This study systematically investigated the effect of various Al/Cu ratios on the optical, electrical, and electrochemical properties of sputter-deposited CuAlx−0.2Ca0.2O (x = 1, 1.25, 1.5, 1.75, 2) thin films. The as-sputtered Cu–Al–Ca–O films were all amorphous regardless of Al/Cu ratio. Both the transmittance and resistivity of the Cu–Al–Ca–O films increased with increasing Al/Cu ratios. Heterojunction diodes fabricated using Al-doped ZnO, ZnO, and Cu–Al–Ca–O thin films of various Al/Cu ratios exhibited high breakdown-voltage values when Cu–Al–Ca–O thin films with high Al/Cu ratios. Optimal rectification ratio and electrical properties were determined in diodes that had CuAl1.3Ca0.2O as their p-layer deposited film. Cu–Al–Ca–O films with high Al/Cu ratios exhibited low average corrosion current density (icorr) values and high breakdown-voltages (Ebr) values, indicating that the corrosion resistance of Cu–Al–Ca–O film increases with increasing Al/Cu ratios.