ASP.NET - C# - set gridview cell as text and export to excel as text

protected void ExportToExcel(DataTable dt)
    {
        string sFileName = "Data_" + DateTime.Now.Year + DateTime.Now.Month + DateTime.Now.Day + ".xls";

        GridView gv = new GridView();
        gv.AllowPaging = false;
        gv.DataSource = dt;
        gv.DataBind();

        Response.Clear();
        Response.ClearContent();
        Response.ClearHeaders();
        Response.Buffer = true;
        Response.AddHeader("content-disposition", string.Format("attachment; filename={0}", sFileName));
        Response.ContentType = "application/vnd.ms-excel";
        //Response.ContentType = "application/vnd.openxmlformats";
        StringWriter sw = new StringWriter();
        HtmlTextWriter htw = new HtmlTextWriter(sw);
        //Change the Header Row Foreground to white color
        gv.HeaderRow.Style.Add("color", "#FFFFFF");
        //Applying stlye to gridview header cells
        for (int i = 0; i < gv.HeaderRow.Cells.Count; i++)
        {
            //Change to Blue
            gv.HeaderRow.Cells[i].Style.Add("background-color", "#507CD1");
        }
        int j = 1;
        //This loop is used to apply stlye to cells based on particular row
        foreach (GridViewRow gvrow in gv.Rows)
        {
            gvrow.BackColor = Color.White;
            if (j <= gv.Rows.Count)
            {
                if (j % 2 != 0)
                {
                    for (int k = 0; k < gvrow.Cells.Count; k++)
                    {
                        gvrow.Cells[k].Style.Add("background-color", "#EFF3FB");
                    }
                }
            }
            j++;
        }
        gv.RenderControl(htw);
        Response.Write(sw.ToString());
        Response.Flush();
        Response.End();
    }