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

(N/A) All three elements,lithium $(Li)$,potassium $(K)$,and cesium $(Cs)$,are alkali metals.
However,$K$ and $Cs$ are used in photoelectric cells,whereas $Li$ is not.
This is because $Li$ has a smaller atomic size compared to $K$ and $Cs$,which results in a higher ionization enthalpy,making it difficult to remove an electron.
On the other hand,$K$ and $Cs$ have lower ionization enthalpies,allowing them to lose electrons easily when exposed to light.
This property of $K$ and $Cs$ is utilized in photoelectric cells.