Zinc　oxide　(ZnO)　acting　as　a　versatile　material　for　electronic　and　optoelectronic　devices　shows　promising　applications　in　triboelectric　nanogenerator　(TENG)-based　self-powered　electronics.　However,　the　output　performances　of　ZnO-based　TENG　are　relatively　low　and　the　origin　of　contact-electrification　in　the　ZnO-based　TENG　is　unclear.　Here,　we　fabricate　a　TENG　by　using　ZnO　film　and　polyimide　(PI)　acting　as　positive　and　negative　friction　layers,　respectively.　The　work　function　of　the　ZnO　film　is　modified　by　doping　with　elemental　Al,　and　the　conductivity　of　the　ZnO　film　is　additionally　enhanced　by　the　presence　of　graphene-oxide　sheets.　The　output　performance　is　greatly　improved　and　it　is　sensitive　to　the　work　function　of　ZnO　film.　These　results　reveal　that　electron　transfer　is　the　dominant　process　in　contact-electrification　for　ZnO-based　TENGs.　This　work　provides　significant　insight　into　understanding　the　contact-electrification　properties　of　ZnO,　thus　allowing　optimization　of　ZnO-based　self-powered　devices.