Photoelectric and Electrochemical Performance of Al-Doped ZnO Thin Films Hydrothermally Grown on Graphene-Coated Polyethylene Terephthalate Bilayer Flexible Substrates

Abstract

Al-doped zinc oxide thin films with Al content in the range of 0–15% were synthesized on graphene-coated polyethylene terephthalate (PET–GR) flexible substrates by a hydrothermal method at 90 °C for 5 h, and their performance as a p–n heterojunction electrode was evaluated. The results revealed that the Al atoms doped into the ZnO lattice were present in the form of an oxidized state, which together with the PET–GR substrate affected the internal stress and lattice constant of the ZnO. The effect of Al content on the morphological evolution of the product was investigated, and the growth mechanism of Al-doped ZnO nanorods (AZO NRs)/PET–GR was also analyzed. The photoluminescence spectra of AZO/PET–GR showed a wide near-ultraviolet emission peak at about 390 nm, and the mechanism of free exciton recombination luminescence was discussed. AZO/PET–GR exhibited excellent mobility; the highest mobility was 154.109 cm2 V–1 s–1 when the Al doping content was 12%, and its optimal resistivity and carrier concentration were 0.656 Ω·cm and 4.668 × 1017 cm–3, respectively. The photogenerated current and electrochemical impedance spectroscopy measurement of the electrode indicated that the photocurrent densities of 9% Al-doped and undoped ZnO/PET–GR were 0.7 and 0.4 μA/cm2, the internal resistance of these two electrodes were 7 and 13 Ω, and the slope of the impedance spectra curves in the low-frequency region were 4.66 and 2.00, which illustrated that the incorporation of Al could promote the further separation of photoinduced electron–hole pairs and that the AZO/PET–GR electrode had better charge conductivity and interface bonding than ZnO/PET–GR.