Photovoltaic and capacitance performance of low-resistance ZnO nanorods incorporated into carbon nanotube-graphene oxide nanocomposites

Nanocomposites of hexagonal ZnO nanorods incorporated into carbon nanotube-graphene (ZnO/CNT-GO) were prepared using a simple hydrothermal approach. The structure, light absorption, surface texturing, and morphology characterizations have established the strong interaction between ZnO, carbon nanotubes, and graphene oxide components because of the presence of well-dispersed ZnO nanorods with an aspect ratio of 3.4 (L × D = 75 × 22 nm). The assembled dye-sensitized solar cell of a ZnO/CNT-GO photoanode exhibited a superior performance, with a power conversion efficiency of 7.73%, surpassing those of its ZnO/CNT (5.07%) and ZnO/GO (3.96%) counterparts. Although CNT-GO exhibited a higher incident photon-to-current conversion efficiency (IPCE) and lower photoluminescence (PL) intensity than those of the ZnO/CNT-GO composite, it presented a lower conversion efficiency of 5.60%. This can be attributed to the presence of structural defects at the CNT/GO interface, which resulted in lower electronic conductivity (0.25 × 10⁻³ Ω⁻¹ cm⁻¹ vs. 0.8 × 10⁻³ Ω⁻¹ cm⁻¹ for ZnO/CNT-GO) and optical performance. The ZnO/CNT-GO composite showed a superior I_sc value of 17.6 mA/cm² but a lower V_oc value of 0.63 V in comparison to those of its ZnO/GO analog (3.9 mA/cm², 0.8 V); it also showed a manifested increase in surface area and pore radius values (227.5 m²/g, 58.9 Å) toward the dye absorption. The electrochemical capacitance performance obtained via using charge-discharge and impedance spectroscopy revealed an excellent specific capacitance in the order of ZnO/CNT-G (290 F/g) > CNT-G (275 F/g) > ZnO/G (230 F/g) > ZnO/CNT (183 F/g) at 1.0 A g⁻¹. This is attributed to the synergistic effect of ZnO within the CNT/GO nanocomposite and affirms its charge collection efficiency toward the improvement of dye-sensitized solar cells. The formation of a p/n junction between CNT/G and ZnO nanorods, as evidenced by the Mott-Schottky analysis, played a vital role in the improvement of solar cell devices based on the narrowing of the band gap (E_g = 2.64 eV), ease of electronic transfer, transient photocurrent density, as well as framework mesoporosity.