Singlet-singlet and singlet-heat annihilations in fluorescence-based organic light-emitting diodes under steady-state high current density

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Abstract
We observed a significant decrease in electroluminescence (EL) in the high-current-density region ( J > 1 A ∕ cm 2 ) in organic light-emitting diodes(OLEDs). The decreased external quantum efficiency ( η ext ) in an OLED with a cathode diameter of d = 50 μ m was in excellent agreement with the singlet–singlet annihilation (SSA) model. In contrast, the decreased η ext in an OLED with a cathode diameter of d = 1000 μ m coincided well with the singlet-heat annihilation (SHA) model. These results suggest that large OLEDs generate significant Joule heat, which causes additional excitondissociation in addition to SSA. We also used a very thermally conductive sapphire substrate ( 46 k ∕ W m − 1 K − 1 ) combined with a small cathode diameter of 50 μ m in an OLED to alleviate the η ext roll off characteristics and achieved injection at a much higher current density. With this device configuration, a maximum current density of J max = 447 A ∕ cm 2 was achieved. However, decreased η ext based on SHA was observed in the high current region, indicating that significant Joule heat was generated under a few hundred A ∕ cm 2 that induced additional excitondissociation even when the sapphire substrate was used.