Organic Light Emitting Devices (OLEDs) have reached industrial maturity in display technology, since OLEDs provide salient advantages such as high brightness, fast response, wide viewing angle, mechanical flexibility, and low cost manufacturing. Due to the ability of electroluminescence (EL) from triplet excited states as well as singlet excited states, phosphorescent OLEDs (PHOLEDs) have a potential to achieve 100% internal quantum efficiency. Therefore, PHOLEDs can offer a competitive external quantum efficiency. However, the operational stability of PHOLEDs is relatively poor. Several...