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

Explanation

The ionization enthalpy of potassium (419k J `mol^(-1)`) and caesium (376kJ `mol^(-1)`) is less than lithium (520kJ `mol^(-1)`). Because of high ionization enthalpy, lithium does not emit electrons on exposure to the light. But both potassium and caesium having less ionization enthalpy emit electrons on exposure to light. Thus, both potassium and caesium rather than lithium used in photoelectric cells.