A transparent electrode is an essential component that has a strong influence on the extraction of light from organic light-emitting diodes (OLEDs) due to its effect on both electrical and optical performance. In this work, we present theoretical studies, full wave simulations, and experimental results to evaluate the influence of the thickness of epsilon negative tri-metal layer (TML) electrodes on the performance of red phosphorescent OLEDs (PHOLEDs) via an optical microcavity effect. The results show that the external quantum efficiency of the optimized TML-based red PHOLED of 17.6% is significantly improved, and it is approximately 40% higher than that of the conventional indium tin oxide (ITO)-based red PHOLED of 12.5%.