Solar cells are devices that turn sunlight (photons) into electricity (moving electrons). So we want to know how materials for solar cells behave electrically and optically (the photons again).
To do this you can wire up your solar cells to a machine that can apply voltages (energy given to the electrons) and measure the resulting current (flow of electrons). By measuring these values as you change other things, or even fire lasers at the solar cell, you can work out how it behaves, or it’s “optoelectronic properties”. By understanding how these different features of the materials interact to produce power, you can try to understand why some materials are better than others, and hope to design better ones for the future.
Lots of physics research involves electrons and photons.
Almost everything interesting about the world is due to electrons (everything you see, touch, taste and feel is electrons interacting with each other), and almost everything we know about the world comes from looking at light coming off things.