In this work, ZnO-based metal–semiconductor–metal photodetectors with and without a ZnO cap layer were fabricated on flexible substrates of poly(ethylene terephthalate) (PET) for comparative analysis. The ZnO films were prepared by a low-temperature sputtering process. The photodetector with a ZnO cap layer (stack structure: ZnO/Ag/ZnO/PET) shows a much higher UV-to-visible rejection ratio of 1.56 ×103 than that without. This can be attributed to the photocurrents that are not only significantly increased in the UV region but also slightly suppressed in the visible region for such a novel structure. With an incident wavelength of 370 nm and an applied bias of 3 V, the responsivities of both photodetectors with and without a ZnO cap layer are 3.80 ×10-2 and 2.36 ×10-3 A/W, which correspond to quantum efficiencies of 1.13 and 0.07%, respectively. The Schottky barrier height at the Ag/ZnO interface is also determined to be 0.782 eV.