We present phosphorescent WOLEDs fabricated simply by inserting an ultrathin nondoped orange layer within the blue emissive zone, where an efficient exciplex system is applied as the host. The resulting WOLED shows maximum power efficiency of 75.3 and 63.1 lm/W at the luminance of 1000 cd/m(2). The exciton density profile in the emitting layer, the operational mechanism and the quenching process at high luminance are systematically investigated by experimental and theoretical methods, from which it is concluded that the efficient utilization of excitons via exciplex host and the wide recombination zone are the key factors for the prominent achievement of high efficiency and greatly reduced efficiency roll-off.