Triplet fusion (TF) and singlet fission (SF) are two important spin-coupled exciton interactions that occur in rubrene-based organic light-emitting diodes (OLEDs). TF produces additional singlets, which increases fluorescence efficiency, while SF consumes singlets and lowers the fluorescence efficiency. In an effort to adjust the SF and TF processes in rubrene-based OLEDs, we changed the average molecular spacing (d) of rubrene by doping it at varying concentrations in the high triplet energy material 1,3-bis(9-carbazolyl)benzene (mCP). Using magneto-electroluminescence (MEL), we observed that TF increased, while SF decreased at ambient temperature as d was increased from 1.8 to 5.0 nm. This was further confirmed using MEL at different temperatures and current intensities. We found that the efficiency of rubrene-based OLEDs was improved by altering the value of d, with the highest efficiency being observed at d = 3.8 nm because of complete conversion of SF to TF (SF → TF). The SF → TF was explained using a model that describes Dexter- and Förster-energy transfer in SF and TF processes with functions that have a different dependence on d. This difference causes the rate constant of SF to decrease more rapidly than that of TF. The TF will be primary when d goes between the Dexter and Förster radii, leading to complete SF → TF at ambient temperature. This work presents a promising approach to improve the efficiency of rubrene-based OLEDs.
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