Further development and commercialization of the new generation of optoelectronic and photovoltaic is often limited by the necessity of fabrication of low-cost and efficient transparent conductive electrodes. The development so far is hindered by the conveniently used indium tin oxide (ITO), which suffers from high cost and not high enough availability to support mass production. The zinc oxide is emerging as a convenient replacement for ITO for solar cell and light-emitting diode applications. The optical functions of aluminum-doped zinc oxide thin films are determined using optical spectroscopy measurements from 300 to 1100 nm. The dopant range studied varies from intrinsic ZnO to 5% Al content. Below the direct band gap there is a residual enhancement of the optical absorption coefficient by Al dopants, which is not related to surface roughness. We determined the dielectric functions and absorption coefficient evolution as the dopant concentration increases, as well as the free-carrier concentration.