Try This
 
Response.Clear()
            Response.Charset = ""
            Response.AddHeader("content-disposition", "attachment;filename=report.xls")
            Response.ContentType = "application/vnd.xls"
            Response.Cache.SetCacheability(HttpCacheability.NoCache)
            Dim stringWrite As New System.IO.StringWriter()
            Dim htmlWrite As System.Web.UI.HtmlTextWriter = New HtmlTextWriter(stringWrite)
            Excel_Word_details_view.RenderControl(htmlWrite)
            Response.Write(stringWrite.ToString())
            Response.End()
          Catch ex As Exception