Solution-processable charge injection materials have gathered considerable attention in the field of organic optoelectronics owing to their ability to enhance device efficiency. Of those candidates, polyethylenimine (PEI) polymers have been widely explored, but their effect on the device physics in organic light-emitting diodes (OLEDs) are not completely elucidated yet. In this work, the impact of ZnO:PEI hybrid nanocomposite as an electron injection layer (EIL) of OLEDs on the device performance is investigated. Two different PEIs with the different spatial molecular configuration of the polymer, either linear or branched-type, are employed. Both PEI polymers have the ability to effectively reduce the work-function of ZnO. The ZnO:linear-PEI EIL shows higher device efficiency in comparison to the ZnO:branched-PEI EIL in the charge-balanced OLEDs, whereas the results are opposite in the charge-imbalanced device setup. This is mainly due to the different role required of EIL materials upon different device working mechanisms. This work shows a series of experimental results of the detailed device characteristics using ZnO:PEI hybrid EILs in order to provide a systematic understanding of the device physics by combining two key concepts together; (1) control of the molecular configuration, and (2) charge carrier dynamics.