用C#在Excel中画矩形，如何去掉矩形的填充色

代码如下：
            Microsoft.Office.Interop.Excel.Application excel = new Microsoft.Office.Interop.Excel.ApplicationClass();
            excel.DefaultFilePath = System.Windows.Forms.Application.StartupPath + @"\file";
            Microsoft.Office.Interop.Excel.Workbook workbook = excel.Workbooks.Add(Type.Missing);            Microsoft.Office.Interop.Excel.Sheets sheet = workbook.Worksheets;
            Microsoft.Office.Interop.Excel.Worksheet worksheet = (Microsoft.Office.Interop.Excel.Worksheet)sheet.get_Item(1);            //画矩形
            Microsoft.Office.Interop.Excel.Shape orgChart2 = worksheet.Shapes.AddShape(Microsoft.Office.Core.MsoAutoShapeType.msoShapeRectangle, 116, 44, 200, 200);
            Microsoft.Office.Interop.Excel.XlFileFormat Version = Microsoft.Office.Interop.Excel.XlFileFormat.xlExcel8;
            Microsoft.Office.Interop.Excel.XlSaveAsAccessMode Mode = Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlNoChange;            workbook.SaveAs("dd.xls", Version, System.Type.Missing, Type.Missing, false, false, Mode, Type.Missing, Type.Missing, Type.Missing,
                Type.Missing, Type.Missing);            workbook.Close(false, Type.Missing, Type.Missing);