I know that solar cells efficiency increase with lower temperatures, but why? I found only equations on efficiency using temperature coefficient, but I want to know the physics behind it. Thank for reply.
There are (at least) two answers to that, one that is a “first principles” answer and the other that is a “mechanistic” answer.
The “first principles” answer is that if it were not so then you could make a perpetual motion machine. Just take a photovoltaic of whatever temperature-independent efficiency, and a Carnot engine between two reservoirs such that the Carnot efficiency is less than the photovoltaic efficiency. Then use the photovoltaic to generate electrical power from the hot reservoir and use the some of the power to pump the heat back to the hot reservoir. There would be some electrical power left over. Since PMMs are not possible the photovoltaic efficiency must depend on temperature and must always be less efficient than a Carnot engine.
The “mechanistic” answer has to do with the band gap in the semiconductor. Ideally the photons have enough energy to get current to cross the band gap and nothing goes back the other way. However, in reality charge carriers have thermal energy, and the hotter the photovoltaic is the more charge carriers will have enough energy to cross backwards. This backwards leakage current reduces the efficiency.
I like the first principles answer better, but not everyone likes it.