Maybe I'm misunderstanding - are the solar panels that 'face down' relying on light that passes through the top panels, or just on ambient light, or is some sor

Maybe I'm misunderstanding - are the solar panels that 'face down' relying on light that passes through the top panels, or just on ambient light, or is some sort of reflector required underneath? If so, is this going to be a problem for agrivolatric installations?