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We fabricated inverted polymer solar cells (PSCs) with the structure of ITO/ZnO NPs/Active layer/MoO3/Ag, in which ZnO NPs act as electron transport layer (ETL), poly{4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b0]-dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diy (PTB7) as donor and [6,6]-phenyl-C71-butyric-acidmethyl-ester (PC71BM) as accepter. It is found that the temperature employed during the annealing process of ZnO NPs has significant impact on the efficiency of the resulted PSCs. The optimized PSC with ZnO NPs annealed at 150 °C based on PTB7:PC71BM (1:1.5 w/w) exhibited a power-conversion efficiency of 7.37% with open-circuit voltage (Voc) of 0.722 V, short-circuit current density (Jsc) of 15.38 mA cm−2, and fill factor (FF) of 66.4%. The effects of the annealing temperature on photovoltaic performances were further illustrated with transmission spectrum, atomic force microscopy, X-ray diffraction and PL spectra, meanwhile electrical performance were illustrated with conductivity. Our result indicated that the improved efficiency is due to the optimized ETL/active layer interface, the enhanced transparency and electron transport of the ETLs. As its facile preparation process, this kind of ETL is compatible with roll-to-roll manufacturing of larger area flexible PSCs. [ABSTRACT FROM AUTHOR]