Are you going to wire the solar to a usb connector and plug it into the bank charging port? This is probably the safer way of approaching this.
Also, sometimes the quoted voltage is a "nominal working voltage" kind of deal. Grab a multimeter and see how many volts the solar cell puts out at noon without any load, it might be in the 8v range that could pose an overvoltage problem to your bank (that's expecting 5v ) if it fills up completely.
Believe this was featured in a paper that recently used trig to prove the Pythagorean theorem (previously thought to be a circular definition). I think some highschoolers cracked it as part of a mathematics challenge or something.