Table of contents

This article attempts to present a comprehensive picture of the elementary processes in conjugated polymers. Key experiments using ultrafast techniques are described, which provide insights into excitation energy migration in homopolymers as well as guest–host systems, charge-carrier generation both intrinsic and in donor–acceptor systems, and the various mechanisms leading to triplet states.

We present combined results of femtosecond transient photoluminescence (PL), femtosecond transient absorption and quasi-steady-state photoinduced absorption spectroscopy on the organic semiconductor poly-6, 6', 12, 12'-tetraalkyl-2, 8-indenofluorene (PIF). By control of interchain order via the choice of the side-chain substituents, we have investigated its effect on exciton and polaron dynamics in this model electronic material. We show that interfaces between ordered and disordered domains play a significant role in the photophysics. At high photoexcitation fluence, a high yield (∼10%) of polarons is only observed in the ordered semiconductor. This process arises from two-step photoexcitation, first to the lowest exciton, and then to a high-energy state of opposite symmetry. In contrast, triplet exciton population is generated via sequential excitation with smaller yield (< 1 %) in both ordered and disordered materials. In the low fluence regime, triplet excitons are found to arise from evolution of polarons generated with low efficiency (also < 1 %) by diffusion-limited processes. The triplet generation yield is strongly dependent on order, with the disordered material displaying a higher yield. Polaron decay is found to be thermally activated, with a higher activation energy and lower room-temperature recombination rate in the ordered material. Furthermore, we do not find that emissive keto defects play a defining role in the PL properties of our PIF samples. Instead, absorption features of aggregate-like species, which we believe to lead to sub-gap emission, are evident in the photocurrent action spectrum of the more ordered PIF derivative.

Organic electroluminescent devices (OEDs) emit light when an electric current is applied to a thin film section. They arise from two main technology branches—small molecules and light emitting polymers. Apart from the insight offered into the fundamentals of their physics, which is relevant to topics such as electrical transport in biological systems and molecular computers, understanding how the mobilities in these systems vary with morphology and composition enables the design of improved materials for technological requirements, e.g. fast switching speeds for active matrix displays and polymer field effect transistors. In this review, we have focussed on the models of transport in OEDs that address the unusual nature of this transport and underpin device design. The review concludes with the following point: as new materials for use in OEDs continue to appear, modelling is essential for the prediction of their transport properties, which in turn leads to the establishment of fundamental trends in the behaviour of devices employing them.

We survey the current state of models for electronic processes in conducting polymer devices, especially light-emitting diodes. We pay special attention to several processes that have been somewhat neglected in the previous literature: charge injection from electrodes into a polymer sample, mobility of charge-or energy-carrying defects within a single molecule and (more briefly) transfer of carriers between molecules and the interaction between the charge transport and the mesostructure of the polymer. Within all these areas substantial progress has been made in recent years in elucidating the important physics, but further progress is needed to make quantitative contact with experiment.

An analytic model of the weak-field carrier transport in an energetically disordered and positionally random hopping system is formulated. Within the framework of this model, the carrier mobility can be calculated by either direct averaging of carrier hopping rates or by the use of the effective transport energy concept. It is shown that multiple carrier jumps within pairs of occasionally close hopping sites affect the position of the effective transport level on the energy scale. In good quantitative agreement with experimental data and results of Monte Carlo simulation, the temperature and concentration dependences of the mobility can be almost perfectly factorized, i.e. represented as a product of two functions one of which depends solely upon the temperature while the other governs the dependence upon the density of localized states. The model is also used for the calculation of trap-controlled hopping mobility and for the analysis of hopping transport at high charge-carrier densities.

The device-physics features of organic materials are presented from an engineering point of view. By treating the organic material and the device in a self-consistent manner the unique features of organic devices are revealed. We discuss charge injection and transport relevant to (polymer/small molecule) light-emitting diodes and field-effect transistors.

We present results from a device model in which the current–voltage (I– V) characteristics of an ITO/MEH-PPV/Al organic light emitting device have been simulated over a range of temperatures by fitting the mobilities and barrier heights. Good agreement with experimental data has been achieved at temperatures of 200–300 K at bias voltages exceeding 2 V, but there are some shortcomings of the model at lower temperatures. We have found that a discrete trap level in the simulated device improved the fit of the simulated I– V data in the low field regime at high temperatures. It has also been noted in the experimental data that cooling the device led to improved efficiency, with the ratio of light output to device current increasing by a factor of approximately 50 times when the device was cooled from 300 to 10 K. The model exhibited increased efficiency upon cooling, provided the electron barrier height, ϕ_bn, was decreased at a greater rate than the hole barrier height, ϕ_bp.

The information that can be obtained on the mobility and relaxation kinetics of electronic charge carriers in bulk molecular materials using the pulse-radiolysis time-resolved microwave conductivity technique is illustrated by results on several dialkoxy-substituted phenylene-vinylene polymers. The results demonstrate the sensitivity of the electronic properties of such conjugated polymers to their morphology. Thus, despite having the same conjugated backbone, the mobility and the relaxation kinetics (due to trapping and/or charge recombination) depend strongly on the nature of the alkyl-chain substituents, with in particular a marked difference between symmetrically and unsymmetrically dialkoxy-substituted compounds. For the latter, high-temperature annealing has a substantial positive effect on the mobility and lifetime of mobile carriers. The mobilities found for annealed materials range from a low of 0.0025 cm² V^-1 s^-1 for the methoxy, ethyl-hexoxy derivative, MEH-PPV, to a high of 0.036 cm² V^-1 s^-1 for the di-octadecoxy derivative, (OD)₂-PPV. The latter compound becomes a free-flowing liquid above 190ºC but still displays a high charge-carrier mobility of 0.017 cm² V^-1 s^-1. For all compounds the temperature dependence of the mobility after annealing is only slight over the range from −50 to +150ºC with an energy of activation ≤0.1 eV. Saturation of vinylene residues (breaking the conjugation) results in a marked decrease in the mobility. For very high accumulated doses of radiation the mobility on a nanosecond timescale remains unaffected but the decay of the mobile carriers at longer times becomes faster. This effect is completely reversed on annealing at 150ºC.

Achieving exquisite control of the structural organization in molecular arrangements of π-conjugated (macro)molecules with tailored chemical functionalities and physical properties is an essential prerequisite for the reproducible fabrication of high-performance molecular electronic devices. This paper reports on the exploitation of scanning probe microscopies to investigate π-conjugated oligomeric and polymeric architectures assembled on flat solid substrates. These techniques provide genuine insight into the structure of the molecular arrangements as well as into their physicochemical properties over a wide range of length scales. Moreover, they allow one to manipulate organic adsorbates with a precision on the molecular scale, opening a pathway towards the nanopatterning of surfaces and the development of single-molecule devices.

We have measured the temperature-dependent photoluminescence quantum yields (PLQYs) of poly(9, 9-dioctylfluorene) (PFO) films with four morphologies, namely as-spin-coated (SC) glass, quenched nematic glass, crystalline, and vapour-treated SC glass containing a fraction of 2₁ helix conformation (β-phase) chains. We find that the room temperature PLQYs of the as-SC, crystalline, and quenched films all increase as the temperature is reduced. However, the PLQY of the film containing β-phase chains decreases at temperatures below 150 K. Via temperature-dependent photoinduced absorption measurements, we show that the polaron population in films containing β-phase PFO chains grows as the temperature is reduced, and is significantly larger than in films with any of the other morphologies. Because of the smaller HOMO–LUMO (highest occupied molecular orbital–lowest unoccupied molecular orbital) energy gap of the β-phase chains compared to chains in the surrounding glassy PFO matrix, they act as recombination sites for excitons, and as traps for polarons. Hence at low temperatures, the polarons become strongly localized on these chains, where they quench the singlet excitons and reduce the PLQY.

Muon-spin relaxation (μ SR) experiments on the conducting polymers poly(2, 3-dibutoxy-1, 4-phenylene vinylene) and poly(2, 5-bis(dimethyloctylsilyl)-1, 4-phenylene vinylene) probe the dynamics of the highly mobile polarons created by the muon-implantation process in which muonium reacts with the polymer forming a radical state. The fluctuating spin density induced by the electronic spin defect rapidly diffusing up and down the chain leads to a characteristic relaxation, the temperature and field dependences of which permit the extraction of intrachain and interchain diffusion rates. The intrachain diffusion rate decreases with temperature and can be fitted to a model of phonon-limited transport. The interchain diffusion rate increases with temperature and can be fitted to an activated temperature dependence.

In recent years, first-principles quantum-mechanical simulations have become established as a complementary tool to experiments in the design and characterization of new materials. Here we illustrate this in the case of boron nitride (BN) analogues of conjugated organic polymers which offer a cheap alternative to inorganic semiconductors in the manufacture of electronic devices. By analogy with heterostructures, such as quantum wells and superlattices, currently used by the conventional semiconductor industry, we show how copolymers consisting of sections of carbon and BN can be designed to tune the electronic properties of these new materials.

A chiral, 3-substituted polythiophene with an amino-acid function shows pH-dependent visible, emission and circular dichroism spectra in buffered aqueous solution. At pH equal to the pI of the amino-acid, the backbone adopts a nonplanar right-handed helical conformation and the polymer chains are separated from each other. Increasing pH leads to a more planar conformation of the backbone and an aggregation of the polymer chains occurs. A lower pH will also lead to a more planar conformation of the backbone, but aggregation of the polymer chains appears to be absent. The aggregates are disrupted by increasing ionic strength in alkaline buffer systems, indicating hydrogen bonding is important for aggregation. On the other hand, ions containing an amino group and one or more hydroxyl groups induce a more planar conformation of the polymer backbone.

This special issue focuses on recent advances in the area of electroactive organic materials, and in particular on conjugated and electroluminescent polymers. In the last 20 years, conjugated molecules and macromolecules have been proposed as a novel class of semiconductors with technological potential for the treatment of information. With respect to conventional inorganic semiconductors, these carbon based materials offer a unique opportunity for looking at a different physics, largely dominated by the formation of a partially delocalised p-orbital. The latter originates by the lateral overlap of the p_z orbitals of adjacent, sp² hybridized, carbon atoms. In the case of polymers, the orbital develops along the polymeric chain, with one-dimensional character, and subsequent lateral confinement of the wavefunction. Further confinement of the excitations, either charged or neutral, arises from self-localization induced by either geometric relaxation of the soft polymeric chains (polarons), or from electron correlation effects. Strong non-linear effects, ultrafast thermalization of optically excitated states, and disorder mediated processes, are other important aspects of the physics of these systems. In spite of the localized extent of the wavefunction, the excitations are mobile in these materials, allowing for both charge and energy transport. Electrically injected charge carriers of opposite sign would drift under the application of an electric field, and mutually capture in order to form luminescent excited states, thus providing a further tool to probe the physics, via analysis of the luminescence, and also significant prospects for applications.

Conjugated, electroluminescent polymers are in fact offering great promise for the development of cheap, large area displays, and many firms are actively working to take them to the market. Similarly, a significant effort is being devoted to developing `all plastic' electronic circuits, capable of logic functions for information treatment. The keen industrial and commercial interest in these materials is not a mere declaration of intents, but is demonstrated with investments that amount to several hundred millions dollars worldwide. Another area of applications is in the fabrication of solar cells. For these, conjugated molecules can take advantage again of cheap and easy fabrication over large areas, and also of relatively high absorption coefficients, deriving from high oscillator strengths for the optical transitions. An interesting prospect for development of the field is related to the chemical analogy of conjugated molecules with those of biological origin, which virtually opens the way to bio-compatible structures and devices, and maybe to artificial organs with complex functions, such as retinas.

The issue starts with a selection of papers introducing the reader to current advances in the understanding of materials photophysics, a rich source of experimental and theoretical challenge, with reference, in particular, to the debate regarding the role of intermolecular interactions and their control. The description of electrical transport in conjugated polymers has proved to be a challenging and stimulating problem, and this is reflected in the several excellent contributions that I feel privileged to be able to introduce in this special issue. Advanced experimental techniques for probing and characterization of the morphological, electrical and optical properties, especially on a micro- or nano-scopic scale are crucial for advancing our understanding of the materials. Scanning probe microscopies and other less common techniques are proving precious tools in this process, as illustrated by different contributions. The issue concludes with two articles at the forefront of explorative research: namely a theoretical paper proposing new hybrid organic-inorganic electroactive polymers, and predicting their properties via density functional theory calculations, and a paper exploring the properties of conjugated polymers functionalized with biomolecular groups.

By focusing on novel, original results, while also introducing some review elements, this special issue aims at stimulating discussion in this lively area, which is still growing fast, thanks to the input from the diverse disciplines that are brought together: physics, chemistry, materials science, electrical and optical engineering. At the dawn of conjugated polymer devices entering the marketplace it is important to catalyse the intellectual involvement of the scientific community (academic and industrial) for several reasons: to gather `critical mass', to develop the materials in the `right' direction, avoiding dead ends, to identify new applications, and to support development of the applications already identified, both in the short and in the long term, by means of an advanced understanding of the basic physics. I hope that this special issue will provide a source of closely interconnected stimuli so that each reader can find a `personal' way of reading single papers and the whole issue, depending on background, interests and motivations. The mentioned role of intermolecular interactions (versus intramolecular ones) is just one of the `threads', or `leit-motifs' that it is possible to identify and use for analysing the issue. Other examples (but this is of course a non-exhaustive list) could be the use and complementarity of different experimental or theoretical techniques, the relation between experiment and theory, the evolution of the materials and of their understanding, microscopic versus macroscopic properties.

Finally, I would just like to thank all the authors for their contributions, and Dr Richard Palmer and the IOPP staff for their precious assistance.

F Cacialli

Guest Editor