The performance of ZnFe2O4 photoanodes largely depends on their nanostructure, crystallinity and n-type doping, though decoupling their impact on photoactivity remains a challenge. Herein, the combined effects of the synthesis temperature and reductive annealing post-treatment on the photoelectrochemical performance of planar ZnFe2O4 photoanodes are investigated in relation to a comprehensive range of film thicknesses, enabled by an optimized sol-gel synthetic approach. By eliminating the effects of the nanostructure, a synergistic effect is revealed between the material crystallinity, controlled by the synthesis temperature, and the n-type doping triggered by the H-2-treatment, which is maximum for the thickest photoactive layers. Intensity modulated photocurrent spectroscopy measurements performed in operando evidence the crucial impact of the synthesis temperature and the reductive treatment on inducing effective surface charge injection and improved bulk charge transport, respectively, to enhance the photoelectrochemical performance of planar ZnFe2O4 films.