  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.
