Luminescent polymer blends in hybrid organic-inorganic polymer light-emitting devices (HyPLEDs)

Abstract

Organic/polymer based light-emitting diodes(OLEDs/PLEDs) have enormous technological significance for low-cost solution processing such as spin coating, dip coating and ink-jet printing. A recent promising approach toward air-stable devices without a low work function metal is to use inorganic metal oxides as charge injection and transport layer. In particular, solution processable n-type ZnO layer as electron injection/transport layer was mainly used in hybrid organic-inorganic polymer light-emitting diodes (HyPLEDs) and organic solar cells (OSCs). Based on HyPLEDs, PLED performance was optimized by blending luminescent polymers with various functional molecules or same fluorescent polymers. Blending approaches are generally considered to offer simpler device fabrication, higher device efficiency and performance. Here, we report a luminescent polymer blend system by mixing fluorescent polymer with ionic salt for hybrid organic-inorganic polymer light-emitting electrochemical cells (HyPLECs), and fluorescent red, green, and blue (RGB) polymers for white PLEDs (WPLEDs). Further development was achieved by attaching cholesteric liquid crystal (CLC) reflector in front of the surface of WPLEDs to obtain white circularly polarized (CP) electroluminescence (EL). This thesis is organized as follow. An introduction of semiconducting polymers, characteristics of OLEDs and hybrid PLEDs with diverse transition metal oxide (TMO) layer are mainly described in the chapter 1. In particular, hybrid organic-inorganic polymer light emitting diode (HyPLEDs) are new type of top-emission device in which device can combine with n-type thin-film transistor (TFTs) for application to active matrix structure. The ZnO layer as a cathode material in the inverted configuration takes a role of the electron-injecting layer as they combine properties such as transparency, low resistance, and air-stability. In the chapter 2, we demonstrate enhanced device performance by using a blend of emissive polymer (“Super Yellow”) and mobile ionic liquid molecules (ILMs) in hybrid organic-inorganic polymeric light-emitting electrochemical cells (HyPLECs) with high air stability. The mobile anions and cations redistributed near each electrode/active layer interface make ohmic contacts, thereby enhancing current density and electroluminescence efficiency at relatively low operating voltage. Moreover, a luminescent blend of blue-emitting polymer (“M-blue”), orange-emitting dye (DCM), and ILMs was investigated to achieve white emission in HyPLECs. By using ILMs, we can observe the characteristics of LECs with low operating voltage and air stability of HyPLECs by introducing ZnO layer. Finally, we investigate RGB ternary blend in single active layer for white emission. WPLEDs using polymer blends showed low turn-on voltage, high brightness, efficiency, and color stability. Furthermore, we observed CP-EL by combination of WPLED and cholesteric liquid crystal (CLC) reflector.