All the three, lithium, potassium, and caesium, are alkali metals. Still, K and Cs are used in the photoelectric cell and not Li. Potassium and Caesium can emit their electrons much easily than Lithium as they have very low ionisation enthalpy, they are used in photoelectric cells rather than Lithium. Ionisation Enthalpy of both elements is much lower than Lithium, so they can emit electrons easily in sunlight too.
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