You can find the information on the support page (or so I believe):
It’s 5V and 1A up to a max. of 3A.
The voltage, as far as I know, is not relevant for quick-charging, it’s the amperage (if that is a valid term?).
Edit:
As I really love mine, I use the chance to do some advertising again.
The Waka-Waka solar power-banks have an output of >2.1A for quick-charge.
So they might not be as fast as you can get, but they share the ethics of Fairphone (as well as the origins, 'cause they are from the Netherlands):
https://waka-waka.com/de/