Get Html in my exported csv file ASP .NET C#

Question

I want to export gridview to CSV file using C#, but I get some html code in my exported file. I don't know why this is happening. My someone help me please. I think that my error provide from Response.

Here is my code:

 protected void ExportCSV(){                   
var now = DateTime.Now;
            string fileName = Page.ResolveClientUrl("~/downloads/" + now.Day + "_" + now.Month + "_" + now.Year + '-' + now.Hour + "_" + now.Minute + "_" + now.Second + ".csv");
            string fullName = HttpContext.Current.Server.MapPath(fileName);


                GV1.AllowPaging = false;
                StringBuilder sb = new StringBuilder();

                DataTable dt = new DataTable();
                int colCount = GV1.Columns.Count - 1;
                for (int i = 0; i < colCount; i++)
                {
                    dt.Columns.Add(GV1.Columns[i].ToString());
                }
                foreach (GridViewRow row in GV1.Rows)
                {
                    DataRow dr = dt.NewRow();
                    for (int j = 0; j < colCount; j++)
                    {
                        if (!row.Cells[j].Text.Equals("&nbsp;"))
                        {
                            if(j==0)
                                dr[GV1.Columns[j].ToString()] = (row.Cells[0].FindControl("lknbtnRowSelected") as LinkButton).Text;
                            else
                                dr[GV1.Columns[j].ToString()] = row.Cells[j].Text;
                        }
                    }

                    dt.Rows.Add(dr);
                }

                foreach (DataColumn col in dt.Columns)
                {
                    sb.Append(col.ColumnName + ';');
                }

                sb.Remove(sb.Length - 1, 1);
                sb.Append(Environment.NewLine);

                foreach (DataRow row in dt.Rows)
                {
                    for (int i = 0; i < dt.Columns.Count; i++)
                    {
                        sb.Append(row[i].ToString() + ";");
                    }

                    sb.Append(Environment.NewLine);
                }

                sb.ToString().Normalize();


                using (StreamWriter writer = new StreamWriter(fullName, true, Encoding.GetEncoding("iso-8859-1")))
                {
                    writer.WriteLine(sb.ToString());
                    FileInfo info = new FileInfo(fullName);
                }



                    Response.Clear();
                    Response.ContentType = "application/vnd.ms-excel";
                    Response.AddHeader("content-disposition", "attachment;filename=" + "file" + now.Day + "_" + now.Month + "_" + now.Year + '-' + now.Hour + "_" + now.Minute + "_" + now.Second + ".csv");

                    Response.WriteFile(fullName);
                    HttpContext.Current.ApplicationInstance.CompleteRequest();


            }
            catch(Exception ex) {  }

        }