Polo wrote: A superimposition of layers of gallium-indium-phosphorus, gallium arsenide and germanium enlarges the spectrum of light converted into electricity. Why use these materials?
The materials are chosen according to their prohibited bandwidth.
On multi-junction cells, the top cell converts the most energetic photons, so we choose a semiconductor material with a high gap (ex AlGaInP) and the bottom cell converts the least energetic radiation corresponding, for example to near IR (GaInNAs for example).