Spin polarised injection in sexithienyl thin films

Abstract

Electroluminescence in organic semiconductors strongly depends on the relative population of excited singlet and triplet excitonic states resulting from recombination of injected carriers. In conventional organic light-emitting diodes (OLED) optical emission is usually based on fluorescence from excited singlet states1, although phosphorescence from triplets is also investigated2. Governing the spin statistics of the injected carriers would open the way to amplify a chosen electronic transition increasing therefore the OLED efficiency. Direct injection of carriers with high degree of spin polarisation along a given axis should lead to preferential population of either singlet or triplet excited states, depending on the relative electrodes polarisation. Here we report a first experimental evidence of direct spin polarised (SP) injection in sexithienyl (T6), a prototypical organic semiconductor, from half-metallic manganites. The spin penetration depth in T6 is about 250-300 nm at room temperature. The results are discussed taking into account possible spin-flip mechanisms in organic material and interface effects.