Carrier injection and transport in the quantum dot (QD) layer was modulated by tuning the Schottky injection barrier and mobility of charge transport layers. The analyses indicate that stages of the J-V curve for hole injection must match with those of electron injection to achieve the charge balance in the QD layer. In addition, it was demonstrated that not only the parameters investigated but also other parameters such as charge trap density and energy levels must be considered to enhance the efficiency of the QLEDs. With the improved charge balance in the QD layer, the current efficiency of the quantum dot light-emitting diode display was improved from 26 to 40 cd/A.