Transient electroluminescence spikes in small molecular organic light-emitting diodes

Abstract

A comprehensive study of transient ns electroluminescence (EL) spikes that exceed the dc level and μs-long EL tails following a bias pulse in guest-host small molecular organic light-emitting diodes (SMOLEDs), including relatively efficient devices, which elucidates carrier and exciton dynamics in such devices, is presented. The transient EL is strongly dependent, among other parameters, on device materials and structure. At low temperatures, all measured devices, with the exception of Pt octaethylporphyrin (PtOEP)-doped tris(8-hydroxyquinoline) Al (Alq 3 ) SMOLEDs, exhibit the spikes at ~70-300 ns. At room temperature (RT), however, only those with a hole injection barrier, carrier-trapping guest-host emitting layer, and no strong electron-transporting and hole-blocking layer (such as 4,7-diphenyl-1,10-phenanthroline (BPhen)) exhibit strong spikes. These narrow and appear earlier under post-pulse reverse bias. To further elucidate the origin of the spikes, we monitored their dependence on the pulsed bias width and voltage, the doped layer thickness, and its location within the OLED structure. The characteristics of the μs-long tails were also

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“…31 Therefore, direct charge recombination on 2CzPN in the EML should occur. In addition, the recombination should also occur on the host because the emission of mCP is observed from the EL spectra, indicating that the energy of 2CzPN emission is also transferred from the host.…”