Increasing the horizontal orientation of transition dipole moments in solution processed small molecular emitters.

Alessia Senes,Katsuaki Suzuki,Horst Greiner,Joanne S. Wilson,René A. J. Janssen,Stefan C. J. Meskers,Chihaya Adachi,Hironori Kaji

doi:10.1039/c7tc01568b

Abstract

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.

Highlights

Preferential orientation of the transition dipole moment of small molecular emitters in the horizontal plane of the organic layer for organic light emitting diodes (OLEDs) has currently only been observed for layers deposited by thermal evaporation in high vacuum.[6,13]
Rather different, molecular emitters which are each embedded in the polymeric host (Fig. 1) via solution processing to determine if PFO has the capability to orient the transition dipole moment of small molecular emitters
The results demonstrate that the emitters with the most elongated backbone (OPV6 and OPV7) show the highest degree of horizontal orientation of the transition dipole moment

Summary

Introduction

Preferential orientation of the transition dipole moment of small molecular emitters in the horizontal plane of the organic layer for OLEDs has currently only been observed for layers deposited by thermal evaporation in high vacuum.[6,13] a significant. For films of PFO deposited from solution by spin coating, preferential horizontal orientation of the polymer chains has been reported.[23] We investigate four, rather different, molecular emitters which are each embedded in the polymeric host (Fig. 1) via solution processing to determine if PFO has the capability to orient the transition dipole moment of small molecular emitters. The guests represent characteristic fluorescent and phosphorescent molecular emitters with relevance for OLED applications They possess very different shapes and the comparison between them enables assessing the influence of the aspect ratio of the molecules on the orientation of their transition dipole moment in solution processed films. For the blend PFO:tBu-DACT-II, the two components were mixed in a 91 : 9 weight ratio, dissolved in toluene 0.5 w/v%, and deposited by spin coating at a speed of 800 rpm on quartz substrates The comparison of the experimental data with the optical simulations allows one to quantify the degree of orientation of the transition dipole moments of the emitters in the host

Results and discussion

Discussion

Conclusion