Highly efficient tandem OLED based on C₆₀/rubrene: MoO₃ as charge generation layer and LiF/Al as electron injection layer

Highlights: • Highly efficient blue fluorescent tandem OLEDs are fabricated. • The optimal tandem OLED consists of C₆₀/rubrene: MoO₃ as a CGL and LiF/Al as an EIL. • Current efficiency and power efficiency of optimal tandem OLED is markedly enhanced. • The turn-on and driving voltages of optimal tandem OLED is obviously reduced. - Abstract: Tandem organic light-emitting diodes (OLEDs) have received much attention in solid-state lighting due to their high current efficiency, long lifetime and excellent stability. The highly efficient blue fluorescent tandem OLEDs based on the charge generation layer (CGL) of C₆₀/rubrene: MoO₃ and the electron injection layer (EIL) of LiF/Al were fabricated. The ultra-thin Al layer in EIL was introduced to further increase electron injection from CGL to the emission unit. We found that the maximal current efficiency and power efficiency of optimal tandem device can reach to 43.1 cd/A and 15.1 lm/W, respectively, which are approximately 2.8 and 1.9 times compared with those of single-emissive-unit device. Moreover, compared with the traditional tandem device, the driving voltage of the optimal device is reduced by 6 V, and the turn-on voltage is reduced by 2.4 V. We analyzed the mechanism and characterization of these tandem devices. The effective charge separation and transport of C₆₀/rubrene: MoO₃, and excellent electron injection ability of ultra-thin Al layer are the main factors for the remarkable enhancement in both current efficiency and power efficiency of tandem OLEDs.