Efficiency improvement in solution-processed multilayered phosphorescent white organic light emitting diodes by silica coated silver nanocubes

Localized surface plasmon resonance (LSPR) effect of metal nanoparticles (MNs) has been widely applied in organic light-emitting diodes (OLEDs) to improve the radiation of excitons. The LSPR wavelength and intensity of MNs and the coupling between MNs and excitons greatly affect the LSPR effect on exciton radiation. In this work, silica coated silver nanocubes (Ag@SiO2 NCs) were doped in the electron transport layer (ETL) of a solution-processed multilayered white OLED (WOLED). Due to the sharp edges and corners, Ag NCs have strong LSPR effect and can effectively enhance the radiation of nearby excitons. With an appropriate concentration of Ag@SiO2 NCs, the WOLED achieved two fold improvement in the current efficiency comparing with the control device without Ag@SiO2 NCs incorporated. The working mechanism of the Ag@SiO2 NCs based WOLED was investigated in detail. For the solution-processed OLED, excitons usually form and recombine near the interface of emission layer and electron transport layer (EML/ETL) because the commonly used host material (such as polyvinylcarbazole, PVK) has the unipolar hole transport property. So the Ag@SiO2 NCs in ETL greatly enhanced the radiation of the excitons located near the EML/ETL interface, which mostly contributed to the performance enhancement of the Ag@SiO2 NCs based WOLED. Study on a group of devices with Ag@SiO2 NCs doped in different locations indicated that Ag@SiO2 NCs in ETL showed more effective LSPR effect than those in hole injection layer. Electroluminescence and photoluminescence spectra of the WOLEDs declared that the Ag@SiO2 NCs simultaneously improved the radiation intensities of the blue and yellow excitons and helped the WOLED maintain the good chromaticity stability, which was mainly attributed to the wide LSPR wavelength range (450-650 nm) of the Ag@SiO2 NCs. The SiO2 coating layer of the Ag@SiO2 NCs played the important role in the LSPR enhanced emission. On the one hand, it formed an appropriated distance between the Ag NCs and the extions, helping to generate the strong coupling between them. On the other hand, it suppressed the effect of Ag NCs on charge trapping, keeping the stability of the carrier transport in the device. Our research demonstrate MNs can effectively improve the performance of OLEDs by carefully designing the device structure.