Cambridge Display Technology

The basic principle of operation of 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.

The structure of a basic P-OLED display device can be extremely simple, consisting of a sandwich containing:

• A transparent conducting anode with a large work function. Indium tin oxide (ITO) is commonly used, coated on a substrate
• A conducting polymer layer which transports and injects holes into the active layers (Hole Injection / Transport Layer)
• A thin organic interlayer material sometimes referred to primer layer developed by CDT to improve efficiency and lifetime
• A thin light emitting polymer layer less than 100nm thick (emissive Layer)
• A metallic cathode with a low work function, such as a barium/aluminium bi-layer.

The simplicity and solution processability of P-OLED materials together make P-OLEDs an exciting prospect for applications from small mobile displays though to large screen TVs and large area lighting panels.

Technology developments

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), while 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 an 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 is maximized, and exciton quenching by either cathode or anode is 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.