Suppression of roll-off characteristics of electroluminescence at high current densities in organic light emitting diodes by introducing reduced carrier injection barriers

We experimentally investigated suppression of the roll-off characteristics of the electroluminescence efficiency at high current densities in organic light emitting diodes (OLEDs). To increase exciton density, we propose a nonheterostructure OLED that consists of a single emitting layer of 4,4′-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) and layers locally with doped donors/acceptors on the cathode and anode sides. The OLED exhibited suppression of the roll-off characteristics at high current densities over 100 A/cm2 with balanced bipolar injection and transport, resulting in the high exciton density of 1024 cm−3 s−1. Furthermore, amplified spontaneous emission with a relatively low threshold of Eth=24 μJ/cm2 was obtained by optically pumping the single-layer device. However, to realize electrical excitation it was necessary to reduce the lasing threshold by two orders of magnitude or to inject a 100 times higher current density.