Abstract
Nonradiative triplets in fluorescent organic light emitting diodes (OLEDs) can lead to increased efficiency through triplet-triplet annihilation, or to decreased efficiency due to singlet-triplet annihilation. We study the tradeoff between the two processes from the electroluminescence transients of an OLED comprising a tetraphenyldibenzoperiflanthene (DBP) doped rubrene emissive layer, whose emission spectrum peaks at a wavelength of 610 nm. The electroluminescent transients in the current density range, 4 mA/cm(2)<J<57 A/cm(2), are modeled based on singlet and triplet density dynamics. Our analysis shows that triplets positively contribute to the OLED efficiency at J<2.2 A/cm(2), while decreasing the efficiency at higher J. The high OLED peak external quantum efficiency of 6.7% and rapid efficiency roll-off with J are quantitatively explained by the tradeoff between triplet-triplet and singlet-triplet annihilation. The model suggests optimal materials properties needed for achieving high efficiency at high brightness in fluorescent OLEDs.
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