Improved impedance characteristics of all-water-processable triple-stacked hole-selective layers in solution-processed OLEDs

We herein report an investigation of the device performance capabilities and impedance characteristics of solution-processed organic light-emitting devices (OLEDs) with all-water-processable triple-stacked hole-selective layers (HSLs) on an indium-tin-oxide (ITO) anode, fabricated using a simple coating technique. Highly smooth and homogeneous triple-stacked layers were deposited via horizontal-dip-(H-dip-) coating using aqueous dispersions of graphene oxide (GO), molybdenum oxide (MoO3), and poly(ethylenedioxy thiophene): poly(styrene sulfonate) (PEDOT: PSS). From the triple-stacked GO/MoO3/PEDOT: PSS HSLs used as hole-injection layers (HILs) in the OLEDs, which outperform a conventional single HIL of PEDOT: PSS, it was found that OLEDs with triple-stacked HILs exhibited characteristic impedance properties, including low parallel resistance with trap-free space-charge-limited conductivity. Furthermore, it was shown that the relaxation frequency of a sample OLED with triple-stacked GO/MoO3/PEDOT: PSS HILs was much higher than that of a reference device with a single PEDOT: PSS HIL. These impedance behaviors indicate that carrier (hole) injection in the sample OLED is more efficient than that in any of the other devices tested here. The results presented here clarify that the triple-stacked GO/MoO3/PEDOT: PSS layers can act as efficient HILs on an ITO anode, representing a remarkable advance in relation to the mass production of high-performance solution-processable OLEDs. (C) 2016 Optical Society of America.