Direct Phone Charge from Solar Cells via USB
Typical phones are very "economical devices". If their charge environment is revolving around standard USB (which is 5 V by default), no-one will design the internal charger that is tolerant to 19V. Most typical line of charging ICs from Texas Instruments has maximum input level of 6-7 V, (say, BQ24xxx line), and the circuit will shut down if this level is exceeded. 

Only most recent, Power-Delivery capable phones might have circuits that are 20-25 V tolerant, but again, you have to engage into some complex protocol to allow this input to be functional.

So in short, you need to provide a step-down converter to 5V if you want to charge a "typical phone" from raw solar panel.