Getting Posts without a Thumbnail

<?php
$spotlight_args = array(
    'orderby'  => 'post_date',
    'order'    => 'DESC'
    'meta_query' =>
    array(
        'relation' => 'AND',
        array('key' => '_thumbnail_id',
            'value' => '',
            'compare' => 'NOT EXISTS')
    )
);

 $spotlight_posts = get_posts( $spotlight_args );