Large contribution of triplet excitons to electro-fluorescence in small molecular organic light-emitting diodes

Singlet exciton formation in working fluorescent devices was investigated by analyzing the magnetic field effects on electroluminescence intensity based on a rate model. Two magnetic field sensitive processes including charge recombination and triplet–triplet annihilation (TTA) are suggested to be involved in the generation of singlet excitons. It reveals that TTA process produces considerable extra singlets which account for as high as 19% of total singlets at 20 K in non-doped device, 34% at 20 K and 17% at room temperature in doped device, causing the total singlet generation yields exceeding the classical 0.25 spin statistics limit.

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► Singlet exciton formation in working fluorescent devices was investigated by analyzing the magneto-electroluminescence.
► Charge recombination and triplet-triplet annihilation are suggested to be involved in the generation of singlet excitons.
► Triplet-triplet annihilation produces considerable extra singlets, causing total singlet generation yields exceeding 0.25.