Yii2 kartik export to excel cell format

I'm using kartik export widget for exporting my grid view in excel. I am able to export all the data but there is an issue, I have a column name imsi which is in text, but after exporting it this column is having all the same numbers i.e. all the IMSI numbers are same. See below

Grid Columns

 $gridColumns =[
                [
                    'class'=>'kartik\grid\SerialColumn',

                ],
                 'id',
                ['attribute'=>'imsi','format'=>'text'],
                'sim_number'
  ]

How can I set the correct format? Any help would be highly appreciated.