I think that the biggest problem with trying to use plant-based photosynthesis for electrical power generation is that photosynthesis is the creation of sugars through the combination of CO2 and water, utilizing energy from the sunlight.

Those sugars are great for animals to produce heat energy, but not good at generating electricity directly.

Meaning, we would need to translate the sunlight into sugar, then use the sugar to generate one kind of energy, then use that energy to generate power. Each step involves a decrease in efficiency.

Plants overcome that efficiency issue by growing larger and taller, to get more sunlight. Animals overcome that issue by eating more plants or eating more plant-eaters.

Photosynthetic power plants would have to become exponentially larger to overcome three or more steps of inefficiency.