Horizontal molecular orientation of light-emitting oligofluorenes in spin-coated glassy organic thin films

Horizontal orientation of light-emitting chromophores plays a critical role in the light outcoupling efficiency and the overall performances of organic light-emitting diodes. Here, we demonstrate that such a horizontal orientation of light-emitting molecules can be achieved in solution-processed blends containing highly fluorescent oligofluorenes dispersed into various small molecule host materials. The influences of the host material, the solvent used for the spin-coating, the doping concentration of the oligofluorenes and the oligofluorene chemical structure on these orientational processes are investigated using variable angle spectroscopic ellipsometry and angle-dependent photoluminescence measurements. By clarifying the roles played by these parameters in the horizontal orientation of the emitters, the results provide crucial insights into these molecular orientational processes in spin-coated glassy organic thin films, which should be relevant for the development of high-efficiency solution-processed organic light-emitting diodes in the near future.