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

(N/A) All three elements,$Li$,$K$,and $Cs$,are alkali metals. However,$K$ and $Cs$ are used in photoelectric cells,while $Li$ is not.
This is because $Li$ has a smaller atomic size compared to $K$ and $Cs$,which results in a higher ionization energy,making it difficult to remove an electron.
On the other hand,$K$ and $Cs$ have lower ionization energies,allowing them to lose electrons easily when exposed to light. This property makes them suitable for use in photoelectric cells.