StartDate and EndDate data select in datagridview

I have MS access database and form C# windows application managed by visual studio 2015. PaymentDate is a column name. I want to select from datagridview StartDate and EndDate with text box or datetimepicker using search button. But within a month it is good. But when I select one month to another month then it select full database. Here is my code:

private void btnSearch_Click(object sender, EventArgs e)

{

con = new OleDbConnection(conString);

string sql = "SELECT * FROM LalData WHERE PaymentDate between '" + txtStartDate.Text.ToString() + "' AND '" + txtEndDate.Text.ToString() + "'";

con.Open();

adapter = new OleDbDataAdapter(sql, con);

dt = new DataTable();

adapter.Fill(dt);

dataGridView1.DataSource = dt;

dataGridView1.CurrentCell = dataGridView1.Rows[dataGridView1.Rows.Count - 1].Cells[0];

btnSubmit.Enabled = false;

}