To investigate the influence of varying Al content on structural, optical, and electrical properties of ZnO thin films, Al-Cu co-doped ZnO thin films with fixed Cu content at 1 wt. % and different Al contents (1, 3, and 5 wt. %) were successfully synthesized on glass substrates using a sol-gel process. The results indicated that the varying Al content affects not only the grain size and band gap but also the electrical conductivity of the films, and a linear relationship was found between the band gap and strain values of the films. The temperature-dependent electrical conductivity data of the films demonstrated that electron transport was mainly controlled by the grain boundaries at intermediate and high temperatures, whereas it was governed by Mott-variable range hopping at low temperatures. Additionally, 3 wt. % Al content improved the electrical conductivity of Al-Cu co-doped ZnO by lowering the trap density and enhancing the hopping probability. Published by AIP Publishing.