Red and green phosphorescent OLED (organic light-emitting devices) were prepared using four types of blocking layer (BL) material with different charge/exciton-blocking characteristics. The luminance, emission spectrum, luminous efficiency, and lifetime under accelerated conditions of each device were determined. The results show that energy-level differences can determine the charge-carrier trapping behavior and also affect the distribution of the exciton formation zone inside the EML and blocking layers. It was also observed that the excitons can be formed at various positions but the major recombination region decides the color spectrum, efficiency and lifetime of the device. The red phosphorescent devices, compared to the green phosphorescent devices show different light-emitting characteristics when the BL material was changed. The green phosphorescent device shows decay curves for all the BL materials used.