In 1939, physicist ,Edmund Bequerel discovered photoelectric effect i.e. certain materials produces electricity when exposed to light.In 1905 , Albert Einstein described the nature of light(photon releasing electrons) and the photoelectric effect on which photovoltaic(PV) technology was evolved.
Space industry has started using this technology in their space craft and space programs, energy crisis drove this PV technology to non space applications.
Solar cell which are made of semiconductor materials like silicon. When solar/sun light strikes the solar cell , electrons are loosed from atoms, and positive and negative side forms a electric current. This electric current or DC(direct current), can be used to power the appliances or save it in battery. collection of solar cell is termed as solar module , and collection of solar module is solar array . So when someone says solar panel, it means reference is made to solar array.
For example - in the picture
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