The charge recombination process is integral to the operating mechanism of organic light-emitting diodes (OLEDs). In particular, the fraction of recombination events resulting in singlet excited states sets an upper limit on operating efficiency. Furthermore, one explanation of the effects of small magnetic fields on the efficiency OLEDs involves magnetic effects on the recombination process. Here, we use the common phenomenon of delayed luminescence in fluorescent conjugated polymers due to recombination of photogenerated charge pairs as a tool to investigate the recombination process. By deliberate addition of small gold nanoparticles, we are able to measure both fluorescence and phosphorescence in the long-lived emission of F8BT (Poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl) films that persist after the directly photogenerated singlet state has decayed. We observe power law decay of the fluorescence on the microsecond timescale due to charge pair recombination and study its excitation pulse energy and magnetic field dependences. Taking advantage of spin-orbit coupling provided by gold nanoparticles incorporated into the conjugated polymer film, we are also able to track the dynamics and magnetic field dependence of the phosphorescence. Our data support an overall picture where charge recombination must be mediated by a charge transfer state.

Gold nanoparticle assisted delayed luminescence in conjugated polymers

253rd National Meeting (2017)

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