Increasing the horizontal orientation of transition dipole moments in solution processed small molecular emittersElectronic supplementary information (ESI) available: Synthesis of tBu-DACT-II; optical constants of TPBi and PFO; fluorescence emission of PFO, PFO:OPV7 (2%) and TPBi:OPV7 (2%); AFM height scan of PFO:OPV7 (2%), not annealed and annealed layers; angular dependence of p-polarised photoluminescence of TPBi:Ir(MDQ)2(acac) evaporated layer (graph and table with value); description of Ori

The efficiency of organic light emitting diodes (OLEDs) can be improved by controlling the orientation of the transition dipole moment of the emitters. Currently, no effective methods exist for orienting the transition dipole moments in solution processed active layers for OLEDs. We investigate the orientation of the transition dipole moment of small molecular emitters in a host matrix of poly(9,9-dioctylfluorene) (PFO) by means of angle dependent luminescence intensity measurements. The polymer chains of the host orient predominantly in the plane of the film. Fluorescent p -phenylenevinylene oligomers with 6 and 7 repeat units (OPV6, OPV7) are found to also orient preferentially horizontally. The orientation of the emitters can be improved by thermal annealing with up to 90% of transition dipole moments oriented in the plane of the film. The phosphorescent emitter Ir(MDQ) 2 (acac) shows a degree of horizontal orientation in the polymeric host matrix lower than that which is observed for oligomers, but as high as is observed for the same emitter in evaporated layers. A carbazole derivative capable of thermally activated delayed fluorescence shows a small preference for vertical orientation within the polymer host. The strong orientation of OPV6 and OPV7 in the oriented polymer host is rationalized in terms of their high aspect ratios. The use of PFO as host material in host/guest systems allows achieving horizontal orientation of transition dipole moments in solution processed oligomers and small molecular emitters. The transition dipole moments of the small molecular emitters in a polyfluorene host matrix align strongly in the plane of the film, when deposited from solution and even more when thermally annealed.