Nowadays, phosphorescent organic light-emitting diodes (PhOLEDs) is a widespread technology, in which all the high-performance devices are constructed on a stack of different organic layers called multi-layer devices (ML-PhOLEDs). Thanks to these functional layers, the injection, the transport, and the recombination of holes and electrons in the emissive layer (EML) are significantly improved, allowing to reach high performances. In this technology, the ideal devices are the single-layer PhOLEDs (SL-PhOLEDs), with a very simple stack only constituted by the electrodes and the EML. These devices are simple, very easy to fabricate, and can hence significantly decrease their costs. Nevertheless, removing the functional layers of an OLED drastically decreases the performances and there is, so far, only a few examples of high-performance SL-PhOLEDs. Thus, in SL-PhOLEDs, the role of the functional layer should be performed by the EML, which should allow an excellent injection, transport, and recombination of holes and electrons. In this work, thanks to a rational molecular design of the EML, we report a green-emitting SL-PhOLED, displaying a very high external quantum efficiency of 22.7%. The EML of this device is constructed on the barely studied Ir(ppy)2acac phosphor and a high efficiency host material possessing a Donor-spiro-Acceptor design. This performance is, to the best of our knowledge, the highest reported to date for SL-PhOLEDs (all colors considered). Through a structure/property/device performance relationship study combining morphological (AFM), photophysical (time-resolved spectroscopy) and charge transport studies, we show that the EML presents all the required characteristics such as smooth surface, quick radiative deactivation, and ambipolarity. In addition, the comparison with Ir(ppy)3, the most famous green emitter used in PhOLEDs, highlights the high potential of Ir(ppy)2acac. The impact of the phosphorescent emitter on the ambipolarity of the charge transport is particularly evidenced.