Try setting the format string input to the array to spreadsheet primitive to "%s" instead of an empty string.
As a side note, the write characters to file VI opens the file, writes the characters, then closes the file. This is very inefficient to do inside a loop. Consider opening the file before the loop and closing afterwards. Pass the references in with shift registers. You can find the VIs to do this by simply opening the write characters to file VI and copying the contents to the appropriate places in your VI.