewicz, H. de Waardt, E. J. Geluk, S.-H. Kwon, Y.-H. Lee, R. N¨ otzel and M. K. Smit, Nat. Photonics, 2001, 1, 589–594. 243 F. Hide, M. A. DiazGarcia, B. J. Schwartz, M. R. Andersson, Q. B. Pei and A. J. Heeger, Science, 1996, 273, 1833–1836. 244 N. Tessler, G. J. Denton and R. H. Friend, Nature, 1996, 382, 695–697. 245 M. D. McGehee, R. Gupta, S. Veenstra, E. Kirk Miller, M. A. D´ıaz-Garc´ıa and A. J. Heeger, Phys. Rev. B: Condens. Matter Mater. Phys., 1998, 58, 7035–7039.

This journal is ª The Royal Society of Chemistry 2013