A ZnO film with a c-axis preferred orientation was prepared on quartz using the radio frequency (RF) magnetron sputtering technique. Then, a metal-semiconductor-metal (MSM)-structured ultraviolet (UV) photodetector was fabricated on the film. It was found that the cutoff wavelength of the photocurrent redshifted from 361 to 379 nm when the bias increased from 5 to 30 V. The origin of the redshift has been interpreted in terms of a qualitative model considering the declining band gap caused by the bias. This method opens up the possibility of tuning the cutoff redshift of ZnO UV photodetectors. (C) 2015 Elsevier B.V. All rights reserved.