The basic principle of operation of polymer light emitting diodes, or P-OLEDs, is as follows:

• An amorphous film of the P-OLED material is sandwiched between two electrodes forming the anode and cathode on a transparent substrate
• Electronic charges are transported and 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.

P-OLEDs 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 P-OLEDs work - Device Structure

Device efficiencies have been improved via a combination of polymer and device modifications. The light emitting polymer (LEP) has been modified to increase its photoluminescence quantum yield (PLQY) whereas devices have been improved by adding a thin polymeric layer between the hole transport layer material (typically PEDOT-PSS) and the LEP. This additional layer is commonly referred to as a interlayer/primer layer (IL).
The LEP has been designed to exhibit higher electron mobility than hole mobility. The interlayer/primer layer, on the other hand, is designed with hole transport in mind and possesses a higher hole mobility than electron mobility, i.e. electron transport is favoured in the LEP while hole transport is favoured in the primer layer. The net effect of this is that electron and hole charges accumulate at the LEP:iL interface and away from either the cathode or anode. In this way, charge balance within the devices can be obtained, exciton (electron-hole pair in the excited state) formation maximized, and exciton quenching by either cathode or anode avoided.

Using this approach we have demonstrated External Quantum Efficiencies (EQE) of 5% or above for a range of colors from blue to deep red.

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 P-OLEDs:  Download our Technical Brief - Introduction to P-OLEDs

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