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

We observed a significant decrease in electroluminescence (EL) in the high-current-density region (J>1A∕cm2) in organic light-emitting diodes (OLEDs). The decreased external quantum efficiency (ηext) in an OLED with a cathode diameter of d=50μm 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=1000μm 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 (46k∕Wm−1K−1) combined with a small cathode diameter of 50μm 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=447A∕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.