Non-phosphor-doped fluorescent/phosphorescent hybrid white organic light-emitting diodes with a sandwiched blue emitting layer for simultaneously achieving superior device efficiency and color quality

In this study, an efficient deep-blue fluorescent OLED with the maximum EQE of 5.92% was achieved by employing a sandwiched blue emitting layer (EML) of 4P-NPD/4P-NPD : Bepp₂ (1 : 1)/Bepp₂. It was demonstrated that the blue light emission simultaneously originated from 4P-NPD and Bepp₂, and the main carrier recombination zone was located at the co-doped 4P-NPD : Bepp₂ (1 : 1) layer. Based on the above facts, by incorporating complementary phosphorescent ultra-thin EMLs (UEMLs) into different positions of EML of the above blue device, a series of non-phosphor-doped two- and three-color hybrid white OLEDs (WOLEDs) were fabricated, of which the utilization and the energy transfer of singlet and triplet excitons were investigated in detail. Finally, several proposed non-phosphor-doped four-color hybrid WOLEDs with complementary phosphorescent UEMLs simultaneously incorporated into the center and the two sides of the sandwiched blue EML were developed. The resulting WOLEDs showed high external quantum efficiencies (EQEs) ranging from 18.88% to 21.46%, demonstrating complete exciton utilization. Also, the optimized WOLED simultaneously achieved a high maximum EQE and a color rendering index of 19.35% and 93, respectively. Such a superior device performance can be due to (i) the highly efficient blue fluorescence emission realized by the sandwiched blue EML and (ii) effective manipulation and exploitation of singlet and triplet excitons by the novel device structure.