What phenomenon did Alexandre Edmond Becquerel observe back in 1839?

The photovoltaic effect

Alexandre Edmond Becquerel is noted for making an earlier observation in the history of solar power, specifically noticing the photovoltaic effect back in 1839. This observation predated the practical application of silicon cells by over a century. While foundational to the science, the photovoltaic effect itself is the physical principle where light creates an electrical current, which is distinct from the later engineering achievement of building a working cell that could power small devices, which was accomplished by the Bell Labs team.