Effect of the charge balance on high-efficiency inverted polymer light-emitting diodes

In this work, an ultra-thin insulating layer, poly(methylmethacrylate) (PMMA), is incorporated between the emissive layer (EML) and hole transporting layer (HTL) within the inverted polymer light-emitting diodes (PLEDs). Such a structure helps to reduce the hole injection and balance the electrons and holes in the EML. PLEDs with optimal PMMA thickness of around 5.6 nm is observed to obtain a maximum current efficiency of 14.33 cd A−1, which corresponds to 7.07% improvement compared to that of the device without PMMA layer (13.5 cd A−1). The device with best performance exhibits a superior low driving voltage of 2.5 V at 1 cd m−2 and shows a lower efficiency roll-off current efficiency which sustains 85% of the maximum value when the current density reaches 140 mA cm−2.