Titanium dioxide (TiO₂) thin films were deposited on fluorine-doped tin oxide (FTO) substrates by atomic layer deposition (ALD) at 300 °C using 300, 400, and 500 cycles. The study investigated the influence of ALD cycle number on the surface morphology, elemental distribution, and optical properties of the resulting TiO₂/FTO structures. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and UV–Vis spectroscopy were used for characterization. ALD enables precise control of thin-film growth, while changes in cycle number and film development can influence microstructure and optical response [1–3].

SEM revealed clear morphological evolution with increasing cycle number. The 300-cycle film exhibited a densely covered surface consisting of relatively small, irregular features with distinct boundaries. At 400 cycles, the surface became more coalesced and compact, with larger interconnected features and reduced separation. Further deposition to 500 cycles produced a more developed and continuous morphology, characterized by larger interconnected surface structures. This evolution is consistent with previous observations that increasing TiO₂ deposition can promote nanostructural development and formation of more continuous coatings [1]. EDS mapping confirmed the presence and distribution of titanium and oxygen, while the detected tin originated from the underlying FTO substrate. Similar studies have shown that TiO₂ structure and surface characteristics can influence the optical response of TiO₂/FTO systems [4].

UV–Vis measurements demonstrated a clear dependence of optical transmission on ALD cycle number. The 300-cycle sample showed greater variation across 300–1100 nm, reaching approximately 80% transmittance in the blue-visible region before decreasing to approximately 69% at 700–800 nm and subsequently increasing at longer wavelengths. In contrast, the 400- and 500-cycle films exhibited higher and more stable transmission. Maximum transmittance reached approximately 86% for 400 cycles and 87% for 500 cycles, with both maintaining approximately 84–87% across substantial portions of the measured spectrum. Absorbance followed the expected inverse relationship with transmittance [2,3,5].

Overall, increasing the ALD cycle number from 300 to 400 cycles substantially improved optical transmission, while 500 cycles maintained similarly high transmission. The response therefore did not exhibit a simple monotonic decrease with increasing deposition. The observed behavior may reflect combined effects of film development, refractive-index contrast, thin-film interference, and intrinsic optical attenuation [2,4,5]. The 400- and 500-cycle films combined developed morphology with high optical transmission, indicating potential suitability as transparent TiO₂ functional layers in dye-sensitized solar-cell architectures. Further AFM, thickness, and photovoltaic measurements are required to establish relationships between film structure and DSSC performance [6,7].