Thin TiO2 films were deposited by atomic layer deposition (ALD) at 150 and 250 °C on FTO and Si/SiO2 substrates to examine the effect of deposition conditions on morphology, structure, chemical stability, and photoelectrochemical performance. Films grown at 150 °C were amorphous and crystallised into anatase after annealing at 500 °C, accompanied by nanoscale morphological rearrangement. In contrast, films deposited at 250 °C were amorphous and non-stoichiometric (TiO2−x) with Ti3+ self-doping; annealing reduced the doping level without inducing crystallisation. The films degraded in 0.1 M HClO4 within 72 h but remained stable in alkaline media (pH 8). Electrochemical studies using the [Fe(CN)6]3−/4− redox couple showed that low-temperature ALD TiO2 layers (8–50 nm) effectively blocked charge transfer, whereas this approach was unsuitable for high-temperature ALD films due to self-doping. The as-deposited high-temperature ALD TiO2/FTO exhibits higher photoelectrochemical (PEC) efficiency than low-temperature films due to Ti3+ self-doping. The as-deposited low-temperature ALD TiO2/FTO shows negligible PEC efficiency, which increases significantly after annealing owing to the formation of the anatase phase.