How to power a 12V fan simply using a PV panel?
The Fan is rated at 0.53A at 12V. Voltage range of 7-13.8V. If you select a nominal 12V solar panel with an output short-circuit current of around 0.5A (and certainly no more than 0.53) you will be OK. Yes, the maximum panel Voltage may be 20V or more, but the panel cannot achieve that voltage when a load is attached.

The solar panel has a V-I curve. In other words, the current and voltage of the panel are always on a well-defined curve (under constant lighting conditions). Likewise, the motor will have a V-I curve. When you connect them together, they will find the single point of intersection of their V-I curves.

As long as the voltage at that single point of intersection does not exceed the maximum motor rated voltage (13.8), then you are OK. Unfortunately, we don't have the V-I curve for the motor or the solar panel.

But, we know that the motor will consume 0.53 Amps at 12V. And, every solar panel publishes its short-circuit current (aka, Isc). So if you use a solar panel which can only supply 0.53A (Isc &lt;= 0.53A), you don't need to worry. Because at any lower current, the voltage will be below 12V.

The panel you linked to has an Isc of 0.69A. So if you use that panel, there is some potential for problems. There is still a chance that it will be OK, but you would have to try it to find out. If you can find a panel with a lower Isc, that will be safer. If you can find a panel with a published V-I curve, you can check the current at 12V to see if it is OK. If you want to use the panel you linked to, you should be prepared to burn up some extra power somehow (if needed), using one or more power diodes in series or resistors in series, or using an LDO regulator. Just enough to keep the voltage under 13.8 in full sun. Personally, I would not use a DC-DC converter in this case just because of the complexity and cost. I am also not totally sure how the DC-DC will behave when the lighting is low. It may due some weird thing where it cycles on and off in an irritating way.