We observed a significant decrease in electroluminescence (EL) in the high-current-density region (J>1 A cm2) in organic light-emitting diodes (OLEDs). The decreased external quantum efficiency (ηext) in an OLED with a cathode diameter of d=50 μm was in excellent agreement with the singlet-singlet annihilation (SSA) model. In contrast, the decreased ηext in an OLED with a cathode diameter of d=1000 μm coincided well with the singlet-heat annihilation (SHA) model. These results suggest that large OLEDs generate significant Joule heat, which causes additional exciton dissociation in addition to SSA. We also used a very thermally conductive sapphire substrate (46 kW m-1 K-1) combined with a small cathode diameter of 50 μm in an OLED to alleviate the ηext roll off characteristics and achieved injection at a much higher current density. With this device configuration, a maximum current density of Jmax =447 A cm2 was achieved. However, decreased ηext based on SHA was observed in the high current region, indicating that significant Joule heat was generated under a few hundred A cm2 that induced additional exciton dissociation even when the sapphire substrate was used.
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