Organic light-emitting diodes have been developed successfully in recent years for application in a wide range of displays. However the ultimate energy efficiency (lumen/W) had been considered limited to 25% because only 25% of electron hole captures events could lead to light-emitting excited states. Recent discoveries, however, indicate that this limit can be greatly exceeded. If this is achieved in polymer LEDs this will greatly extend energy efficiency and hence the range of applications, including both small hand-held displays and also very large displays. The STEPLED programme will develop the basic science in close coordination with display development (display and drivers), and will deliver polymer displays with at least 50 lumen/W efficiency (double presently available performance with polymers).
The objectives of STEPLED are to develop the basic understanding of the science which controls the spin states of polymer-based LEDs, and to use this understanding to provide direction for the next phase of industrial development. STEPLED will establish whether the better route to achieving higher efficiency is through the use of triplet-emitting 'dopants' or with singlet-emission, through engineering of the singlet-triplet ratio. The programme can be judged at its termination on whether or not it has made a decisive impact in the technological directions taken by European industry in the next phase of development. It will achieve this if it shows how to raise efficiencies in polymer-based LEDs significantly above present levels, and the target set is 50 lumen/W for green (or equivalent efficiencies for red and blue).
The first phase of the programme is to compare critically the two approaches to better efficiency which recent discoveries have indicated as promising:
1) Development of triplet-emitting polymer-based LEDs. Routes will be explored to place appropriate 'guest' triplet-emitting species in polymer diodes, either distributed in solid solution, or directly incorporated into the polymer, either within the main chain or on a side chain. The necessary range of measurements will be carried out to establish detailed models for operation, and the output will be clear guidance for processing of diodes, which are both efficient and realistically manufactured and long-lived in operation;
2) Development of singlet-emitting polymer LEDs. The materials-dependence of the singlet-triplet ratio will be measured systematically, and compared with quantum chemical modelling. Novel electrodes capable of injecting spin-polarised electrons will also be used to establish whether the singlet:triplet ratio can be controlled in this way. The output from this approach should be a clear basic understanding of the means to optimise efficiency for PLEDs with singlet exciton emitters through control of the spin physics. This should allow decisions to be taken as to whether singlet emission is viable, or indeed superior, to triplet emission (particularly important for blue PLEDs). The second phase of the programme then extends the work to downstream activities undertaken by the industrial partners. In particular, the choice between singlet and triplet emission has important implications for the system integration of the display. Triplet-emitting diodes currently suffer from saturation in emission at high brightness and this restricts their use in passive matrix-addressed displays (though not for active-matrix addressing). An important later output from the programme is therefore an assessment of the practicality of different drive schemes for these higher efficiency schemes.
At 12 months, new materials will have been developed to enable polymer LEDs with efficient triplet-emitting dopants, and in parallel, to enable measurement and optimisation of singlet-emitting polymers;
At 24 months, clear strategies for optimising high efficiency operation under different drive conditions (active and passive matrix), and efficiencies above 50 lumen/W will be delivered. Demonstrator matrix displays using these materials will be available.
Funding SchemeCSC - Cost-sharing contracts
CB3 0HJ Cambridge, Cambridgeshire
65926 Frankfurt Am Main
5621 JG Eindhoven