Effects of Cu₂O Deposition on Cracked TiO₂ Nanoflower Photoanodes in Dye-Sensitized Solar Cells

Achieving a crack-free, high-surface-area photoanode is essential for maximizing the efficiency of dye-sensitized solar cells (DSSCs). In this work, rutile titanium dioxide (rTiO₂) nanoflowers were synthesized hydrothermally and then conformally coated with copper(I) oxide (Cu₂O) by RF magnetron sputtering to seal pre-existing cracks and to create a nanothorn surface favorable for dye adsorption. Systematic control of the sputtering time identified 60 min as optimal condition, yielding a photoanode thickness of about 6.8 μm and a root-mean-square roughness of 0.557 μm, conditions that maximize dye adsorption and light scattering without blocking pore pathways. The optimized coating increased the dye adsorption density to 3.89 × 10^-7 mol cm^-2 and narrowed the optical band gap from 3.00 to 1.96 eV, extending absorption into the visible. The incident photon to current efficiency broadened to 400-700 nm, and the integrated photocurrent rose from 6.89 mA cm^-2 to 18.50 mA cm^-2. Under AM 1.5 G, the device delivered open-circuit voltage of 0.79 V, short-circuit current density of 18.50 mA cm^-2, fill factor of 57.1%, and overall power conversion efficiency of 8.34%, nearly three times higher than that of the unmodified rTiO₂ nanoflower cell (2.86%). Electrochemical impedance spectroscopy further confirmed this performance enhancement, showing a pronounced reduction in charge-transfer resistance from 52.21 Ω for bare rTiO₂ to 2.89 Ω at the optimized sputtering duration. Collectively, these results demonstrated that a single Cu₂O sputtering step offers an effective approach for crack mitigation, band gap engineering, and interfacial optimization in TiO₂-based DSSCs.