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

Explanation

The ionisation enthalpy of lithium `(520 kJ mol^(-1))` is much higher than those of potassium `(419 kJ mol^(-1))` and caesium `(376 kJ mol^(-1))`. Due to high ionisation enthalpy, lithium does not emit electrons on exposure to light. On the other hand, potassium and caesium do so. Therefore, K and Cs rather than Li are used in photoelectric cells.