The basic principle of operation of polymer light emitting diodes, or PLEDs, is as follows:

• An amorphous film of the PLED material is sandwiched between two metallic electrodes forming the anode and cathode on a transparent substrate
• Electronic charges are injected into the polymer from the electrodes: electrons from the cathode, and 'holes' from the anode
• The electrons and holes 'capture each other' through electrostatic interaction
• Radiative recombination of electron and hole generates light
• The wavelength of this emitted light depends on the band gap of the polymer used.

PLEDs can be used to produce light of a very wide range of wavelengths - including light outside the visible range - by modifying the precise structure of the polymer used.

Internal device efficiencies have been improved by modifying the polymer material to be more or less electron withdrawing and therefore to have higher or lower electron affinity. 

 How PLEDS work - Device Structure

In the example shown below, the cyano group is electron withdrawing and therefore pushes down the barrier to electron injection. The cyanoPPV layer therefore acts as an electron transport layer. Because the bandgap of the cyanoPPV layer is lower than the PPV layer, recombination takes place in this layer. In addition to the increase in efficiency, emission from blue to the near-infrared has been obtained from a range of polymers, all at efficiencies of over 1%.

The figure shows a schematic of an efficient light emitting structure.  Click to enlarge

Heterostructures can be designed using polymer material. Organic synthesis allows additional degrees of freedom in tuning bandgap and work function of semiconductors

For a more detailed look at the chemistry of PLEDs:  Download our Technical Brief - Introduction to PLEDs 

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