Edmond Alexandre Becquerel, ...
... father of the discoverer of spontaneous radioactivity, comes across in 1839 with the "inner" photo effect, where electrons freely move, like they do it in a conductor (i.e. a current may flow) - when illuminating the substance with the proper light. Such materials, like selenium or silicon, are coined "optical semiconductors".

A scant fifty years later, in 1886, Hertz discovers the "outer" photo effect where electrons are completely released from metal surfaces under illumination. He induces his student Hallwachs to experimentally explore the effect, that's why the phenomenon is sometimes called "Hallwachs effect". After Hertz' early death another of his former students, Lenard, takes up the theme and discovers contradictions to the generally accepted wave nature of light, which direct Einstein to formulate his light quantum hypothesis in 1905.

The energy of the electrons released only depends on the frequency, not on the intensity of the incoming light. This holds likewise for the inner photo effect. Einsteins explanation of the outer photo effect becomes thus an essential part of the theoretical basis of modern photovoltaics.

No doubt however, pioneer of this development is Becquerel.

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