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

Nakanotani, Hajime; Sasabe, Hiroyuki; Adachi, Chihaya
May 2005
Applied Physics Letters;5/23/2005, Vol. 86 Issue 21, p213506
Academic Journal
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 k/Wm-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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