faveo/vendor/maatwebsite/excel/docs/export/format.md

Column formatting

To tell Excel how it should interpret certain columns, you can use ->setColumnFormat($array).

// Format column as percentage
$sheet->setColumnFormat(array(
    'C' => '0%'
));

// Format a range with e.g. leading zeros
$sheet->setColumnFormat(array(
    'A2:K2' => '0000'
));

// Set multiple column formats
$sheet->setColumnFormat(array(
    'B' => '0',
    'D' => '0.00',
    'F' => '@',
    'F' => 'yyyy-mm-dd',
));

Go to the reference guide to see a list of available formats.