Emission from organic materials is usually fluorescence from decay of singlet states, but in LEDs a majority of the excited states generated are triplet states which can only decay by phosphorescence or by thermally-activated delayed fluorescence (TADF). To improve the potential maximum efficiency of LEDs, it is necessary to incorporate into the emissive material chromophores which are phosphorescent or which show TADF. The ways in which such units can be incorporated into polymers are described and compared and the device results to date and prospects for future development discussed.

This chapter discusses how to tune the orbital energy levels and bandgaps of copolymers containing arylene and/or heteroarylene units, so as to obtain materials for high efficiency LEDs, TFTs and OPVs. By careful selection of the structures, and optimisation of the molar masses, polymers have been made which show very high charge carrier mobilities due to efficient charge transport. Here control of the solid-state packing is important but high crystallinity does not seem to be necessary. Transistors can be made with mobilities superior to that of amorphous silicon, though their commercial viability remains unproven. Careful control of bandgaps, molar masses and solid-state order combined with the development of new acceptor molecules has led to the fabrication of OPV devices with efficiencies close to 20%, which is better than many commercial solar cells. The commercial viability of OPVs remains to be demonstrated with device lifetimes still needing improvement, but these results combined with the low cost of making and processing conjugated polymers suggests such devices could be competitive with current ones with further optimisation.

Intermediate between PPPs and LPPPs in structure are stepladder polymers in which the monomers contain two or more phenylene units which are connected by one or two atom bridges. The simplest and most widely studied of these are poly(dialkylfluorene)s (PDAFs) whose monomers are biphenyl units linked by one carbon bridges. These were developed as blue-emitting materials, but their emission is unstable due to formation of emissive ketone defects by oxidation of monoalkylfluorene impurities. This problem can be overcome by replacing the alkyl groups with aryl groups or by making the monomers by routes which give only fully dialkylated compounds. The efficiency of the devices can be improved by incorporation of charge-transporting groups, while the emission colour is tunable by incorporation of emissive dye units. The emission from PDAFs is a violet-blue, but pure blue emission has been obtained by making polymers from monomers containing a larger number of linked phenylene rings. Also discussed are the synthesis and properties of other step-ladder polymers such as polycarbazoles which are analogous to PDAFs but contain nitrogen instead of carbon bridges.

Methods for making films of insoluble poly(para-phenylene) (PPP) are described and its potential as a blue-emitting polymer discussed. Efficient methods for making soluble PPP derivatives have been developed but these polymers suffer from undesirable changes in their emission due to twisting of the polymer backbones caused by steric interactions between the solubilising side-chains or by the formation of emissive aggregates in the solid state. To overcome this, ladder-type PPPs (LPPPs) made from precursor polymers have been made and their structure–property relationships and potential utility in devices are discussed. Stable blue emission from LPPPs has proven to be difficult to obtain due to the formation of emissive defects, while their wide bandgaps and unsuitable frontier orbital energies have made them of limited use in other devices.

Unlike standard conjugated polymers which may contain a range of conjugation lengths in their emissive chromophores, polymers can be made in which there are isolated chromophores of identical size and properties. This chapter describes the various types of such polymers that can be made, the routes to their synthesis and their device performances. Their advantages and disadvantages compared to standard polymers are discussed.

The state of the art in the use of conjugated polymers in electronic devices is summarised and the prospects for their further development discussed.

The methods for synthesising by precursor routes films of insoluble poly(phenylene vinylene) (PPV), the prototypical poly(arylene vinylene) (PAV) are described and compared and its properties discussed. Methods for preparing soluble substituted PPVs are described and their structure–property relationships discussed. By suitable choice of structure, PAVs with emission colours ranging from the blue to the near infra-red have been made and tested in light-emitting diodes. The choice of substituents has also been used to enhance the charge accepting and transporting properties of PAVs, thus improving their efficiency in devices. The efficiency of polymer-based LEDs is also affected by the presence of defects in the polymer structures and methods have been developed to minimise these, enabling commercially-viable LEDs to be made using PAVs. The potential use of PAVs in OTFTs and OPVs is also discussed.

The methods for synthesising polyacetylene are discussed and compared. As unsubstituted polymer is insoluble, precursor methods must be used to make films suitable for use in devices. While the fact that doped polyacetylene is conducting is of scientific interest, its instability and lack of luminescece has made it useless for practical applications. Substituted polyacetylenes can be made which are both soluble and luminescent, making them potentially useful in LEDs.The synthesis and properties of such polymers are discussed as well as their structure–property relationships and potential for use in devices.

Polythiophenes are the most widely studied class of heteroarene-based polymers. The properties of poly(3-alkylthiophene)s have been shown to depend upon the degree of regioregularity in the polymer backbone. Routes have been developed to make almost completely regioregular polymers with nearly 100% head-to-tail couplings. These regioregular polymers show much better chain packing in the solid state and significantly better charge carrier mobilities, making them suitable for use in OTFTs. They show less promise as LED materials due to low emission efficiencies, but are promising as solar cell materials. A combination of regioregular poly(3-hexylthipophene) and a fullerene acceptor is the most widely studied donor–acceptor pair in OPVs, with device efficiencies of over 5% combined with a relatively inexpensive synthesis, making it potentially commercially viable.

White electroluminescence is required for lighting applications. This is obtainable by either blending two materials with complementary colours (usually blue and red or orange) or by obtaining simultaneous emission from independent chromophores with complementary colours. The designs of polymers that have been used to achieve this are described and compared and examples of the best performing materials given.

Conjugated polymers are semiconductors, which if doped can become conducting. Their electronic properties make them suitable for use in organic electronic devices such as transistors (OTFTs), light-emitting diodes (OLEDs) and solar cells (OPVs). The operating principles of these devices are discussed. Each of these devices have different requirements for their active materials. Among the important parameters which must be considered to optimise device performance are the energy difference between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) (known as the bandgap) which controls which colours of light can be absorbed or emitted, the energy levels of the HOMO and LUMO, which control the rate at which charges can be injected and extracted and the mobility of the charge carriers within the material. These parameters must be considered in designing or selecting suitable materials for use in these devices.

Methods for preparing soluble poly(arylene ethynylene)s (PAEs) and PAE-PAV copolymers are described and compared. The structure–property relationships in such polymers are described and their potential applications in devices such as LEDs and sensors discussed.

Non-linear polymers, including hyperbranched and star polymers, and dendrimers, which contain emissive chromophores, have been made. Routes to make them are described and compared and their utility in LEDs is discussed.

Light emitting diodes (LEDs): photon flux, injection efficiency, internal quantum efficiency. Double-heterostructure LEDs. Burstein-Moss shift. Carrier overflow. Carrier leakage over the barrier. External efficiency of a LED: transmission efficiency and external quantum efficiency. Output power, responsivity, and wall-plug efficiency. Emission pattern of LEDs. Luminous efficiency. Blue LEDs.