Cathode interfacial engineering using stearic-acid-mediated polyethylenimine ethoxylated for high-performance solution-processed organic light-emitting diodes

Solution-processed organic light-emitting diodes (OLEDs) have attracted significant attention for use in nextgeneration displays and light sources. Developing solution-processable electron transport/injection layers is crucial for the realisation of high-speed and low-cost completely solution-processed OLEDs. In this study, we demonstrate an interface optimisation strategy using stearic-acid-modified polyethylenimine ethoxylated (SAPEIE) at the cathode interface for highly efficient solution-processed OLEDs. The organic layers, including the emissive, hole transport, and electron injection layers, are completely solution-processed, allowing a multilayer structure. The work function of cathode significantly shifts from 4.20 to 3.96 eV by introducing SA-PEIE, which minimizes the electron injection barrier at the interface. The SA-PEIE-based completely solution-processed OLEDs improve the device efficiency and reduce the operating voltage compared to a control device with PEIE. The synergistic effects of the optimised SA-PEIE layer considerably enhance the device performance. In addition, the effective interfacial properties enhance the long-term stability and mechanical durability under the cyclic bending of flexible OLEDs. Furthermore, an outstanding light-emitting performance up to 9 cm2 is observed in the SA-PEIE-based device. Thus, we believe that interface engineering with SA-PEIE can lead to the development of high-performance and low-cost completely solution-processed OLEDs with superior device efficiency and long-term mechanical stability.