Electronics

Polymers light the way

Using the methods of polymer deposition that are employed in making integrated circuits, light-emitting polymers can be patterned for application in flat-screen, full-colour displays.

Liquid-crystal devices dominate the market for the flat-panel displays used in laptops, personal organizers and mobile telephones. They have their drawbacks, however, and light-emitting polymers are showing great promise as a complementary technology. Processing such polymers to produce a colour, pixelated display is one of the challenges. As they describe on page 829 of this issue, it is a challenge that Müller et al.1 have tackled in a new way.

The disadvantage of liquid-crystal devices is that the light must pass through various colour and polarizing filters before it reaches the eye. So, as everyone knows who has travelled in an aircraft with personal video screens, they can be viewed conveniently only if the screen is at right angles to the viewer. For flat-panel screens, one solution is to use organic fluorescent materials, which are themselves the actual light source and in principle visible from a much wider range of viewing angles. The emissive material can be a thin film of either an organic molecule or a polymer; fluorescence (electroluminescence) is induced by the injection of charge into a film of the emitter sandwiched between oppositely charged electrodes (ideally) powered by a small battery. Good red, green and blue electroluminescent materials are now available, and car radio and multicolour mobile telephone displays using small-molecule 'organic light-emitting diodes' (OLEDs) are on the market2. The drawback is that such materials can only be deposited using vacuum (sublimation) deposition techniques, in combination with a shadow mask to control where the molecule is deposited. This presents a problem of scale in large-area displays, although prototype television screens have been fabricated.

By contrast, fluorescent polymer light-emitting diodes (PolyLEDs) can be assembled by deposition from solution. Here the problems are to avoid impurities (in the polymer and the solvent) and not to dissolve away a film during deposition of another layer. One elegant method of delivering a polymer droplet of the right colour to a small dot (pixel) in the display is to use ink-jet printing3, and rapid progress has been made towards television-size prototype displays using ink-jet printing onto specially prepared wafers of polysilicon (Fig. 1). Simple monochrome PolyLED products are now also on the market, as demonstrated by the display in the electric shaver used by Pierce Brosnan in the latest James Bond movie Die Another Day. Müller et al.1 now describe a completely different way of solution-processing coloured displays, one that involves a clever chemical crosslinking method.

Figure 1: An ink-jet show. This full-colour, light-emitting polymer display, printed by the ink-jet technique using polyfluorenes, is 2.8 inches in size, diagonally.
figure1

It has 100 dots per inch, and displays 64 colours plus grey scale. This is the current standard for solution-processing technology. (Picture courtesy of the Seiko–Epson Corp.)

Electroluminescent devices operate by forcing positive and negative charges from oppositely charged electrodes into a sandwich device containing a thin film of the fluorescent organic or polymeric material4. The charges migrate in opposite directions through the material until they annihilate and cause fluorescence from the excited state. One of the most powerful families of stable light-emitting polymers is the polyfluorenes, which can conveniently be prepared in good yield and high molecular weight by the Suzuki reaction. Generically, this involves carbon-bond formation between aryl halide and boron compounds. In the case of producing polyfluorenes, it is the palladium-mediated polycondensation of a bis-boronate ester with an appropriate dibromo-substituted aromatic compound5.

The reaction schemes used by Müller et al. are outlined in Fig. 1 of their paper on page 830. They obtained the three primary polymers (red, green and blue) by 'tuning' the Suzuki copolymerization6,7 of the bis-boronate monomer with the comonomer containing reactive oxetane end-groups and various dibromo-substituted aromatic comonomers. To form a patterned device, each polymer was crosslinked using the standard photoresist techniques that are employed to make integrated-circuit patterns on silicon chips. Thus, solution deposition of the first polymer onto a transparent electrode (precoated with a conducting polythiophene derivative) in the presence of the photo-acid generator, followed by irradiation of the film through a shadow mask (diffraction grating), released photochemically generated acid in the regions under irradiation.

The acid released in the film caused the strained-ring oxetane end-group to undergo a ring-opening cationic polymerization, leading to crosslinked material. Washing with solvent removed the material that had not become crosslinked, and further gentle baking left the polymer in a well-defined pattern. The two remaining layers of emissive polymers were then deposited in the same way, followed by vacuum deposition of the top electrode, to give a device that showed good resolution and characteristics.

It might have been expected that release of acid and crosslinking would adversely affect the performance of the light-emitting device. Certainly, some of the polymers that were crosslinked showed reduced efficiency compared with those that were not, but others shone more brightly and could be operated at higher 'drive' voltages, apparently as a result of crosslinking. All of the crosslinked devices exhibited lower 'turn-on' voltages, which may have been a sign of the 'protonic doping' that often reduces the barriers to charge injection at the electrode interface. In general it seems likely that some of the other, more poorly performing features will be improved when devices are optimized.

This work is an exciting demonstration of the possibilities that can emerge from collaboration between polymer chemists and device physicists, chemists and engineers. But it remains to be seen whether devices patterned in this way can match the operating lifetimes of more than 10,000 hours exhibited by their ink-jet-printed counterparts, and whether their operation will be affected by the residual photo-acid residing in the polymer films.

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Correspondence to Andrew Holmes.

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Holmes, A. Polymers light the way. Nature 421, 800–801 (2003). https://doi.org/10.1038/421800a

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