The optical and electronic characteristics of thick OLEDs were examined by varying the thickness of the carrier transport layers (NPB and Alq<sub>3</sub>) for the first time. For having better mode guiding, more than 285nm thickness device was studied in this paper. The experimental results show the threshold voltage is proportional to the total thickness but the breakdown electric fields decrease. I-V curves can be fitted with a second-order polynomial form, and these fit-ting curves can be used for modeling the carrier transport in thick OLEDs. Eventually, a comprehensive model is pro-posed to describe the carrier transport due to the trap effect in bulk organic materials. It shows this model is in a good agreement with our measured I-V curves.