Minimum charging voltage for an iPhone 6
I'm not entirely sure you're asking the right question, because voltage is different to current.

All iOS devices (iPhones, iPads, etc) charge at 5 volts. So, if all you need to know is voltage, then 5 volts is the official requirement for charging.

However, the rate at which a device will charge is partly dependant on the rate at which the charge is flowing (known as current). I say 'partly' because the state of your iOS device also has an impact (e.g. it will charge quicker if it's not being used, it will charge slower if you're watching YouTube videos, etc). 

So, in summary, iPhone chargers are typically rated at 5 volts and 1 amp while iPad chargers are rated at 5 volts and 2.1 amps.