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Three FTO-BiVO 4 photoelectrodes are fabricated modifying the BiVO 4 thickness, and systematically evaluating the influence of FTO substrate on the optical, electrical properties, and photoelectrochemical performance of BiVO 4 semiconductor. The catalysts are characterized using two light sources with back-side or front-side irradiations, to investigate the impacts of different energy sources and configuration illumination on the FTO-BiVO 4 photoactivity. This analysis reveals the existence of an additional charge transfer resistance increasing with thickness film subjected to front-side illumination, while the resistance remarkably diminishes when this interface is directly irradiated under back-side illumination. The highest photocurrent is achieved with the LED lamp under back-side illumination, condition selected to compare the degradation of 20 mg L−1 ciprofloxacin (CIP) in 0.05 M NaCl through electrocatalysis, photocatalysis, and photoelectrocatalysis using front-side or back-side illuminations. In these evaluations, modified FTO contributes to the photogeneration of reactive chlorine species, whence it cannot be considered as a simple substrate. Back-side illumination presents a higher CIP elimination in comparison with front-irradiation. A schematic energy band diagram relying on Tauc and Mott-Schottky plots, and incorporating FTO as a photoactive semiconductor is established to rationalize the formation of oxidant species in the system. A degradation mechanism is established based on HPLC measurements of the different treatment methods. [Display omitted] • Light source affects the formation of a true FTO-BiVO 4 heterojunction. • FTO beyond a current collector influencing the photoelectrochemical performance. • Charge transfer resistance increasing with FTO-BiVO 4 thickness at front radiation. • FTO contributes to photogenerate reactive chlorine species (non-substrate). • Back-side illumination presents higher Ciprofloxacin elimination than front one. [ABSTRACT FROM AUTHOR]