Carbon Nanotube-Assisted NiO–ZnO Heterojunction Counter Electrodes for Enhanced Photovoltaic Performance in Pt-Free Dye-Sensitized Solar Cells

This study focuses on bandgap tuning and charge-transport properties of NiO, carbon nanotube (CNT)–NiO, and CNT–Ni_0.5Zn_0.5O nanocomposite counter electrodes for Pt-free dye-sensitized solar cells (DSSCs). This work clarifies how material composition and nanostructure influence optical characteristics, charge-transport pathways, and DSSC performance. The structural and microstructural analysis yielded distinct relationships between microstructure development and electronic performance. The ultraviolet–visible (UV–Vis) analysis showed that pristine NiO possessed a wide bandgap of 3.26 eV, consistent with uniform grains and limited photocarrier generation. Upon CNT incorporation, the bandgap narrowed to 2.95 eV due to the formation of a conductive percolation network that suppressed recombination. The ternary CNT–Ni_0.5Zn_0.5O nanocomposite showed further bandgap reduction (2.75 eV) and enhanced light absorption due to Zn²⁺-induced orbital hybridization and the high-mobility CNT framework. Photovoltaic measurements indicated continuous enhancements in J_SC and fill factor, with CNT–Ni_0.5Zn_0.5O achieving a maximum efficiency of 12.1%, surpassing NiO (8.37%) and CNT–NiO (8.76%). The enhanced performance is attributed to improved electron mobility, decreased interfacial resistance, and prolonged carrier lifetime resulting from the hierarchical heterointerfaces. These findings show that CNT-assisted mixed metal oxides offer a promising platform for developing efficient, low-cost, and scalable Pt-free counter electrodes for next-generation DSSCs.