Why in a photocell the cathode is coated with alkali metals

The working of photocells is dependent on photoelectric effect.Photons are bombarded on the electrons which results in electrons being knocked out of the atoms.Now the photons must possess energy greater than the threshold energy of the material.The value of threshold energy or work function for alkali metals is small.So, the electrons in alkali metals can be knocked out with lesser quantity of energy.Thats the reason for coating the cathode in photocells with alkali metals.