On my commuter bike, my front dynamo hub charges a small 18650 battery that I then can tap to provide power to my smart phone. You need to have an intermediary battery because most dynamos only produce the German government mandated 6VAC / 3 watts minimum, which is around 0.5 amps (500 milliamps) -- and this is only when you're going faster than 15 km/h. There are conversion losses going from 6VAC to 5VDC so you won't get the full 500 milliamps.
This is made worst because at speeds less than 15km/h, the voltage and available current drop to non-usable levels and the phone will stop charging and then start charging again -- on my phone the screen turns on and off each time there's a charge/non-charge event, which drains the battery tremendously.
In any case, most cell phones will want 5VDC @ 1A if not 2.1A, so the hub clearly can't produce enough power by itself for a steady charge. This is why you need the intermediary battery.
I designed and made the circuit** that I use for myself. You can buy something similar off the shelf here:
** I'd share the circuit except it's a bastard/hack and anyone who knows how to solder would be able to hack their own using a DC rectifier that takes the 0~6VAC and feeds 0~6 vdc into a LiPO charging circuit that charges the 18650; and subsequent boost regulator that takes the 18650's 3-4vdc to 5 vdc stabilized to a USB female A.