Remember you always need a blocking diode when connecting a solar cell to a battery as
an unlit solar cell will drain away the battery rapidly.
Such a diode means that too little voltage at the cell means no current can flow from it to
charge the battery.
Many devices designed to run from USB are expecting 4.75 to 5.25V range, what
happens outside this range depends on the device. Overvoltage is always bad news.
Undervoltage usually causes malfunction, but not usually damage.