Size-Dependent PhotoelectricProperties of GrapheneOxide for a Phosphorescent Green Organic Light-Emitting Diode Achievinga High Device Efficiency

In this work, size-dependent photoelectric properties for graphene oxide (GO) are investigated in detail by breaking pristine large-sized GO to different sheet diameters using the tip ultrasonic treatment method. Clearly, the average size for pristine GO is about 700–800 nm, which is decreased to about 100–200 nm after 7–9 h ultrasonic treatment. The reduced diameter for GO induces a better film-forming property and slightly improved transmittance. Meanwhile, small size GO possesses more boundaries, in which abundant oxygen-containing functional groups are introduced, causing a relatively higher work function of about 5.20 eV for 7–9 h ultrasonically treated GO. To explore its potential application, Ir(ppy)₃-based green organic light-emitting diodes (OLEDs) were fabricated using different size GO as a hole injection layer (HIL). Optimized green devices with 7 h ultrasonically treated GO HIL show a lower turn-on voltage of 2.9 V and achieve the record-breaking current efficiency and external quantum efficiency of 105.98 cd/A and 30.56%, respectively, which are significantly higher than these for pristine GO (79.06 cd/A and 22.87%)- and PEDOT:PSS (73.26 cd/A and 21.01%)-based reference devices. Moreover, it is demonstrated that a small size GO-based device also exhibits obviously better operational stability, further supporting its huge potential application in developing high-performance solution-processed OLEDs.