Influences of molecular orientations on stimulated emission characteristics of oligofluorene films

Efficient luminescence and interesting thermal properties of oligofluorenes provide the opportunity to investigate influences of molecular orientational distributions on characteristics of stimulated emission in organic semiconducting films. While vacuum-deposited terfluorene films exhibit preferential in-plane molecular orientation, they are reoriented and become isotropic with annealing around or above its glass transition temperature. Studies of stimulated emission (amplified spontaneous emission) show that the anisotropic terfluorene films exhibit higher gain and lower threshold than the isotropic films, due to larger confinement of the waveguided mode and larger cross section of stimulated emission. A net gain as high as 78 cm−1 (at a pump intensity of ∼30 kW/cm2) and a threshold as low as 0.7 kW/cm2 were observed. The results suggest that the gain and threshold of stimulated emission in organic semiconducting films could be substantially improved beyond the isotropic case if molecules and their transition dipole moments can be well aligned with the polarization of stimulated emission yet without causing significant loss and emission quenching.