datagridview导出到excel后，怎么让某一列在excel中强制处理为字符型

比如有个datagridview，有数据如下
编码 名称
01   aa
02   bb
导出到excel后，编码列为数字型，我想让他导出后为字符型
导出代码如下：using Excel = Microsoft.Office.Interop.Excel;public static int DataGridViewToExcel(System.Windows.Forms.DataGridView GridView, string strExcelFileName, out string strError)
        {
            strError = "";
            int nRet = 0;            Excel.Application xlApp = new Excel.Application();
            Excel.Workbooks workbooks = xlApp.Workbooks;
            Excel.Workbook workbook = workbooks.Add(Excel.XlWBATemplate.xlWBATWorksheet);
            Excel.Worksheet worksheet = (Excel.Worksheet)workbook.Worksheets[1];//取得sheet1            try
            {
                //~~
                // 写字段名
                int z = 0;
                for (int i = 0; i < GridView.Columns.Count; i++)
                {
                    if (GridView.Columns[i].Visible == true)
                    {
                        worksheet.Cells[1, z + 1] = GridView.Columns[i].HeaderText.ToString();
                        z++;
                    }
                }                // 写记录
                for (int i = 0; i < GridView.Rows.Count; i++)
                {
                    z = 0;
                    for (int j = 0; j < GridView.Columns.Count; j++)
                    {
                        if (GridView.Columns[j].Visible == true)
                        {
                            worksheet.Cells[i + 2, z + 1] = GridView.Rows[i].Cells[j].Value.ToString();
                            z++;
                        }
                    }
                }                worksheet.Columns.EntireColumn.AutoFit();//自动适应每列的宽度 add by sunny.li
                Excel.Range rg = worksheet.get_Range(worksheet.Cells[1, 1], worksheet.Cells[1, GridView.Columns.Count]);
                rg.Font.Bold = true;
                workbook.Saved = true;
                workbook.SaveCopyAs(strExcelFileName);                // 关掉内存中的进程
                xlApp.Quit();
                nRet = 0;
            }
            catch (Exception ex)
            {
                strError = ex.ToString();
                nRet = -1;
            }            return nRet;
        }