Between with field type varchar

I have two fields where a start and end date is set:

<form name="filtroClientes" action="consulta_contas.php" method="POST">

            <div class="form-group">
                <label for="nomefiltrar" style="color:#E4CDAC; font-size: 17px; font-family:Arial">Observação</label>
                <input type="text" class="teste" id="nomefiltrar" name="nomefiltrar" />
                <p>
                <label for="dataemissaoinicialfiltrar" style="color:#E4CDAC; font-size: 17px; font-family:Arial">Data Emissao Inicial</label>
                <input type="date" id="dataemissaoiniciofiltrar" class="teste" name="dataemissaoiniciofiltrar" />
                <p>
                <label for="dataemissaofiltrar" style="color:#E4CDAC; font-size: 17px; font-family:Arial">Data Emissao Final</label>
                <input type="date" class="teste" id="dataemissaofinalfiltrar" name="dataemissaofinalfiltrar" />             



                <input type="hidden" name="acao" value="filtrar"/>

            </div>


            <button type="submit" class="btn btn-custom btn-roxo">Filtrar</button>
    </div>
    </form>

Through a function I retrieve the value of these fields and perform a query to filter according to the values placed in this field: registra_contas.php:

function filtraContas(){
    $contas = '';
    $banco = abrirBanco();
    if(isset($_POST['nomefiltrar'])){
        $nomefiltrar = $_POST['nomefiltrar'];
    }else{
        $nomefiltrar = '';
    }

    if(isset($_POST['dataemissaoiniciofiltrar'])){
        $dataemissaoinicio = $_POST['dataemissaoiniciofiltrar'];

    }else{
        $dataemissaoinicio = '';
    }

    if(isset($_POST['dataemissaofinalfiltrar'])){
        $dataemissaofinal = $_POST['dataemissaofinalfiltrar'];
    }else{
        $dataemissaofinal = '';
    }

    $sql = " SELECT c.*, c.dataemissao as dataemissaoformatada, c.datavencimento as datavencimentoformatada, p.nome, e.nome_especie FROM contas c INNER JOIN pessoa p ON (c.id_fornecedor = p.id) INNER JOIN especies e ON (c.id_especie = e.id) WHERE observacao LIKE '%$nomefiltrar%' OR c.dataemissao BETWEEN STR_TO_DATE('$dataemissaoinicio', '%d/%m/%Y') AND STR_TO_DATE('$dataemissaofinal ', '%d/%m/%Y') ";
    $resultado = $banco->query($sql);
    $banco->close();
    while($row = mysqli_fetch_array($resultado)){
        $contas[] = $row;
    }
    return $contas;
}

However, the entre is not being applied. Why does this occur? The date field is a field of type varchar in the database. I noticed that even if I change the field from "date" to "text" and remove the STR_TO_DATE, the entre is still not performed, I believe it should work since the bank is type varchar and in that field would be catching in text ?? format