High efficiency polymeric light-emitting diodes with a blocking layer

Abstract

In this paper, we describe a simple method for fabrication of high efficiency multilayered polymeric light-emitting diodes (PLEDs). In the device configuration of ITO/HIL/BL/EML/BaF2:Ca:Al (ITO: indium tin oxide, BL: blocking layer, HIL: hole injection layer as PEDOT:PSS, EML: emitting layer as poly(alkylfluorene) derivative), poly(9-vinyl carbazole) (PVK) was selected as an BL because it has a hole transporting property and a relatively higher band gap, especially a lower LUMO level than the emitting polymer and higher HOMO level than PEDOT:PSS. As a result, the PVK layer could effectively block the electrons from metal cathode and confine them in the emitting polymer layer. In addition, it could properly control the amount of holes from ITO anode by partial blocking. The charge balance was achieved and the device with a PVK layer exhibits a higher brightness and luminance efficiency than those in devices without PVK. Also, the thickness of PVK was easily controlled by solvents for emitting polymer and the device with a thicker PVK layer shows better performance.