Usb input current requirements for a power bank
Usb battery packs have built in LIPO charging circuits, for 5V input to constant current charging the Lipo. They also have boost circuits for the 5v  output. 

Mainly,  if you don't have a stable, 250 mA or higher,  5v +- 10% voltage source,  then it won't charge properly.

Depending on the actual circuit it uses internally, it might have better tolerances,  or even allow you to connect the solar panel directly. 

That said your solar panel can only produce those 6W 6V  at full sunlight.