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In this study, n-type CdyZn1−yO (y = 0, 0.5, 1, 1.5 at%) thin films were deposited on p-Si and soda lime glass substrates using sol–gel spin coating technique. From the X-ray diffraction patterns, it was observed that all thin films had wurtzite structure with (002) orientation. Scanning electron micrograph results revealed that the morphology varied with higher Cd doping in ZnO. The bandgap was decreased from 3.29 eV (undoped ZnO) to 3.26 eV (y = 1.5 at%) as Cd content increased. The electronic parameters of the CdyZn1−yO (y = 1 at%) thin film heterojunction such as ideality factor (η) and barrier height (ΦB) were obtained using the thermionic emission theory as 2.53 and 0.70 eV, respectively. CdyZn1−yO (y = 1 at%) showed responsivity 0.01 A/W on 3.2 mW/cm2 exposure of UV light (365 nm) at 4 V bias voltage. The heterojunction so formed seems to be an excellent candidate to be used as an optical sensor (UV photodetector) in optoelectronics. [ABSTRACT FROM AUTHOR]