Organic light emitting diodes (OLEDs) are considered as the next generation of light sources for illumination. They are lighter, thinner, more efficient and easier for large area applications than usual LEDs. In addition, OLEDS offer wide tunability over a broad spectral coverage, by varying the chemical structure of the emissive species or by modifying the device structures. This work identifies for the first time the fundamental process occurring in polymeric light emitting devices by exploiting an advanced time resolved spectroscopic technique which allows monitoring the dynamics of opposite charges encounters and coalescence.
The technique, namely field-assisted pump-probe spectroscopy, has been developed in our laboratories in order to extend the valuable and specific characterization provided by femtosecond spectroscopic techniques (namely: pump-probe) to semiconducting layers in devices structures under operational conditions, i.e. in presence of electric field, current and interface potentials. This has important fallout in the understanding of device performances.
In organic LEDs opposite charge carriers are injected from the electrodes and after migration get captured to form excitons, expected to be in the singlet state (emissive state) for 25% of the encounters from theoretical arguments related to fundamental arguments (spin multiplicity). Femtosecond UV pulses generate pairs of charges in poly-fluorene, the material under investigation, defining, a trigger (t=0) for the time resolution measurements of the device dynamics. Observation is taking place by sending a broad band probe pulse through the device optical window at variable delays with respect to the pump.
We find that free charges coalesce into singlets and triplets states. Our results indicate that the efficiency of singlet formation of the order of 70% is much higher than the expected one of 25% (from simple state degeneracy arguments). Since the singlets are the emitting species, our results point out that organic semiconductors can aim to reach a much higher efficiency than other materials.
The work has been done in collaboration with the Imperial College of London (UK), the group of Prof. Donal Bradley, which is well known for the contribution to the organic electronic field, and the Christian Doppler laboratory in Graz (Austria). The fruitful collaboration with both institutions is continuing in the area of science and technology of organic materials.
The ability of reproducing in a controlled way real device-situations of high technological impact is great value for understanding and improving device performance.
Funding was provided by the European Community through different contracts: Marie Curie fellowship and EUROLED RTN network.
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