What is the mechanism of photosynthesis?
I'm definitely not going to outline light harvesting in the light harvesting complexes (LHC), the role of the quinones in electron transfer, etc.

But let me give a short answer on your last question:-

The molecular aggregates, such as the LHCs, involved in the different steps of photosynthesis are not optimized for duration. In contrast to man-made machines, they don't have to! Remember that all these fragile molecular assemblies are part of a living system. If parts break, they will resynthesized and replaced.

While it is partly possible to mimic photosynthesis in the lab, it is the self-repair mechanisms that we are unable to replicate in artificial systems.