We investigate the mechanism of operation of hybrid organic/colloidal quantum dot light emitting devices (QD-LEDs). Novel quantum dot (QD) deposition methods allow us to change the location of an emissive QD monolayer within a QD-LED multilayer structure. We find that the quantum efficiency of devices improves by >50% upon imbedding QD monolayers into the hole transporting layer <10nm away from the interface between hole and electron transporting layers. We consider two possible mechanisms responsible for this improvement: one based on a charge injection model of the device operation and the other based on an exciton energy-transfer model. In order to differentiate between the two suggested mechanisms, we fabricate a set of structures that enable control over charge injection into colloidal QDs. We find that the dominant process limiting QD-LED efficiency is charging of the QDs by trapped electrons. We demonstrate that with the set of organic materials implemented in this study, device efficiency is increased by maximizing energy transfer from organics to QDs and by limiting direct charge injection that contributes to QD charging.