70‐1: Invited Paper: Capacitance Reduction of Red OLED through Device Optimization

In this work, we demonstrate a novel device structure with a dual‐electron blocking layer (2EBL) configuration to improve the capacitance‐voltage (CV) behavior without compromising other critical performances of bottom‐emission red organic light‐emitting diode (OLED). We started the work from studying the influence of electron blocking materials (EBMs) on various device characteristics including capacitance behavior. It is found that with conventional single EBL configuration, EBM with a deep highest occupied molecular orbital (HOMO) level tends to have high voltage yet low capacitance, while EBM with a shallow HOMO has the opposite behavior. To combine the advantages of low voltage and low capacitance, we investigate the employment of 2EBL configuration. 2 EBMs were developed, where EBM1 has a HOMO of ‐5.17 eV and a faster hole mobility of 21×10‐5 cm2/Vs, while EBM2 has a deeper HOMO level of ‐5.22 eV and a slower mobility of 14×10‐5 cm2 /Vs. Bottom emission red OLEDs A‐D with single EBM1, single EBM2, EBM1/EBM2, and EBM2/EBM1 were fabricated, respectively. The characterization of OLEDs A‐D reveals that OLED C reaches a balanced performance, at 10 mA/cm2 achieving a low voltage of 3.5 V, a low maximum capacitance per unit area (u‐Cmax) of 36 nF/cm2 and an accumulated charges per unit area (u‐Q) of 8 nC/cm2, while maintaining a high EQE of 30.6% and a long lifetime LT95 of 11,300 h. Moreover, Electrochemical Impedance Spectroscopy (EIS) measurements highlight the differences in bulk resistance among the OLED configurations, emphasizing the importance of EBL composition and arrangement in optimizing OLED performance. Low capacitance of OLED ensures a quick signal response and hence quick refresh rate, especially at low gray scale scenarios. Reduction of capacitance through EBL optimization is believed to be the best strategy, comparing to tuning emitter and hosts which is more likely to affect other key performances of OLED. Although this work is based on study of red OLED, the same scheme can be extended to other colors.