I'm looking for possibilities to charge a power bank with a 12V solar panel, is that possible to connect both of them directly?

 

you can use buck converter to drop the volts from 18v dc down to 5.2v dc to change your power bank.

 

Yes, very . . . very slowly (← but this again is not the best charging profile for Li-Ion)

 

If you try that just like I once did, your powerbank will blow up. So, create a buck converter or use a buck converter Ic. Like mc34063. Good Luck!