Amide-Functionalized Small Molecules as Solution-Processed Electron Injection Layers in Highly Efficient Polymer Light-Emitting Diodes

An efficient polymer light-emitting diode (PLED) is demonstrated using the solution-processed octadecanamide (ODAD) as an electron injection layer (EIL). The ability for the EIL of ODAD to lower the electron injection barrier is systematically investigated by the analysis of structural analogs, photovoltaic measurements, X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). Devices with ODAD/Al and 2-phenylacetamide/Al cathodes show the higher luminance efficiency (8.5 and 11.0 cd A(-1)) than that with the Ca/Al cathode (5.9 cd A(-1)). On the contrary, the device with the octadecylamine/Al cathode shows the performance similar to that of the device with the Al cathode. It correlates the amide functional group with the enhanced electron injection. The corresponding photovoltaic measurements suggest the amide functional group be essential to decrease the work function (WF) of Al. XPS and UPS verify the mechanism for lowering electron injection barrier: the amide functional group forms the C-O-Al complex at the ODAD/Al interface to create the interfacial dipole for decreasing the WF of Al. This is the first demonstration that efficient PLEDs are achieved using amide-functionalized small molecules as EILs. The work here paves the way for developing interfacial materials with the amide functional group to achieve efficient organic/polymer optoelectronics.