Phosphorescent white organic light-emitting devices (Ph-WOLEDs) fabricated utilizing carbazole/thioxanthene-S,S-dioxide (EBCz-ThX) host material were investigated to enhance their efficiency by understanding the energy transfer process from the host to the dopant materials. Time-resolved photoluminescence (PL) spectra at room and low temperatures demonstrated that the energy transfer occurs between the EBCz-ThX host and the bis(2-benzo[b]thiophen-2-ylpyridine)(acetylacetonate)-iridium(III) (Ir(bt)2acac) orange dopant or the (bis(2-(4,6-difluorophenyl)pyridyl-N,C2′)iridium(III) picolinate (FIrpic) blue dopant. The energy transfer process was inferred from the PL decay curves of Ir(bt)2acac doped EBCz-ThX with doping concentrations of 5, 10, and 20%. The quantum efficiency in the Ph-WOLEDs with 5% Ir(bt)2acac doped EBCz-ThX is the highest since its energy transfer process from EBCz-ThX to Ir(bt)2acac involves the charge transfer states and is most efficient. The quantum efficiency of the Ph-WOLEDs with 5% Ir(bt)2acac doped EBCz-ThX emission layers at 1,000 cd/m2 showed the highest value of 10.2%, and the commission internationale de l'Eclairage (CIE) coordinates were (0.45, 0.46), indicative of the warm white emission.