Buck converter MOSFET getting hot
The upper MOSFET is N channel and requires a voltage at the gate several volts greater than the solar panel voltage in order to turn it on properly. This then ensures that you get a low volt drop between drain and source when it's conducting: -



The lower half of the picture above will be a nightmare when you fix the upper MOSFET gate drive because any switching regulator needs tight control of interconnections between components or it will not perform adequately.

The next problem you might face is saturation of the inductor when operating at such a low operating frequency given the perceived requirements for performance of a battery charger.