Understanding the Mechanism of Piezoelectric Organic Light-Emitting Diodes

Piezoelectric organic light-emitting diodes (p-OLEDs)are acousto-opticdevices that enable direct visualization of ultrasound intensity profilesand pixel-free ultrasound imaging. However, there have been no reportssince the initial report because the underlying physics has not beenfully explained. In this study, we report the mechanism of p-OLEDsby elaborating on an alternating-current (AC)-driven operating environment.Strong light emission was observed from the p-OLED, although the ultrasoundfrequency (590 kHz) was significantly higher than the cutoff frequency(45 kHz) of our phosphorescent OLED. Such a contradictory result canbe explained by an occurrence of a direct-current (DC) voltage offsetor exceptionally large AC voltage generation in the OLED. We discoveredthat an AC voltage amplitude (V (AC)) aslarge as 16 V could be induced in the OLED by applying only 60 V of V (AC) to the transmitter, revealing the originof the strong acousto-optical coupling, while the DC offset was notobserved during the measurement. Based on this mechanism, we demonstratethat p-OLEDs made on the single-crystalline [Pb(Mg1/3Nb2/3)O-3](1-x ) [PbTiO3] (x) substrate with a higher piezoelectriccoefficient exhibit a considerably lower turn-on voltage and higherluminance compared to the case with the polycrystalline Pb(Zr (x) Ti1-x )O-3 substrate.