To investigate the electronic structure change during the UV laser annealing process, we analyzed the optical absorption spectra, as shown in Fig. 1b. We plotted the Tauc plot with the squared optical absorption coefficient multiplied by the photon energy under the assumption of being an indirect bandgap semiconductor, i.e., indirect bandgap in SnO2 and direct bandgap in ZnO. The as-coated ZTO sol-gel film and the 1p sample did not show an inter-band absorption edge near 4 eV, whereas the 30p and 20,000p samples showed bandgap absorption edges at 4.0 and 3.9 eV, respectively. These observations indicate that only a few laser pulses (~ 30 pulses) were needed to convert the sol-gel film into crystallized bonding states with bandgap formation. Between the 30p and 20,000p samples, the extended laser annealing may have increased the fraction of laser annealed crystalline phases, which is discussed below in detail.