See the note about "Cell notation". The $format parameter is optional

 In general it is sufficient to use the "write()" method.

rite_string($row, $column, $string, $format)
 Write a string to the cell specified by $row and $column:

     $worksheet->write_string(0, 0, 'Your text here' );
     $worksheet->write_string('A2', 'or here' );