How to multiply a voltage when its lower than 5V?
Assuming you want to get 620 mA @ 12 V as an end result ...that means approximately 7.5 W

At 1 V input and assuming ~80 % efficiency you need to supply 9.5 A from your source.
Not an impossible scenario but quite a challenge.

Re-visit your specs and work out exactly what you can provide from your source and you'll be able to figure out what can be done in the convertor and what power you might deliver.