LEDs produce light due to the recombination of negative and positive charges inside semiconductor materials. In organic LEDs (OLEDs), this process occurs in a layer of carbon-based compounds such as polymers. These polymers may be printed directly onto a substrate, significantly simplifying manufacture and potentially making the device flexible. They are now commonly used as mobile phone displays, and in 2007 the first commercial OLED television was unveiled.

One commonly-used polymer is called poly(p-phenylene vinylene) (PPV) and Chang-Tao Hsiao and Shih-Yuan Lu of National Tsing-Hua University, Taiwan, have now found a simple way to add zinc atoms to PPV to significantly improve its performance.1

According to Lu, conventional chemical routes to such metal-polymer complexes are tedious and complicated, particularly the purification steps. Here, the scientists used a one-step process called chemical vapour deposition polymerization (CVDP), where the raw materials for the polymer were allowed to combine with zinc in a hot furnace. The product then condensed onto a substrate to form a thin film. ‘The process can be readily conducted in a continuous fashion, which is beneficial for mass production,’ says Lu.

The scientists say that this is the first time the process was applied to metal-polymer complexes, and also the first time that a zinc-PPV film was used in an OLED device.

Fig. 1: Metal-polymer complexes containing a higher concentration of zinc produced a brighter, bluer light.

Hsiao and Lu were able to change the amount of zinc in the complex by moving the source of the metal between different temperature zones within the furnace. PPV itself normally emits yellow light and here, adding more zinc switched that to bluer light (see picture), as the metal changed the energy levels available to charge carriers in the polymer.

Adding zinc also made the polymer 71 times more efficient at converting electricity into light —perhaps because the metal atoms enhanced recombination of positive and negative charges or reduced the interference from neighbouring polymer chains on the light emission processes, the scientists suggest.