The problem is that the photosynthetic machinery has evolved to turn solar energy into a hydrogen gradient across a very thin membrane.
Also, only one of the versions of the photosystems is capable of using water as an electron acceptor and this is composed of two seperate cores, surrounded by a massive light harvesting complex and dependant on a large amount of tertiary subcomponents both in the membrane and on both sides. And these components themselves shuttle around electrons they get from/donate to other parts in the cell.
All in all this requires about 100 differnent proteins in balanced amounts. Given the high energy involved in this process these proteins are also very easily damaged and need to be constantly replenished.
In a plant cell or bacterium this is not a problem as the energy produced is used to facilitate these processes, with just enough left over to be able to grow (slowly).
So while the photosynthesis process in plants looks to be incredibly efficient when looked at on its own, in reality the majority of the energy captured is used just to maintain the status quo. If we'd want to use it, there are three sort of options.
The first is to somehow put electrodes in plant chloroplasts or cyanobacteria and try to syphon off some of the energy, which would most likely kill the plants used, but they could be replaced with new growth. But this process would not be easy as the membranes are extremely small and fragile and individually are very small
The second would be to create some form of massive membrane studded with photosystems and feed it the necessary components somehow, which would need to be produced individually. But again, the membrane is very thin and we'd need to be able to somehow reproduce the normal repair mechanisms.
Third would be to somehow create artificial reproductions from sturdier materials, but the entire photosystem uses the pretty unique properties of carbon to capture and transport electrons, which is hard, if not impossible to mimic with other materials.
Finally, a hydrogen gradient is hard to directly translate into electrical current. The energy is normally stored as high energy sugars, which is also the way CO2 is captured from the atmosphere. The photosystem alone just turns water into hydrogen and oxygen.
