QLED panels are backlit, which means they need some light source to brighten up. Probably a light source such as bulbs (not those household ones) is used. As a result, a QLED TV would be thicker than any crystal UHD TV. The added thickness is to accommodate the light source at the back of the QLED panel in a TV. However, being brightened up by a separate light source also allows increased brightness, which serves as an ideal feature if the ambient receives sufficient light.