The popularity of colloidal quantum dot (CQD) solar cells has increased owing to their tunable bandgap, multiple exciton generation, and low-cost solution processes. ZnO nanoparticle (NP) layers are generally employed as electron transport layers in CQD solar cells to efficiently extract the electrons. However, trap sites and the unfavorable band structure of the as-synthesized ZnO NPs have hindered their potential performance. Herein, we introduce a facile method of ZnO NP annealing in the colloidal state. Electrical, structural, and optical analyses demonstrated that the colloidal-annealing of ZnO NPs effectively passivated the defects and simultaneously shifted their band diagram; therefore, colloidal-annealing is a more favorable method as compared to conventional film-annealing. These CQD solar cells based on colloidal-annealed ZnO NPs exhibited efficient charge extraction, reduced recombination and achieved an enhanced power conversion efficiency (PCE) of 9.29%, whereas the CQD solar cells based on ZnO NPs without annealing had a PCE of 8.05%. Moreover, the CQD solar cells using colloidal-annealed ZnO NPs exhibited an improved air stability with 98% retention after 120 days, as compared to that of CQD solar cells using non-annealed ZnO NPs with 84% retention.