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Salt improves polymer emissions

17 Jun 2002

Production of organic electro-chemical cells is made easier by blending a salt with the light-emitting polymer.

By blending a molten salt with an electroluminescent polymer, researchers in France have developed a new method of producing organic light-emitting electrochemical cells (OLECs) - a low-cost and more efficient alternative to organic light-emitting diodes (OLEDs) (App. Phys. Lett. 80 679).

In an OLED, the light-emitting polymer is in its intrinsic undoped state and luminescence originates from the recombination of holes and electrons. In an OLEC, ionic compounds are blended into the light-emitting polymer and the mobile ions enhance the efficiency of the recombination process.

Most light-emitting polymers, however, are immiscible substances and conventional methods of adding ions involve synthesizing new polymers with solvating side-chains.

The French group however, has found a molten salt that can be blended with a polymer, thus eliminating the tedious synthesis process.

Olivier Stéphan and his colleagues from Joseph Fourier University in Grenoble used a trifluoromethylsulfonyl imide salt and the active polymers polyphenylenevinylene (PPV) and polyfluorene to make their devices. These emitted red light for the PPV derivative and blue light when using polyfluorene. The salt was chosen because it is molten at ambient temperature.

Stéphan told Optics.org: "There have been previous attempts worldwide to develop OLECs. These attempts involved complex synthesis of material capable of solvating salts. We have found an efficient way of obtaining OLECs with regular semiconducting polymers."

Using the polymer-salt mixture in a chloroform solution, an active layer is obtained by spin coating. The active layer has a thickness ranging from 100 to 400 nm depending on the amount of the salt.

The blue electroluminescence of the devices was measured using a calibrated photodiode. As the amount of salt was increased up to a maximum of 50% weight-for-weight, the emission intensity reached 4 µW/cm2 .

The initial applications for these devices include low-cost displays, for example for mobile phones.

Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.

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