Device performance enhancement in quantum dot light-emitting diodes (QLEDs) is realized by adding a small amount of insulating polymer polymethyl methacrylate (PMMA) into the emitting quantum dot layer. Pool-Frenkel effect is observed through temperature-dependent current density-voltage experiments, indicating the important role of trap states, and the addition of the insulating PMMA helps to reduce the Poole-Frenkel barrier hence the trap depth. Reduced density and depth of trap states with PMMA are indeed observed through further capacitance measurements. This work contributes to a better understanding on the effects of traps in QLEDs.