Photodetectors based on colloidal quantum dots and methods of making the photodetectors are provided. Also provided are methods for doping films of colloidal quantum dots via a solid-state cation exchange method. The photodetectors include multi-band photodetectors composed of two or more rectifying photodiodes stacked in aback-to-back configuration. The doping methods rely on a solid-state cation exchange that employs sacrificial semiconductor nanoparticles as a dopant source for a film of colloidal quantum dots.