Enhanced Photovoltaic Efficiency in Dye-Sensitized Solar Cells Using rGO/Cu ²⁺ :ZnO/Polymer (PMMA/PPy/PANI) Ternary Photoanodes

Cu²⁺-doped ZnO/reduced graphene oxide (rGO) ternary nanocomposites were synthesized via a facile oxalate-assisted wet-chemical precipitation route and systematically investigated as photoanodes for dye-sensitized solar cells (DSSCs). Progressive incorporation of Cu²⁺ (0-5 atom %) into the ZnO lattice resulted in effective band gap narrowing from 3.28 to 3.05 eV, accompanied by enhanced visible-light absorption (450-800 nm) and suppressed charge carrier recombination, as confirmed by photoluminescence quenching. Electron microscopy revealed a morphology evolution from irregular crystallites to hierarchical porous microspheres (∼2.4 μm) assembled from nanoscale building blocks (50-80 nm), while structural analyses confirmed the preservation of the wurtzite ZnO phase without secondary copper oxide formation. Hall effect measurements demonstrated a marked enhancement in charge transport properties, with carrier mobility increasing from 26.5 to 42.8 cm²·V^-1·s^-1 and electrical conductivity from 19.2 to 23.2 Ω^-1·cm^-1 upon Cu²⁺ doping. When implemented as DSSC photoanodes, the optimized 5% Cu²⁺:ZnO/rGO system delivered a power conversion efficiency (PCE) of 4.9%, significantly outperforming that of pristine ZnO/rGO (3.1%). Further hybridization with polymer matrices (PMMA, PPy, and PANI) revealed a synergistic improvement in interfacial charge transport and film stability. Among them, the 5% Cu²⁺:ZnO/rGO/PANI photoanode exhibited the highest efficiency of 5.6% (<i>J</i> _SC = 14.8 mA·cm^-2, <i>V</i> _OC = 0.72 V, and FF = 0.53) at an optimal film thickness of ∼18 μm. The enhanced photovoltaic performance is attributed to the combined effects of Cu²⁺-induced band structure modulation, rGO-mediated rapid electron extraction, and PANI-facilitated interfacial charge transport. These results demonstrate an effective ternary hybrid strategy for developing efficient, low-cost, and Pt-free ZnO-based DSSC photoanodes.