Loads of things actually.
First of all the “band gap” defines the energy of photons wich can be absorbed. Photons with lower energy won’t produce any voltage, photons with more energy will basically waste the excess. This puts a general limit since sunlight is a wild mix of photons of different energies.
Then the conductivity of the material, no semiconductor is a perfect insulator, so a part of the current will immediately return where it came from without doing anything usefull. This kind of loss increases with temperature and also with defects in the material (wich can be created by aging)
And then dirt also plays a huge rule and blocks sunlight from hitting the panel in the first place
Another limit is how the individual cells are internally connected. They form stacks of serial connections to step up the voltage. But that means when one cell in a stack breaks the entire stack can’t contribute any current anymore
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