Export to excel from girdview satisfy conditions

I have a LoadData function according to the search criteria and make girdview, I want to export data to excel from this girdview the path they have chosen.This is LoadData function

  private void LoadData(string idPB, string StartDate, string ToDate)
      {
          hmweb.Services.cChuyenMuc mChiPhi = new hmweb.Services.cChuyenMuc();
          DataTable dtb = new DataTable();
          dtb = mChiPhi.ChiPhi_Search(idPB, StartDate, ToDate);
          if (dtb.Rows.Count > 0)
          {
              pnlDanhSach.GroupingText = "Tìm th?y " + dtb.Rows.Count.ToString() + " mã chi phí";
              grvDanhSach.DataSource = dtb;
              grvDanhSach.DataBind();
              //ViewState["dtList"] = dtb;
          }
          else
          {
              pnlDanhSach.GroupingText = "Tìm th?y 0 mã chi phí";
          }
          mChiPhi = null;
      }

and this is Find

  protected void lbtFind_Click(object sender, EventArgs e)
      {
          grvDanhSach.DataSource = null;
          grvDanhSach.DataBind();
          LoadData(cboPhongBan.SelectedValue, txtStartDate.Text.ToString(), txtToDate.Text.ToString());
      }

he program is written in asp.net c #. Does anyone know detailed help. Thank you