In this paper, we report different methods to reduce the sheet resistance of ZnO:Al (AZO) films on flexible substrates without degrading the optical transmittance in the visible range. Under proper bias, AZO films deposited on Al2O3-buffered flexible substrates showed a significant decrease of sheet resistance when compared with those deposited on bare polymer. The films with resistivity as low as 8.4 x 10(-4) ohm cm and the optical transmittance about 80% have been obtained by improved methods. By calculating the Al doping efficiency and the mean free path of electrons, ionized impurity scattering was considered to be the dominant factor for the transport mechanism of carriers. (c) 2005 Elsevier B.V. All rights reserved.