5V 1A Solar panel only delivering low power
If you open the black box you will probably find that each of the Data+ and Data- lines are each connected to ground with a 15 kilo-ohm resistor. The phone is supposed to detect these resistors, know that it is connected to a "standard downstream port" (SDP) USB port, and configure itself to draw at most 100 milliamps of current. 

If you measure the IV curve of the solar panel at 100 milliamps current, try a variable resistor or about 50 ohms fixed resistor, you will maybe measure 5 volts. Thus allowing the manufacturer to claim that the solar panel will charge a mobile phone (even though it will take about 50 hours of full sun, or about 12 days allowing for 4 hours a day of noonday sun).