It is possible to use a 100W 12V solar panel directly to charge a laptop, using a buck converter for example?
It might be possible, but not very reliable.  

The reason is the input impedance is very non-linear to any DC-DC converter such as those used on Mobiles and laptops.   This means the inability to match impedances from source to load (maximum power transfer theorem) which is the condition for MTTP.  Worse yet, the surge start current might shunt the PV into a low power low voltage start condition that fails to draw sufficient current to reach an equilibrium , assuming sufficient power exists for a matched linear load.

PV’s are current sources with Vmpt ~ 82% of Voc with voltage limits.

This means the load instead of appearing as a positive resistance can start as a negative incremental impedance and result in unstable cycle of little or no charging.  Boost DC-DC converters work with voltage source inputs 
it will then have a CC, CV cutoff profile.
with low ESR and not with current sources.

Your best bet to make a linear load using the DCR of a battery pack using an array with 24Voc (open circuit and charging a 19V. A good charger with MTTP will match the charger impedance to the battery chemistry and you have a backup to To charge your laptop while charging the other .

Check around for battery specs or purchase a 19 to 20V roughly batt Pack to charge your laptop and emulate the 19.5V universal charger. Then find a PV charger that works on that.

If you plan to go off grid, get some 1kg jars of Vit C with C&amp;L Lycene and Mg.  5,000 IU’s/ day and more if ill.  There ought to be enough sun for Vit D. (10,000 IU tab/day )

A battery load is linear and works but a DCDC converter does not unless there is a huge capacitor in between.  Your test mileage may. Vary.

(Batteries are 10k Farads approx)