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.

Jacob van Heemskerck

SHIPS WITH THE NAME JACOB VAN HEEMSKERCK

The first ship with this name was the yacht on which Abel Tasman sailed when he discovered the island which he called Van Diemensland which later became Tasmania.

During this voyage he also discovered New Zealand in December 1642, the Fiji Islands and the Friendship Islands in 1643. This Heemskerck is considered to be a merchant vessel and therefore not

included in the number of navy ships named after Admiral J. van Heemskerck.

The second Heemskerck was a schooner built in 1803 which served as flagship for admiral VerHuell and operated against Dunkirk. It was broken up in 1821.

The third Heemskerck was a heavily armed coastal defence ship launched in 1906. In 1908 she took part in operations against Venezuela. At the start of the First World War she remained in the Caribbean because of the presence of the German cruiser Karlsruhe. At the outbreak of the Second World War she served as ‘battery ship’ in the harbour of IJmuiden and was scuttled by her crew. She was raised after the war.

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.