Improved Characteristics of Fluorene‐Type Polymer Light‐Emitting Devices Based on Multilayer Formation from Polymers and Solution‐Processable Wide‐Bandgap Inorganic Copper(I) Thiocyanate with p‐Type Conduction and High Refractive Index

Abstract

The functional solution‐processable multilayer formation is a significant factor in the improved characteristics of polymer light‐emitting devices. The improved characteristics of direct current and alternating current (AC)‐driven fluorene‐type polymer light‐emitting devices based on multilayer structures utilizing polymers and solution‐processable inorganic wide‐bandgap semiconductor, copper(I) thiocyanate, CuSCN, which exhibits p‐type conduction and high refractive index, are investigated. For the polymer light‐emitting diodes based on poly(9,9‐dioctylfluorene‐co‐benzothiadiazole), F8BT with CuSCN, the plateau of current efficiency indicates the carrier injection and transport to be balanced at lower current density and the balance to be maintained through the wide current range. CuSCN acts as the hole transport and electron‐blocking layer, although CuSCN acts as a fluorescence quencher for F8BT. The introduction of inorganic/organic hybrid dielectric mirrors, which consists of the alternating layers of inorganic CuSCN and insulating poly(vinylidene fluoride‐trifluoroethylene), P(VDF‐TrFE), into the AC voltage‐driven polymer electroluminescent device (ACEL) structure, is an effective way to achieve both the spectral narrowing and color tunability. Using high‐refractive‐index CuSCN, the color‐tunable emission for ACEL with distributed Bragg reflector mirrors and the electroluminescent (EL) emission from a weak microcavity effect are achieved.