The operation of a photovoltaic cell is possible by the action of

A photovoltaic cell (solar cell) turns light directly into electricity, and it can only do so because it is built from semiconductor material, typically silicon doped to form a p-n junction. In a semiconductor the valence and conduction bands are separated by a small energy gap, of the order of 1 eV. A photon of visible light carries just enough energy to lift an electron across that gap, creating a free electron and leaving a positive hole behind. The built-in electric field at the junction then sweeps the electron one way and the hole the other, and this separation of charge is what produces the cell's e.m.f. and drives current through an external circuit.

That mechanism explains why the other materials cannot do the job. In a metallic conductor the conduction band is already full of free electrons and there is no energy gap and no internal junction field, so light-generated charge carriers recombine at once and no sustained potential difference builds up. In an insulator the gap is far too wide, so ordinary light photons lack the energy to release any electrons at all. A chemical action is the basis of a primary or secondary cell, where energy comes from a redox reaction rather than from incident light, so it is not the operating principle of a photovoltaic cell.

Keep the distinction sharp between the two light-and-electron effects on the syllabus: in photoemission (the photoelectric effect) electrons are ejected completely from a metal surface into a vacuum, whereas in the photovoltaic effect electrons stay inside the semiconductor and are simply moved across a junction to create a voltage.