Simplified multilayer green light phosphorescent organic light emitting diodes (PHOLED) with the structure of ITO/MoO3 (1 nm)/CBP(20 nm)/CBP:Ir(ppy)(3) (1 wt%) (15 nm)/TPBi(60 nm)/LiF(0.5 nm)/Al were fabricated via thermal evaporation and in situ thermal treatment (heating the OLED substrates to certain temperatures during the thermal evaporation of the organic materials) was performed. The effect of the in situ thermal treatment on the electroluminescence (EL) performance of the PHOLED was investigated. It was found that the OLED exhibited strong EL dependence on the thermal treatment temperatures, and their current efficiency was improved with the increasing temperature from room temperature (RT) to 69 degrees C and deteriorated with the further increasing temperature to 105 degrees C. At the brightness of 1000 cd/m(2), over 80% improvement of the current efficiency at the optimal thermal treatment temperature of 69 degrees C (64 cd/A) was demonstrated compared to that at RT (35 cd/A). Meanwhile, the tremendous influences of the in situ thermal treatment on the morphology of the multilayer CBP/CBP:Ir(ppy)(3)/TPBi were also observed. At the optimal thermal treatment temperature of 69 degrees C, the improvement of the EL performance could be ascribed to the enhancement of the electron and hole transporting in the CBP:Ir(ppy)(3) emitting layer, which suppressed the triplets self-quenching interactions and promoted the charge balance and excitons formation. The working mechanism responsible for such EL dependence was discussed in detail. (C) 2015 Elsevier B.V. All rights reserved.