Optimizing the Photocatalytic Performance of n-Type Cu 2 O/FTO Photoanodes by Controlling Electrodeposited Film Thickness

n-Type Cu 2 O is a promising photocatalyst due to its direct bandgap, enabling strong visible-light absorption. When used as a photoelectrode, however, its performance depends critically on film thickness, which must balance light absorption with efficient charge transport. This study systematically optimized the thickness of electrodeposited Cu 2 O films on FTO substrates by varying deposition time from 5 to 40 min. As deposition time increased, the films evolved from pyramidal nanostructures to dense polycrystalline layers, with thicknesses ranging from ~150 nm to ~1200 nm. Thicker films exhibited improved visible-light absorption and a slight bandgap narrowing from 2.35 to 2.25 eV. All samples showed stable anodic photocurrents characteristic of n-type behavior. The highest photocurrent 0.118 mA occurred for the film deposited for 30 min, corresponding to an optimal thickness of ~750 nm. At this thickness, the film effectively absorbs incident photons while still allowing photogenerated carriers to reach the interface before recombining. Beyond this optimal point, photocatalytic activity declined sharply despite continued gains in light absorption, due to the thickness exceeding the charge-carrier diffusion length, which increased recombination and transport resistance. These results highlight the importance of precise thickness control for maximizing Cu 2 O/FTO photoanode efficiency.