Study on charge dynamics and recombination process via transient electroluminescence analysis in blue fluorescence organic light-emitting diodes

Abstract

Fluorescent organic light-emitting diodes (FOLEDs) exhibit substantially different efficiency-lifetime characteristics depending on dopant materials even in the same host material. Thus, obviously charge and exciton dynamics and recombination process may vary depending on the dopants. However, an accurate understanding of charge and exciton dynamics with dopants in FOLEDS is lacking so far. Herin, triplet–triplet-fusion-based FOLEDs with two blue dopants, 2,5,8,11-tetra-tert-butylperylene (TBPe) and 2,12-di-tert-butyl-5,9-bis(4-(tert-butyl)phenyl)-5,9-dihydro-5,9-diaza-13b-boranaphtho[3,2,1-de] anthracene (t-DABNA) were investigated via transient electroluminescence (TrEL). Interestingly, the TrEL of the blue FOLEDs elucidated different charge profiles and recombination processes within the emission layers. The TrEL spikes after pulse-off and the characteristic times of EL intensity after pulse-on revealed the accumulated charge profile concerning the charge balance. In temperature-dependent TrEL, the delayed fluorescence, exciton decay time, and EL spikes after pulse-on reflected the emission mechanisms as well. The results showed that the electrical properties and the device performances of blue FOLEDs can be significantly impacted by the alignment of the energy levels of hosts and dopants, even with a low doping concentration. Furthermore, depending on the charge trap conditions, it was found that efficiency roll-off can be attributed to Dexter energy transfer from host to dopant and singlet–triplet annihilation by direct recombination on dopant molecules.