Transparent and conducting Ga-doped ZnO films on flexible substrates prepared by sol-gel method

Transparent and conducting Zn1-x GaxO thin films with different nominal Ga content (x = 0-4%) on flexible polyimide (PI) substrates were successfully synthesized by conventional sol-gel spin coating method. All the films retained a hexagonal wurtzite structure of ZnO and showed preferential c-axis orientation after air and vacuum annealing and good adhesion to flexible PI substrates. Generally, the particles in the films were small and the connectivity between the particles was good, irrespective of doping concentration and treatment ambient. The average transmittances reached 70% for all the films above a cut-off wavelength of similar to 500 nm originated from PI substrates. The average sheet resistances decreased suddenly after Ga doping, while increased slightly from x = 3-4%. According to Hall measurement, the broad minimum resistivity of 1-3% Ga doping concentration might be due to the competition of dopant atoms at lattice sites and the segregated ones at the grain boundaries, which could be demonstrated by decreased Hall mobility and increased carrier concentration with increasing Ga doping content.