What is the best way to limit voltage of a solar panel?
The simplest is a shunt voltage clamp. The power from the panel is free, the panel is a high impedance output, and the power is low enough for a reasonable sized heatsink on the clamp to do the job. That way you lose no power at all while the output voltage is below the clamp voltage.

Use a TL431 (or TLV431 the low voltage version) as the voltage sensor to drive a PNP clamp transistor.







simulate this circuit &ndash; Schematic created using CircuitLab

Choose R2 and R3 to deliver the 431's threshold voltage, 2.495V, or 1.24V for the V version, at your chosen clamp voltage. Note the V version needs a lower current at this port, so you can use larger resistors. Read the datasheet for whichever you're using, and calculate resistors accordingly.

What threshold voltage? 6.2V is the absolute maximum, but as Ron Beyer and Dave Tweed point out, 5v may be rather better. You shouldn't plan run any device at its absolute maximum rating, and the lower voltage results in a lower worst case dissipation in the clamp.

R4 is optional, it will shift some of the dissipation away from Q1, but make sure it's small enough, less than a few ohms, to conduct enough current to shunt the maximum current.

Q1 and R4 must have enough power handling and heatsinking to absorb the full output of the panel on a bright day.

All TL431s appear to be the same, but TLV431 is a bit of an oddity. The TI make is only rated to 6v, whereas the On Semi and the Diodes Inc brands are rated to 16v, beware.