Question

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

Answer 1

All the three, lithium, potassium, and caesium, are alkali metals. But only $K$ and $Cs$ are used in the photoelectric cell rather than $Li$.
This is because as compared to $Cs$ and $K$, $Li$ is smaller in size and therefore, requires high energy to lose an electron. While $K$ and $Cs$ have low ionization energy.

Hence, they can easily lose electrons. This property of $K$ and $Cs$ is utilized in photoelectric cells.

$\text{Final answer}:$ Due to low ionization energy of $K$ and $Cs$ over $Li$.