Ha 
A solar panel would be great, indeed!
But type-C so called “Power Delivery” (PD) is not as simple as former USB charging where the source just provided 5V DC and was done with it - eat or die. PD is very complex and like it was already mentioned here requires the source and the sink to negotiate the terms, the negotiate a so called “contract”, where the source tells the sink which different voltage levels it can provide and also at which rate (current). Then the sink, in this case the laptop, picks one that suits best and confirms this back to the source. If both are happy, current flows.
In the case of the Librem14 the source needs to be able to provide at least 19V voltage, else it will not work at all. The current should total up to about at least 40W to 50W as a minimum, the DC charger that comes with the laptop provides 65W and the battery charger and power consumption control is tuned for 65W, so I would recommend a type-C source with 60W or more.
Cheers
nicole