Note that, in spite of the name, the output is a list of (unicode) code points, i.e. integers - not characters, i.e. atoms of length 1.

?- with_output_to_chars(write("Hello"),Codes).
Codes = [72, 101, 108, 108, 111].

It's probably better to use with_output_to/2 which can generate codes, chars, strings or atoms.