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 > 1 A/cm(2)) in organic light-emitting diodes (OLEDs). The decreased external quantum efficiency (eta(ext)) in an OLED with a cathode diameter of d=50 mu m was in excellent agreement with the singlet-singlet annihilation (SSA) model. In contrast, the decreased eta(ext) in an OLED with a cathode diameter of d=1000 mu m 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 k/Wm(-1) K-1) combined with a small cathode diameter of 50 mu m in an OLED to alleviate the eta(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 eta(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 exciton dissociation even when the sapphire substrate was used. (c) 2005 American Institute of Physics.