Why does the intensity of light in LEDs not increase with current after a particular value?
Not all recombinations result in the emission of a visible-light photon. Only ones that occur within the P-N junction of the LED itself have the energy for that, and this volume can become "saturated" at high current levels. When this happens, some of the electrons and holes pass all the way through the junction before recombining in the bulk material on either side, where they do so with reduced energy, resulting in the release of longer-wavelength (heat) photons.