Spin polarised electrodes for organic light emitting diodes

Electroluminescence in organic semiconductors strongly depends on the relative population of singlet and triplet excitonic states, i.e. on the carrier spin statistics. In conventional organic light-emitting diodes (OLED) the optical emission is usually based on fluorescence from excited singlet states, while triplet states provide phosphorescent radiation. Radiative emission from singlet excitons has a very large transition probability providing the main radiative pathway. Quantum constraints determine the statistics of singlet-triplet exciton formation from charge recombination to a 1:3 partition. Controlling the spin statistics by injecting carriers with the desired spin polarisation would open the way to enhance a chosen electronic transition and therefore increase the device efficiency. We show that spin polarised materials can successfully replace conventional electrodes in OLEDs. Electrical and optical characterisations of Alq3/TPD based OLEDs for both normal and spin polarised electrodes are presented. Epitaxial thin films of the manganite La0.7Sr0.3MnO3 were used as spin polarised hole injectors, while iron and cobalt films were used as spin polarised electron injectors. The results are a first step towards the fabrication of devices where the light emission can be tuned by controlling the spin injection.