Photoelectric effect: It is the phenomenon of emission of electrons from the surface of metals, when radiations of suitable frequency fall on them. The emitted electrons are called photoelectrons and the current so produced is called photoelectric current.This occurs in alkali metals, as alkali metals have low ionization energies, which allows the electrons to be easily ejected when exposed to light. Among alkali metals, cesium has lowest ionization energy and hence it can show photoelectric effect to the maximum extent.
Lithium does show this effect but to less extent than other alkali metals.
We hope that clarifies your query.