The functional sequence of hydrogen in electronic conduction of sol–gel derived Al-doped ZnO films as determined via impedance spectroscopy

Sol–gel derived Al-doped ZnO (AZO) films were annealed under various degrees of hydrogen reduction treatments. Three sequential equivalent circuits were obtained by simulating the impedance spectra of the AZO films. By interpreting the equivalent circuit elements, the functional sequence of hydrogen in AZO films was disclosed. Hydrogen destroys the energetic barriers at grain boundaries and releases free electrons, which are shown by the presence of a parallel resistor–capacitor circuit, a constant phase element and a short Warburg element in equivalent circuits.