16 June: Blue light shines on polymer light emitting diodes

Separate research groups have discovered families of stable blue-light emitting materials that could overcome a longstanding drawback of polymer light-emitting diodes (PLEDs).

PLEDs, which are based on polymeric materials that light up when an electric current passes, have yet to revolutionise a display industry dominated by cathode-ray, liquid crystal and plasma-screen technologies. The problem is that in full-colour PLED displays (consisting of red, green and blue-emitting pixels), the blue pixels have a relatively limited lifespan. 

PLED displays have many advantages over current technologies. They need no additional backlights and fewer filters, unlike LCDs, thus providing clearer, brighter colour images with faster response times (even at low temperatures), and almost 180^o viewing angles. PLEDs are also robust, highly flexible, cheap to build, and potentially more energy efficient than other types of display. As well as flexible displays of any size or purpose, PLEDs could be used for efficient low-voltage lighting. 

They use special polymers with extended arrays of conjugated carbon-carbon double bonds, which form a semiconductor-like band structure within the polymer. This band structure allows negative and positive charges to be injected and conducted through the polymer when a thin film is sandwiched between metallic electrodes and a potential applied. Opposite charges neutralise when they meet, giving off light with a colour that depends on the energy gap between the PLED’s bands: the larger the band gap, the bluer the emitted light. 

To be truly competitive, PLEDs will need lifetimes in excess of 100 000 hours. Red- and green-emitting PLEDs achieve this, but it is only recently that Cambridge Display Technology (CDT; see Chemistry World, April, p42) achieved such performance with a blue device. This is because blue light is the most energetic of the three wavelengths. Unfortunately, the blue-emitting industry standard - poly(9,9-di-octyl)fluorene (PF) -  degrades to a green-emitting ketonic product during device operation and in air. 

Two independent teams of chemists - one based at the University of Cambridge, UK; the other a collaboration between Donetsk University, Ukraine and the University of Durham, UK, have produced new stable variants of PF. 

Essentially, the carbon atom at the PF monomer’s 9-position is replaced with a substituted second-row element. The Cambridge team, in conjunction with CDT, made 2,7-dibenzosilole and then polymerised it;¹ The Durham-Donetsk collaboration replaced the carbon at PF’s 9-position with SO₂to prepare dibenzothiophene-S,S-dioxide, which was then formed into co-oligomers with 9,9-dihexylfluorene.²

’PF is good at transporting positive charges but poor at electron transport. Conceptually, our approach is similar to the Cambridge group’s in that we both needed to lower the energy of the semiconductor-like bands in the polymer,’ said Martin Bryce at the University of Durham, who coordinated the Durham-Donetsk project. ’This would improve PF’s electron transporting properties and stop it degrading. Both our approaches achieve this: the Cambridgegroup’s enhanced colour stability of their new polymer over PF is impressive,’ he told Chemistry World.

Andrew Holmes, professor of chemistry at the Universityof Melbourne, who led the Cambridge group when he was based there, agrees: ’Our approach completely avoids this problem of ketone formation by being a better electron acceptor than PF. Our new polymer is also thermally stable and shows good blue-light emission in a PLED device. I am also delighted at the excellent work of Martin Bryce’s Durham-Donetsk collaboration, which achieves the same goal with its co-oligomers - a stable blue-light emitting polymer.’

Last year technology firm Seiko-Epson demonstrated a 40-inch PLED display. Now Holmes predicts the arrival of PLED screens in hand-held personal organisers ’in the very near future.’ Lionel Milgrom.