The electronic structure of the interfaces formed after deposition of MoO3 hole-injection layers on top of a polymer light-emitting material, poly(dioctylfluorene-alt-benzothiadiazole) (F8BT), is studied by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy and metastable atom electron spectroscopy. Significant band bending is induced in the F8BT film by MoO3 “acceptors” that spontaneously diffuse into the F8BT “host” probably driven by kinetic energy of the deposited hot MoO3. Further deposition leads to the saturation of the band bending accompanied by the formation of MoO3 overlayers. Simultaneously, a new electronic state in the vicinity of the Fermi level appears on the UPS spectra. Since this peak does not appear in the bulk MoO3 film, it can be assigned as an interface state between the MoO3 overlayer and underlying F8BT film. Both band bending and the interface state should result from charge transfer from F8BT to MoO3, and they appear to be the origin of the hole-injection enhancement by the insertion of MoO3 layers between the F8BT light-emitting diodes and top anodes.