Piezoelectric organic light-emitting diodes (p-OLEDs) are acousto-optic devices that enable direct visualization of ultrasound intensity profiles and pixel-free ultrasound imaging. However, there have been no reports since the initial report because the underlying physics has not been fully explained. In this study, we report the mechanism of p-OLEDs by elaborating on an alternating-current (AC)-driven operating environment. Strong light emission was observed from the p-OLED, although the ultrasound frequency (590 kHz) was significantly higher than the cutoff frequency (45 kHz) of our phosphorescent OLED. Such a contradictory result can be explained by an occurrence of a direct-current (DC) voltage offset or exceptionally large AC voltage generation in the OLED. We discovered that an AC voltage amplitude (VAC) as large as 16 V could be induced in the OLED by applying only 60 V of VAC to the transmitter, revealing the origin of the strong acousto-optical coupling, while the DC offset was not observed during the measurement. Based on this mechanism, we demonstrate that p-OLEDs made on the single-crystalline [Pb(Mg_1/3Nb_2/3)O₃]_1–x [PbTiO₃]_x substrate with a higher piezoelectric coefficient exhibit a considerably lower turn-on voltage and higher luminance compared to the case with the polycrystalline Pb(Zr_xTi_1–x)O₃ substrate.