A Novel Triplet Green Host System and Charge Balance Tuning for High Performance Singlet Blue Devices

Our former findings on the charge balance in blue fluorescent devices show a clear connection between the basic performance parameters: Efficiency and lifetime both show an optimum with respect to voltage. Unfortunately, for devices using a conventional hole injection and NPB as hole transport material, the optimum values do not occur at the same voltage. Moreover, the optimum voltage for lifetime is much higher than that for optimum efficiency. After investigating the underlying charge balance issues, we developed new stack layouts and hole transport materials to shift the lifetime optimum to lower voltage values. Using an appropriate hole injection layer and one of our latest hole transport materials, we can now combine excellent lifetime, low voltage and good efficiency in one fluorescent blue device. In addition, we obtain 10% EQE by carefully tuning the hole transport side in fluorescent blue devices. No special hole blocking layer is required to obtain this high efficiency. For green phosphorescent devices, we developed a thermally stable co-host material which allows for the use of low emitter concentrations in the range of 5% without sacrificing device performance. We combine this material with an electron transporting host that shows a slight initial rise in lifetime when a low emitter concentration is employed. Due to these features, the tuning of the host mixing ratio results in excellent LT95 values in combination with good voltage, efficiency and low efficiency rolloff for high luminance.