Bandgap-graded materials present varying spectral responses at different positions, making them possible to be used as an alternative to photoactive materials array in multi-spectral responsive devices, thus miniaturizing the apparatus. However, the preparation of bandgap-graded materials usually requires complicated deposition process. Here we report a facile low-temperature solution process to make films with lateral bandgap gradients, which form spontaneously via self-spreading and interdiffusion of solutions. We show lead halide perovskite films with MAPbCl₃-MAPbBr₃ and MAPbBr₃-MAPbI₃ gradients, which exhibit light absorption onsets ranging from 410 to 781 nm. The bandgap-graded films were used to make self-powered multiband photodetectors, which show different spectral responses at different positions without applying bias voltage. Furthermore, self-powered spectrometers were made by using the multiband photodetectors.