I know that, but I don't think every material has the interesting properties that Si does for image sensors, i.e. it needs to basically turn photons of the right wavelength range into electrons.
> As a photodiode, an LED is sensitive to wavelengths equal to or shorter than the predominant wavelength it emits. For example, a green LED is sensitive to blue light and to some green light, but not to yellow or red light.
If you want to measure the light of all visible wavelength, you must use a semiconductor with a low band gap, for example the named in the other comments: Germanium, Silicon or Gallium Arsenide
