The metal mainly used in devising photoelectric cells is:

The correct option is B

Cs

Explanation for correct option:

B. $\mathrm{Cs}$:

Photoelectric cells:

1. In order to broadcast scenes, TV cameras use photoelectric cells.
2. In cinema, these cells are employed to reproduce both the sound and the image.
3. In counting devices, photocells are employed.
4. These cells are also utilized in fire alarm and burglar alarm systems.
5. The temperature of stars is also measured using these cells.

Caesium $\left(\mathrm{Cs}\right)$:

1. The final member of the alkali metal family is Caesium.
2. As a result, Caesium has a lower ionization potential than the other metals and emits or ejects electrons more readily from the group of metals when exposed to electromagnetic radiation.
3. Caesium is utilized in photoelectric cells because it has the lowest ionization energy of all the elements and is the most electropositive.

Explanation for incorrect options:

A. $\mathrm{Li}$:

1. Alkali metals lose electrons when they are exposed to light.
2. The photoelectric effect is what we refer to as. Due to $\mathrm{Li}$'s tiny size, the electrons are tightly bound to the nucleus and must be released with a lot of energy.
3. We also know that lithium has a high ionization enthalpy.
4. $\mathrm{Li}$ must therefore expend a lot of energy to lose electrons.
5. Hence, it is not a photoelectric cell.

C. $\mathrm{Rb}$:

1. $\mathrm{Rb}$, which is an abundant isotope (72.2 percent), is neither radioactive nor unstable.
2. $\mathrm{Rb}$'s ionization energy is between that of Cs and K, making it very simple and energy-efficient to extract electrons from $\mathrm{Rb}$ as well. $\mathrm{Rb}$'s ionization energy is between that of Cs and K, making it very simple and energy-efficient to extract electrons from Rb as well.
3. Hence, it is not a photoelectric cell.

D. $\mathrm{Na}$:

1. An electron tube with a photosensitive cathode that produces electrons when lit is what a photoelectric cell, also known as an electric eye or a phototube, is essentially made of.
2. The released electrons are collected by the anode.
3. Hence, it is not a photoelectric cell.

The metal mainly used in devising photoelectric cells is, Cs.

Hence, option B is the correct answer.