Why are potassium and caesium, rather than lithium used in photoelectric cells ?

Since alkali metals emit electrons when radiation strikes on their surfaces. This phenomenon of emission of electrons when electromagnetic radiation strikes against them is calledphotoelectric effect. Therefore, alkali metals exhibit photoelectric effect.
One more thing-->Alkali metals used to have low ionisation enthalpies and therefore, the electrons are easily ejected when exposed to light.Among alkali metals,as compared to Cs and K, Li is
smaller in size and therefore, requires high energy to lose an electron.
On the other hand, K and Cs have low ionization energy. Hence,
they can easily lose electrons. This property of K and Cs is utilized in
photoelectric cells.

Pottasium and caesium have much lower ionization enthalpy than that of lithium as a result, these metals exposure to light, easily emit electrons but lithium doesn`t therefore potassium and cesium rather than lithium used in photoelectric effect.

Potasium and caesium have much lower ionization enthalpy than that of lithium. as a result, these metals exposure to light, easily emit electrons but lithium doesn`t therefore potassium and cesium rather than lithium used in photoelectric effect.