Electron-injecting interlayers (ILs) which are stable in air, inject electrons efficiently, block holes, and block quenching of excitons, are very important to realize efficient inverted polymer light-emitting diodes (IPLEDs). Two air-stable polymer electron-injecting interlayers (ILs), branched polyethyleneimine (PEI) and polyethyleneimine ethoxylated (PEIE) for use in IPLEDs are introduced, and the roles of the ILs in IPLEDs comparing these with a conventional Cs2CO3 IL are elucidated. These polymer ILs can reduce the electron injection barrier between ZnO and emitting layer by decreasing the work function (WF) of underlying ZnO, thereby effectively facilitating electron injection into the emitting layer. WF of ZnO covered by PEI is found to be lower than that covered by PEIE due to higher [N+]/[C] ratio of PEI. Furthermore, they can block the quenching of excitons and increase the luminous efficiency of devices. Thus, IPLEDs with PEI IL of optimum thickness (8 nm) show current efficiency (13.5 cd A-1), which is dramatically higher than that of IPLEDs with a Cs2CO3 IL (8 cd A-1).