A hydrophilic monodisperse conjugated starburst macromolecule with multidimensional topology as electron transport/injection layer for organic electronics

Abstract

We present the design and synthesis of a hydrophilic monodisperse conjugated starburst macromolecule, TrOH, grafted with diethanolamine groups on its side chains, as efficient electron transport/injection layer (ETL) for solution-processed organic light emitting diodes (OLEDs), which allows the use of high work-function metal Al as the cathode instead of the use of conventional environmentally unstable cathode. Multilayer OLEDs with device configuration of ITO/PEDOT:PSS/emissive layer/TrOH/Al have been facilely constructed by solution processing due to the orthogonal solubility of the ETL and the active layer. For comparison, devices with Al, Ca/Al cathodes and a conjugated polymer grafted with the same polar pendant groups (PFN-OH) as ETL with Al as cathode have also been fabricated. The device based on TrOH/Al cathode showed superior luminous efficiency (LE) of 3.01 cd A⁻¹ to those using Al cathode (0.08 cd A⁻¹), Ca/Al cathode (1.32 cd A⁻¹) and PFN-OH/Al (2.00 cd A⁻¹). In order to understand the role of hydrophilic TrOH as ETL on improving the device performance, open-circuit voltage (V_oc) of the devices, the water contact angles (θ), and atomic force microscopy (AFM) were used to investigate the surface properties before and after deposition of ETL. The results confirmed that the electron injection barrier from the Al electrode could be effectively decreased by inserting hydrophilic TrOH as ETL. The uniform well-defined nanostructured surface morphology of TrOH appeared to be an important factor for improving device performance, which will likely have a positive effect on improving interface contact and/or increase the local electric field intensity. This study thus opens a broad idea on designing highly efficient ETL materials with monodisperse characteristics and novel molecular topology for organic electronics.